The motor impulses for voluntary movement are mainly generated in the precentral gyrus of the frontal lobe (primary motor cortex, Brodmann area 4) and in the adjacent cortical areas (first motor neuron). They travel in the long fiber pathways (mainly the corticonuclear and corticospinal tracts/pyramidal pathway), passing through the brainstem and down the spinal cord to the anterior horn, where they make synaptic contact with the second motor neuron—usually by way of one or more intervening interneurons.
The nerve fibers emerging from area 4 and the adjacent cortical areas together make up the pyramidal tract, which is the quickest and most direct connection between the primary motor area and the motor neurons of the anterior horn. In addition, other cortical areas (especially the premotor cortex, area 6) and subcortical nuclei (especially the basal ganglia, cf. ▶Chapter 8) participate in the neural control of movement. These areas form complex feedback loops with one another and with the primary motor cortex and cerebellum; they exert an influence on the anterior horn cells by way of several distinct fiber pathways in the spinal cord. Their function is mainly to modulate movement and to regulate muscle tone.
Impulses generated in the second motor neurons of the motor cranial nerve nuclei and the anterior horn of the spinal cord pass through the anterior roots, the nerve plexuses (in the cervical and lumbosacral regions), and the peripheral nerves on their way to the skeletal muscles. The impulses are conveyed to the muscle cells through the motor end plates of the neuromuscular junction.
Lesions of the first motor neuron in the brain or spinal cord usually produce spastic paresis, while lesions of the second motor neuron in the anterior horn, anterior root, peripheral nerve, or motor end plate usually produce flaccid paresis. Motor deficits rarely appear in isolation as a result of a lesion of the nervous system; they are usually accompanied by sensory, autonomic, cognitive, and/or neuropsychological deficits of various kinds, depending on the site and nature of the causative lesion.
The central portion of the motor system for voluntary movement consists of the primary motor cortex (area 4) and the adjacent cortical areas (particularly the premotor cortex, area 6), and the corticobulbar and corticospinal tracts to which these cortical areas give rise (▶Fig. 3.1 and ▶Fig. 3.2).
The primary motor cortex (precentral gyrus, ▶Fig. 3.1) is a band of cortical tissue that lies on the opposite side of the central sulcus from the primary somatosensory cortex (in the postcentral gyrus) and, like it, extends upward and past the superomedial edge of the hemisphere onto its medial surface. The area representing the throat and larynx lies at the inferior end of the primary motor cortex; above it, in sequence, are the areas representing the face, upper limbs, trunk, and lower limbs (▶Fig. 3.2). This is the inverted motor homunculus, corresponding to the “somatosensory homunculus” of the postcentral gyrus (see ▶Fig. 9.19).
Motor neurons are found not only in area 4 but also in the adjacent cortical areas. The fibers mediating fine voluntary movements, however, originate mainly in the precentral gyrus. This is the site of the characteristic, large pyramidal neurons (Betz cells), which lie in the fifth cellular layer of the cortex and send their rapidly conducting, thickly myelinated axons (▶Fig. 3.3) into the pyramidal tract. The pyramidal tract was once thought to be entirely composed of Betz cell axons, but it is now known that these account for only 3 to 4% of its fibers. The largest fiber contingent in fact originates from the smaller pyramidal and fusiform cells of Brodmann areas 4 and 6. Axons derived from area 4 make up about 40% of all pyramidal tract fibers; the remainder come from other frontal areas, from areas 3, 2, and 1 of the parietal somatosensory cortex (sensorimotor area), and from other areas in the parietal lobe (▶Fig. 3.1). The motor neurons of area 4 subserve fine, voluntary movement of the contralateral half of the body; the pyramidal tract is, accordingly, crossed (see ▶Fig. 3.4). Direct electrical stimulation of area 4, as during a neurosurgical procedure, generally induces contraction of an individual muscle, while stimulation of area 6 induces more complex and extensive movements, e.g., of an entire upper or lower limb.
