Spinal cord: internal organization

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CHAPTER 18 Spinal cord: internal organization

The spinal cord provides innervation for the trunk and limbs via spinal nerves and their peripheral ramifications. It receives primary afferent fibres from peripheral receptors located in widespread somatic and visceral structures, and sends motor axons to skeletal muscle. It also contains the cell bodies of all the preganglionic neurones responsible for the sympathetic innervation of cardiac and smooth muscle and secretory glands, and for the parasympathetic innervation of smooth muscle in the distal part of the hind gut, the pelvic viscera and the erectile tissues of the external genitalia. Many bodily functions are regulated at an unconscious level by intraspinal reflex connections between afferent and efferent neurones. Profuse ascending and descending pathways link the spinal cord with the brain, allowing higher centres to monitor and perceive external and internal stimuli and to modulate and control spinal efferent activity.

EXTERNAL FEATURES AND RELATIONS

The topographical anatomy of the spinal cord, its external features and relations are described in more detail in Chapter 43. In brief, the cord lies within the vertebral canal. It is continuous rostrally with the medulla oblongata, just below the level of the foramen magnum (Fig. 28.11) and it terminates caudally as the conus medullaris, which is continuous with the filum terminale and is anchored to the dorsum of the coccyx. The cord is ensheathed by spinal meninges which are continuous with the cranial meninges through the foramen magnum. Although it is approximately circular in cross-section, the diameter of the spinal cord varies according to level; it bears two enlargements, cervical and lumbar.

The spinal cord is essentially a segmental structure, giving rise to 31 bilaterally-paired spinal nerves. These attach to the cord as linear series of smaller dorsal and ventral nerve rootlets. Dorsal rootlets contain afferent nerve fibres and ventral rootlets contain efferent fibres (see Fig. 15.4). Groups of adjacent rootlets coalesce to form dorsal or ventral nerve roots that cross the subarachnoid space and unite to form functionally mixed spinal nerves as they pass through the intervertebral foramina. The dorsal roots bear dorsal root ganglia that contain the cell bodies of primary afferent neurones.

INTERNAL ORGANIZATION

In transverse section, the spinal cord is incompletely divided into symmetrical halves by a dorsal (posterior) median septum and a ventral (anterior) median sulcus (Fig. 18.1). It consists of an outer layer of white matter and an inner core of grey matter; their relative sizes and configuration vary according to level. The amount of grey matter reflects the number of neurones present; it is proportionately largest in the cervical and lumbar enlargements, which contain the neurones that innervate the limbs. The absolute amount of white matter is greatest at cervical levels, and decreases progressively at lower levels, because descending tracts shed fibres as they descend and ascending tracts accumulate fibres as they ascend.

Fig. 18.1 Transverse sections through the spinal cord at representative levels. Approximately ×5. (Figure enhanced by B Crossman.)

A diminutive central canal, lined by columnar, ciliated epithelium (ependyma) and containing cerebrospinal fluid (CSF), extends the whole length of the spinal cord lying in the centre of the spinal grey matter. Rostrally, the central canal extends into the caudal half of the medulla oblongata and then opens into the fourth ventricle.

SPINAL GREY MATTER

In three dimensions, the spinal grey matter is shaped like a fluted column (Fig. 43.1F). In transverse section the column is often described as being ‘butterfly-shaped’ or resembling the letter ‘H’ (Fig. 18.1). It consists of four linked cellular masses, the right and left dorsal and ventral horns, that project dorsolaterally and ventrolaterally towards the surface respectively. The grey matter that immediately surrounds the central canal and unites the two sides constitutes the dorsal and ventral grey commissures. The dorsal horn is the site of termination of the primary afferent fibres that enter the cord via the dorsal roots of spinal nerves. The tip of the dorsal horn is separated from the dorsolateral surface of the cord by a thin fasciculus or tract (of Lissauer) in which primary afferent fibres ascend and descend for a short distance before terminating in the subjacent grey matter. The ventral horn contains efferent neurones whose axons leave the spinal cord in ventral nerve roots. A small intermediate, or lateral, horn is present at thoracic and upper lumbar levels; it contains the cell bodies of preganglionic sympathetic neurones.

Spinal grey matter (Fig. 18.2) is a complex mixture of neuronal cell bodies, their processes and synaptic connections, neuroglia and blood vessels. Neurones in the grey matter are multipolar. They vary in size and features such as the length and the arrangement of their axons and dendrites. Neurones may be intrasegmental, i.e. contained within a single segment, or intersegmental, i.e. their ramifications spread through several segments.

Fig. 18.2 Transverse section of spinal cord at a midlumbar level. The larger motor neurones in the ventral grey column are visibly grouped. Stained with cresyl fast violet.

Neuronal cell groups of the spinal cord

Viewed from the perspective of its longitudinal columnar organization, the grey matter of the spinal cord consists of a series of discontinuous cell groupings associated with their corresponding segmentally arranged spinal nerves. At any particular cross-sectional level these cell groupings are often considered to correspond approximately with one or more of ten cell layers, known as Rexed’s laminae. These laminae are defined on the basis of neuronal size, shape, cytological features and density and are numbered in a dorsoventral sequence.

Laminae I–IV correspond to the dorsal part of the dorsal horn, and are the main site of termination of cutaneous primary afferent terminals and their collaterals. Many complex polysynaptic reflex paths (ipsilateral, contralateral, intrasegmental and intersegmental) start from this region, as also do many long ascending tract fibres which pass to higher levels. Lamina I (lamina marginalis) is a very thin layer with an ill-defined boundary at the dorsolateral tip of the dorsal horn. It has a reticular appearance, reflecting its content of intermingling bundles of coarse and fine nerve fibres. It contains small, intermediate and large neuronal somata, many of which are fusiform in shape. The much larger lamina II consists of densely packed small neurones, responsible for its dark appearance in Nissl-stained sections. With myelin stains, lamina II is characteristically distinguished from adjacent laminae by the almost total lack of myelinated fibres. Lamina II corresponds to the substantia gelatinosa. Lamina III consists of somata which are mostly larger, more variable and less closely packed than those in lamina II. It also contains many myelinated fibres. Some workers consider that the substantia gelatinosa contains part or all of lamina III as well as lamina II. The ill-defined nucleus proprius of the dorsal horn corresponds to some of the cell constituents of laminae III and IV. Lamina IV is a thick, loosely packed, heterogeneous zone permeated by fibres. Its neuronal somata vary considerably in size and shape, from small and round, through intermediate and triangular, to very large and stellate.

Laminae V and VI lie at the base of the dorsal horn. They receive most of the terminals of proprioceptive primary afferents, profuse corticospinal projections from the motor and sensory cortex and input from subcortical levels, suggesting their involvement in the regulation of movement. Lamina V is a thick layer, divisible into a lateral third and medial two-thirds. Both have a mixed cell population but the former contains many prominent well-staining somata interlaced by numerous bundles of transverse, dorsoventral and longitudinal fibres. Lamina VI is most prominent in the limb enlargements. It has a densely staining medial third of small, densely packed neurones and a lateral two-thirds containing larger, more loosely packed, triangular or stellate somata.

Laminae VII–IX show a variety of forms in the limb enlargements. Lamina VII includes much of the intermediate (lateral) horn. It contains prominent neurones of Clarke’s column (nucleus dorsalis, nucleus thoracis, thoracic nucleus) and intermediomedial and intermediolateral cell groupings (Fig. 18.3). The lateral part of lamina VII has extensive ascending and descending connections with the midbrain and cerebellum (via the spinocerebellar, spinotectal, spinoreticular, tectospinal, reticulospinal and rubrospinal tracts) and is thus involved in the regulation of posture and movement. Its medial part has numerous propriospinal reflex connections with the adjacent grey matter and segments concerned both with movement and autonomic functions. Lamina VIII spans the base of the thoracic ventral horn but is restricted to its medial aspect in limb enlargements. Its neurones display a heterogeneous mixture of sizes and shapes from small to moderately large. Lamina VIII is a mass of propriospinal interneurones. It receives terminals from the adjacent laminae, many commissural fibres from the contralateral lamina VIII, and descending connections from the interstitiospinal, reticulospinal and vestibulospinal tracts and the medial longitudinal fasciculus. The axons from these interneurones influence α motor neurone activity bilaterally, perhaps directly but more probably by excitation of small γ motor neurones supplying efferent fibres to muscle spindles. Lamina IX is a complex array of cells consisting of α and γ motor neurones and many interneurones. The large α motor neurones supply motor end-plates of extrafusal muscle fibres in striated muscle. Recording techniques have demonstrated tonic and phasic α motor neurones. The former have a lower rate of firing and lower conduction velocity and tend to innervate type S muscle units. The latter have higher conduction velocity and tend to supply fast twitch (type FR, FF) muscle units. The smaller γ motor neurones give rise to small-diameter efferent axons (fusimotor fibres), which innervate the intrafusal muscle fibres in muscle spindles. There are several functionally distinct types of γ motor neurone. The ‘static’ and ‘dynamic’ responses of muscle spindles have separate controls mediated by static and dynamic fusimotor fibres, which are distributed variously to nuclear chain and nuclear bag fibres.

