Arising from the cerebral cortex (primarily the precentral motor cortex, or area 4, and the premotor area, or area 6) is a large bundle of myelinated axons that descends through the brain stem via a tract called the medullary pyramid and then largely crosses over (decussates) downward into the lateral white columns. These tracts contain more than 1 million axons; the majority are myelinated.
The corticospinal tracts contain the axons of upper motor neurons (ie, neurons of the cerebrum and subcortical brain stem that descend and provide input to the anterior horn cells of the spinal cord). These anterior horn cells, which project directly to muscle and control muscular contraction, are called lower motor neurons.
The great majority of axons in the corticospinal system decussate in the pyramidal decussation within the medulla and descend within the lateral corticospinal tract (Fig 5–13 and Table 5–3). These fibers terminate throughout the ventral gray column and at the base of the dorsal column. Some of the LMNs supplying the muscles of the distal extremities receive direct monosynaptic input from the lateral corticospinal tract; other LMNs are innervated by interneurons (via polysynaptic connection).
Schematic illustration of the course of corticospinal tract fibers in the spinal cord, together with cross sections at representative levels. This and the following schematic illustrations show the cord in an upright position.
TABLE 5–3Descending Fiber Systems in the Spinal Cord. ||Download (.pdf) TABLE 5–3 Descending Fiber Systems in the Spinal Cord.
|System ||Function ||Origin ||Ending ||Location in Cord |
|Lateral corticospinal (pyramidal) tract || |
Fine motor function (controls distal musculature)
Modulation of sensory functions
|Motor and premotor cortex ||Anterior horn cells (interneurons and lower motor neurons) ||Lateral column (crosses in medulla at pyramidal decussation) |
|Anterior corticospinal tract ||Gross and postural motor function (proximal and axial musculature) ||Motor and premotor cortex ||Anterior horn neurons (interneurons and lower motor neurons) ||Anterior column (uncrossed until after descending, when some fibers decussate) |
|Vestibulospinal tract ||Postural reflexes ||Lateral and medial vestibular nucleus ||Anterior horn inter-neurons and motor neurons (for extensors) ||Ventral column |
|Rubrospinal ||Motor function ||Red nucleus ||Ventral horn interneurons ||Lateral column |
|Reticulospinal || |
Modulation of sensory transmission (especially pain)
Modulation of spinal reflexes
|Brain stem reticular formation ||Dorsal and ventral horn ||Anterior column |
|Descending autonomic ||Modulation of autonomic functions ||Hypothalamus, brain stem nuclei ||Preganglionic autonomic neurons ||Lateral columns |
|Tectospinal ||Reflex head turning ||Midbrain ||Ventral horn inter-neurons ||Ventral column |
|Medial longitudinal fasciculus ||Coordination of head and eye movements ||Vestibular nuclei ||Cervical gray ||Ventral column |
The lateral corticospinal tract is relatively new in phylogenetic terms, present only in mammals, and most highly developed in primates. It provides the descending pathway that controls voluntary, highly skilled, and fractionated movements.
In addition to the lateral corticospinal tract, which decussates and is the largest descending motor pathway, there are two smaller descending motor pathways in the spinal cord. These pathways are uncrossed.
About 10% of the corticospinal fibers that descend from the hemisphere do not decussate in the medulla but rather descend uncrossed in the anterior (or ventral) corticospinal tract and are located in the anterior white matter column of the spinal cord. After descending within the spinal cord, many of these fibers decussate, via the anterior white commissure, and then project to interneurons (which project to LMNs) but connect directly to LMNs of the contralateral side.
A small fraction (0–3%) of the corticospinal axons descend, without decussating, as uncrossed fibers within the lateral corticospinal tract. These axons terminate in the base of the posterior horn and the intermediate gray matter of the spinal cord. They provide synaptic input (probably via polysynaptic circuits) to LMNs controlling axial (ie, trunk and proximal limb) musculature involved in maintaining body posture.
B. Vestibulospinal Tracts
There are two major components to the vestibulospinal tracts. Fibers of the lateral vestibulospinal tract arise from the lateral vestibular nucleus in the brain stem and course downward, uncrossed, in the ventral white column of the spinal cord. Fibers of the medial vestibulospinal tract arise in the medial vestibular nucleus in the brain stem and descend within the cervical spinal cord, with both crossed and uncrossed components, to terminate at cervical levels. Fibers of both vestibulospinal tracts provide synaptic inputs to interneurons in Rexed's laminae VII and VIII, which project to both alpha and gamma LMNs. Fibers of the vestibulospinal tracts provide excitatory input to the LMNs for extensor muscles. The vestibulospinal system facilitates quick movements in reaction to sudden changes in body position (eg, falling) and provides control of antigravity muscles.
This fiber system arises in the contralateral red nucleus in the brain stem and courses in the lateral white column. The tract projects to interneurons in the spinal gray columns and plays a role in motor function (see Chapter 13).