This tract originates in the motor cortex and travels through the cerebral white matter (corona radiata), the posterior limb of the internal capsule (where the fibers lie very close together), the central portion of the cerebral peduncle (crus cerebri), the pons, and the base (i.e., the anterior portion) of the medulla, where the tract is externally evident as a slight protrusion called the pyramid. The medullary pyramids (there is one on either side) give the tract its name. At the lower end of the medulla, 80 to 85% of the pyramidal fibers cross to the other side in the so-called decussation of the pyramids. The fibers that do not cross here descend the spinal cord in the ipsilateral anterior funiculus as the anterior corticospinal tract; they cross farther down (usually at the level of the segment that they supply) through the anterior commissure of the spinal cord (▶Fig. 3.6). At cervical and thoracic levels, there are probably also a few fibers that remain uncrossed and innervate ipsilateral motor neurons in the anterior horn, so that the nuchal and truncal musculature receives a bilateral cortical innervation.
The majority of pyramidal tract fibers cross in the decussation of the pyramids, then descend the spinal cord in the contralateral lateral funiculus as the lateral corticospinal tract. This tract shrinks in cross-sectional area as it travels down the cord, because some of its fibers terminate in each segment along the way. About 90% of all pyramidal tract fibers end in synapses onto interneurons, which then transmit the motor impulses onward to the large α motor neurons of the anterior horn, as well as to the smaller γ motor neurons (▶Fig. 3.4).
Some of the fibers of the pyramidal tract branch off from the main mass of the tract as it passes through the midbrain and then take a more dorsal course toward the motor cranial nerve nuclei (▶Fig. 3.4 and ▶Fig. 4.54). The fibers supplying these brainstem nuclei are partly crossed and partly uncrossed (for further details, cf. Chapter 4, ▶“Cranial Nerves”). The nuclei receiving pyramidal tract input are the ones that mediate voluntary movements of the cranial musculature through cranial nerves V (the trigeminal nerve), VII (the facial nerve), IX, X, and XI (the glossopharyngeal, vagus, and accessory nerves), and XII (the hypoglossal nerve).
Corticomesencephalic tract. There is also a contingent of fibers traveling together with the corticonuclear tract that arises, not in areas 4 and 6, but rather in area 8, the frontal eye field (▶Fig. 3.1 and ▶Fig. 3.4). The impulses in these fibers mediate conjugate eye movements (see ▶Conjugate Eye Movements, Chapter 4), which are a complex motor process. Because of its special origin and function, the pathway originating in the frontal eye fields has a separate name (the corticomesencephalic tract), though most authors consider it a part of the corticonuclear tract.
The corticomesencephalic tract runs in tandem with the pyramidal tract (just rostral to it, in the posterior limb of the internal capsule) and then heads dorsally toward the nuclei of the cranial nerves that mediate eye movements, i.e., cranial nerves III, IV, and VI (the oculomotor, trochlear, and abducens nerves). Area 8 innervates the eye muscles exclusively in synergistic fashion, rather than individually. Stimulation of area 8 induces conjugate gaze deviation to the opposite side. The fibers of the corticomesencephalic tract do not terminate directly onto the motor neurons of cranial nerve nuclei III, IV, and VI; the anatomical situation here is complicated and incompletely understood, it is discussed further in ▶Midbrain, Chapter 4.
A number of central pathways beside the pyramidal tract play major roles in the control of motor function (▶Fig. 3.5). One important group of fibers (the corticopontocerebellar tract) conveys information from the cerebral cortex to the cerebellum, whose output in turn modulates planned movements (cf. Chapter 5, ▶“Cerebellum”). Other fibers travel from the cortex to the basal ganglia (mainly the corpus striatum = caudate nucleus and putamen), the red nucleus, the substantia nigra, the brainstem reticular formation, and other nuclei (e.g., in the midbrain tectum). In each of these structures, the impulses are processed and conveyed onward, via interneurons, to efferent tracts that project to the motor neurons of the anterior horn—the tectospinal, rubrospinal, reticulospinal, vestibulospinal, and other tracts (▶Fig. 3.6). These tracts enable the cerebellum, basal ganglia, and brainstem motor nuclei to influence motor function in the spinal cord. (For further details, see ▶Chapter 4 and ▶Chapter 8.)
Lateral and medial motor tracts in the spinal cord. The motor tracts in the spinal cord are anatomically and functionally segregated into two groups: a lateral group, comprising the corticospinal and rubrospinal tracts, and a medial group, comprising the reticulospinal, vestibulospinal, and tectospinal tracts.