Fig. 18.3 The groups of nerve cells in the grey columns of the spinal cord. The relative positions of these columnar groups and their extent through spinal segments are indicated.

Lamina X surrounds the central canal and consists of the dorsal and ventral grey commissures.

Considered longitudinally, ventral horn neurones are arranged in elongated groups, and form a number of separate columns, which extend through several segments. These are seen most easily in transverse sections. The ventral horn may be divided into medial, central and lateral cell columns, which all exhibit subdivision at certain levels, usually into dorsal and ventral parts (Fig. 18.3). The medial group extends throughout the cord, but may be absent in the fifth lumbar and first sacral segments. In the thoracic and the upper four lumbar segments, it is subdivided into ventromedial and dorsomedial groups. In segments cranial and caudal to this region, the medial group has only a ventromedial moiety, except in the first cervical segment, where only the dorsomedial group exists.

The central group of cells is the least extensive, and is found only in some cervical and lumbosacral segments. The centrally situated phrenic nucleus, containing the motor neurones that innervate the diaphragm, lies in the third to seventh cervical segments. An irregular accessory group of neurones in the upper five or six cervical segments at the ventral border of the ventral horn give rise to axons that are thought to enter the spinal accessory nerve (Fig. 18.3).

The lateral group of cells in the ventral horn is subdivided into ventral, dorsal and retrodorsal groups, largely confined to the spinal segments which innervate the limbs. The nucleus of Onuf, which is thought to innervate the perineal striated muscles, is a ventrolateral group of cells in the first and second sacral segments.

The motor neurones of the ventral horn are somatotopically organized. The basic arrangement is that medial cell groups innervate the axial musculature, and lateral cell groups innervate the limbs. The basic building block of the somatic motor neuronal populations is represented by a longitudinally disposed group of neurones, which innervate a given muscle, and in which the α and γ motor neurones are intermixed. The various groups innervating different muscles are aggregated into two major longitudinal columns, medial and lateral. In transverse section these form the medial and lateral cell groups in the ventral horn (Fig. 18.4).

Fig. 18.4 The approximate location of motor cell groups at C8 segmental level of the spinal cord.

The medial longitudinal motor column extends throughout the length of the spinal cord. Its neurones innervate epaxial and hypaxial muscle groups. Basically, epaxial muscles include the erector spinae group (which extend the head and vertebral column), while hypaxial muscles include prevertebral muscles of the neck, intercostal and anterior abdominal wall muscles (which flex the neck and the trunk). The epaxial muscles are innervated by branches of the dorsal primary rami of the spinal nerves, and the hypaxial muscles by branches of the ventral primary rami. In the medial column, motor neurones supplying epaxial muscles are sited ventral to those supplying hypaxial muscles.

The lateral longitudinal motor column is found only in the enlargements of the spinal cord. The motor neurones in this column in the cervical and lumbar enlargements innervate muscles of the upper and lower limbs, respectively. In the cervical enlargement, motor neurones which supply muscles intrinsic to the upper limb are situated dorsally in the ventral grey column, and those innervating the most distal (hand) muscles are sited further dorsally. Motor neurones of the girdle muscles lie in the ventrolateral part of the ventral horn. There is a further somatotopic organization in that the proximal muscles of the limb are supplied from motor cell groups located more rostrally in the enlargement than those supplying the distal muscles. For example, motor neurones innervating intrinsic muscles of the hand are sited in segments C8 and T1, while motor neurones of shoulder muscles are in segments C5 and 6. A similar overall arrangement of motor neurones innervating lower limb muscles applies in the lumbosacral cord (Fig. 18.5).

Fig. 18.5 The segmental arrangement of motor neurones innervating muscles of the lower limb.

The main afferent connections to motor neurones are: direct monosynaptic connections from proprioceptive dorsal root afferents in the same or nearby segments; connections from axonal collaterals of dorsal horn and other interneurones; direct monosynaptic connections from the vestibulospinal and corticospinal tracts.

Spinal reflexes

The intrinsic connections of the spinal cord and brain stem subserve a number of reflexes by which the functions of peripheral structures are modulated in response to afferent information in a relatively automatic or autonomous fashion. The fundamental components of such reflex ‘arcs’ are, thus, an afferent and an efferent neurone. However, in all but the simplest of reflexes, interneurones intervene between the afferent and efferent components, conferring increased versatility and complexity on reflex responses. Reflexes, by their very nature, are relatively fixed and stereotyped in form. Nevertheless, they are strongly influenced and modulated by descending connections. In the case of spinal reflexes these descending controls come from both the brain stem and the cerebral cortex. Pathology of descending supraspinal pathways commonly causes abnormalities of spinal reflex activity, which are routinely tested for in neurological examination. During development, descending control mechanisms suppress what may be regarded as ‘primitive’ spinal reflex responses, such as the extensor plantar reflex and the grasp reflex. When the higher control mechanisms become damaged, these reflexes are released and reappear as a sign of CNS pathology (e.g. the Babinski reflex).

Stretch reflex

The stretch reflex is the mechanism by which stretch applied to a muscle elicits its reflex contraction. It is essential for the maintenance of both muscle tone and an upright stance (via the innervation of the postural muscles of the neck, back and lower limbs). Anatomically it is the simplest of reflexes, since it is mediated solely by an afferent and an efferent neurone. The afferent component arises from stretch receptors associated with intrafusal muscle fibres located within muscle spindles. The primary or annulospiral endings of these receptive cells give rise to primary afferent fibres which enter the spinal cord, where they make excitatory synaptic contact directly onto α motor neurones innervating the same muscle (Fig. 18.6). The α motor neurones of antagonistic muscles are simultaneously inhibited via collateral connections to inhibitory interneurones.

Fig. 18.6 The stretch reflex.

(By permission from Crossman AR, Neary D 2000 Neuroanatomy, 2nd edn. Edinburgh: Churchill Livingstone.)

Gamma reflex

As well as α motor neurones innervating extrafusal muscle fibres, muscles also receive γ motor neurones, which innervate intrafusal muscle fibres. Activation of γ motor neurones increases the sensitivity of the intrafusal fibres to stretch (Fig. 18.7). Therefore, changes in γ activity have a profound effect upon the stretch reflex and upon muscle tone. Like α motor neurones, γ motor neurones are under the influence of descending pathways from the brain stem and cerebral cortex. Changes in the activity of the stretch reflex and of muscle tone are commonly found in disorders of the CNS as well as the PNS.

Fig. 18.7 The gamma reflex.

(By permission from Crossman AR, Neary D 2000 Neuroanatomy, 2nd edn. Edinburgh: Churchill Livingstone.)

Flexor reflex

Painful stimulation of the limbs leads to flexion withdrawal, that is mediated by a polysynaptic reflex (Fig. 18.8) in which interneurones link the afferent and efferent neurones. Thus, activation of nociceptive primary afferents indirectly causes activation of limb flexor motor neurones. Collateralization of fibres to nearby spinal segments mediates flexion of a limb at several joints, depending on the intensity of the stimulus. Decussating connections to the contralateral side of the cord activate α motor neurones innervating corresponding extensor muscles, which produces the so-called crossed extensor reflex. In principle, virtually any cutaneous stimulus has the potential to induce a flexor reflex, but, other than in the case of noxious stimuli, this response is normally inhibited by descending pathways. When descending influences are lost, even harmless cutaneous stimulation can elicit flexion of the limbs. The Babinski (extensor plantar) reflex, which is generally regarded as pathognomonic of damage to the corticospinal tract, is part of a flexion withdrawal of the lower limb in response to stimulation of the sole of the foot.

