The Spinal Cord Is Segmented

Segments of the spinal cord (Fig. 10-1)—8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal—are defined by the spinal nerves formed from dorsal and ventral roots attached bilaterally to each segment. The spinal cord has a cervical and a lumbar enlargement, serving the needs of the arms and legs respectively, and ends at the pointed conus medullaris. The conus medullaris is located at vertebral level L1/L2, even though the dural sac surrounding the spinal cord extends to vertebral level S2. So dorsal and ventral roots from progressively more caudal levels need to travel progressively longer distances through spinal subarachnoid space before reaching their intervertebral foramina of entry or exit. The collection of spinal nerves in subarachnoid space caudal to the conus medullaris is the cauda equina (Latin for “horse’s tail”).

Figure 10-1 Overview of spinal cord gross anatomy.

Spinal nerves C1-C7 use the foramen above the corresponding vertebra, C8 uses the foramen between vertebrae C7 and T1, and all others use the foramen below the corresponding vertebra.

All Levels of the Spinal Cord Have a Similar Cross-Sectional Structure

The gray matter core of the spinal cord is roughly in the shape of an H with a dorsal-ventral orientation at all levels (Fig. 10-2). The dorsally directed limbs of the H are the posterior (or dorsal) horns, and the ventrally directed limbs are the anterior (or ventral) horns. The posterior horn, derived from the alar plate of the neural tube, is a sensory processing area that receives most of the afferents that arrive in ipsilateral dorsal roots. The anterior horn is derived from the basal plate and contains the motor neurons whose axons form the ventral roots. The intermediate gray matter between the anterior and posterior horns is a mixture of interneurons and tract cells in sensory and motor circuits; at some levels it also contains autonomic motor neurons with axons that leave in the ventral roots.

Figure 10-2 General cross-sectional anatomy of the spinal cord. At all levels, large-diameter (1) and small-diameter (2) sensory fibers enter through the dorsal root and feed into ascending pathways and reflex arcs; axons of lower motor neurons (3) leave through ventral roots. Some levels, as described a little later, also contain preganglionic autonomic neurons (4) with axons that leave through ventral roots. AH, Anterior horn; IG, intermediate gray; PH, posterior horn.

The anterior and posterior horns divide the surrounding spinal white matter into anterior, lateral, and posterior funiculi (funiculus is Latin for “string”).

As dorsal roots approach the spinal cord, the afferent fibers sort themselves out so that large-diameter fibers enter medial to small-diameter fibers. The large-diameter fibers, carrying touch and position information, have some branches that travel rostrally in the posterior funiculus and others that end in deeper portions of the posterior horn. The small-diameter fibers, primarily carrying pain and temperature information, travel in the dorsolateral fasciculus (Lissauer’s tract) to termination sites in a superficial zone of the posterior horn called the substantia gelatinosa.

The Spinal Cord Is Involved in Sensory Processing, Motor Outflow, and Reflexes

The wiring principles discussed in Chapter 3 are pretty apparent in the spinal cord. Central processes of primary afferents (cell bodies in dorsal root ganglia) are the only routes through which sensory information from the body can reach the spinal cord. They give rise to branches that feed into reflex circuits, into pathways to the thalamus, and into pathways to the cerebellum. These branches end in ipsilateral gray matter, mostly but not entirely in the posterior horn. Lower motor neurons in the anterior horn receive inputs from reflex circuits, as well as through descending pathways (i.e., axons of upper motor neurons), and project to ipsilateral muscles. They are the only routes through which the spinal cord can tell skeletal muscles to contract, and loss of lower motor neurons or their axons is followed by flaccid paralysis of the muscles they used to innervate—profound weakness, loss of tone and reflexes, and atrophy.

Spinal Gray Matter Is Regionally Specialized

Some parts of the spinal gray matter (e.g., the substantia gelatinosa) are present in all segments. Others are either present at only some levels or emphasized at some levels in ways that make functional sense (THB6 Figure 10-8, p. 236). The most prominent examples of the former are preganglionic sympathetic neurons, present in the T1-L3 intermediate gray and forming a pointy lateral horn in thoracic segments; preganglionic parasympathetic neurons, present in the S2-S4 intermediate gray; and Clarke’s nucleus, at the base of the posterior horn from T1-L2 and particularly prominent at lower thoracic levels. As an example of level-specific emphasis, an anterior horn is present at all levels but is enlarged laterally in the cervical and lumbar enlargements to accommodate all the lower motor neurons for distal muscles supplied by these levels.