The lateral tracts mainly project to the distal musculature (especially in the upper limbs) and also make short propriospinal connections. They are primarily responsible for voluntary movements of the forearms and hands, i.e., for precise, highly differentiated, fine motor control.
The medial tracts, in contrast, innervate motor neurons lying more medially in the anterior horn and make relatively long propriospinal connections. They are primarily responsible for movements of the trunk and lower limbs (stance and gait).
Views differ on the significance of the rubrospinal tract. Many neuroanatomists consider it to be an insignificant pathway in human that ends in the cervical spinal cord; others disagree. The rubrospinal tract originates not only from large nuclei, but rather mainly from small nuclei within the red nucleus.
Pathogenesis of central spastic paresis. In the acute phase of a lesion of the corticospinal tract, the deep tendon reflexes are hypoactive and there is flaccid weakness of the muscles. The reflexes return a few days or weeks later and become hyperactive, because the muscle spindles respond more sensitively to stretch than normal, particularly in the upper limb flexors and the lower limb extensors. This hypersensitivity is due to a loss of descending central inhibitory control of the fusimotor cells (γ motor neurons) that innervate the muscle spindles. The intrafusal muscle fibers are, therefore, permanently activated (prestretched) and respond more readily than normal to further stretching of the muscle. A disturbance of the regulatory circuit for muscle length probably occurs, in which the upper limb flexors and lower limb extensors are set to an abnormally short target length. The result is spastic increased tone and hyperreflexia, as well as so-called pyramidal tract signs and clonus. Among the pyramidal tract signs are certain well-known findings in the fingers and toes, such as the Babinski sign (tonic extension of the big toe in response to stroking of the sole of the foot).
Spastic paresis is always due to a lesion of the central nervous system (brain and/or spinal cord) and is more pronounced when both the lateral and the medial descending tracts are damaged (e.g., in a spinal cord lesion). The pathophysiology of spasticity is still poorly understood, but the accessory motor pathways clearly play an important role, because an isolated, purely cortical lesion does not cause spasticity. Changes in the irritability of the motor neurons themselves and changes in the activity of spinal interneurons and muscle fibers all play a role in the development and maintenance of spasticity.
A lesion involving the cerebral cortex (a in ▶Fig. 3.7), such as a tumor, an infarct, or a traumatic injury, causes weakness of part of the body on the opposite side. Hemiparesis is seen in the face and hand (brachiofacial weakness) more frequently than elsewhere, because these parts of the body have a large cortical representation. The typical clinical finding associated with a lesion in site (a) is a predominantly distal paresis of the upper limb, most serious functional consequence of which is an impairment of fine motor control. The weakness is incomplete (paresis rather than plegia), and it is flaccid, rather than spastic, because the accessory (nonpyramidal) motor pathways are largely spared. An irritative lesion at site (a) can cause focal (jacksonian) seizures (which are described further in neurology textbooks).
Fig. 3.7 Sites of potential lesionsof the pyramidal tract. For the corresponding clinical syndromes, see Localization of Lesions in the Central Motor System. (a) Cortical lesion, (b) lesion of the internal capsule, (c) lesion of the cerebral peduncle, (d) pontine lesion, (e) lesion of the medullary pyramids, (f) lesion of the pyramidal tract in the cervical spinal cord, (g) lesion of the pyramidal tract in the thoracic spinal cord, (h) lesion of a peripheral nerve.
If the internal capsule (b in ▶Fig. 3.7) is involved (e.g., by hemorrhage or ischemia), there will be a contralateral spastic hemiplegia—lesions at this level affect both pyramidal and nonpyramidal fibers, because fibers of the two types are in close proximity here. The corticonuclear tract is involved as well, so that a contralateral facial palsy results, perhaps accompanied by a central hypoglossal nerve palsy. No other cranial nerve deficits are seen, however, because the remaining motor cranial nerve nuclei are bilaterally innervated. The contralateral paresis is flaccid at first (in the “shock phase”) but becomes spastic within hours or days because of concomitant damage to nonpyramidal fibers.
Lesions at the level of the cerebral peduncle (c in ▶Fig. 3.7c), such as a vascular process, a hemorrhage, or a tumor, produce contralateral spastic hemiparesis, possibly accompanied by an ipsilateral oculomotor nerve palsy (Weber’s syndrome).
