Centrally the butterfly-shaped grey matter of the cord is divided in the midline by the central canal. The grey matter passing dorsally to the central canal is referred to as the posterior grey commissure and the grey matter passing ventrally to the central canal is referred to as the anterior grey commissure. Arising from the area of the ventral lateral sulci are the ventral roots of the spinal cord, which just as they exit the vertebral foramina combine with the afferent axons of the dorsal root ganglion neurons as they enter the vertebral foramina to form the root of the spinal nerve. Entering the spinal cord at the dorsal lateral sulcus are the sensory dorsal roots, completing their journey to the cord from the dorsal root ganglion cells (Fig. 7.5). The areas of grey matter that give rise to or receive the afferent and efferent input resemble the shape of a horn and are thus termed the anterior and posterior horns. The anterior horn does not extend through the anterior funiculus and reach the surface of the cord. The posterior horn projects much more deeply into the dorsal funiculus and except for a small band of translucent neurons, the substantia gelatinosa, it would extend to the posterior surface of the cord. A small angular projection from the intermediate areas of the cord forms the lateral horn of grey matter that only occurs between the levels of the first thoracic to the second lumbar segment. This lateral outgrowth of grey matter houses many of the presynaptic neurons of the sympathetic nervous system.

Figure 7.5 The various structures and nerve fibre pathways in a cross-sectional view of the spinal cord.

The neurons of the grey matter form a complex intermingled array involving multiple synaptic connections with many of the axons crossing the midline via the anterior and posterior commissures. Some of the neurons are intrasegmental and their axons and dendrites remain within the same segment of the spinal cord as the neuron soma. Other neurons are intersegmental and their axons and dendrites spread over many segments both rostrally and caudally. In many parts of the neuraxis groups of neurons, usually with a related functional activity, cluster together into nuclei or, when large enough, ganglia. Several nuclei have been identified in the grey matter of the spinal cord. The most predominant neurons in the ventral grey areas are the large multipolar neurons whose axons emerge from the spinal cord to form the anterior horn, and contribute to the spinal nerves, to ultimately innervate the skeletal muscles of the body. These neurons are also referred to as alpha-efferents or alpha motor neurons. Also present in large numbers in the anterior horn are slightly smaller neurons whose axons supply the intrafusal fibres of the muscle spindle called gamma-efferents or gamma motor neurons (Fig. 7.6).

Figure 7.6 The various locations of grey matter nuclei of the spinal cord.

The neuron groupings in the posterior horns involve four main nuclei, two of which extend through the length of the cord and two that are present only at selective levels of the cord. The substantia gelatinosa of Rolando extends throughout the cord and composes the extreme tip of the dorsal horn. These neurons are involved with signal processing of afferent information from the dorsal root ganglion neurons and are thought to play an essential role in the initial processing of pain due to extensive connections with incoming axons destined to form the spinothalamic tracts (Fig. 7.6).

A second nuclear group that extends throughout the spinal cord is located ventral to the substantia gelatinosa and is referred to as the dorsal funicular group or the nucleus proprius (Fig. 7.6). Lying ventral to the nucleus proprius in the basal region of the dorsal horn and extending from the eighth cervical region of the cord to the fourth lumbar region of the cord is the nucleus dorsalis or Clark’s nucleus (Fig. 7.6). Finally, a small group of nuclei known as the visceral grey area, or nucleus centrobasalis, is present only in the lower cervical and lumbosacral segments of the cord.

The intermediate region of the grey matter is composed of relatively small neurons that function as the presynaptic sympathetic neurons of the autonomic nervous system. Two regions are usually identified, the intermediolateral group (IML), which houses the presynaptic sympathetic neurons and the intermediomedial group (IMM), where similar small neurons to the IML reside and probably act as multimodal integrators for the output IML neurons. Neurons from both the IML and IMM send axons via the white rami communicants to the paravertebral ganglia that extend from T1 to L2 vertebral levels.

Rexed’s laminae can be used to classify functional aspects of grey matter

In the 1950s and early 1960s an architectural scheme was developed to classify the structure of the spinal cord, based on the cytological features of the neurons in different regions of the grey substance. It consists of nine laminae (I–IX) that extend throughout the cord, roughly paralleling the dorsal and ventral columns of the grey substance, and a tenth region (lamina X) that surrounds the central canal (Rexed 1964) (Fig. 7.8).

Figure 7.8 The laminar divisions of the spinal grey matter as per Rexed.

Quick facts 7.4

Alar and basal plate development

Figure 7.7 During embryonic development the alar or roof plate of the spinal cord develops into the neurons that receive afferent information from the dorsal root ganglion cells. The basal or floor plate develops into the motor output neurons of the spinal cord.

