Types of spinal neurons

The smallest neurons (soma diameters 5–20 µm) are propriospinal, being entirely contained within the cord. Some are confined within a single segment; others span two or more segments by way of the neighboring propriospinal tract. Many of the smallest neurons participate in spinal reflexes. Others are intermediate cell stations interposed between fiber tracts descending from the brain and motor neurons projecting to the locomotor apparatus. Others again are so placed as to influence sensory transmission from lower to higher levels of the CNS.

Medium-sized neurons (soma diameters 20–50 µm) are found in all parts of the gray matter except the substantia gelatinosa. Most are relay (projection) cells receiving inputs from posterior root afferents and projecting their axons to the brain. The projections are in the form of tracts, a tract being defined as a functionally homogeneous group of fibers. As will be seen, the term ‘tract’ is often used loosely because many projections originally thought to be ‘pure’ contain more than one functional class of fiber.

The largest neurons of all are the alpha motor neurons (soma 50–100 µm) for the supply of skeletal muscles. Scattered among them are small, gamma motor neurons supplying muscle spindles. In the medial part of the anterior horn are Renshaw cells, which exert tonic inhibition upon alpha motor neurons.

Spinal reflex arcs originating in muscle spindles and tendon organs have been described in Chapter 10, and the withdrawal reflex in Chapter 14.

In thick sections of the spinal cord, the nerve cells exhibit a laminar (layered) arrangement. True lamination is confined to the posterior horn (Figure 15.3), but 10 laminae of Rexed have been defined in the gray matter as a whole in order to correlate findings from animal research in different laboratories.

Figure 15.3 Laminae (I–X) and named cell groups at mid-thoracic level.

Spinal ganglia

The spinal or posterior (dorsal) root ganglia are located in the intervertebral foramina, where the anterior and posterior roots come together to form the spinal nerves. Thoracic ganglia contain about 50,0000 unipolar neurons, and those serving the limbs contain about 100,000. The individual ganglion cells are invested with modified Schwann cells called satellite cells (Figure 15.4). The common stem axon of each cell bifurcates, sending a centrifugal process into one or other ramus of the spinal nerve (or into the recurrent branch) and a centripetal (‘center-seeking’) process into the spinal cord. Following stimulation of the peripheral sensory receptors, trains of nerve impulses traverse the point of bifurcation without interruption, although the cell body is also depolarized. The initial segment of the stem axon does not normally generate impulses but it may do so if the adjacent part of the posterior root is compressed, e.g. by a prolapsed intervertebral disc.

Figure 15.4 Posterior root ganglion. In bottom of figure, note T-shaped bifurcation of stem fibers.

Traditionally, the centripetal axons of all spinal ganglion cells have been thought to enter posterior nerve roots. It is now known that many visceral afferents (in particular) enter the cord by way of ventral roots and work their way to the posterior gray horn (Ch. 13). This feature accounts for the frequent failure of posterior rhizotomy (surgical section of posterior roots) to relieve pain originating from intra-abdominal cancer.

Central terminations of posterior root afferents (Figure 15.5)

In the dorsal root entry zone close to the surface of the cord, the afferent fibers become segregated into medial and lateral streams. The medial stream comprises medium and large fibers which divide within the posterior funiculus into ascending and descending branches. The branches swing into the posterior gray horn and synapse in laminae II, III, and IV. The largest ascending fibers run all the way to the posterior column nuclei (gracilis/cuneatus) in the medulla oblongata. These long fibers form the bulk of the gracile and cuneate fasciculi.

Figure 15.5 Targets of primary afferent neurons in the posterior gray horn.

The lateral stream comprises small (Aδ and C) fibers which, upon entry, divide into short ascending and descending branches within the posterolateral tract of Lissauer. They synapse upon neurons in the marginal zone (lamina I) and in the substantia gelatinosa (lamina II); some fibers synapse upon dendrites of cells belonging to laminae III–V.

Categories of sensation

In accordance with the flowchart in Figure 15.6, neurologists speak of two kinds of sensation, conscious and nonconscious (unconscious). Conscious sensations are perceived at the level of the cerebral cortex. Nonconscious sensations are not perceived; they have reference to the cerebellum (see later).

Figure 15.6 Categories of sensation. ^aExteroceptors can be categorized as telereceptors receiving from a distance (retina and cochlea) and somatic receptors on the body surface (touch, pain, etc.)

^bEnteroceptors (Gr. enteron, ‘gut’) are strictly a subdivision of interoceptors, a term signifying all of the viscera. In pathological states they may produce conscious visceral/viscerosomatic sensations.