Pontine lesions involving the pyramidal tract (d in ▶Fig. 3.7; e.g., a tumor, brainstem ischemia, a hemorrhage) cause contralateral or possibly bilateral hemiparesis. Typically, not all of the fibers of the pyramidal tract are involved, because its fibers are spread over a wider cross-sectional area at the pontine level than elsewhere (e.g., at the level of the internal capsule). Fibers innervating the facial and hypoglossal nuclei have already moved to a more dorsal position before reaching this level; thus, an accompanying central facial or hypoglossal palsy is rare, though there may be an accompanying ipsilateral trigeminal nerve deficit or abducens palsy (see ▶Fig. 4.66 and ▶Fig. 4.68). A lesion of the medullary pyramids (e in ▶Fig. 3.7; usually a tumor) can damage the pyramidal tract fibers in isolation, as the nonpyramidal fibers are further dorsal at this level. Flaccid contralateral hemiparesis is a possible result. The weakness is less than total (i.e., paresis rather than plegia), because the remaining descending pathways are preserved.
Lesions of the pyramidal tract in the spinal cord. A lesion affecting the pyramidal tract at a cervical level (f in ▶Fig. 3.7; e.g., a tumor, myelitis, trauma) causes ipsilateral spastic hemiplegia: ipsilateral because the tract has already crossed at a higher level, and spastic because it contains nonpyramidal as well as pyramidal fibers at this level. A bilateral lesion in the upper cervical spinal cord can cause quadriparesis or quadriplegia.
A lesion affecting the pyramidal tract in the thoracic spinal cord (g in ▶Fig. 3.7; e.g., trauma, myelitis) causes spastic ipsilateral monoplegia of the lower limb. Bilateral involvement causes paraplegia.
A lesion at the level of the peripheral nerve (h in ▶Fig. 3.7) causes flaccid paralysis.
The peripheral portion of the motor system comprises the motor cranial nerve nuclei of the brainstem, the motor anterior horn cells of the spinal cord, the anterior roots, the cervical and lumbosacral nerve plexuses, the peripheral nerves, and the motor end plates in skeletal muscle.
Anterior horn cells (α and γ Motor Neurons). The fibers not only of the pyramidal tract but also of the nonpyramidal descending pathways (the reticulospinal, tectospinal, vestibulospinal, and rubrospinal tracts, among others), as well as afferent fibers from the posterior roots, terminate on the cell bodies or dendrites of the larger and smaller α motor neurons. Fibers of all of these types also make synaptic contact with the small γ motor neurons, partly directly and partly through intervening interneurons and the association and commissural neurons of the intrinsic neuronal apparatus of the spinal cord (▶Fig. 3.6). Some of these synapses are excitatory, while others are inhibitory. The thin, unmyelinated neurites of the γ motor neurons innervate the intrafusal muscle fibers. In contrast to the pseudounipolar neurons of the spinal ganglia, the anterior horn cells are multipolar. Their dendrites receive synaptic contact from a wide variety of afferent and efferent systems (▶Fig. 3.6).
The functional groups and nuclear columns of neurons in the anterior horn are not separated from one another by anatomically discernible borders (▶Fig. 2.5b). In the cervical spinal cord, the motor neurons for the upper limbs lie in the lateral portion of the gray matter of the anterior horn; those for the truncal muscles lie in its medial portion. The same somatotopic principle applies in the lumbar spinal cord, where the lower limbs are represented laterally and the trunk is represented medially.
Inhibition of anterior horn cells by Renshaw cells. Among the various types of interneurons of the anterior horn, the Renshaw cells deserve special mention (▶Fig. 2.11). These small cells receive synaptic contact from collateral axons of the large α motor neurons. Their axons then project back onto the anterior horn cells and inhibit their activity. Renshaw inhibition is an example of a spinal negative feedback loop that stabilizes the activity of motor neurons.
Anterior roots. The neurites of the motor neurons exit the anterior aspect of the spinal cord as rootlets (fila radicularia) and join together, forming the anterior roots. Each anterior root joins the corresponding posterior root just distal to the dorsal root ganglion to form a spinal nerve, which then exits the spinal canal through the intervertebral foramen.