Fig. 18.8 The flexor reflex and crossed extensor reflex.

(By permission from Crossman AR, Neary D 2000 Neuroanatomy, 2nd edn. Edinburgh: Churchill Livingstone.)

SPINAL WHITE MATTER

The spinal white matter surrounds the central core of grey matter. The white matter consists primarily of longitudinally running nerve fibres. Fibres subserving related functions, or those with common origins or destinations, are generally grouped together anatomically to form tracts (fasciculi), which may be ascending, descending and propriospinal. Ascending tracts consist either of primary afferent fibres, that enter the cord via the dorsal roots of spinal nerves, or fibres derived from intrinsic spinal neurones, that carry afferent impulses to supraspinal levels. Descending tracts contain fibres that descend from the cerebral cortex or brain stem nuclei to control the activity of spinal neurones. Propriospinal tracts contain the axons of neurones that are localized entirely to the spinal cord: they contain both ascending and descending components, collectively mediating intersegmental coordination.

The spinal white matter is conventionally described as being arranged into three large, bilaterally paired masses, the dorsal, lateral and ventral funiculi, each of which contains a number of predominantly specific tracts (see Fig. 18.1 and Fig. 18.9). Narrow dorsal and ventral white commissures run between the two halves of the cord.

Fig. 18.9 The approximate positions of nerve fibre tracts in the spinal cord at mid-cervical (A) and lumbar (B) levels.

Fibres in the spinal white matter vary in calibre. Many are small and lightly, or non-, myelinated. Most regions contain a wide spectrum of fibre diameters, from 1 μm or less to 10 μm. Some tracts typically contain only small fibres, e.g. the dorsolateral tract, fasciculus gracilis and central part of the lateral funiculus. The fasciculus cuneatus, anterior funiculus and peripheral zone of the lateral funiculus all contain many large-diameter fibres.

Whilst the ascending and descending tracts are to a large extent discrete and regularly located, significant overlap between adjacent tracts does occur. Their general disposition is shown in Fig. 18.9.

Ascending pathways

Dorsal columns

The dorsal funiculus consists of two large ascending tracts, the fasciculus gracilis and fasciculus cuneatus (Fig. 18.10), that are also known as the dorsal columns. They are separated by a posterointermediate septum. The dorsal columns contain a high proportion of myelinated fibres carrying proprioceptive (position sense and kinaesthesia), exteroceptive (touch-pressure) and vibratory sensation to higher levels. These fibres come from several sources: long primary afferent fibres which enter the cord in the dorsal roots of spinal nerves and ascend to the dorsal column nuclei in the medulla oblongata; shorter primary afferent fibres projecting to neurones of Clarke’s column and other spinal neurones; axons from secondary neurones of the spinal cord ascending to the dorsal column nuclei. The dorsal columns also contain axons of propriospinal neurones.

Fig. 18.10 The dorsal columns. Primary afferent fibres from different levels and their associated second- and third-order neurones are depicted in different colours.

The fasciculus gracilis begins at the caudal end of the spinal cord. It contains long ascending branches of primary afferents, which enter the cord through ipsilateral dorsal spinal roots and ascending axons of secondary neurones in laminae IV to VI of the ipsilateral dorsal horn. As the fibres ascend, they are joined by axons of successive dorsal roots. Fibres entering in coccygeal and lower sacral regions are shifted medially by successive additions of fibres entering at higher levels.

The fasciculus gracilis lies medial to the fasciculus cuneatus in the upper spinal cord (Fig. 18.9). At upper cervical levels the fasciculus gracilis contains a larger proportion of afferents from cutaneous receptors than from deep proprioceptors because many of the latter leave the fasciculus at lower segments to synapse in Clarke’s column. Indeed, proprioception from the lower limb mostly reaches the thalamus by relaying in Clarke’s column and then again in nucleus Z (p. 280). Axons of the fasciculus gracilis, from both primary and secondary neurones, terminate in the nucleus gracilis of the dorsal medulla.

The fasciculus cuneatus (Fig. 18.9) begins at midthoracic level and lies lateral to the fasciculus gracilis. It is composed mostly of primary afferent fibres of the upper thoracic and cervical dorsal roots. At upper cervical levels it contains a large population of afferents from both deep and cutaneous receptors of the upper limb. In addition, some of its axons arise from secondary neurones in laminae IV–VI of the ipsilateral dorsal horn. Many axons (both primary and secondary) that ascend in the fasciculus cuneatus terminate in the nucleus cuneatus of the dorsal medulla. Some also end in the lateral (external or accessory) cuneate nucleus; neurones in this nucleus project to the cerebellum via the cuneocerebellar pathway.

Many ascending fibres of the fasciculus gracilis and fasciculus cuneatus terminate by synapsing on neurones of the dorsal column nuclei (nucleus gracilis and nucleus cuneatus, respectively) in the medulla oblongata. (The connections of the dorsal column nuclei are described further with the medulla oblongata, p. 280.) Axons arising from neurones in the dorsal column nuclei arch ventromedially round the central grey matter of the medulla as internal arcuate fibres (Fig. 19.5) and decussate in the great sensory decussation (decussation of the medial lemniscus) to form the medial lemniscus itself. They ascend to the ventral posterolateral nucleus of the thalamus, from where neurones project to the somatosensory cortex in the postcentral gyrus of the parietal lobe (areas 3, 1 and 2). Some neurones of the dorsal column nuclei form posterior external arcuate fibres which enter the cerebellum.

The high degree of somatotopic organization that is present in the dorsal columns is preserved as the pathways ascend through the dorsal column nuclei and thalamus to reach the primary somatosensory cortex. In the dorsal column nuclei, the lower limb is represented in the nucleus gracilis, the upper limb in the nucleus cuneatus, and the trunk is represented in an intermediate position between them. Fibres are also segregated by modality in the dorsal columns: fibres from hair receptors are most superficial, while those from tactile and vibratory receptors lie in deeper layers.

Spinocerebellar tracts

There are two principal spinocerebellar tracts: dorsal (posterior) and ventral (anterior). They occupy the periphery of the lateral aspect of the spinal white matter (Fig. 18.9, Fig. 18.11) and carry proprioceptive and cutaneous information to the cerebellum for the coordination of movement. Both tracts contain large-diameter myelinated fibres, but there are more in the dorsal tract. Finer-calibre fibres are associated with the ventral tract.

Fig. 18.11 The spinocerebellar tracts.

(Redrawn from Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore: Williams and Wilkins, by permission of author and publisher.)

The dorsal spinocerebellar tract lies lateral to the lateral corticospinal tract. It begins about the level of the second or third lumbar segment and enlarges as it ascends. Axons of the tract originate ipsilaterally from the larger neurones of Clarke’s column, in lamina VII throughout spinal segments T1–L2. Clarke’s column receives input from collaterals of long ascending primary afferents of the dorsal columns and terminals of shorter ascending primary afferents of the dorsal columns. Many of these afferent fibres ascend from segments caudal to L2. In the medulla, the dorsal spinocerebellar tract passes through the inferior cerebellar peduncle to terminate ipsilaterally in the rostral and caudal parts of the cerebellar vermis.

The ventral spinocerebellar tract lies immediately ventral to the dorsal tract. The cells of origin are in laminae V–VII of the lumbosacral cord and the tract carries information from the lower limb. Most of the axons forming the tract decussate, but some remain ipsilateral. The tract begins in the upper lumbar region and ascends through the medulla oblongata to reach the upper pontine level, from where it descends in the dorsal part of the superior cerebellar peduncle to terminate, mainly contralaterally, in the anterior cerebellar vermis.

The spinocerebellar tracts are organized such that fibres from lower spinal segments are most superficial. Both tracts convey proprioceptive and exteroceptive information, but they are functionally different. Neurones of Clarke’s column are excited monosynaptically by Ia and Ib primary afferent fibres (from muscle spindles and tendon organs, respectively) and also by group II muscle afferents, and cutaneous touch and pressure afferents. The proprioceptive impulses often arise from a single muscle or from synergistic muscles acting at a common joint. Thus, the dorsal spinocerebellar tract transmits modality-specific and space-specific information that is used in the fine coordination of individual limb muscles. On the other hand, the cells of the ventral tract are activated monosynaptically by Ib afferents and transmit information from large receptive fields that include different segments of a limb. The ventral tract lacks subdivisions for different modalities and transmits information for the coordinated movement and posture of the entire lower limb.