A brief description of the functional characteristics of these laminae follows.

Laminae I–IV

These areas are considered the main receiving junctions for primary afferent information. This region is characterised by complex multisynaptic networks of both intra- and intersegmental neurons. Many of the pathways cross the midline of the cord and ascend or descend in contralateral tracts (Fig. 7.9).

Figure 7.9 Another view of the grey matter laminae which illustrates the groupings of neurons forming small nuclear units.

Laminae V and VI

Quick facts 7.5

Various rexed laminae can be referred to by more than one name

Laminae	Names
I	Lamina marginalis
	Layer of Waldeyer
II and III	Substantia gelatinosa
III and IV	Nucleus proprius
V	Deiter’s nucleus
VII	Clark’s column
	Thoracic nucleus
X	Substantia gelatinosa centralis

These areas receive proprioceptive primary afferent information as well as descending collateral input from axons of the corticospinal and reticulospinal tracts. This area is probably very involved in multimodal integration and regulation of movement (Fig. 7.9).

Lamina VII

The lateral aspect of this area receives extensive efferent and afferent connections from the cerebellum, spinocerebellar, spinotectal, spinoreticular, and rubrospinal tracts and is involved in the multimodal integration of posture and movement.

The medial aspect of this area contains numerous complex networks of connections between propriospinal neurons. This area is probably involved in the integration of the complex propriospinal reflex network of the spinal cord concerned with both movement and autonomic function (Fig. 7.9).

Lamina VIII

This area receives collateral projections from adjacent laminae, medial longitudinal fasciculus, and vestibulospinal and reticulospinal tracts and profuse projections from the contralateral lamina VIII region. Output of this area influences both ipsilateral and contralateral neuron pools through both direct projections to the alpha motor neurons and projections to the gamma motor neuron pools (Fig. 7.9).

Lamina IX

This area is populated with both alpha and gamma motor neurons as well as interneurons.

Lamina X

This area comprises the grey matter in close approximation to and surrounding the central canal. This area also consists of the dorsal and ventral commissures and the central gelatinous substance.

The spinal cord itself consists of columns of cells and axon fibre tracts that allow communication throughout the length of the spinal cord and with supraspinal levels of the neuraxis. It is convenient to describe the axon fibre tracts of the spinal cord with respect to the funiculus in which they are located.

Axon fibre tracts of the dorsal funiculus

The dorsal columns are composed of the medially located fasciculus gracilis and the more laterally located fasciculus cuneatus (Figs 7.10, 7.11, 7.12). These pathways transport information from receptors in the periphery about fine and discriminative touch, conscious proprioception, pressure, two-point discrimination, and vibration sense. The primary afferent axons enter the spinal cord grey matter through the dorsal horn and synapse on the neurons in laminae V and VI. The secondary afferents ascend in the ipsilateral dorsal columns. Information from the lower limb and trunk is carried in the gracile funiculus and information from the upper limb and hand by the cuneate funiculus and synapse ipsilaterally in the gracile and cuneate nuclei of the caudal medulla.

Figure 7.10 The projections of the dorsal columns to the neurons in the cuneate and gracile nuclei in the caudal medulla region of the neuraxis.

Figure 7.11 The fibre tracts in a cross-sectional view of the spinal cord.

Figure 7.12 The pathway of the dorsal column/medial lemniscal system through the various levels of the neuraxis.

Neurons then decussate in the caudal medulla as the internal arcuate fibres and ascend to the contralateral thalamus via the medial lemniscus of the brainstem. Some fibres contained in the cuneate fasciculus arising from proprioceptive afferents project to the cerebellum. These projection fibres form the external arcuate fibres and form the cuneocerebellar tract.

Axon fibre tracts of the lateral funiculus

The anterolateral system contains the fibre tracts of the spinothalamic tract and some of the fibres comprising the spinoreticular and spinomesencephalic tracts. These last two pathways provide the afferent limb for neuroendocrine and limbic responses to nociception (Figs 7.11 and 7.13).

Figure 7.13 The pathway of the anterior and lateral spinal thalamic tracts (anterolateral system) through the various levels of the neuraxis.