Peripheral nerve and motor end plate. There is one pair of spinal nerves for each segment of the body. The spinal nerves contain afferent somatosensory fibers, efferent somatic motor fibers, efferent autonomic fibers from the lateral horns of the spinal gray matter, and afferent autonomic fibers (cf. ▶Peripheral Nerve, Dorsal Root Ganglion, Posterior Root, Chapter 2). At cervical and lumbosacral levels, the spinal nerves join to form the nerve plexuses, which, in turn, give rise to the peripheral nerves that innervate the musculature of the neck and limbs (▶Fig. 3.31, ▶Fig. 3.32, and ▶Fig. 3.34).
The thick, myelinated, rapidly conducting neurites of the large α motor neurons are called α1 fibers (▶Fig. 2.11). These fibers travel to the working musculature, where they divide into a highly variable number of branches that terminate on muscle fibers. Synaptic impulse transmission occurs at the neuromuscular junctions (motor end plates).
Motor unit. An anterior horn cell, its neurites, and the muscle fibers it innervates are collectively termed a motor unit. Each motor unit constitutes the final common pathway for movement-related impulses arriving at the anterior horn cell from higher levels: its activity is influenced by impulses in a wide variety of motor tracts that originate in different areas of the brain, as well as by impulses derived from intrasegmental and intersegmental reflex neurons of the spinal cord. All of these movement-related impulses are integrated in the motor unit, and the result of this integration is transmitted to the muscle fibers.
Muscles participating in finely differentiated movements are supplied by a large number of anterior horn cells, each of which innervates only a few (5–20) muscle fibers; such muscles are thus composed of small motor units. In contrast, large muscles that contract in relatively undifferentiated fashion, such as the gluteal muscles, are supplied by relatively few anterior horn cells, each of which innervates 100 to 500 muscle fibers (large motor units).
Flaccid paralysis is caused by interruption of motor units at any site, be it in the anterior horn, one or more anterior roots, a nerve plexus, or a peripheral nerve (h in ▶Fig. 3.7). Motor unit damage cuts off the muscle fibers in the motor unit from both voluntary and reflex innervation. The affected muscles are extremely weak (plegic), and there is a marked diminution of muscle tone (hypotonia), as well as a loss of reflexes (areflexia) because the monosynaptic stretch reflex loop has been interrupted. Muscle atrophy sets in within a few weeks, as the muscle is gradually replaced by connective tissue; after months or years of progressive atrophy, this replacement may be complete. Thus, the anterior horn cells exert a trophic influence on muscle fibers, which is necessary for the maintenance of their normal structure and function.
The lesion can usually be localized more specifically to the anterior horn, the anterior root(s), the nerve plexus, or the peripheral nerve with the aid of EMG and electroneurography (nerve conduction studies). If paralysis in a limb or limbs is accompanied by somatosensory and autonomic deficits, then the lesion is presumably distal to the nerve roots and is thus located either in the nerve plexus or in the peripheral nerve. Flaccid paralysis is only rarely due to a cortical lesion (see ▶Lesions of Central Motor Pathways); in such cases, the reflexes are preserved or even exaggerated, and the muscle tone is normal or increased.
Damage to individual components of the nervous system generally does not cause an isolated motor deficit of the kind described up to this point. Rather, motor deficits are usually accompanied by somatosensory, special sensory, autonomic, cognitive, and/or neuropsychological deficits of variable type and extent depending on the site and extent of the lesion. The complex clinical syndromes due to lesions in specific regions of the brain (telencephalon, diencephalon, basal ganglia, limbic system, cerebellum, and brainstem) will be described in the corresponding chapters. In this section, we will present the typical syndromes arising from lesions of the spinal cord, nerve roots, plexuses, peripheral nerves, motor end plates, and musculature.
Because the spinal cord contains motor, sensory, and autonomic fibers and nuclei in a tight spatial relationship with one another, lesions of the spinal cord can cause a wide variety of neurological deficits, which can be combined with each other in many different ways. Careful clinical examination usually enables highly precise localization of the lesion, but only if the examiner possesses adequate knowledge of the anatomy of the relevant motor, sensory, and autonomic pathways. Thus, this section will begin with a brief discussion of clinical anatomy. The individual spinal pathways have already been discussed in ▶Posterior and Anterior Spinocerebellar Tracts, Chapter 2, Corticospinal Tract (Pyramidal Tract), and ▶Corticonuclear (Corticobulbar) Tract.