Since Clarke’s column diminishes rostrally (Fig. 18.3) and does not extend above the lowest cervical segment, it follows that the dorsal spinocerebellar tract carries information from the trunk and lower limb. Proprioceptive and exteroceptive information from the upper limb travel in primary afferent fibres of the fasciculus cuneatus. These fibres end somatotopically in the accessory (external or lateral) cuneate nucleus and the adjoining part of the cuneate nucleus situated in the medulla oblongata. Cells of these nuclei give rise to the posterior external arcuate fibres that form the cuneocerebellar tract (Fig. 18.11), which enters the cerebellum via the ipsilateral inferior cerebellar peduncle. The accessory cuneate nucleus and the lateral part of the cuneate nucleus are considered to be homologous to the cells of Clarke’s column. The cuneocerebellar tract is, therefore, functionally allied to the dorsal spinocerebellar tract, and is its upper limb equivalent.

Axons of all the spinocerebellar tracts and the cuneocerebellar tract form part of the ‘mossy-fibre system’. They end in the cerebellar cortex in a highly organized, somatotopical and functional pattern (p. 302).

Spinothalamic tracts

The spinothalamic tracts (Fig. 18.9) consist of second-order neurones which convey pain, temperature, coarse (non-discriminative) touch and pressure information to the somatosensory region of the thalamus. The cells of origin lie in various laminae of all segments of the spinal cord. Fibres decussate in the ventral white commissure to reach the contralateral spinothalamic tracts: pain and temperature fibres do so promptly, within about one segment of their origin, whilst fibres carrying other modalities may ascend for several segments before crossing. Spinothalamic fibres mostly ascend in the white matter ventrolateral to the ventral horn, partly intermingled with ascending spinoreticular fibres and descending reticulospinal fibres. Some authorities describe two spinothalamic tracts (lateral and ventral) with more-or-less distinct anatomical locations and functions. However, physiological studies in animals support the notion that these tracts may be best considered as a structural and functional continuum.

The lateral spinothalamic tract (Fig. 18.12) is sited in the lateral funiculus, lying medial to the ventral spinocerebellar tract. Clinical evidence indicates that it subserves pain and temperature sensations. The ventral spinothalamic tract (Fig. 18.13) lies in the anterior funiculus medial to the point of exit of the ventral spinal nerve roots and dorsal to the vestibulospinal tract, which it overlaps. On the basis of clinical evidence, it subserves coarse tactile and pressure modalities.

Fig. 18.12 The lateral spinothalamic tract.

(Redrawn from Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore: Williams and Wilkins, by permission of author and publisher.)

Fig. 18.13 The ventral (anterior) spinothalamic tract.

(Redrawn from Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore: Williams and Wilkins, by permission of author and publisher.)

A dorsolateral spinothalamic tract has been described in animals. The axons arise mainly from neurones in lamina I, and cross to ascend in the contralateral dorsolateral funiculus. These neurones respond maximally to noxious, mechanical and thermal cutaneous stimuli. Examples of clinical pain relief following dorsolateral cordotomy suggest that a similar projection exists in man.

On reaching the lower brain stem, spinothalamic tract axons separate. Axons in the ventral tract join the medial lemniscus. Axons in the lateral tract continue as the spinal lemniscus.

There is clear somatotopic organization of the fibres in the spinothalamic tracts throughout their extent. Fibres crossing at any cord level join the deep aspect of those that have already crossed, which means that both tracts are segmentally laminated (Fig. 18.14). Somatotopy is maintained throughout the medulla oblongata and pons. In the midbrain, fibres in the spinal lemniscus conveying pain and temperature sensation from the lower limb extend dorsally, while those from the trunk and upper limb are more ventrally placed. Both lemnisci ascend to end in the thalamus. The major spinothalamic projections in man are to the ventral posterolateral nucleus, and also to the centrolateral intralaminar nucleus.

Fig. 18.14 General plan of the segmental organization of fibres in the dorsal funiculus, the lateral corticospinal tract and the spinothalamic tracts. The probable cross-sectional areas of these tracts are schematically enlarged. This general plan applies to all segmentally organized tracts whether ascending, descending, ipsilateral or contralateral.

Neurones of the spinothalamic tracts

The specific localization of spinothalamic tract cell bodies is poorly documented in man. In animals, about a third are localized to the upper three cervical segments. About 20% are located in lower cervical segments, 20% in the thoracic region (mostly in segments T1–3), 20% in the lumbar region and 10% in the sacrococcygeal cord. Cells are located in laminae I and IV–VIII, the greatest concentration being in laminae VI and VII. Cell bodies giving rise to spinothalamic tract axons are predominantly contralateral, although a relatively small number (10%), mostly in the upper three cervical segments, are ipsilateral.

Neurones of the spinothalamic tracts have very different receptive fields. Specificity of separate channels, as it exists in the dorsal column nuclei, is absent in the laminae of the cord. Convergence of different functional types of afferent fibres onto an individual tract cell is a common feature in the cord. On the basis of laminar site, functional properties, and specific thalamic termination of their axons, spinothalamic tract neurones may be divided into three separate groups. These are the apical cells of the dorsal grey column (lamina I), deep dorsal column cells (laminae IV–VI), and cells in the ventral grey column (laminae VII, VIII). There are species differences and the description below is derived from studies in non-human primates.

Lamina I cells which project to the thalamus show the following characteristics. In essence they respond maximally to noxious or thermal cutaneous stimulation, and consist mainly of high-threshold, but also some wide-dynamic-range, units. Their receptive fields are usually small, representing a part of a digit or a small area of skin involving several digits. Lamina I spinothalamic tract neurones receive input from Aδ and C fibres, and some respond to convergent input from deep somatic and visceral receptors. Spinothalamic tract cells in the thoracic cord display marked viscerosomatic convergence. Lamina I spinothalamic tract neurones project preferentially to the ventral posterolateral nucleus of the thalamus, with limited projections to the centrolateral and mediodorsal thalamic nuclei. The population of deep dorsal column (laminae IV–VI) spinothalamic neurones of the lumbar cord contains wide-dynamic-range (60%), high-threshold (30%), and low-threshold (10%) type units. They can code accurately both innocuous and noxious cutaneous stimuli. Some cells also respond to input from deep somatic and visceral receptors. In the lumbar cord their receptive fields are small or medium sized; they are larger than the area of the foot, but smaller than the entire leg. In the thoracic cord the fields of these laminar cells are larger, and often include the entire upper limb plus part of the chest. Many of the deep dorsal grey column spinothalamic tract neurones in the thoracic segments receive convergent input from sympathetic afferent fibres. Laminae IV–VI spinothalamic tract units project either to the ventral posterolateral nucleus or to the centrolateral nucleus of the thalamus, and sometimes to both. Units projecting to the ventral posterolateral nucleus receive input from all classes (Aβ, A δ and C) of cutaneous fibres.

Ventral grey column (laminae VII and VIII) spinothalamic tract cells respond mainly to deep somatic (muscle and joint) stimuli, but also to innocuous and/or noxious cutaneous stimuli. In the thoracic regions of the spinal cord they also receive convergent input from visceral sources. The majority of laminae VII and VIII spinothalamic tract neurones have large, complex receptive fields (often bilateral), which encompass widespread areas of the body. Cells of this group, which project exclusively to the medial thalamus, receive input from Aβ, Aδ and C classes of afferent fibres, and many respond to convergent input from receptors of deep structures. This population of neurones contains wide-dynamic-range (25%), high-threshold (63%), and low-threshold (12%) units. Most of the spinothalamic tract cells in the ventral grey column project to the intralaminar nuclei of the thalamus. Wide-dynamic-range type neurones are particularly effective for discriminating between different intensities of painful stimulation.

Pain mechanisms

The ascending connections through which sensory information reaches higher centres are not simple relays, since it is known that they are subject to modulation by intraspinal connections and by descending pathways from the brain stem and cerebral cortex. This is particularly important in relation to the spinothalamic and spinoreticular pathways and the perception of pain.

