3. Anatomy of the Spinal Cord
Part 3 of 4

Nachum Dafny, Ph.D.
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Surrounding the gray matter is white matter containing myelinated and unmyelinated nerve fibers. These fibers conduct information up (ascending) or down (descending) the cord. The white matter is divided into the dorsal (or posterior) column (or funiculus), lateral column and ventral (or anterior) column (Figure 3.8). The anterior white commissure resides in the center of the spinal cord, and it contains crossing nerve fibers that belong to the spinothalamic tracts, spinocerebellar tracts, and anterior corticospinal tracts. Three general nerve fiber types can be distinguished in the spinal cord white matter: 1) long ascending nerve fibers originally from the column cells, which make synaptic connections to neurons in various brainstem nuclei, cerebellum and dorsal thalamus, 2) long descending nerve fibers originating from the cerebral cortex and various brainstem nuclei to synapse within the different Rexed layers in the spinal cord gray matter, and 3) shorter nerve fibers interconnecting various spinal cord levels such as the fibers responsible for the coordination of flexor reflexes. Ascending tracts are found in all columns whereas descending tracts are found only in the lateral and the anterior columns.

Four different terms are often used to describe bundles of axons such as those found in the white matter: funiculus, fasciculus, tract, and pathway. Funiculus is a morphological term to describe a large group of nerve fibers which are located in a given area (e.g., posterior funiculus). Within a funiculus, groups of fibers from diverse origins, which share common features, are sometimes arranged in smaller bundles of axons called fasciculus, (e.g., fasciculus proprius [Figure 3.8]). Fasciculus is primarily a morphological term whereas tracts and pathways are also terms applied to nerve fiber bundles which have a functional connotation. A tract is a group of nerve fibers which usually has the same origin, destination, and course and also has similar functions. The tract name is derived from their origin and their termination (i.e., corticospinal tract - a tract that originates in the cortex and terminates in the spinal cord; lateral spinothalamic tract - a tract originated in the lateral spinal cord and ends in the thalamus). A pathway usually refers to the entire neuronal circuit responsible for a specific function, and it includes all the nuclei and tracts which are associated with that function. For example, the spinothalamic pathway includes the cell bodies of origin (in the dorsal root ganglia), their axons as they project through the dorsal roots, synapses in the spinal cord, and projections of second and third order neurons across the white commissure, which ascend to the thalamus in the spinothalamic tracts.

The spinal cord white matter contains ascending and descending tracts.

Ascending tracts (Figure 3.8). The nerve fibers comprise the ascending tract emerge from the first order (1°) neuron located in the dorsal root ganglion (DRG). The ascending tracts transmit sensory information from the sensory receptors to higher levels of the CNS. The ascending gracile and cuneate fasciculi occupying the dorsal column, and sometimes are named the dorsal funiculus. These fibers carry information related to tactile, two point discrimination of simultaneously applied pressure, vibration, position, and movement sense and conscious proprioception. In the lateral column (funiculus), the neospinothalamic tract (or lateral spinothalamic tract) is located more anteriorly and laterally, and carries pain, temperature and crude touch information from somatic and visceral structures. Nearby laterally, the dorsal and ventral spinocerebellar tracts carry unconscious proprioception information from muscles and joints of the lower extremity to the cerebellum. In the ventral column (funiculus) there are four prominent tracts: 1) the paleospinothalamic tract (or anterior spinothalamic tract) is located which carry pain, temperature, and information associated with touch to the brain stem nuclei and to the diencephalon, 2) the spinoolivary tract carries information from Golgi tendon organs to the cerebellum, 3) the spinoreticular tract, and 4) the spino-tectal tract. Intersegmental nerve fibers traveling for several segments (2 to 4) and are located as a thin layer around the gray matter is known as fasciculus proprius, spinospinal or archispinothalamic tract. It carries pain information to the brain stem and diencephalon.

Descending tracts (Figure 3.9). The descending tracts originate from different cortical areas and from brain stem nuclei. The descending pathway carry information associated with maintenance of motor activities such as posture, balance, muscle tone, and visceral and somatic reflex activity. These include the lateral corticospinal tract and the rubrospinal tracts located in the lateral column (funiculus). These tracts carry information associated with voluntary movement. Other tracts such as the reticulospinal vestibulospinal and the anterior corticospinal tract mediate balance and postural movements (Figure 3.9). Lissauer's tract, which is wedged between the dorsal horn and the surface of the spinal cord carry the descending fibers of the dorsolateral funiculus (DFL), which regulate incoming pain sensation at the spinal level, and intersegmental fibers. Additional details about ascending and descending tracts are described in the next few chapters.

Information from the skin, skeletal muscle and joints is relayed to the spinal cord by sensory cells located in the dorsal root ganglia. The dorsal root fibers are the axons originated from the primary sensory dorsal root ganglion cells. Each ascending dorsal root axon, before reaching the spinal cord, bifurcates into ascending and descending branches entering several segments below and above their own segment. The ascending dorsal root fibers and the descending ventral root fibers from and to discrete body areas form a spinal nerve (Figure 3.10). There are 31 paired spinal nerves. The dorsal root fibers segregate into lateral and medial divisions. The lateral division contains most of the unmyelinated and small myelinated axons carrying pain and temperature information to be terminated in the Rexed laminae I, II, and IV of the gray matter. The medial division of dorsal root fibers consists mainly of myelinated axons conducting sensory fibers from skin, muscles and joints; it enters the dorsal/posterior column/funiculus and ascend in the dorsal column to be terminated in the ipsilateral nucleus gracilis or nucleus cuneatus at the medulla oblongata region, i.e., the axons of the first-order (1°) sensory neurons synapse in the medulla oblongata on the second order (2°) neurons (in nucleus gracilis or nucleus cuneatus). In entering the spinal cord, all fibers send collaterals to different Rexed lamina.

Axons entering the cord in the sacral region are found in the dorsal column near the midline and comprise the fasciculus gracilis, whereas axons that enter at higher levels are added in lateral positions and comprise the fasciculus cuneatus (Figure 3.11). This orderly representation is termed “somatotopic representation”.

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