General anatomical preliminaries. The spinal cord, like the brain, is composed of gray matter and white matter. The white matter contains ascending and descending fiber tracts, while the gray matter contains neurons of different kinds: the anterior horns contain mostly motor neurons, the lateral horns mostly autonomic neurons, and the posterior horns mostly somatosensory neurons participating in a number of different afferent pathways (see below and ▶Chapter 2). In addition, the spinal cord contains an intrinsic neuronal apparatus consisting of interneurons, association neurons, and commissural neurons, whose processes ascend and descend in the fasciculus proprius (▶Fig. 2.9).
In the adult, the spinal cord is shorter than the vertebral column: it extends from the craniocervical junction to about the level of the intervertebral disk between the first and second lumbar vertebrae (L1–L2) (▶Fig. 2.4; this must be borne in mind when localizing the level of a spinal process). The segments of the neural tube (primitive spinal cord) correspond to those of the vertebral column only up to the third month of gestation, after which the growth of the spine progressively outstrips that of the spinal cord. The nerve roots, however, still exit from the spinal canal at the numerically corresponding levels, so that the lower thoracic and lumbar roots must travel an increasingly long distance through the subarachnoid space to reach the intervertebral foramina through which they exit. The spinal cord ends as the conus medullaris (or conus terminalis) at the L1 or L2 level (rarely at L3). Below this level, the lumbar sac (theca) contains only nerve root filaments, the so-called cauda equina (“horse’s tail”; ▶Fig. 3.22).
The fanlike filaments of the nerve roots still display the original metameric structure of the spinal cord, but the cord itself shows no segmental division. At two sites, however, the spinal cord is somewhat swollen, namely at the cervical and lumbar enlargements. The former contains the segments corresponding to the upper limbs (C4–T1), which form the brachial plexus; the latter contains the ones for the lower limbs (L2–S3), which form the lumbosacral plexus (▶Fig. 2.4).
Spinal cord lesions. These occasionally affect only the white matter (e.g., posterior column lesions) or only the gray matter (e.g., acute poliomyelitis), but more often affect both. In the following paragraphs, the manifestations of typical spinal cord syndromes will be presented from a topical point of view. For completeness, a number of syndromes characterized primarily or exclusively by somatosensory deficits will also be presented here.
Syndromes due to Lesions of Individual Spinal Tracts and Nuclear Areas and the Associated Peripheral Nerves
Syndrome of the dorsal root ganglion. Infection of one or more spinal ganglia (▶Fig. 3.8) by a neurotropic virus occurs most commonly in the thoracic region and causes painful erythema of the corresponding dermatome(s), followed by the formation of a variable number of cutaneous vesicles. This clinical picture, called herpes zoster, is associated with very unpleasant, stabbing pain and paresthesiae in the affected area. The infection may pass from the spinal ganglia into the spinal cord itself, but, if it does, it usually remains confined to a small area within the cord. Involvement of the anterior horns causing flaccid paresis is rare, and hemiparesis or paraparesis is even rarer.
EMG can demonstrate a segmental motor deficit in up to two-thirds of all cases, but, because herpes zoster is usually found in the thoracic area, the deficit tends to be functionally insignificant, and may escape the patient’s notice. In some cases, the cutaneous lesion is absent (herpes sine herpete). Herpes zoster is relatively common, with an incidence of 3 to 5 cases per 1,000 persons per year; immunocompromised individuals (e.g., with AIDS, malignancy, or immunosuppression) are at elevated risk. Treatment with topical dermatological medication as well as aciclovir, or another specific virustatic agent, is recommended. Even with appropriate treatment, postherpetic neuralgia in the affected area is not an uncommon complication. It can be treated symptomatically with various medications, e.g., gabapentin and pregabalin.
Posterior root syndrome. If two or more adjacent posterior roots (▶Fig. 3.9) are completely divided, e.g., owing to traumatic injury, sensation in the corresponding dermatomes is partially or totally lost. Incomplete posterior root lesions (e.g., due to syphilis or other infectious diseases) affect different sensory modalities to variable extents, with pain sensation usually being most strongly affected. Because the lesion interrupts the peripheral reflex arc, the sensory deficit is accompanied by hypotonia and hyporeflexia or areflexia in the muscles supplied by the affected roots. These typical deficits are produced only if multiple adjacent roots are affected.