Presynaptic inhibition influences many, possibly all, primary afferent terminals. A much-investigated site of presynaptic effects is the substantia gelatinosa. It has been proposed that impulses from cutaneous (and other) afferents are here subject to tonic control by presynaptic modulation of primary afferent terminals, mediated by small neurones of the substantia gelatinosa.

The ‘gate control theory’ (Melzack & Wall 1965) proposed a mechanism for the modulation of inflow of information along nociceptive and other afferent pathways (Fig. 18.15). The hypothesis was that large-diameter afferents (e.g. from hairs and touch corpuscles) are excitatory to the large neurones of lamina IV, from which spinothalamic fibres arise, and to interneurones in the substantia gelatinosa. In contrast, fine non-myelinated afferents are excitatory to tract cells but inhibitory to the interneurones. The axons of substantia gelatinosa interneurones are presumed to inhibit presynaptically the terminals of all afferents that synapse with tract cells. In such a system, low activity in the fine afferents inhibits the interneurones, and so prevents them inhibiting tract cells. Hence the ‘gate’ to cells in lamina IV is opened to transmit intermittent, small volleys of impulses from the large fibres. A prolonged high-frequency volley of impulses in the large-diameter afferents would be transmitted to lamina IV tract cells initially, but this would soon cease as activity in the interneurones closed the gate. Conversely, a persistent high activity in the fine afferents would open the gate resulting in massive bombardment of neurones of lamina IV (which include some neurones of high threshold that are only activated by such bombardment). It was assumed that onward transmission in the lateral spinothalamic tract would evoke the perception of pain at supraspinal levels. Pain, therefore, would result from an imbalance between the varieties of afferent impulses when there was a disproportionately large traffic along the fine afferents. In fact, inhibition of spinothalamic tract neurones can be produced by electrical stimulation of peripheral nerves, the most effective being volleys which activate Aδ fibres.

Fig. 18.15 The basic arrangement of the sensory ‘gate’ mechanism in the dorsal laminae of the grey matter of the spinal cord.

The activity of spinothalamic tract neurones may also be selectively modulated by pathways descending from the brain to the spinal cord. Many studies have shown that the response of spinothalamic tract cells to noxious stimuli is inhibited by stimulation of certain brain regions. This is obviously of considerable clinical interest in the treatment of chronic, intractable, pain.

In the brain stem, the regions inducing such effects correspond to a number of midbrain and rhombencephalic nuclei which, with their connections, constitute an endogenous analgesic system. In the midbrain, these regions are the periaqueductal grey matter, dorsal raphe nucleus and part of the cuneiform nucleus. Neurones in these sites contain serotonin (5-HT), γ-aminobutyric acid (GABA), substance P, CCK, neurotensin, enkephalin and dynorphin. The periaqueductal grey matter receives afferents from the frontal somatosensory and cingulate neocortex, the amygdala, numerous local reticular nuclei and the hypothalamus. Afferents from the latter are separate bundles, which carry histamine, luteinizing hormone-releasing hormone (LHRH), vasopressin, oxytocin, adrenocorticotrophic hormone (ACTH), melanocyte-stimulating hormone (γ-MSH), endorphin, and angiotensin II. Some fibres descend from the periaqueductal grey matter to rhombencephalic centres, others pass directly to the spinal cord.

In the rhombencephalon, the nucleus raphe magnus and the medial reticular column constitute an important multineuromediator centre. Neurones in these sites contain serotonin, substance P, CCK, thyrotrophin-releasing hormone (TRH), enkephalin and dynorphin: some neurones contain two or even three neuromediators. Descending bulbospinal fibres pass to the nucleus of the spinal tract of the trigeminal nerve and its continuation, the substantia gelatinosa. The latter extends throughout the length of the cord and contains populations of neurones that express many different neuromediators, e.g. GABA, substance P, neurotensin, enkephalin and dynorphin. There is abundant physiological and pharmacological evidence that all of these regions are intimately concerned with the control of nociceptive (and probably other modality) inputs.

Stimulation of forebrain sites including the periventricular grey matter, the ventral posterolateral nucleus of the thalamus, and the primary sensory (SI) and posterior parietal cortices, inhibit spinothalamic tract cells. In contrast, some spinothalamic tract cells are excited by stimulation of the medullary reticular formation, and the primary motor cortex (the latter effect probably being mediated by the corticospinal tract).

The perception of pain is further described in Chapter 23.

Spinoreticular pathway

Spinoreticular fibres are intermingled with those of the spinothalamic tracts, and ascend in the ventrolateral quadrant of the spinal cord (Fig. 18.16). Evidence from animal studies suggests that cells of origin occur at all levels of the spinal cord, particularly in the upper cervical segments. Most neurones are in lamina VII, some are in lamina VIII, and others are in the dorsal horn, especially lamina V. Most axons in the lumbar and cervical enlargements cross the midline, but there is a large uncrossed component in cervical regions. Most axons are myelinated. The pattern of anterograde degeneration, in both human postmortem studies and in experimental animals following anterolateral cordotomy, indicates the existence of spinoreticular projections to many nuclei of the medial pontomedullary reticular formation. There is also a projection to the lateral reticular nucleus (a precerebellar relay nucleus). These projections do not appear to be somatotopically organized. Spinoreticular neurones respond to inputs from the skin or deep tissues. Innocuous cutaneous stimuli may inhibit or excite a particular cell, whereas noxious stimuli are often excitatory. A spino-reticulo-thalamo-cortical pathway has been proposed as an important route serving pain perception. Like other ascending pathways, the tract cells are influenced by descending control. For example, electrical stimulation of the periaqueductal grey matter inhibits the responses of certain spinoreticular cells to input from cardiopulmonary afferents. Stimulation of the reticular formation also alters the activity of spinoreticular neurones.

Fig. 18.16 Reticular tracts.

(Redrawn from Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore: Williams and Wilkins, by permission of author and publisher.)

Spinocervicothalamic pathway

The lateral cervical nucleus is small in man. It lies in the lateral funiculus, ventrolateral to the dorsal horn in the upper two cervical segments. In some human cord specimens the nucleus is not distinctly defined, and is possibly incorporated into the dorsal horn. It receives axons from the spinocervical tract, which ascends in the dorsolateral funiculus. The tract cells are found in laminae III–V at all levels of the spinal cord, ipsilateral to the nucleus. Most neurones of the nucleus project to the contralateral thalamus via the medial lemniscus, and some project to the contralateral midbrain. Specific thalamic targets include the ventral posterolateral nucleus and part of the posterior complex. Spinocervical tract neurones respond to hair movement, pressure, pinch and thermal stimuli and to high-threshold muscle input; many also respond to noxious stimuli. Like tract cells of other ascending pathways, they are under tonic descending inhibitory control.

Spinomesencephalic pathway

The spinomesencephalic pathway consists of a number of tracts ascending from the spinal cord to various regions of the midbrain. It includes the spinotectal tract projecting to the superior colliculus, neurones synapsing in the periaqueductal grey matter, and other spinal cord projections which terminate in the parabrachial nucleus, the pretectal nuclei and the nucleus of Darkschewitsch. Cells of origin are located throughout the length of the spinal cord, particularly in the cervical segments and the lumbosacral enlargement, mostly in lamina I, but they are also present in laminae IV–VIII, where they are concentrated in lamina V. Most are contralateral, but a prominent ipsilateral group is also found at upper cervical levels. Spinomesencephalic fibres are mostly myelinated and ascend in the white matter of the ventrolateral quadrant of the spinal cord, in association with the spinothalamic and spinoreticular tracts.

Spinomesencephalic neurones are of low-threshold, wide-dynamic-range, or high-threshold classes. Their receptive fields may be small, or very complex and encompass large surface areas of the body. Many spinomesencephalic cells are nociceptive and are likely to be involved in the motivational–affective component of pain. Electrical stimulation of their site of termination in the periaqueductal grey matter results in severe pain in man. Furthermore, the cells of the deeper layers of the superior colliculus, where spinotectal fibres synapse, are activated by noxious stimuli.

