Spinal Cord

Each Spinal Cord Segment Innervates a Dermatome

As the neural tube closes, the adjacent mesoderm also segments, here into a series of somites (see Fig. 2.3 ) that will give rise to skin, muscle, and bone. Each spinal nerve retains its relationship with a somite during development, with the result that spinal cord segments are related systematically to areas of skin, to muscles, and in some instances to bones (e.g., vertebrae). Therefore each spinal nerve (except C1, which typically has only a rudimentary dorsal root) innervates a single dermatome ( Fig. 10.4 ). This dermatomal arrangement is particularly apparent in the trunk, where pairs of dermatomes form bands that encircle the chest and abdomen; outgrowth of limb buds during development makes the dermatomal arrangement somewhat more complex in the upper and lower extremities. ^a

a Dermatomes are generally not demarcated from one another as abruptly as Fig. 10.4 seems to indicate. When neighboring areas of skin are innervated by consecutive spinal segments (e.g., T6 and T7), the territories innervated by the two segments overlap considerably. However, when neighboring areas of skin are innervated by nonconsecutive segments (e.g., C4 and T2 dermatomes), the overlap between the two territories is limited.

Similarly, the innervation of skeletal muscles is related systematically to spinal segments ( Table 10.1 ).

Knowledge of the segmental innervation of muscles and cutaneous areas ( Table 10.2 ) can be extremely helpful in diagnosing the site of damage in or near the spinal cord. For example, compression of a dorsal root can cause pain in its dermatome, allowing pain caused by root compression to be differentiated from pain caused by peripheral nerve damage. In addition, the highest level of a sensory or motor deficit may allow deductions about the segmental level of a suspected spinal cord lesion (see Fig. 10.31 ).

The Spinal Cord Is Shorter Than the Vertebral Canal

The spinal cord approaches its adult length before the vertebral canal does. Until the third month of fetal life, both grow at about the same rate, and the cord fills the canal. Thereafter the body and the vertebral column grow faster than the spinal cord does, so at the time of birth the spinal cord ends at the third lumbar vertebra. A small additional amount of differential growth in the vertebral column occurs subsequent to this, and by a few months of age the cord ends at about the level of the first lumbar vertebra. However, the spinal nerves still exit through the same intervertebral foramina as they did early in development, and each dorsal root ganglion remains at the level of the appropriate foramen. Proceeding from cervical to sacral levels, the dorsal and ventral roots become progressively longer because they have longer and longer distances to travel before reaching their sites of exit from the vertebral canal (see Fig. 10.3A ). The lumbar cistern, from the end of the spinal cord at vertebral level L1-L2 to the end of the dural sheath at vertebral level S2, is filled with this collection of dorsal and ventral roots, collectively referred to as the cauda equina (Latin for “horse's tail”; Fig. 10.5E and F ). Therefore a needle carefully inserted into the lumbar cistern will pass harmlessly among nerve roots, allowing safe sampling of cerebrospinal fluid.

Each of the first seven cervical nerves leaves the vertebral canal above the corresponding vertebra; for instance, the first cervical nerve leaves between the occiput and the first cervical vertebra (the atlas), the second leaves between the first and second cervical vertebrae (the atlas and the axis), and so on. However, because there are only seven cervical vertebrae, the eighth cervical nerve leaves between the seventh cervical and first thoracic vertebrae, and each of the subsequent nerves leaves below the corresponding vertebra.

The meningeal coverings of the spinal cord are described in Chapter 4 (see Fig. 4.13 ). The cord is suspended within an arachnoid-lined dural tube by the denticulate ligaments ( Fig. 10.6A ), which are extensions of the pia-arachnoid, similar to but more substantial than arachnoid trabeculae. In addition, the caudal end of the cord is anchored to the end of the dural tube by the filum terminale (see Fig. 10.6B ), an extension of the pial covering of the conus medullaris. The filum terminale then acquires a dural outer layer and in turn is anchored to the coccyx.