Posterior column syndrome. The posterior columns (▶Fig. 3.10) can be secondarily involved by pathological processes affecting the dorsal root ganglion cells and the posterior roots. Lesions of the posterior columns typically impair position and vibration sense, discrimination, and stereognosis; they also produce a positive Romberg’s sign, as well as spinal ataxia that worsens significantly when the eyes are closed (unlike cerebellar ataxia, which does not). Posterior column lesions also often produce hypersensitivity to pain. Possible causes include vitamin B12 deficiency (e.g., in “funicular myelosis”), AIDS-associated vacuolar myelopathy, and spinal cord compression (e.g., in cervical spinal stenosis). Tabes dorsalis due to syphilis is rare in North America and Western Europe but is an increasingly common type of posterior column disturbance in other parts of the world.
Posterior horn syndrome. Posterior horn syndrome (▶Fig. 3.11) can be a clinical manifestation of syringomyelia, hematomyelia, and some intramedullary spinal cord tumors, among other conditions. Like posterior root lesions, posterior horn lesions produce a segmental somatosensory deficit; yet, rather than impairing all sensory modalities like posterior root lesions, posterior horn lesions spare the modalities subserved by the posterior columns, i.e., epicritic and proprioceptive sense. “Only” pain and temperature sensation are lost in the corresponding ipsilateral segments, because these modalities are conducted centrally through a second neuron in the posterior horn (whose axon ascends in the lateral spinothalamic tract). Loss of pain and temperature sensation with sparing of posterior column sense is called a dissociated somatosensory deficit. There may be spontaneous pain (deafferentation pain) in the analgesic area.
Pain and temperature sensation are intact below the level of the lesion, as the lateral spinothalamic tract, lying in the anterolateral funiculus, is undamaged and continues to conduct these modalities centrally.
Gray matter syndrome. Damage to the central gray matter of the spinal cord (▶Fig. 3.12) by syringomyelia, hematomyelia, intramedullary spinal cord tumors, or other processes interrupts all of the fiber pathways passing through the gray matter. The most prominently affected fibers are those that originate in posterior horn cells and conduct coarse pressure, touch, pain, and temperature sensation; these fibers decussate in the central gray matter and then ascend in the anterior and lateral spinothalamic tracts. A lesion affecting them produces a bilateral dissociated sensory deficit in the cutaneous area supplied by the damaged fibers.
Syringomyelia is characterized by the formation of one or more fluid-filled cavities in the spinal cord; the analogous disease in the brainstem is called syringobulbia. The cavities, called syringes, can be formed by a number of different mechanisms and are distributed in different characteristic patterns depending on their mechanism of formation. Some syringes are an expansion of the central canal of the spinal cord, which may or may not communicate with the fourth ventricle; others are a hollowing-out of the parenchyma and are separate from the central canal. The term hydromyelia is sometimes used loosely for communicating syringes of the central canal, but it properly refers to an idiopathic, congenital variant of syringomyelia in which the syrinx communicates with the subarachnoid space, and should only be used in this sense. Syringomyelia most commonly affects the cervical spinal cord, typically producing loss of pain and temperature sensation in the shoulders and upper limbs. A progressively expanding syrinx can damage the long tracts of the spinal cord, producing spastic (para)paresis and disturbances of bladder, bowel, and sexual function. Syringobulbia often causes unilateral atrophy of the tongue, hypalgesia or analgesia of the face, and various types of nystagmus depending on the site and configuration of the syrinx.
The syndrome of combined lesions of the posterior columns and corticospinal tracts (funicular myelosis). See ▶Fig. 3.13 and ▶Case Presentation 2.1. This condition is most commonly produced by vitamin B12 deficiency due to a lack of gastric intrinsic factor (e.g., in atrophic gastritis), and is known in such cases as “subacute combined degeneration.” Foci of demyelination are found in the cervical and thoracic regions in the posterior columns (70–80%), and somewhat less commonly in the pyramidal tracts (40–50%), while the gray matter is usually spared. Posterior column damage causes loss of position and vibration sense in the lower limbs, resulting in spinal (deafferentation-type) ataxia and a positive Romberg’s sign (unstable stance with eyes closed). The accompanying pyramidal tract damage causes spastic paraparesis with hyperreflexia and bilateral Babinski’s signs.