Spino-olivary tract

The spino-olivary tract is described in animals as arising from neurones in the deeper laminae of grey matter. Axons forming the tract cross and then ascend superficially at the junction of the anterior and lateral white funiculi, to end in the dorsal and medial accessory olivary nuclei. The tract carries information from muscle and tendon proprioceptors, and also from cutaneous receptors. A functionally similar route, the dorsal spino-olivary tract, ascends in the dorsal white funiculi, and relays in the dorsal column nuclei to the contralateral inferior olivary nucleus. Information on these tracts in primates is scant, but postmortem evidence following cordotomies in man has revealed degenerating axonal terminals in the inferior olivary nucleus.

Descending tracts

Descending pathways to the spinal cord originate primarily from the cerebral cortex and from numerous sites within the brain stem (Fig. 18.16, Fig. 18.17, Fig. 18.18). They are concerned with the control of movement, muscle tone and posture, the modulation of spinal reflex mechanisms and the transmission of afferent information to higher levels. They also mediate control over spinal autonomic neurones.

Fig. 18.17 The corticospinal tracts.

(Redrawn from Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore: Williams and Wilkins, by permission of author and publisher.)

Fig. 18.18 A simplified scheme of some of the major descending pathways of the spinal cord including their overlapping zones of termination in the grey matter. Within the grey matter the dotted lines show the laminar pattern, while within the white matter they are an approximate guide to the topography of the tracts.

Corticospinal and corticobulbar tracts

Corticospinal and corticobulbar fibres arise from neurones in the cerebral cortex. They project, in a somatotopically organized fashion, to neurones that are mostly located in the contralateral side of the spinal cord or brain stem respectively (Fig. 18.17). The majority of corticospinal and corticobulbar fibres arise from cells situated in the primary motor cortex (area 4) and the premotor cortex (area 6). A small contribution comes from cells in the postcentral gyrus (somatosensory cortex; areas 3, 1, and 2) and the adjacent parietal cortex (area 5). In the monkey, 30% of corticospinal fibres arise from area 4, 30% from area 6, and 40% from the parietal regions. Cells of origin of corticospinal and corticobulbar fibres vary in size in the different cortical areas and are clustered into groups or strips. The largest cells (giant pyramidal neurones, or Betz cells) are located in the primary motor cortex of the precentral gyrus.

Corticospinal and corticobulbar fibres descend through the subcortical white matter to enter the genu and posterior limb of the internal capsule. They then pass through the ventral part of the midbrain in the crus cerebri. As they continue caudally through the pons they are separated from its ventral surface, and fragmented into fascicles, by transversely running pontocerebellar fibres. Corticobulbar fibres leave to terminate in association with the cranial nerve motor nuclei of the midbrain, pons and medulla. In the medulla oblongata, the residual corticospinal fibres form a discrete bundle, the pyramid (Fig. 19.2) which forms a prominent longitudinal column on the ventral surface of the medulla. The corticospinal tract is, therefore, also referred to as the pyramidal tract. Each pyramid contains about a million axons of varying diameter. The majority (70%) are myelinated: most (90%) have a diameter of 1–4 μm; 9% have diameters of 5–10 μm; and less than 2% have diameters of 11–22 μm. The largest diameter axons arise from the giant Betz cells.

Just rostral to the level of the spinomedullary junction, approximately 75–90% of the corticospinal fibres in the pyramid cross the median plane in the pyramidal decussation (decussation of the pyramids) and continue caudally as the lateral corticospinal tract. The rest of the fibres continue uncrossed as the ventral corticospinal tract. The lateral tract also contains some uncrossed corticospinal fibres. The lateral corticospinal tract (Fig. 18.17) descends in the lateral funiculus throughout most of the length of the spinal cord. It occupies an oval area, ventrolateral to the dorsal horn and medial to the dorsal spinocerebellar tract (Fig. 18.9). In the lumbar and sacral regions, where the dorsal spinocerebellar tract is absent, the lateral corticospinal tract reaches the dorsolateral surface of the cord. As it descends, the tract diminishes in size as its fibres terminate in progressively lower spinal segments until about the fourth sacral segment: its axons terminate on ipsilateral spinal neurones.

The smaller ventral corticospinal tract (Fig. 18.17) descends in the ventral funiculus. It lies close to the ventral median fissure, and is separated from it by the sulcomarginal fasciculus (Fig. 18.9). The ventral corticospinal tract diminishes as it descends and usually disappears completely at midthoracic cord levels. It may either be absent or, very rarely, contain almost all the corticospinal fibres. Near their termination, most fibres of the tract cross the median plane in the ventral white commissure to synapse on contralateral neurones. The vast majority of corticospinal fibres, irrespective of the tract in which they descend, therefore terminate in the spinal cord on the side contralateral to their cortical origin.

Knowledge of the detailed termination of corticospinal fibres is based largely upon animal studies, but is supplemented by data from postmortem studies on human brains using anterograde degeneration methods. Most corticospinal fibres are believed to terminate contralaterally on interneurones in the lateral parts of laminae IV–VI and both lateral and medial parts of lamina VII. Some are also distributed to lamina VIII bilaterally. Terminals are also associated with contralateral motor neuronal cell groups in lamina IX, in the dorsolateral group and the lateral parts of both central and ventrolateral groups (Fig. 18.18).

Corticospinal fibres from the frontal cortex, including motor and premotor areas 4 and 6, terminate mostly on interneurones in laminae V–VIII, with the densest concentration ending in the lateral part of lamina VI. They influence α and γ motor neurones of lamina IX, via these interneurones. Because the widespread dendrites of multipolar neurones in lamina IX penetrate lamina VII, direct monosynaptic axodendritic contacts also occur on large α motor neurones. Direct termination on motor neurones is most abundant in the spinal enlargements.

Experimental evidence shows that precentral corticospinal axons influence the activities of both α and γ motor neurones, facilitating flexor muscles and inhibiting extensors, which are the opposite effects to those mediated by lateral vestibulospinal fibres. Evidence from animal studies shows that direct projections from the precentral cortical areas to spinal motor neurones are concerned with highly fractionated, precision movements of the limbs. Accordingly, in primates, precentral corticospinal fibres are mainly distributed to motor neurones supplying the distal limb muscles. Corticospinal projections may use glutamate or aspartate, often co-localized, as excitatory neurotransmitters.

Corticospinal fibres from parietal sources end mainly in the contralateral dorsal horn, in the lateral parts of laminae IV–VI and lamina VII. Phylogenetically these fibres represent the oldest part of the corticospinal system. Axons from the sensory cortex terminate chiefly in laminae IV and V. They are concerned with the supraspinal modulation of the transmission of afferent impulses to higher centres, including the motor cortex.

Experimental studies in primates indicate that isolated transection of corticospinal fibres at the level of the pyramid (pyramidotomy) results in flaccid paralysis or paresis of the contralateral limbs and loss of independent hand and finger movements. Destruction of corticospinal fibres at the level of the internal capsule, commonly caused by a cerebral vascular accident or ‘stroke’, results in a contralateral hemiplegia. The paralysis is initially flaccid, but later becomes spastic, and is most marked in the distal muscles of the extremities, especially those concerned with individual movements of the fingers and hand. Associated signs on the paralysed side are: hyperactive deep tendon reflexes; hypertonicity; the loss of superficial abdominal and cremasteric reflexes; and the appearance of dorsiflexion of the toes (Babinski’s sign) in response to stroking the sole of the foot. The latter is usually interpreted as pathognomonic of corticospinal damage, but it is not always present in patients with confirmed corticospinal lesions. Moreover, Babinski’s sign is normally present in human infants up to about 2 years of age – its subsequent disappearance may reflect the completion of myelination of the corticospinal fibres and/or the establishment of direct cortical connections to lower motor neurones.

Some of the sequelae of stroke damage in the internal capsule, in particular hyperreflexia and hypertonia, are due to the involvement of other pathways in addition to the corticospinal tract. These include descending cortical fibres to brain stem nuclei, such as the vestibular and reticular nuclei, that themselves give rise to descending projections which influence motor neurone activity.

Rubrospinal tract

The rubrospinal tract arises from neurones in the caudal magnocellular part of the red nucleus (an ovoid mass of cells situated centrally in the midbrain tegmentum (p. 290)). This part of the nucleus contains some 150–200 large neurones, interspersed with smaller neurones.