The spinothalamic pathway, from a clinical standpoint, carries pain and temperature sensation from the entire body, excluding trigeminal distributions, to the thalamus. Primary afferent fibres have cell bodies located in the dorsal root ganglion (DRG) and their central processes synapse in the dorsal horn laminae I, II, and V, predominately. Secondary afferents, which form the spinothalamic tract proper, decussate (cross the spinal cord) about 2–3 levels higher in the spinal cord and ascend in the anterolateral funiculus to the ventral posterior lateral (VPL) nucleus of the thalamus. Sensory modulation can occur in the brain, thalamus, or spinal cord, especially in lamina II, and is also influenced by visceral afferents in lamina V where convergence of afferent information can result in referred pain phenomena.

The spinocerebellar pathways include the ipsilateral dorsal and the contralateral ventral pathways. These pathways convey unconscious proprioception mainly from the joint receptors and muscle spindle fibres of the muscles and joints of the body and integrated data from multimodal neuron systems in the spinal grey matter to the cerebellum.

The ventral spinocerebellar pathway conveys information about the ongoing status of interneuronal pools in the spinal cord to the cerebellum. It therefore provides continuous monitoring of ascending and descending information concerning locomotion and posture. The neurons of this tract originate in laminae V–VII between L2 and S3. Their projection axons decussate to the other side so that they ascend in the contralateral anterolateral funiculus. These fibres then decussate again via the superior cerebellar peduncle to synapse on neurons in the anterior part of the ipsilateral cerebellum (see Fig. 7.11).

The dorsal spinocerebellar tract neurons originate medially to the IML column of the spinal cord between C8 and L2/3.

The primary afferent cell bodies are located in the DRG and their central processes synapse with the above-mentioned neurons near the entry level or after ascending for a short distance in the dorsal columns. Secondary afferents ascend in the ipsilateral dorsolateral funiculus, lateral to the corticospinal tracts, and enter the ipsilateral cerebellum via the inferior cerebellar peduncle. Via this pathway, the cerebellum is provided with ongoing information about joint and muscle activity in the trunk and limbs. The cuneocerebellar pathway carries the same type of information from the upper limb and cervical spine via the cuneate fasciculus of the dorsal columns (see Fig. 7.11).

The anterior and lateral corticospinal tracts are important spinal tracts in the control of volitional movement. The fibre tracts are composed of axons from many different areas of the cortex as well as about 50% of their axons from unidentified areas. These tracts contain about 50% of their axon projections from the large pyramidal neurons of the primary motor cortex and association motor cortex. The lateral corticospinal tract descends in the spinal cord anterolateral to the posterior horn of grey matter and medial to the posterior spinocerebellar tract (Figs 7.11 and 7.14). It contains a large number of motor axon projections from cortical areas 1–4 and 6 to the hands, arms, legs, and feet. Its defining role is to convey motor signals to the ventral horn cells (VHCs) at the lower aspect of the cervical and lumbosacral enlargements of the spinal cord, thus controlling distal limb movements and coordinating distal and proximal muscles to achieve specific trajectories in space. Each corticomotoneuronal cell can achieve these complex goals by synapsing on interneuronal cells that communicate with whole groups of VHCs.

Figure 7.14 The pathway of the anterior (ventral) and lateral corti-cospinal tracts through the various levels of the neuraxis.

Axons from the projection neurons in the cortex descend in the internal capsule of the cerebrum through the cerebral peduncle of the mesencephalon and continue through the ventral areas of the pons until they enter the pyramids of the medulla oblongata. As the fibres descend in the medulla, about 68% of the fibres cross to form the lateral corticospinal tracts in the lateral funiculus of the contralateral side of the spinal cord; about 30% of the fibres do not cross and form the anterior corticospinal tracts in the ventral funiculus of the ipsilateral side of the spinal cord. The remaining fibres continue as uncrossed fibres of the lateral corticospinal tracts (Fulton & Sheenan 1935).

The anterior corticospinal tract runs adjacent to the anterior median fissure and descends to about the middle of the thoracic region. It contains most of the uncrossed motor projection fibres from the cortex and is thought to modulate axial musculature involved in strut stabilisation and postural control.

Traditionally, the anterior and lateral corticospinal tracts were referred to as the pyramidal tracts. These tracts have the distinction among motor pathways of forming a continuous non-interrupted pathway from the cortex to the grey matter of the spinal cord. A number of other tracts also involved in motor control, including the rubrospinal, vestibulospinal, and other tracts that form intermediate synaptic connections in the brainstem, are referred to as the extrapyramidal tracts.

