Chapter 47: Spinal Cord & Spinal Nerves Gross Anatomy

Spinal Cord & Spinal Nerves Gross Anatomy anatomical chapter explores the gross structure of the spinal cord and its extensions, the spinal nerves, housed within the vertebral canal and exiting through the intervertebral foramina. The spinal cord occupies the upper two-thirds of the canal, beginning below the foramen magnum and terminating caudally as the conus medullaris, typically situated near the middle third of the first lumbar vertebra in adults. Structural adaptations include the cervical enlargement (C3 through T2 segments) and the lumbar enlargement (L1 through S3 segments), which correlate with the large nerves supplying the upper and lower limbs, respectively. The cord's external features include the ventral median fissure and the posterior median sulcus, while internally, gray matter columns are surrounded by white matter organized into funiculi. Protection is provided by the three layers of meninges—dura, arachnoid, and pia mater—which collectively form the theca. The epidural space, located external to the dura, is loosely packed with fat, connective tissue, and a significant venous plexus. Distal to the conus medullaris, the pia mater extends as the filum terminale, which has internal and external components. There are 31 pairs of spinal nerves, each formed by the union of ventral roots (efferent/motor) and dorsal roots (afferent/sensory), with the latter housing the spinal ganglia. Because the vertebral column lengthens more rapidly than the cord during development, the caudal spinal roots descend obliquely to form the cauda equina. Spinal nerves immediately divide into ventral rami (which supply the anterolateral trunk and form plexuses for the limbs) and dorsal rami (which retain segmental distribution to the back musculature and skin, such as the greater occipital nerve (C2)). The cord's arterial supply is reinforced segmentally by vessels like the great anterior radiculomedullary artery of Adamkiewicz (T9-L2), which is crucial as it may supply the lower two-thirds of the cord, making the mid-thoracic region (T4–T9) a critical vascular zone liable to ischemia. Venous drainage occurs via intramedullary veins leading to the surface coronal plexus and longitudinal channels. Clinical relevance of this anatomy is highlighted in spinal cord injuries, where injury terminology (e.g., tetraplegia, paraplegia) and specific syndromes (e.g., central cord, Brown-Séquard) are defined. Procedures like lumbar puncture safely access cerebrospinal fluid (CSF) within the subarachnoid space, typically below the cord's termination at the L3/L4 or L4/L5 interspace. Finally, pain diagnosis differentiates between sharp, radiating radicular pain (caused by nerve root compression) and deep, aching somatic referred pain (caused by musculoskeletal structures).