10 Spinal Cord
The spinal cord is pretty small, but it’s important out of proportion to its size. It’s the home of all the motor neurons that work your body, and of a large percentage of the autonomic motor neurons as well. It’s also the recipient of nearly all the sensory information taken in by your body. Beyond that, many of the organizing principles of spinal cord reflexes and pathways apply to other parts of the CNS.
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”).
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.
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.
