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.

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.

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.

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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.

(Modified with the permission of Simon & Schuster from Correlative Anatomy of the Nervous System by E. Crosby, T. Humphrey, E. Lauer. Copyright © 1962 Macmillan Publishing Company.)

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

Dorsal horn

The dorsal horn is a major zone of termination of primary afferent fibres, which enter the spinal cord through the dorsal roots of spinal nerves. Dorsal root fibres contain numerous molecules, which are either known, or suspected, to fulfil a neurotransmitter or neuromodulator role. These include glutamic acid, substance P, calcitonin gene-related peptide (CGRP), bombesin, vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), somatostatin, dynorphin and angiotensin II. Dorsal root afferents carry exteroceptive, proprioceptive and interoceptive information. Laminae I–IV are the main cutaneous receptive areas; lamina V receives fine afferents from the skin, muscle and viscera; lamina VI receives proprioceptive and some cutaneous afferents. Most, if not all, primary afferent fibres divide into ascending and descending branches on entering the cord. These then travel for variable distances in the tract of Lissauer, near the surface of the cord, and send collaterals into the subjacent grey matter. The formation, topography and division of dorsal spinal roots have all been confirmed in man.

The lamina marginalis is a thin lamina of neurones at the dorsolateral tip of the dorsal horn, deep to the tract of Lissauer. Beneath it lies the substantia gelatinosa (laminae II and III), which is present at all levels, and consists mostly of small neurones, together with some larger neurones. The substantia gelatinosa receives afferents via the dorsal roots, and its neurones give rise to fibres that form the contralateral spinothalamic tract. The large propriospinal neurones of the nucleus proprius lie ventral to the substantia gelatinosa; they link segments for the mediation of intraspinal coordination (Fig. 18.3).

Clarke’s column lies at the base of the dorsal horn. At most levels, it is near the dorsal white funiculus and may project into it. In the human spinal cord, it can usually be identified from the eighth cervical to the third or fourth lumbar segments. Neurones of Clarke’s column vary in size, but most are large, especially in the lower thoracic and lumbar segments. Some send axons into the dorsal spinocerebellar tracts and others are interneurones.

Ventral horn

Neurones in the ventral horn vary in size. The largest cell bodies, which may exceed 25 μm in diameter, are those of α motor neurones, the axons of which emerge in ventral roots to innervate extrafusal fibres in striated skeletal muscles. Large numbers of smaller neurones, 15–25 μm in diameter, are also present. Some of these are γ motor neurones, which innervate intrafusal fibres of muscle spindles, and the rest are interneurones. Motor neurones utilize acetylcholine as their neurotransmitter.

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).

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).

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).

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.

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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.

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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.

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.