The Posterior Horn Contains Sensory Interneurons and Projection Neurons

The posterior horn consists mainly of interneurons whose processes remain within the spinal cord and of projection neurons whose axons collect into long, ascending sensory pathways. This area of gray matter contains two prominent parts, the substantia gelatinosa and the body of the posterior horn, both present at all spinal levels.

The substantia gelatinosa is a distinctive region of gray matter that caps the posterior horn ( Fig. 10.8 ). In myelin-stained preparations this region looks pale compared with the rest of the gray matter because it deals mostly with finely myelinated and unmyelinated sensory fibers that carry pain and temperature information. Between the substantia gelatinosa and the surface of the cord is a relatively pale-staining area of white matter called Lissauer's tract. ^d

d Lissauer's tract is an unusual case in which an eponym is becoming more commonly used rather than fading away. For many years Lissauer's tract was also known by the descriptive term dorsolateral fasciculus. However, as described elsewhere in this and the next chapter, the dorsal part of the lateral funiculus is now known to contain some distinctively important ascending and descending pathways. As a result, many now refer to the latter area of spinal white matter as the dorsolateral fasciculus or funiculus. To avoid ambiguity, Lissauer's tract is probably best referred to by its eponymous name.

This tract stains more lightly than the rest of the white matter because it contains the finely myelinated and unmyelinated fibers with which the substantia gelatinosa deals.

The body of the posterior horn consists mainly of interneurons and projection neurons that transmit various types of somatic and visceral sensory information. In this respect it functionally overlaps parts of the intermediate gray matter.

The Anterior Horn Contains Motor Neurons

The anterior horn contains the cell bodies of the large motor neurons that supply skeletal muscle ( Fig. 10.9 ). These alpha motor neurons, also referred to as lower motor neurons, ^e

e They are also referred to simply as anterior horn cells, even though other cell types also live in the anterior horn.

are the only means by which the nervous system can exercise control over body movements, whether voluntary or involuntary; a number of different parts and pathways of the nervous system can influence these lower motor neurons, but they alone can elicit muscle contraction. Destruction of the lower motor neurons supplying a muscle or interruption of their axons therefore causes complete paralysis of that muscle. Lower motor neuron lesions cause paralysis of a type called flaccid paralysis, indicating that the muscle is limp and uncontracted. Reflex contractions can no longer be elicited, and the muscle slowly atrophies (because of a lack of trophic factors normally delivered to it by motor axons; see Chapter 24 ). This occurs, for example, in poliomyelitis (a viral disease that attacks the motor neurons of the anterior horn) and in injuries in which ventral roots are damaged.

Alpha motor neurons occur in longitudinally oriented, cigar-shaped groups, each group innervating an individual muscle. Therefore in cross sections they appear to be arranged in clusters ( Fig. 10.10 ), separated from one another by areas of interneurons; the clusters that innervate axial muscles are medial to those that innervate limb muscles. In the cervical and lumbar enlargements, which innervate the limbs, the anterior horns are enlarged laterally to accommodate the additional motor neurons (see Fig. 10.8 ). Smaller gamma motor neurons are interspersed with alpha motor neurons in all such groups. They innervate the intrafusal muscle fibers of muscle spindles, so they are also referred to as fusimotor neurons.

Two columns of motor neurons in the anterior horn of the cervical cord are recognized as separate entities. The spinal accessory nucleus extends from the caudal medulla to about C5. The axons of these motor neurons emerge from the lateral surface of the spinal cord just posterior to the denticulate ligament as a separate series of rootlets that form the accessory nerve (see Fig. 3.18 ). The phrenic nucleus, containing the motor neurons that innervate the diaphragm, is located in the medial portion of the anterior horn in segments C3 to C5. This makes injuries to the upper cervical spinal cord a matter of grave concern, because destruction of the descending pathways that control the phrenic nucleus and other respiratory motor neurons renders a patient unable to breathe.

The Intermediate Gray Matter Contains Autonomic Neurons

The gray matter that is intermediate to the anterior and posterior horns has some characteristics of both and also contains the spinal preganglionic autonomic neurons. In addition, at some levels it includes a distinctive region called Clarke's nucleus.