Most of the axons of both the anterior and lateral corticospinal tracts synapse on interneurons located in laminae IV to VII of the grey matter of the spinal cord (Nyberg-Hansen 1969). Most physiological evidence suggests that the majority of the corticospinal fibres of both tracts act to facilitate flexor groups of muscles and inhibit extensor groups of muscles, which is the opposite effect observed by projections of the vestibulospinal tracts. Injuries involving the corticospinal tracts affect the motor control of the peripheral muscles differently at different levels of the neuraxis. Injuries above the medulla decussation affect the contralateral peripheral muscles. Injuries below the decussation affect the peripheral muscles ipsilateral to the lesion. It must be remembered that not all of the corticospinal fibres cross the midline so that a lesion to a motor cortical area on one side of the corticospinal tract above the decussation will affect the motor control on both sides of the body to a certain extent. This contributes to the understanding of the ipsilateral pyramidal paresis observed when a decreased cortical function (hemisphericity) occurs.

Vestibulospinal tract

The vestibulospinal tracts, lateral and medial, descend in the ventral funiculus, and mediate reflexes (vestibulospinal) that enable an individual to maintain balance and posture despite the effect of gravity and changes in the centre of mass due to movement of the trunk, head, and limbs. The lateral segments, the lateral vestibulospinal tract, descend from the lateral vestibular nucleus uncrossed and exert modulatory effects on the ipsilateral anterior column neurons in the grey matter through the length of the cord. The medial segments of this pathway, the medial vestibulospinal tracts, arise from the medial and inferior vestibular nuclei and descend first in the medial longitudinal fasciculus before entering the vestibulospinal tracts of the cord. This pathway is both crossed and uncrossed and only projects to the cervical and thoracic levels of the cord and as such is probably only involved with upper limb and neck movements (Figs 7.11 and 7.15).

Figure 7.15 The pathway of the vestibulospinal tracts through the various levels of the neuraxis.

The vestibulospinal tracts transport afferent information from the vestibular apparatus of the inner ear and descending efferent information from the inferior and lateral vestibular nuclei. The pathway descends in the ipsilateral ventral funiculus of the spinal cord, dorsal to the tectospinal tract and immediately adjacent to the anterior median fissure. The axon projections of both pathways synapse predominately on alpha and gamma VHCs of laminae VII and VIII. The physiological evidence to date suggests that this pathway has a facilitory effect on extensor muscles and an inhibitory effect on flexor groups.

Tectospinal tract

The fibres of this tract originate from the deep layers of the contralateral superior colliculus in the dorsal midbrain (mesencephalon). The tectospinal tract crosses in the dorsal tegmental decussation of the midbrain, which is ventral to the oculomotor nucleus and the medial longitudinal fasciculus (MLF). It maintains a close relationship with the MLF until it reaches the level of the internal arcuate fibres at the decussation of the medial lemniscus. At this point it passes laterally so that it comes to lie in the ventral lateral white matter of the spinal cord. It then descends in the contralateral medial ventral funiculus of the spinal cord, synapsing on interneurons in laminae VI to VIII that communicate with the alpha and gamma motor neurons of the cervical spine (Szentagothai 1948) (Fig. 7.16).

Figure 7.16 The pathways of the mesencephalic, ponto, and medullary reticulospinal tracts (green) and the tectospinal tract (blue) through the various levels of the neuraxis.

The superior colliculus is a remnant of the optic lobe in primitive animals and is involved in visual reflexes in addition to integration of somatic (especially neck and head), auditory, and visual afferents for spatial orientation of incoming stimuli and associated reflex head and eye movements. It facilitates accurate head and eye movements in response to sound and light stimuli.

Rubrospinal tract

The rubrospinal pathway has been rumoured to be vestigial in humans because of the evolutionary advancement of the corticospinal pathways; however, its presence in primates probably indicates it will eventually be found in humans as well and an open mind needs to prevail. It originates mainly from the large (magnocellular) neurons of the red nucleus in the rostral mesencephalon, decussates slightly more caudally, and descends just ventrally to the corticospinal fibres in the dorsolateral funiculus of the spinal cord contralaterally (Fig. 7.17).

Figure 7.17 The pathway of the rubrospinal tract through the various levels of the neuraxis.

Neurons of the red nucleus share extensive interaction with the cerebellum and basal ganglia and partly mediate their control over spinal motor output. The red nucleus is composed of a magnocellular or large cell and parvicellular or small cell components. Magnocellular components are homologous to the large diameter neurons of the primary motor cortex, while the parvicellular components are homologous to the premotor and supplementary motor areas of the cerebral cortex. The latter component acts as a relay and modulatory centre for feedforward connections between the cerebellum and the cortex.

The rubrospinal tract acts much like the corticospinal tracts in that it affects enhancement flexor tone and inhibition of extensor tone, especially in the proximal limb muscles.

