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FIGURE 1 Organization and internal structure of the spinal cord. Pairs of spinal nerves leave the vertebral column through spaces between the vertebra. These nerves are classified by the region of the body they innervate: cervical, thoracic, lumbar, or sacral. (A) Lateral and (B) dorsal views of the entire spinal cord. The spinal cord is shown in blue, and spinal neurons are in black. Note that the spinal cord is shorter than the vertebral column, leaving a space at the caudal end of the vertebral column that contains only spinal nerves. (C) Cross sections of the spinal cord at different levels. Note that the white matter fiber tracts (shown in white) increase in size from caudal to rostral levels. Also, the areas of gray matter, containing cell bodies and many synaptic connections (shown in blue), are larger in the cervical and lumbar regions because of the many motor and sensory fibers that innervate arms and legs respectively.

segments are divided into four groups: cervical (segments C1-C8); thoracic (segments T1-T12); lumbar (segments L1-L5); and sacral (segments S1-S5). At higher levels of the cord, nerves and vertebral notches are fairly well aligned, whereas at lower levels the alignment is lost because the cord is considerably shorter than the vertebral column. Long projections of spinal nerves called the cauda equina (Latin for ''horse's tail'') fill the vertebral column at lumbar and sacral levels, providing clinicians with a relatively safe area from which to extract cere-brospinal fluid, as described in Chapter 48.

Cell bodies within each dorsal root ganglion collectively innervate one area of the body designated as a dermatome (Table 1). Although not traditionally included in illustrations of dermatome distributions (including Figs. 2 and 3), central projections from each ganglion usually overlap two or three spinal cord levels, resulting in significant overlap between adjacent dermatomes. Thus, sensory sensation can be reasonably maintained if only a single ganglion is lesioned.

Once the spinal nerves leave the bony protection of the vertebrae or skull, they reorganize according to their peripheral destinations. For example, the majority of the neurons from cervical nerves C4 to C8 and thoracic nerves T1 and T2 coalesce to form the brachial plexus. They then redivide into anterior and posterior branches and finally segregate into individual peripheral nerves such as the radial nerve, composed of both sensory and

seventh cervical segment (C7)

FIGURE 2 Internal structure of the spinal cord. Primary neurons carrying information about proprioception and discriminatory touch enter the spinal cord and ascend uncrossed in the dorsal columns to the gracilus or cuneate nuclei in the brain stem. Fibers are somatopically organized, with cervical and thoracic fibers projecting in the fasciculus cuneatus, and lumbar and sacral fibers projecting in the fasciculus gracilus. Primary neurons carrying information about pain, temperature, and crude touch enter the spinal cord and make one or more synaptic connections in the dorsal horn before crossing the midline and ascending in the anteriolateral tract or the ventral spinocerebellar tract.

seventh cervical segment (C7)

FIGURE 2 Internal structure of the spinal cord. Primary neurons carrying information about proprioception and discriminatory touch enter the spinal cord and ascend uncrossed in the dorsal columns to the gracilus or cuneate nuclei in the brain stem. Fibers are somatopically organized, with cervical and thoracic fibers projecting in the fasciculus cuneatus, and lumbar and sacral fibers projecting in the fasciculus gracilus. Primary neurons carrying information about pain, temperature, and crude touch enter the spinal cord and make one or more synaptic connections in the dorsal horn before crossing the midline and ascending in the anteriolateral tract or the ventral spinocerebellar tract.

motor fibers. This major nerve collects sensory information from the back of the arm and carries motor commands to extensor-supinator muscles of the arm.

Within the internal structure of the spinal cord, there are no apparent markers of segmental boundaries. Cell bodies within the spinal cord are concentrated in a continuous central core of gray matter that bulges anteriorly to form the ventral horns associated with motor systems and posteriorly to form the dorsal horns associated with sensory systems. White matter, consisting of afferent and efferent fibers, fills in the contours of the gray matter and helps accentuate the butterfly pattern of gray matter seen in cross sections of the cord. From a three-dimensional perspective, the white matter extends as a series of columns within the spinal cord. Ascending sensory fibers are found in the dorsal and lateral columns; descending motor fibers are concentrated in lateral and anterior columns. Although there are no anatomic boundaries between segments, variations in internal organization reflect functional differences among the four levels of the spinal cord. For example, cervical and lumbar segments are slightly enlarged overall and have pronounced ventral horns because of the large numbers of motor neurons required for innervation of muscles in the extremities, whereas the thoracic cord has very narrow ventral horns. Likewise, the number of efferent fibers ascending within the spinal cord increases at higher levels. The cervical cord contains sensory projections from the entire body and thus has the largest proportion of white matter, whereas the sacral level contains only fibers from sacral dermatomes and has correspondingly less white matter.