The preganglionic sympathetic neurons for the entire body lie in segments T1 through L3, most of them located in a column of cells called the intermediolateral cell column, which forms a pointy lateral horn on the spinal gray matter (see Fig. 10.8 ). Their axons leave through the ventral roots. Cells in a corresponding location in segments S2 to S4 constitute the sacral parasympathetic nucleus but do not form a distinct lateral horn. Their axons leave through the ventral roots and synapse on the postganglionic parasympathetic neurons for the pelvic viscera.

The posterior thoracic nucleus (i.e., Clarke's nucleus, or the nucleus dorsalis of Clarke) is a rounded collection of large cells located on the medial surface of the base of the posterior horn from about T1 to L2. It is particularly prominent at lower thoracic levels (see Fig. 10.8 ). This is an important relay nucleus for the transmission of information to the cerebellum and may also play a role in forwarding proprioceptive information from the leg to the thalamus. Because of its prominent role in sensory processing, it is treated as part of the posterior horn.

The remainder of the intermediate gray matter is a collection of various projection neurons, sensory interneurons, and interneurons that synapse on motor neurons.

Spinal Cord Gray Matter Is Arranged in Layers

In 1952 Rexed devised a system for subdividing the gray matter of the cat's spinal cord into layers, or laminae. The same system has since been applied to the cords of other mammals, including humans (see Fig. 10.7B ). Lamina I (also called the marginal zone ) is a thin layer of gray matter that covers the substantia gelatinosa, lamina II is the substantia gelatinosa, and laminae III through VI are the body of the posterior horn; lamina VII roughly corresponds to the intermediate gray matter (including Clarke's nucleus) but also includes large extensions into the anterior horn; lamina VIII comprises some of the interneuronal zones of the anterior horn, whereas lamina IX consists of the clusters of motor neurons embedded in the anterior horn; lamina X is the zone of gray matter surrounding the central canal.

This terminology has proved useful for experimental anatomists and physiologists because the histological differences among the laminae correspond to functional differences ( Table 10.3 ). For example, the functional dichotomy between large- and small-diameter peripheral nerve fibers is maintained to a great extent in the patterns of termination of these fibers in the spinal gray matter: there are prominent (although not exclusive) terminations of pain and temperature afferents in laminae I and II, tactile afferents from cutaneous nerves in lamina III, and Ia muscle spindle afferents in laminae VI, VII, and IX.

Muscle Stretch Leads to Excitation of Motor Neurons

All skeletal muscles (except perhaps some muscles in the head) contract to at least some extent in response to being stretched. The reflex arc responsible for this contraction uses the simplest possible route through the CNS because it involves only two neurons and a single intervening synapse. It is therefore sometimes referred to as the monosynaptic reflex or the myotatic reflex (from two Greek words meaning “muscle stretch”). The afferent limb of the arc is a Ia afferent with its associated muscle spindle primary ending. Central processes of the Ia afferent make synapses within the spinal cord directly on the alpha motor neurons that innervate the muscle containing the stimulated spindle ( Fig. 10.11 ).

The stretch reflex is commonly used for clinical testing purposes. Tapping the patellar tendon, as in the familiar knee-jerk reflex, stretches the quadriceps slightly. Ia endings in quadriceps muscle spindles are excited and in turn excite quadriceps alpha motor neurons; these cause the quadriceps to contract, completing the reflex. Similarly, tapping the Achilles tendon stretches the gastrocnemius slightly, thereby causing a reflex contraction. Testing a variety of stretch reflexes can provide valuable clinical information about the integrity not only of peripheral nerves but also of predictable spinal cord segments ( Table 10.4 ). Because stretch reflexes are usually elicited by tapping a tendon, they are often referred to as deep tendon reflexes (sometimes abbreviated as DTRs ). Even though the reflex is studied in this manner, the responsible receptors are in the muscles attached to the tapped tendons.

Stretch reflexes are thought to be important for the constant automatic corrections performed during movements and postures (although other reflexes may be more important for this function). For example, when standing still and upright, a person sways to and fro a bit. Each time that person sways in one direction, muscles are stretched and the resulting reflex contraction helps return him or her toward the desired position.