Reflex Circuitry Is Built into the Spinal Cord

Reflexes are involuntary, stereotyped responses to sensory inputs, and every kind of sensory input is involved in reflex circuitry of one or more types. The simplest kind of reflex imaginable (other than the axon reflex) involves a primary afferent and a lower motor neuron, with a synapse in the CNS connecting the two. This is the circuit underlying stretch reflexes (Fig. 10-3), through which a muscle contracts in response to being stretched. (Stretch reflexes are tested by tapping tendons and so they’re often called deep tendon reflexes, even though the receptors that initiate them are located in muscle spindles and not tendons.)

Figure 10-3 The stretch reflex arc.

Stretch reflexes are the only monosynaptic reflexes. All others involve one or more interneurons. An example is the flexor reflex (or withdrawal reflex), through which a limb is removed from a painful stimulus (Fig. 10-4). This reflex is considerably more complex than a stretch reflex because all the muscles of a limb, and therefore motor neurons in several spinal segments, come into play.

Figure 10-4 The flexor (withdrawal) reflex arc, which involves connections, through interneurons, in multiple spinal segments.

Reflexes are often depicted as isolated responses to a stimulus, but typically there are associated reciprocal and crossed effects that enhance their effectiveness. Tapping the patellar tendon stretches the quadriceps, which contracts in response; at the same time, hamstrings motor neurons are inhibited, making it easier for the quadriceps to shorten. As one foot is withdrawn from a painful stimulus, extensor motor neurons for the contralateral leg are automatically excited, making it easier for that leg to support the body.

Ascending and Descending Pathways Have Defined Locations in the Spinal White Matter

There are only three pathways of major clinical significance in the spinal cord: the posterior columns (touch and position), spinothalamic tract (pain and temperature), and lateral corticospinal tract (commands for voluntary movements). Each has a consistent location at all cord levels (see Fig. 10-9 later in this chapter).

The Posterior Column–Medial Lemniscus System Conveys Information about Touch and Limb Position

Key Concepts

Information about the location and nature of a stimulus is preserved in the posterior column–medial lemniscus system.

Damage to the posterior column–medial lemniscus system causes impairment of proprioception and discriminative tactile functions.

Large-diameter primary afferents, carrying touch and position information, have collaterals that ascend through the ipsilateral posterior funiculus. Incoming collaterals add on lateral to those already present in the posterior column, so by cervical levels each posterior column is subdivided into a medial fasciculus gracilis, representing the leg, and a more lateral fasciculus cuneatus, representing the arm (Fig. 10-5). Fasciculi gracilis and cuneatus then terminate in nuclei gracilis and cuneatus, the posterior column nuclei of the caudal medulla. Second order fibers from neurons of the posterior column nuclei cross the midline and form the medial lemniscus, which ascends through the brainstem to the thalamus (to a thalamic nucleus called the ventral posterolateral nucleus (VPL), for reasons explained in Chapter 16). VPL neurons then project to somatosensory cortex of the postcentral gyrus and adjacent areas.

Figure 10-5 The posterior column–medial lemniscus pathway. DRG, Dorsal root ganglion; FC, fasciculus cuneatus; FG, fasciculus gracilis; ML, medial lemniscus.

There’s a well-defined, modality-specific somatotopic organization at all levels of this pathway, which makes it easier to keep track of details about the nature and location of stimuli.

The Spinothalamic Tract Conveys Information about Pain and Temperature

Key Concept

Damage to the anterolateral system causes diminution of pain and temperature sensations.

The most important pathway for pain and temperature information is the spinothalamic tract (Fig. 10-6); it also carries some touch information. Spinothalamic neurons (i.e., the second order cells) are located in the posterior horn, so this pathway, unlike the posterior column-medial lemniscus pathway, crosses the midline in the spinal cord. Small-diameter primary afferents, carrying pain and temperature and some touch information, traverse Lissauer’s tract and end on spinothalamic neurons in the posterior horn. Small interneurons of the substantia gelatinosa regulate the transmission of information at this synapse. The axons of spinothalamic neurons then cross the midline and form the spinothalamic tract, which ascends through the brainstem to VPL and to other thalamic nuclei as well (corresponding to pain usually having a more widespread effect on attention and mood than touch does). These thalamic neurons then project to somatosensory cortex of the postcentral gyrus and some other cortical areas (again, more widespread than touch information). Intermingled with these direct spinothalamic fibers are other axons that convey pain and temperature information from the posterior horn to the thalamus indirectly, by way of the reticular formation, and to other sites as well. The entire collection of ascending pain and temperature fibers is often referred to collectively as the anterolateral pathway because of its position in the spinal cord.