This tract arises in the reticular formation of the brain stem and descends in both the ventral and lateral white columns. Both crossed and uncrossed descending fibers are present. The fibers terminating on dorsal gray column neurons may modify the transmission of sensation from the body, especially pain. Those that end on ventral gray neurons influence gamma motor neurons and thus various spinal reflexes.
E. Descending Autonomic System
Arising from the hypothalamus and brain stem, this poorly defined fiber system projects to preganglionic sympathetic neurons in the thoracolumbar spinal cord (lateral column) and to preganglionic parasympathetic neurons in sacral segments (see Chapter 20). Descending fibers in this system modulate autonomic functions, such as blood pressure, pulse and respiratory rates, and sweating.
This tract arises from the superior colliculus in the roof (tectum) of the midbrain and then courses in the contralateral ventral white column to provide synaptic input to ventral gray interneurons. It causes head turning in response to sudden visual or auditory stimuli.
G. Medial Longitudinal Fasciculus
This tract arises from vestibular nuclei in the brain stem. As it descends, it runs close to, and intermingles with, the tectospinal tract. Some of its fibers descend into the cervical spinal cord to terminate on ventral gray interneurons. It coordinates head and eye movements. The last two descending fiber systems descend only to the cervical segments of the spinal cord.
All afferent axons in the dorsal roots have their cell bodies in the dorsal root ganglia (Table 5–4). Different ascending systems decussate at different levels. In general, ascending axons synapse within the spinal cord before decussating.
TABLE 5–4Ascending Fiber Systems in the Spinal Cord. ||Download (.pdf) TABLE 5–4 Ascending Fiber Systems in the Spinal Cord.
|Name ||Function ||Origin ||Ending ||Location in Cord |
|Dorsal column system ||Fine touch, proprioception, two-point discrimination ||Skin, joints, tendons ||Dorsal column nuclei. Second-order neurons project to contralateral thalamus (cross in medulla at lemniscal decussation) ||Dorsal column |
|Spinothalamic tracts ||Sharp pain, temperature, crude touch ||Skin ||Dorsal horn. Second-order neurons project to contralateral thalamus (cross in spinal cord close to level of entry) ||Ventrolateral column |
|DDorsal spinocerebellar tract ||Movement and position mechanisms ||Muscle spindles, Golgi tendon organs, touch and pressure receptors (via nucleus dorsalis [Clarke's column]) ||Cerebellar paleocortex (via ipsilateral inferior cerebellar peduncle) ||Lateral column |
|Ventral spinocerebellar ||Movement and position mechanisms ||Muscle spindles, Golgi tendon organs, touch and pressure receptors ||Cerebellar paleocortex (via contralateral and ipsilateral superior cerebellar peduncle) ||Lateral column |
|Spinoreticular pathway ||Deep and chronic pain ||Deep somatic structures ||Reticular formation of brain stem ||Polysynaptic, diffuse pathway in ventrolateral column |
These tracts, which are part of the medial lemniscal system, convey well-localized sensations of fine touch, vibration, two-point discrimination, and proprioception (position sense) from the skin and joints; they ascend, without crossing, in the dorsal white column of the spinal cord to the lower brain stem (Fig 5–14). The fasciculus gracilis carries input from the lower half of the body, with fibers that arise from the lowest, most medial segments. The fasciculus cuneatus lies between the fasciculus gracilis and the dorsal gray column; it carries input from the upper half of the body, with fibers from the lower (thoracic) segments more medial than the higher (cervical) ones. Thus, one dorsal column contains fibers from all segments of the ipsilateral half of the body arranged in an orderly somatotopic fashion from medial to lateral (Fig 5–15).
The dorsal column system in the spinal cord.
Somatotopic organization (segmental arrangement) in the spinal cord.
Ascending fibers in the gracile and cuneate fasciculi terminate on neurons in the gracile and cuneate nuclei (dorsal column nuclei) in the lower medulla. These second-order neurons send their axons, in turn, across the midline via the lemniscal decussation (also called the internal arcuate tract) and the medial lemniscus to the thalamus. From the ventral posterolateral thalamic nuclei, sensory information is relayed upward to the somatosensory cortex.
Small-diameter sensory axons conveying the sensations of sharp (noxious) pain, temperature, and crudely localized touch course upward, after entering the spinal cord via the dorsal root, for one or two segments at the periphery of the dorsal horn. These short, ascending stretches of incoming fibers that are termed the dorsolateral fasciculus, or Lissauer's tract, then synapse with dorsal column neurons, especially in laminas I, II, and V (Figs 5–11 and 5–16). After one or more synapses, subsequent fibers cross to the opposite side of the spinal cord and then ascend within the spinothalamic tracts, also called the ventrolateral (or anterior) system. These spinothalamic tracts actually consist of two adjacent pathways: The anterior spinothalamic tract carries information about light touch, and the lateral spinothalamic tract conveys pain and temperature sensibility upward.
