Chapter 14: Nervous System: Spinal Cord & Spinal Nerves

Structurally, the adult spinal cord extends approximately 45 centimeters from the foramen magnum to the first lumbar vertebra (L1). Anatomical landmarks include the shallow posterior median sulcus and the deep anterior median fissure. The spinal cord exhibits the cervical enlargement and the lumbosacral enlargement, regions containing an increased mass of gray matter necessary for the complex sensory and motor innervation of the upper and lower limbs, respectively. Inferiorly, the spinal cord narrows to the conus medullaris, from which the fibrous tissue strand known as the filum terminale extends to the coccyx, providing longitudinal stability. The lengthy ventral and dorsal roots extending caudal to the conus medullaris, along with the filum terminale, form the cauda equina. The delicate neural tissue is protected by the specialized spinal meninges (dura mater, arachnoid mater, and pia mater), which offer stability and shock absorption. The outermost dura mater is separated from the vertebral canal by the epidural space, which contains adipose tissue. The middle arachnoid mater is separated from the inner pia mater by the subarachnoid space, which is filled with cerebrospinal fluid (CSF) acting as a shock absorber and diffusion medium. Clinically, CSF samples can be safely obtained via a lumbar puncture (spinal tap) between lumbar vertebrae L3 and L4. The spinal cord is divided into 31 segments, each associated with a pair of dorsal root ganglia containing sensory neuron cell bodies. Sensory fibers from the dorsal root and motor fibers from the ventral root unite to form 31 pairs of mixed spinal nerves. Internally, the gray matter is organized into horns, containing nuclei that process sensory information (posterior/dorsal horns) and issue somatic motor commands (anterior/ventral horns) or visceral motor commands (lateral/intermediate horns, T1–L2). The surrounding white matter is organized into columns (funiculi) which contain bundles of myelinated and unmyelinated axons called tracts. Ascending tracts carry sensory information toward the brain, while descending tracts convey motor commands down the cord. Peripheral nerve fibers distribute via rami, including the dorsal ramus for the back and the ventral ramus for the ventrolateral body surface and limbs. In segments T1-L2, the rami communicantes (white and gray rami) connect to sympathetic autonomic ganglia. The resulting regional distribution of sensory fibers defines dermatomes, areas of skin monitored by a specific pair of spinal nerves. In the neck and limbs, ventral rami merge to form complex interwoven nerve networks called nerve plexuses. The four major plexuses are the cervical plexus (containing the phrenic nerve, vital for breathing), the brachial plexus (innervating the upper limb and pectoral girdle, giving rise to nerves like the radial and median nerves), the lumbar plexus, and the sacral plexus. The latter two often combine into the lumbosacral plexus and contain the largest nerve in the body, the sciatic nerve. Finally, the chapter details neural reflexes, which are immediate, involuntary motor responses essential for homeostasis, occurring along a neural pathway called the reflex arc. Reflexes are classified based on complexity as either the fast, single-synapse monosynaptic reflex (e.g., the stretch reflex like the patellar or knee jerk reflex) or the slower, multiple-synapse polysynaptic reflex, which allows for more complicated, coordinated motor responses.