Quick facts 7.7

The history of motor function of the spinal nerve

The anterior spinal nerve roots contain only motor fibres and posterior roots only sensory fibres.

• Charles Bell’s work of 1811 contains the first reference to experimental work on the motor functions of the ventral spinal nerve without, however, establishing the sensory functions of the dorsal roots. In 1822 François Magendie definitively discovered that the anterior root is motor and that the dorsal root is sensory.

• Magendie announced that ‘section of the dorsal root abolishes sensation, section of ventral roots abolishes motor activity, and section of both roots abolishes both sensation and motor activity’.

• This discovery has been called ‘the most momentous single discovery in physiology after Harvey’. In the same volume of Journal de physiologie expérimentale et de pathologie, Magendie gave experimental proof of the Bell–Magendie Law.

• Magendie proved Bell’s Law by severing the anterior and posterior roots of spinal nerves in a litter of puppies. Stimulation of the posterior roots caused pain. Magendie sums it up: ‘Charles Bell had had, before me, but unknown to me, the idea of separately cutting the spinal roots; he likewise discovered that the anterior influences muscular contractility more than the posterior does. This is a question of priority in which I have, from the beginning, honored him. Now, as for having established that these roots have distinct properties, distinct functions, that the anterior ones control movement, and the posterior ones sensation, this discovery belongs to me’ (F. Magendie (1847) Comptes rendus hebdomadaires des séances de l’Académie des sciences, 24: 3).

Autonomic modulatory projection fibres from supraspinal centres to the preganglionic neurons of the autonomic nervous system are known to exist but have been very difficult to isolate as a solid tract of fibres, probably because they are composed of polysynaptic columns of neurons and interneurons that range throughout wide areas of the white matter. The best indications are that most of these projections are located in the lateral fasciculus, with a smaller number also located in the anterior funiculus. Some findings suggest that some of the pyramidal neurons in the frontal cortex that form the corticospinal tracts are actually autonomic modulatory neurons. Other projections are undoubtedly from various hypothalamic and reticular nuclei.

Local spinal cord reflex circuits are also important in volitional movement in that descending motor pathways converge on interneurons to allow complex movement patterns to occur – i.e. corticomotoneuronal cells of the brain alter the trajectory of a limb in space by activating these reflex circuits involving agonist, antagonist, synergist, and neighbouring joint muscle groups. Feedback and feedforward mechanisms are employed by the cerebellum to assist plastic changes in the brain and spinal cord. Some stereotypical reflexes mediated by the spinal cord are state- or phase-dependent. For example, activation of Golgi tendon organs (GTOs) in the soleus and gastrocnemius muscles will trigger a different set of interneurons in the spinal cord, depending on whether the individual is in a state of locomotion or is non-ambulatory, and whether the individual is in the swing or stance phase of gait (Fig. 7.18).

Figure 7.18 (A) The simple muscle spindle (tendon) reflex. The reflex hammer strikes the tendon and causes a stretch of the muscle spindle fibres. This fires the afferent nerve pathway, which then synapses on the ventral horn cell, causing a depolarisation of the ventral horn neuron, which in turn produces a contraction of the extrafugal muscles innervated by the ventral horn neurons. (B) The complex array of modulating inputs that the ventral horn neuron is exposed to at any given moment. Thus even the simple reflex arc that we are all familiar with is much more complex than first imagined.

This is analogous to the stumbling-correction reflex observed in cats. Sensory stimuli to the dorsum of the foot will activate a different set of interneurons, depending on whether the individual is in the stance or swing phase of gait. For example, during stance, the reflex would result in lower limb extension on that side, while during swing, the reflex would result in powerful flexion withdrawal response.

Flexor reflex afferent (FRA) responses result in a whole limb response of flexion and withdrawal. These FRA responses are stereotypical responses serving a protective function. An example includes the response to plantar stimulation of the foot, which results in the withdrawal of the entire lower limb.

When faced with motor or sensory signs and symptoms, it is important to consider the level of decussation in different pathways to assist in identifying the location of a lesion in the various planes of the neuraxis.

A unilateral spinal cord lesion may affect motor control, joint position sense, and discriminatory sensation ipsilaterally, while pain and temperature sensation may be affected contralaterally.

Laminar organisation in the spinal cord is not as complete or as accurate as previously thought. The traditional understanding of the laminar distribution of pathways in the white matter of the spinal cord was that the projections to and from the most distal areas of the body were more lateral in the spinal cord except in the dorsal columns where the reverse occurs. Some variability of these laminar patterns has been demonstrated; however, the general pattern in dorsal column, spinothalamic, and corticospinal tracts is important to understand from a clinical perspective.