Anterior horn syndrome. Both acute poliomyelitis and spinal muscle atrophy of various types specifically affect the anterior horn cells (▶Fig. 3.14), particularly in the cervical and lumbar enlargements of the spinal cord.
In poliomyelitis (a viral infection), a variable number of anterior horn cells are acutely and irreversibly lost, mainly in the lumbar region, causing flaccid paresis of the muscles in the corresponding segments. Proximal muscles tend to be more strongly affected than distal ones. The muscles become atrophic and, in severe cases, may be completely replaced by connective tissue and fat. It is rare for all of the muscles of a limb to be affected, because the anterior horn cells are arranged in long vertical columns within the spinal cord (▶Fig. 2.10).
Combined anterior horn and pyramidal tract syndrome. In amyotrophic lateral sclerosis (▶Fig. 3.15), a combination of flaccid and spastic paresis is seen as the result of degeneration of both cortical and spinal motor neurons. Muscle atrophy, appearing early in the course of the disease, is generally so severe that the deep tendon reflexes would ordinarily be absent, if only the lower motor neurons were affected. Yet, because of the simultaneous damage of the upper motor neurons (with consequent pyramidal tract degeneration and spasticity), the reflexes often remain elicitable and may even be exaggerated. Accompanying degeneration of the motor cranial nerve nuclei can cause dysarthria and dysphagia (progressive bulbar palsy).
Syndrome of the corticospinal tracts. Loss of cortical motor neurons (▶Fig. 3.16) is followed by degeneration of the corticospinal tracts in a number of different diseases, including primary lateral sclerosis (a variant of amyotrophic lateral sclerosis) and the rarer form of hereditary spastic spinal paralysis. The most common subform of this disease (SPG4) is due to a mutation of the spastin gene; the disease appears in childhood and progresses slowly thereafter. Patients complain initially of a feeling of heaviness, then of weakness in the lower limbs. Spastic paraparesis with a spastic gait disturbance gradually develops and worsens. The reflexes are brisker than normal. Spastic paresis of the upper limbs does not develop until much later, and generally only in complicated forms of the disease.
Syndrome of combined involvement of the posterior columns, spinocerebellar tracts, and (possibly) pyramidal tracts. When a pathological process affects all of these systems (▶Fig. 3.17), the differential diagnosis should include spinocerebellar ataxia of Friedreich type, the axonal form of a hereditary neuropathy (HMSN II), and other ataxias.
Characteristic clinical manifestations are produced by the lesions in each of the involved systems. Friedreich’s ataxia begins before age 25 with loss of dorsal root ganglion cells, leading to posterior column degeneration. The clinical result includes an impairment of position sense, two-point discrimination, and stereognosis, with spinal ataxia and a positive Romberg’s sign. Pain and temperature sense are largely or completely spared. The ataxia is severe, because both the posterior columns and the spinocerebellar tracts are involved; it is evident when the patient tries to walk, stand, or sit, as well as in the finger–nose–finger and heel–knee–shin tests. The patient’s gait is uncoordinated, with festination, and also becomes spastic over time as the pyramidal tracts progressively degenerate. About half of all patients manifest skeletal deformities such as scoliosis or pes cavus (the so-called Friedreich’s foot).
The spinal cord hemisection syndrome. This syndrome, generally called Brown–Séquard syndrome (▶Fig. 3.18), is rare and usually incomplete; its most common causes are spinal trauma and cervical disk herniation. Interruption of the descending motor pathways on one side of the spinal cord causes an initially flaccid, ipsilateral paresis below the level of the lesion (spinal shock), which later becomes spastic and is accompanied by hyperreflexia, Babinski’s signs, and vasomotor disturbances. At the same time, the interruption of the posterior columns on one side of the spinal cord causes ipsilateral loss of position sense, vibration sense, and tactile discrimination below the level of the lesion. The ataxia that would normally be caused by the posterior column lesion cannot be demonstrated because of the coexisting ipsilateral paresis. Pain and temperature sensation are spared on the side of the lesion, because the fibers subserving these modalities have already crossed to the other side to ascend in the lateral spinothalamic tract, but pain and temperature sensation are lost contralaterally below the level of the lesion, because the ipsilateral (crossed) spinothalamic tracts are interrupted.