TABLE 1 Examples of Dermatomal Distributions Dermatome Area of Body Represented

C5 Lateral shoulder

C6 Thumb

C7 Index and middle fingers

C8, T1 Ring, little fingers

T4 Nipple

T10 Navel

L3, L4 Anterior thigh

L5 Dorsal foot

S1 Lateral foot, sole

DORSAL COLUMN/LATERAL LEMNISCAL PATHWAY

Sensory fibers within the dorsal rootlets project into the cord in a spatially precise and modality-specific fashion. A shallow groove, the dorsal sulcus, marks the point of attachment of the spinal nerves on the dorsal surface of the cord. Sensory fibers enter the underlying

FIGURE 3 Dermatomes map the orderly distribution of sensory projects from the skin to the spinal cord. (A) Cell bodies within each dorsal root ganglion collectively innervate one region of skin called a dermatome; the name of each dermatome (from C2 through S5) denotes the ganglion that innervates it, Dermatomes of the face are named to reflect innervation by the three divisions (oral, VO; interposed, VI; caudal, VC) of the trigeminal ganglion. Each dorsal root ganglion sends central fibers primarily to one level of the spinal cord, with some degree of overlap (not shown). However, the fibers that project from each ganglion to the skin for the most part do not travel as separate nerves; rather, they are combined with sensory nerve fibers from other ganglia and also with motor nerves projecting to muscles in the same region. Two examples are shown. (B) The peripheral projections of the radial nerve combine fibers from portions of C6, C7, C8, and T1 dorsal root ganglia. (C) Peripheral projections of the trigeminal ganglion are rearranged into ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve.

FIGURE 3 Dermatomes map the orderly distribution of sensory projects from the skin to the spinal cord. (A) Cell bodies within each dorsal root ganglion collectively innervate one region of skin called a dermatome; the name of each dermatome (from C2 through S5) denotes the ganglion that innervates it, Dermatomes of the face are named to reflect innervation by the three divisions (oral, VO; interposed, VI; caudal, VC) of the trigeminal ganglion. Each dorsal root ganglion sends central fibers primarily to one level of the spinal cord, with some degree of overlap (not shown). However, the fibers that project from each ganglion to the skin for the most part do not travel as separate nerves; rather, they are combined with sensory nerve fibers from other ganglia and also with motor nerves projecting to muscles in the same region. Two examples are shown. (B) The peripheral projections of the radial nerve combine fibers from portions of C6, C7, C8, and T1 dorsal root ganglia. (C) Peripheral projections of the trigeminal ganglion are rearranged into ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve.

cord and segregate into two major pathways. The first pathway, consisting of fibers carrying information about light touch and proprioception, ascends in the dorsal columns. Axons innervating the lower body assume a position along the posterior midline; fibers from the upper body are added sequentially in a mediolateral fashion. In the midthoracic level of the cord, the dorsal column of fibers is divided into two fasciculi. The more medial area, called the gracile fasciculus, contains fibers from sacral, lumbar, and lower thoracic dermatomes. The more lateral area, called the cuneate fasciculus, carries fibers from upper thoracic and cervical dermatomes. The first synaptic contact for these firstorder sensory neurons is in the brain stem, at the level of the medulla in the gracile and cuneate nuclei, respectively. Fibers in the pathway do not cross the midline of the spinal cord as they ascend. Thus, a neuronal lesion affecting only one side of the spinal cord will interrupt information about light touch and proprioception from the ipsilateral side of the body below the level of the lesion. The patient will lose the ability to identify objects by touch and will have difficulty with balanced movement on the affected side. Note that this primary sensory pathway does not make intermediate synaptic contacts with neurons within the spinal cord.

Cells in the gracilis and cuneate nuclei represent second-order neurons within this pathway, and their axons cross the midline to ascend in the medial lemniscal tract to the ventral posterolateral nucleus (VPL) of the thalamus. These thalamic neurons then project to specific regions of the primary somatosensory cortex. The entire four-neuron chain comprises the dorsal column-medial lemniscal pathway (Fig. 4).

Points of synaptic contact within a pathway do not simply pass information along unchanged. Each relay serves as a computing center that allows the information to be processed, amplified, sharpened, suppressed, or otherwise altered. For example, the function of the dorsal column nuclei is to shape tactile information in time and space. These nuclei make edges seem sharper and abrupt pressure changes seem more sudden than they really are. These enhancements of textures are produced by lateral inhibition in much the same way as visual information is altered, as will be discussed in Chapter 51.

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