The spinothalamic (ventrolateral) system in the spinal cord.
The spinothalamic tracts, like the dorsal column system, show somatotopic organization (see Fig 5–15). Sensation from sacral parts of the body is carried in lateral parts of the spinothalamic tracts, whereas impulses originating in cervical regions are carried by fibers in medial parts of the spinothalamic tracts. Axons of the spinothalamic tracts project rostrally after sending branches to the reticular formation in the brain stem and project to the thalamus (ventral posterolateral, intralaminar thalamic nuclei).
The second-order neurons of both the dorsal column system and spinothalamic tracts decussate. The pattern of decussation is different, however. The axons of second-order neurons of the dorsal column system cross in the lemniscal decussation in the medulla; these second-order sensory axons are called internal arcuate fibers where they cross. In contrast, the axons of second-order neurons in the spinothalamic tracts cross at every segmental level in the spinal cord. This fact aids in determining whether a lesion is in the brain or the spinal cord. With lesions in the brain stem or higher, deficits of pain perception, touch sensation, and proprioception are all contralateral to the lesion. With spinal cord lesions, however, the deficit in pain perception is contralateral to the lesion, whereas the other deficits are ipsilateral. Clinical Illustration 5–1 provides an example.
CLINICAL ILLUSTRATION 5–1
A 27-year-old electrician was stabbed in the back at the midthoracic level. On examination, he was unable to move his right leg, and there was moderate weakness of finger flexion, abduction, and adduction on the right. There was loss of position sense in the right leg, and the patient could not appreciate a vibrating tuning fork that was placed on his toes or bony prominences at the right ankle, knee, or iliac crest. There was loss of pain and temperature sensibility below the T2 level on the left.
Magnetic resonance maging showed a hemorrhagic lesion involving the spinal cord at the C8–T1 level, and the patient was taken to the operating room. A blood clot that was partially compressing the cord was removed, and bone fragments were retrieved from the spinal canal. The surgeon observed that the spinal cord had been partially severed, on the right side, at the C8 level. The patient's deficits did not improve.
This case provides an example of Brown–Séquard syndrome resulting from unilateral lesions or transections of the spinal cord, which occurs most commonly in the context of stab injuries or gunshot wounds. Ipsilateral weakness and loss of position and vibration sense below the lesion is a result of transection of the lateral corticospinal tract and dorsal columns. A loss of pain and temperature sensibility manifests a few segments below the level of the lesion because the decussating fibers enter the spinothalamic tract a few segments rostral to the level of entry of the nerve root.
Segregation of second-order sensory axons carrying pain sensibility within the lateral spinothalamic tract is of considerable clinical importance. As might be expected, unilateral interruption of the lateral spinothalamic tract causes a loss of sensibility to pain and temperature, beginning about a segment below the level corresponding to the lesion, on the opposite side of the body. Neurosurgeons occasionally may take advantage of this fact when performing an anterolateral cordotomy in patients with intractable pain syndrome.
D. Spinoreticular Pathway
The ill-defined spinoreticular tract courses within the ventrolateral portion of the spinal cord, arising from cord neurons and ending (without crossing) in the reticular formation of the brain stem. This tract plays an important role in the sensation of pain, especially deep, chronic pain (see Chapter 14).
E. Spinocerebellar Tracts
Two ascending pathways (of lesser importance in human neurology) provide input from the spinal cord to the cerebellum (Fig 5–17 and Table 5–4).
The spinocerebellar systems in the spinal cord.
1. Dorsal spinocerebellar tract
Afferent fibers from muscle and skin (which convey information from muscle spindles, Golgi tendon organs, and touch and pressure receptors) enter the spinal cord via dorsal roots at levels T1 to L2 and synapse on second-order neurons of the nucleus dorsalis (Clarke's column). Afferent fibers originating in sacral and lower lumbar levels ascend within the spinal cord (within the dorsal columns) to reach the lower portion of the nucleus dorsalis.
The dorsal nucleus of Clarke is not present above C8; it is replaced, for the upper extremity, by a homologous nucleus called the accessory cuneate nucleus. Dorsal root fibers originating at cervical levels synapse with second-order neurons in the accessory cuneate nucleus.
The second-order neurons from the dorsal nucleus of Clarke form the dorsal spinocerebellar tract; second-order neurons from the lateral cuneate nucleus form the cuneocerebellar tract. Both tracts remain on the ipsilateral side of the spinal cord, ascending via the inferior cerebellar peduncle to terminate in the paleocerebellar cortex.
2. Ventral spinocerebellar tract
This system is involved with movement control. Second-order neurons, located in Rexed's laminae V, VI, and VII in lumbar and sacral segments of the spinal cord, send axons that ascend through the superior cerebellar peduncle to the paleocerebellar cortex. The axons of the second-order neurons are largely but not entirely crossed.