Figure 10-6 The spinothalamic tract.

Descending Pathways Influence the Activity of Lower Motor Neurons

Key Concept

The corticospinal tracts mediate voluntary movement.

The most important pathway for the control of voluntary movement is the corticospinal tract (also called the pyramidal tract because its fibers travel through the pyramids on the ventral surface of the medulla). Corticospinal neurons are located in the precentral gyrus and adjacent cortical areas. Their axons descend through the internal capsule (bypassing the thalamus) and travel in a ventral location in the brainstem, passing through the cerebral peduncle (on the ventral surface of the midbrain), the basal pons, and the medullary pyramids (Fig. 10-7). Most of these fibers then cross the midline in the pyramidal decussation at the medulla-spinal cord junction and enter the lateral corticospinal tract. (The few uncrossed fibers enter the anterior corticospinal tract in the anterior funiculus.) Corticospinal fibers then end on motor neurons or interneurons of the spinal gray matter. Just as in the case of sensory pathways, there are additional, parallel routes for some of this input to reach the spinal cord, so destruction of the corticospinal tract causes weakness but not total paralysis. For example, projections from the vestibular nuclei of the brainstem to the spinal cord (vestibulospinal tracts) provide one alternate route.

Figure 10-7 The lateral corticospinal tract. IC, Internal capsule.

Neurons that project to lower motor neurons are called upper motor neurons, and damage to them causes a distinctive type of weakness. Reflex circuitry survives and, over time, becomes more influential than normal; hyperreflexia and increased muscle tone result. In addition, some normally suppressed spinal circuitry is unmasked and several pathological reflexes emerge. The most famous of these is Babinski’s sign (dorsiflexion of the big toe and fanning of the others in response to firmly stroking the sole of the foot).

The Autonomic Nervous System Monitors and Controls Visceral Activity

The autonomic nervous system (ANS) does not project efferents from the CNS directly to smooth or cardiac muscles or to glands. Instead, preganglionic autonomic neurons in the CNS project through the ventral roots to postganglionic neurons in ganglia outside the CNS (Fig. 10-8). These postganglionic neurons then project to target organs. (The only exception is the adrenal medulla, which receives autonomic (sympathetic) projections directly from the CNS.) Preganglionic axons are thinly myelinated, while postganglionic axons are unmyelinated.

Figure 10-8 General layout of sympathetic and parasympathetic neurons. ACh, Acetylcholine; NE, norepinephrine.

A Longitudinal Network of Arteries Supplies the Spinal Cord

Each vertebral artery gives rise to an anterior and a posterior spinal artery. The posterior spinal arteries form a plexiform network that travels along the line of attachment of the dorsal roots and supplies the posterior columns, the posterior horns, and part of the lateral corticospinal tract. The two anterior spinal arteries fuse and travel along the midline of the spinal cord, supplying its anterior two thirds. Blood from the vertebral artery only reaches cervical segments; below this level, both anterior and posterior spinal arteries receive blood from segmental arteries.

Spinal Cord Damage Causes Predictable Deficits

Spinal cord damage is typically followed by an acute phase of spinal shock, in which reflexes and muscle tone are depressed. The consistent locations of tracts and cell groups at all levels of the spinal cord (Fig. 10-9) make it possible to predict the subsequent chronic effects of partial cord damage (Fig. 10-10). Sensory deficits will be on the side ipsilateral to the damage if a pathway is affected before it crosses (e.g., posterior column), and on the contralateral side if affected after it crosses (e.g., spinothalamic tract). Weakness will be on the side ipsilateral to the damage because (1) lower motor neurons project to ipsilateral muscles, and (2) most corticospinal fibers cross in the pyramidal decussation (at the spinal cord-medulla junction) and are therefore on their way to ipsilateral lower motor neurons. So after unilateral damage there’s a seemingly weird mix of ipsilateral (touch, position, strength) and contralateral (pain, temperature) deficits.

Figure 10-9 Summary of major spinal cord tracts, as seen at a lower cervical level. The spinothalamic and corticospinal tracts are present at all levels. Below about T6, the posterior column system is represented only by fasciculus gracilis.

Figure 10-10 Effects of damage in various parts of the spinal cord.