The spinal cord and peripheral nerves

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7 The spinal cord and peripheral nerves

Introduction

The spinal cord used to be thought of as simply a conduit for nerve pathways to and from the brain. The most elaborate neural integration thought to occur in the spinal cord was limited to simple reflex muscle servo loops. It is now known that the spinal cord is a complex neural integration system contiguous with the neural structures of the brain and is an essential component of the neuraxis in humans. Long-term plasticity of both excitatory and inhibitory transmission, postsynaptic trafficking and recycling of various receptors, activation of immediate early genes in neurons, and constant changes in synaptic structure and connections are all active processes occurring in the spinal cord.

The spinal cord is the first site of sensory modulation and the last site of motor modulation that influences perception and movement of the body parts.

The brain and brainstem also play a part in modulating other sensory systems that influence motor output via the spinal cord.

The peripheral nerves transmit receptor information to neurons in the spinal cord. The peripheral nerves can vary in transmission speeds, size, and modalities that they transmit. The peripheral nerves are also very sensitive to injury, but have specific healing strategies to recover.

The spinal cord also contributes to the integration and modulation of pain. A variety of peripheral and central processes influence pain processing including receptor mechanisms, peripheral sensitisation, central sensitisation, neural plasticity, and the sympathetic nervous system.

In this chapter we consider all of the above processes.

Anatomy of the spinal cord

The spinal cord develops as a contiguous structure with the rest of the neuraxis, arising from the ventricular layer of ependymal cells and maintains the basic dorsal and ventral segregation of sensory and motor function as the brainstem (see Chapter 2). The result of this is that most afferent (sensory) information arrives in the dorsal aspects of the cord and the efferent (motor) information exits from the ventral aspects of the cord (see Fig. 2.9A). As the spinal cord matures during embryonic development the dorsal/ventral segregation becomes more defined and by about 3 months post-conception two discrete cellular areas can be determined: the alar lamina, which is located dorsally and contains the neurons that will receive the afferent (sensory) information, and the basal lamina, which is located ventrally and contains the neurons that will supply the efferent (motor) outflow from the spinal cord (Fig. 7.1).

The spinal cord is composed of two major types of matter, one consisting of mainly neurons and neuropil, the grey matter, and the other consisting of mainly axons and supporting glial cells, the white matter. The grey matter forms the central regions of the cord and is surrounded by white matter for its entire course in the spinal cord. The spinal cord proper (medulla spinalis) begins at the superior border of the atlas or first cervical vertebra, and extends to the upper border of the second lumbar vertebra.

For the first 3 months of embryonic development the spinal cord and the vertebral column develop at the same pace and are roughly equal in length. During the rest of embryonic development the vertebra column grows in size faster than the spinal cord, resulting in the spinal cord terminating about two-thirds of the way down the vertebral column (Chusid 1982). The length of the spinal cord, which is usually between 42 and 45 cm, can show significant variation between individuals with the end result affecting the level of termination of the spinal cord (Barson 1970). The variation in the termination of the spinal cord can range from the lower third of the twelfth vertebra to the disc between the second and third lumbar vertebra (Jit & Charnalia 1959). The spinal cord terminates by converging into a cylindrical funnel-shaped structure referred to as the conus medullaris, from the distal end of which extends a thin filament, the filum terminale, to its attachment on the first coccygeal segment. The spinal nerve roots radiating from the spinal cord and the dorsal root ganglion neuron’s central projections form a structure referred to as the cauda equina as they traverse the distance, through the spinal canal, between the spinal cord termination point and their exit vertebral foramina in the spinal column (Fig. 7.2).

The volume of the spinal cord is dependent on the number of neurons and axons that it contains at any one point. Because of the increase in afferent input and efferent output that occurs at the level of the cervical and lumbar cord levels, due to the innervation of the arms and legs, the spinal cord expands in circumference, resulting in the cervical and lumbar enlargements.

On cross-sectional views of the spinal cord, the dorsal or posterior median sulcus, which is continuous with a projection of connective tissue that penetrates the posterior aspect of the cord, the dorsal median septum, symmetrically divides the dorsal cord into two halves. Ventrally, the ventral median fissure performs a similar function so that a line connecting it and the dorsal median sulcus effectively divides the spinal cord into left and right symmetrical halves. It is convenient to divide the white matter of the spinal cord into regions referred to as funiculi, so that each half of the spinal cord contains a dorsal or posterior funiculus, a posterior lateral and anterior lateral funiculus, and a ventral or anterior funiculus (Fig. 7.3). In the mature spinal cord the embryonic alar and basal plates, with a few exceptions, maintain their distribution of sensory and motor segregation. These areas can be outlined quite accurately by the funicular divisions just described (Fig. 7.4).

The grey matter of the spinal cord is composed of a high proportion of neurons, neuroglia, and blood vessels

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.

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).

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).

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

The white matter of the spinal cord is composed of axon fibre tracts

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.

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).

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).

Motor pathways

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.

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).

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.

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).

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.

image 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).

Reticulospinal tract

The reticulospinal pathways can be divided into the medial or pontoreticular and lateral or medulloreticular spinal tracts. The pontoreticular neuron projections comprise the medial reticulospinal pathways and are predominately ipsilateral. They project to interneurons of laminae VII and VIII where they act to excite VHCs on the same side of origin. Some fibres do cross one or two spinal segments above their target destinations but the main modulating effects remain ipsilateral to the neurons of origin. The lateral reticulospinal pathways arise from the neurons in the medullary areas of the reticular formation and in particular from the nucleus reticularis gigantocellularis region. The projections have been found in a variety of fibre tracts in the white matter of the cord, but for the most part travel medial to the corticospinal tracts with a small tract occasionally travelling lateral to the corticospinal tracts in the lateral funiculus. In contrast with the medial reticulospinal tracts, the projections of the lateral tract are largely crossed with some ipsilateral representation (see Fig. 7.16). Projections from each half of the medullary reticular formation exert an inhibitory effect on spinal cord neurons bilaterally, probably through the activities of inhibitory interneurons (Renshaw cells) of lamina VII of the spinal cord. These projections also modulate the effects of afferent impulses arriving in these areas of the cord (Nyberg-Hansen 1965). The loss of inhibitory projections to the spinal cord from the cortex has been thought to play an important role in spasticity observed in lesions of the cord, brainstem, or cortex. However, extrapyramidal, reticulospinal inhibitory dysfunction is also thought to be an important contributing factor. The differential activation of VHC groups by reticulospinal projections (e.g. locomotor and inhibitory systems) in combination with the effects of lesions of the corticospinal projections as previously discussed leads to a characteristic weakness or ‘soft’ weakness pattern in the limbs in response to spinal cord lesions, brain damage, or hemisphericity.

Spinal cord reflexes

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).

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.