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The spinal cord is organized into segments with dorsal sensory roots and ventral motor roots that merge into spinal nerves. Gray matter (neurons and synapses) forms a butterfly-shaped core; white matter (axons) surrounds it in ascending and descending tracts. Spinal nerves carry sensory and motor signals between the CNS and body.
The spinal cord is the cable through which the brain communicates with everything below the neck, and understanding its organization unlocks the logic of how neurological damage produces specific, predictable deficits. From your study of brain anatomy, you know the CNS and PNS are distinct divisions; the spinal cord sits at their junction. It extends from the brainstem (at the foramen magnum) to approximately vertebral level L1–L2 in adults, where it tapers into the conus medullaris. Below that, the remaining nerve roots continue as a bundle called the cauda equina (horse's tail) before exiting through their respective vertebral foramina.
The cord is organized into 31 segments, each corresponding to a pair of spinal nerves: 8 cervical (C1–C8), 12 thoracic (T1–T12), 5 lumbar (L1–L5), 5 sacral (S1–S5), and 1 coccygeal. Each segment receives sensory input from and sends motor output to a defined region of the body. Each spinal nerve forms by the union of a dorsal root (carrying sensory/afferent fibers — cell bodies in the dorsal root ganglion just outside the cord) and a ventral root (carrying motor/efferent fibers — cell bodies in the anterior horn of the cord). Bell's law summarizes this: dorsal = sensory, ventral = motor. The merged spinal nerve then divides into a dorsal ramus (supplying the back muscles and skin) and a ventral ramus (supplying the limbs and anterior trunk), with some thoracolumbar segments also contributing to the sympathetic chain via white rami communicantes.
On cross section, the cord shows two concentric regions. The central gray matter — shaped like a butterfly or letter H — contains neuron cell bodies and synapses. The dorsal horns process incoming sensory information; the ventral horns contain alpha motor neurons whose axons exit via the ventral root to directly drive skeletal muscle. Thoracic and upper lumbar segments also have a lateral horn containing preganglionic sympathetic neurons. Surrounding the gray matter, the white matter consists of myelinated axon bundles organized into tracts (funiculi). The dorsal columns carry fine touch, vibration, and proprioception ipsilaterally up to the medulla. The spinothalamic tracts carry pain and temperature, but with a twist — these fibers decussate within 1–2 segments of entry, then ascend contralaterally. The corticospinal tracts carry voluntary motor commands from cortex; most fibers decussate in the medullary pyramids and descend contralaterally in the lateral funiculus.
This anatomy has direct clinical utility through the concept of dermatomes — the skin territory supplied by a single spinal nerve's sensory fibers. A band of numbness around the torso identifies the thoracic level; tingling in the thumb localizes to C6; loss of sensation on the lateral foot points to S1. Similarly, weakness patterns identify motor root levels. Crucially, the different decussation levels of the sensory tracts explain the Brown-Séquard syndrome seen with hemisection of the cord: motor loss and proprioception loss occur ipsilateral to the lesion (dorsal columns haven't crossed yet), while pain and temperature loss occur contralateral (spinothalamic fibers already crossed near entry). This pattern — sensory dissociation with an anatomical logic — is only readable if you know the tract organization.