The origin, localization, termination and functions of rubrospinal connections are poorly defined in man, and the tract appears to be rudimentary. Rubrospinal fibres cross in the ventral tegmental decussation and descend in the lateral funiculus of the cord, where they lie ventral to, and intermingled with, fibres of the lateral corticospinal tract (Fig. 18.9). In animals, the tract descends as far as lumbosacral levels, whereas in man it appears to project only to the upper three cervical cord segments. Rubrospinal fibres are distributed to the lateral parts of laminae V–VI and the dorsal part of lamina VII of the spinal grey matter. The terminal zones of the tract correspond to those of corticospinal fibres from the motor cortex. Animal studies demonstrate that the effects of rubrospinal fibres on α and γ motor neurones are similar to those of corticospinal fibres.

Tectospinal tract

The tectospinal tract arises from neurones in the intermediate and deep layers of the superior colliculus of the midbrain. It crosses ventral to the periaqueductal grey matter in the dorsal tegmental decussation and descends in the medial part of the ventral funiculus of the spinal cord (Fig. 18.9). Fibres of the tract project only to the upper cervical cord segments, ending in laminae VI–VIII. They make polysynaptic connections with motor neurones serving muscles in the neck, facilitating those that innervate contralateral muscles and inhibiting those that innervate ipsilateral ones. In animals, unilateral electrical stimulation of the superior colliculus causes turning of the head to the contralateral side, an effect mainly mediated through the tectospinal tract.

Vestibulospinal tracts

The vestibular nuclear complex lies in the lateral part of the floor of the fourth ventricle, at the level of the pontomedullary junction. It gives rise to the lateral and medial vestibulospinal tracts, which are functionally and topographically distinct (Fig. 18.19).

Fig. 18.19 The vestibulospinal tracts.

(Redrawn from Carpenter MB 1991 Core Text of Neuroanatomy, 4th edn. Baltimore: Williams and Wilkins, by permission of author and publisher.)

The lateral vestibulospinal tract arises from small and large neurones of the lateral vestibular nucleus (Deiters’ nucleus). It descends ipsilaterally, initially in the periphery of the ventrolateral spinal white matter, but subsequently migrating into the medial part of the ventral funiculus at lower spinal levels. Fibres of this tract are somatotopically organized. Thus, fibres projecting to the cervical, thoracic and lumbosacral segments of the cord arise from neurones in the rostroventral, central and dorsocaudal parts, respectively, of the lateral vestibular nucleus. Lateral vestibulospinal fibres end ipsilaterally, mostly in the medial part of the ventral horn in lamina VIII and the medial part of lamina VII. Axons of the lateral vestibulospinal tract are excitatory to motor neurones of extensor muscles of the neck, back and limbs, through both mono- and polysynaptic connections; γ motor neurones are also probably facilitated. Lateral vestibulospinal tract axons also inhibit motor neurones of flexor limb muscles, via inhibitory interneurones.

The medial vestibulospinal tract (Fig. 18.19) arises mainly from neurones in the medial vestibular nucleus, but some are also located in the inferior and lateral vestibular nuclei. The medial vestibulospinal tract descends in the medial longitudinal fasciculus into the ventral funiculus of the spinal cord where it lies close to the midline in the so-called sulcomarginal fasciculus (Fig. 18.9). Unlike the lateral tract it contains both crossed and uncrossed fibres, and does not extend beyond the midthoracic cord level. Fibres of the medial tract project mainly to the cervical cord segments, ending in lamina VIII and the adjacent dorsal part of lamina VII. Data from stimulation of the vestibular nuclei in animals indicate that axons of the medial tract monosynaptically inhibit the motor neurones that innervate axial muscles of the neck and upper part of the back.

Reticulospinal tracts

The reticulospinal tracts pass from the brain stem reticular formation to the spinal cord. Detailed knowledge of their origins and connections has been obtained mainly from studies in animals.

The medial reticulospinal tract (Fig. 18.16) originates from the medial tegmental fields of the pons and medulla. The main sources are the oral and caudal pontine reticular nuclei and the gigantocellular reticular nucleus in the medulla. Pontine fibres descend, mainly ipsilaterally, in the ventral funiculus of the cord. Medullary fibres descend, both ipsilaterally and contralaterally, in the ventral funiculus and the ventral part of the lateral funiculus. These fibres have many collaterals, and two-thirds of the reticulospinal neurones that reach the cervical cord also descend to lumbosacral levels. The terminals of reticulospinal fibres are distributed to lamina VIII, and the central and medial parts of lamina VII. The medullary reticulospinal terminals are more widely distributed, ending additionally in the lateral parts of laminae VI and VII and also directly on motor neurones. Terminations of reticulospinal fibres that originate in the medulla are, in general, more dorsally placed than those that originate in the pons, although there is considerable overlap.

Both α and γ motor neurones are influenced by reticulospinal fibres, through polysynaptic and monosynaptic connections. Physiological evidence shows that reticulospinal fibres from pontine sources excite motor neurones of axial and limb muscles, while medullary fibres excite, or inhibit motor neurones of cervical muscles and excite motor neurones of axial muscles. Functionally, the medial reticulospinal tract is concerned with posture, the steering of head and trunk movements in response to external stimuli, and crude, stereotyped movements of the limbs.

The lateral reticulospinal tract lies in the lateral funiculus of the spinal cord, closely associated with the rubrospinal and lateral corticospinal tracts (Fig. 18.9). Its fibres arise from neurones of the ventrolateral tegmental field of the pons. The fibres cross in the rostral medulla oblongata and project, with a high degree of collateralization, throughout the length of the spinal cord. Axons of this tract terminate in laminae I, V and VI, and also bilaterally in the lateral cervical nucleus. Evidence suggests that this pathway is involved in the control of pain perception and in motor functions.

Interstitiospinal tract

The interstitiospinal tract arises from neurones in the interstitial nucleus (of Cajal) and the immediate surrounding area, and descends via the medial longitudinal fasciculus into the ventral funiculus of the spinal cord. Its fibres project, mainly ipsilaterally, as far as lumbosacral levels, and are mostly distributed to the dorsal part of lamina VIII and the dorsally adjoining part of lamina VII. They establish some monosynaptic connections with motor neurones supplying neck muscles, but their main connections are disynaptic with motor neurones supplying limb muscles.

Solitariospinal tract

The solitariospinal tract is a small group of mostly crossed fibres which arises from neurones in the ventrolateral part of the nucleus solitarius of the medulla. Descending in the ventral funiculus and ventral part of the lateral funiculus of the cord, these axons terminate on phrenic motor neurones supplying the diaphragm and thoracic motor neurones which innervate intercostal muscles. A pathway with somewhat similar course and terminations to that of the solitariospinal tract originates from the nucleus retroambiguus. Both pathways subserve respiratory activities by driving inspiratory muscles, and some descending axons from the nucleus retroambiguus facilitate expiratory motor neurones. There is clinical evidence that bilateral ventrolateral cordotomy at high cervical levels abolishes rhythmic ventilatory movements.

Hypothalamospinal fibres

Hypothalamospinal fibres exist in animals. They arise from the paraventricular nucleus and other areas of the hypothalamus and descend ipsilaterally, mainly in the dorsolateral region of the cord, to be distributed to sympathetic and parasympathetic preganglionic neurones in the intermediolateral column. Fibres from the paraventricular nucleus show oxytocin and vasopressin immunoreactivity. They are also distributed to laminae I and X. Descending fibres from the dopaminergic cell group (A11) situated in the caudal hypothalamus innervate sympathetic preganglionic neurones and neurones in the dorsal horn. That similar pathways exist in man may be inferred from ipsilateral sympathetic deficits (e.g. Horner’s syndrome), which follow lesions of the hypothalamus, the lateral tegmental brain stem, or the lateral funiculus of the cord.

Monoaminergic spinal pathways

Monoaminergic cell groups utilize dopamine, adrenaline (epinephrine), noradrenaline (norepinephrine) and 5-HT (5-hydroxytryptamine, serotonin) as neurotransmitters. They occur widely throughout the brain stem and in the hypothalamus. They project rostrally to many forebrain areas and caudally to the spinal cord and appear to be concerned with the modulation of sensory transmission, and the control of autonomic and somatic motor neuronal activities.

The projections to the spinal cord arise from several sources. Coeruleospinal projections originate from noradrenergic cell groups A4 and A6 in the locus coeruleus complex in the pons and descend via the ventrolateral white matter to innervate all cord segments bilaterally. They end in the dorsal grey matter (laminae IV–VI) and the intermediate and ventral horns. They also project extensively to preganglionic parasympathetic neurones in the sacral cord. Descending noradrenergic fibres, which arise from the lateral tegmental cell groups A5 and A7 of the pons, travel in the dorsolateral white matter. They are distributed to laminae I–III, and particularly to the intermediate grey horn. Descending fibres from adrenergic cell groups C1 and C3 of the medulla oblongata have been traced into the anterior funiculus of the cord and are extensively distributed to the intermediolateral column. Dopaminergic fibres projecting to the spinal cord travel in the hypothalamospinal pathway.

The raphe nuclei pallidus (B1), obscurus (B2) and magnus (B3) in the brain stem give rise to two serotoninergic descending bundles. The lateral raphe spinal bundle, from B3 neurones, is concerned with the control of nociception. It descends close to the lateral corticospinal tract and ends in the dorsal horn (laminae I, II and V). The ventral bundle, composed mainly of axons from B1 neurones, travels in the medial part of the ventral white column and ends in the ventral horn (laminae VIII and IX). It facilitates extensor and flexor motor neurones. Some descending serotoninergic fibres project to sympathetic preganglionic neurones and are concerned with the central control of cardiovascular function.

Propriospinal pathways

Propriospinal pathways (fasciculi proprii) consist of the ascending and descending fibres of intrinsic spinal neurones. They contact other neurones within the same segment and/or in more distant segments of the spinal cord and so subserve intrasegmental and intersegmental integration and coordination. The majority of spinal neurones are propriospinal neurones, most of which lie in laminae V–VIII. Propriospinal fibres are mainly concentrated around the margins of the grey matter (Fig. 18.9), but are also dispersed diffusely in the white funiculi.

The propriospinal system plays important roles in spinal functions. Descending pathways end on specific subgroups of propriospinal neurones and these, in turn, relay to motor neurones and other spinal neurones. The system mediates all those automatic functions which continue after transection of the spinal cord, e.g. sudomotor and vasomotor activities, bowel and bladder functions.

Some propriospinal axons are very short, and span only one segment, while others run the entire length of the cord. The shortest axons lie immediately adjacent to the grey matter, and the longer ones are situated more peripherally. Propriospinal neurones can be categorized according to the length of their axons as long, intermediate, or short neurones. Long propriospinal neurones distribute their axons throughout the length of the cord, mainly via the ventral and lateral funiculi; their cell bodies are in lamina VIII and the dorsally adjoining part of lamina VII. Axons from the long propriospinal neurones of the cervical cord descend bilaterally, whereas those from the corresponding lumbosacral neurones ascend mainly contralaterally. Most of the fibres are fine (less than 3 μm in diameter). Some are the first spinal tract axons to become myelinated. Intermediate propriospinal neurones occupy the central and medial parts of lamina VII and project mainly ipsilaterally. Short propriospinal neurones are found in the lateral parts of laminae V–VIII and their axons run ipsilaterally in the lateral funiculus.

Propriospinal fibres in the different parts of the white funiculi are distributed preferentially to specific regions of the spinal grey matter. In the spinal enlargements, the propriospinal fibres in the dorsolateral funiculus project to the dorsal and lateral parts of the intermediate zone, and also to spinal motor neurones which supply distal limb muscles, especially those of the hand and the foot. The propriospinal fibres in the ventral part of the ventrolateral funiculus are distributed to the central and medial parts of lamina VII and to motor neurones of proximal limb and girdle muscles. Other propriospinal fibres run in the medial part of the ventral funiculus and travel mainly to the ventromedial part of the intermediate zone, which characteristically contains long propriospinal neurones, and to motor neurones innervating axial and girdle muscles.

The tract of Lissauer

The tract of Lissauer, or the dorsolateral tract, lies between the apex of the dorsal horn and the surface of the spinal cord, where it surrounds the incoming dorsal root fibres. It is present throughout the spinal cord, and is most developed in the upper cervical regions.

The tract consists of fine myelinated and non-myelinated axons. Many are the branches of axons in the lateral bundles of the dorsal roots. These axons bifurcate into ascending and descending branches as they enter the cord. The branches travel in the tract of Lissauer for one or two segments and give off collaterals, which end on and around neurones in the dorsal horn. The tract also contains propriospinal fibres, some of which are short axons of small substantia gelatinosa neurones, which re-enter the dorsal horn.

SPINAL CORD LESIONS

Mechanical compression and secondary ischaemic damage to underlying nervous tissue cause surgically relevant spinal cord disease (myelopathy). The site and the level of damage to the cord determine the particular clinical syndrome, e.g. whether the lesion involves the upper or lower cervical, thoracic or lumbosacral spinal cord. At each of these levels, symptoms and signs are determined by direct destruction of segmental tissue, i.e. transversely distributed damage, and disconnection of suprasegmental ascending and descending tracts above and below the level of a lesion, i.e. longitudinally distributed damage (Fig. 18.20). For example, a lower cervical spinal cord lesion damages the segmental sensory and motor contributions to the nerve roots and brachial plexus causing sensory loss, weakness and wasting of the muscles and loss of tendon reflexes in the upper limbs. Disruption of the ascending sensory pathways in the lateral and dorsal columns of the cervical spinal cord leads to loss of sensation to pain and temperature (lateral spinothalamic tracts) and touch and proprioception (dorsal fasciculi) below the ‘sensory level’ corresponding to the segment of the spinal cord.

Fig. 18.20 Lesions of the spinal cord.

(By permission from Crossman AR, Neary D 2000 Neuroanatomy, 2nd edn. Edinburgh: Churchill Livingstone.)

Damage to the descending corticospinal tracts in the lateral columns of the spinal cord produces a spastic paraparesis, i.e. increased tone of the muscles, weakness of movements of flexion, exaggerated tendon reflexes and abnormal superficial reflexes, e.g. extensor plantar responses and absent abdominal reflexes. Descending pathways to the bladder are interrupted, and this produces a ‘neurogenic bladder’.

The same principles apply to lesions at other levels of the spinal cord and they are illustrated in diagrammatic form in Fig. 18.20.

The precise clinical syndrome is determined by anatomical site alone and not by pathology. However, it is of practical use to classify lesions on the basis of their anatomical relationship to the spinal cord and meninges, i.e. whether they are extradural, intradural or intramedullary (see table). This anatomic classification provides a guide to the diagnostic probabilities as well as an aid to neuroradiological interpretation prior to neurosurgical intervention. For example, neurofibromas are common in the cervical spinal canal, meningiomas in the thoracic spinal canal, and ependymomas in the lumbosacral spinal canal. Degenerative disease of the vertebral column is common in the cervical and lumbosacral vertebrae, but rare in the thoracic vertebrae. Discrete anterior and central intramedullary lesions, e.g. due to syringomyelia and angiomas respectively, preferentially destroy the spinothalamic pathways in the anterolateral columns and central areas of the spinal cord. This leads to a characteristic ‘dissociated’ sensory loss, i.e. loss of pain and temperature sensation, but with preservation of touch sensation and proprioception at and below the level of the lesion.

Classification of spinal cord lesions

Extradural lesions

A. Disorder of the vertebral column

B. Abscess

C. Haematoma

Degenerative osteoarthritis (osteophytes and prolapsed intervertebral discs)

Tumour (metastatic carcinoma, myeloma, lymphoma, chordoma, sarcoma)

(Complication of bleeding, diathesis or anticoagulation)

Intradural lesions Meningioma, neurofibroma, lipoma, angioma Intramedullary lesions Syringomyelia (Arnold–Chiari malformation), angioma, glioma, ependymoma, epidermoid tumour

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Melzack R, Wall PD. Pain mechanisms: a new theory. Science. 1965;150:971-979.

Rexed B. The cytoarchitectonic organization of the spinal cord in the cat. J Comp Neurol. 1952;96:415-495.

Schoenen J, Faull RLM. Spinal cord: cytoarchitectural, dendroarchitectural and myeloarchitectural organization. In: Paxinos G, editor. The Human Nervous System. San Diego, CA: Academic Press; 1990:19-53.