October 12, 2026

All forms of somatosensory information from the trunk and limbs enter the spinal cord, which as a core H-shaped region of gray matter where neuronal cell bodies are located. The gray matter is surrounded by white matter formed by myelinated axons that make up both short and long connections. The gray matter on each side of the cord is divided into dorsal (or posterior) and ventral (or anterior) horns.

The dorsal horn contains groups of secondary sensory neurons (sensory nuclei) whose dendrites receive stimulus information from primary sensory neurons that innervate the body’s skin, muscles, and joints. The ventral horn contains groups of motor neurons (motor nuclei) whose axons exit the spinal cord and innervate skeletal muscles. The spinal cord has circuits that mediate behaviors ranging from the stretch reflex to coordination of limb movements.

Interneurons of various types in the gray matter regulate the output of the spinal cord motor neurons Some of these interneurons are excitatory, whereas others are inhibitory. These interneurons modulate both sensory information flowing toward the brain and motor commands descending from the brain to the spinal motor neurons. Motor neurons can also adjust the output of other motor neurons via the interneurons.

The major anatomical features of the spinal cord.

The ventral horn (green) contains large motor neurons, whereas the dorsal horn (orange) contains smaller neurons. Fibers of the gracile fascicle carry somatosensory information from the lower limbs, whereas fibers of the cuneate fascicle carry somatosensory information from the upper body. Fiber bundles of the lateral and ventral columns include both ascending and descending fiber bundles.

The white matter surrounding the gray matter contains bundles of ascending and descending axons that are divided into dorsal, lateral, and ventral columns. The dorsal columns, which lie between the two dorsal horns of the gray matter, contain only ascending axons that carry somatosensory information to the brain stem.

The lateral columns include both ascending and descending axons from the brain stem and neocortex that innervate spinal interneurons and motor neurons.

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Processing tends to be hierarchical: Projections from a lower to a higher processing region are said to be feedforward, while descending projections can modulate spinal reflexes and are considered to be feedback.

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The ventral columns also include ascending and descending axons. The ascending somatosensory axons in the lateral and ventral columns constitute parallel pathways that convey information about pain and thermal sensation to higher levels of the central nervous system. The descending axons control axial muscles and posture.

The internal and external appearances of the spinal cord vary at different levels.

The proportion of gray matter (the H-shaped area within the spinal cord) to white matter is greater at sacral levels than at cervical levels. At sacral levels, very few incoming sensory axons have joined the spinal cord, whereas most of the motor axons have already terminated at higher levels of the spinal cord. The cross-sectional enlargements at the lumbar and cervical levels are regions where the large number of fibers innervating the limbs enter or leave the spinal cord.

Axons projecting from the spinal cord to body structures that develop at the same segmental level join with axons entering the spinal cord in the intervertebral foramen to form spinal nerves. Spinal nerves at the cervical level are involved with sensory perception and motor function of the back of the head, neck, and arms; nerves at the thoracic level innervate the upper trunk; lumbar and sacral spinal nerves innervate the lower trunk, back, and legs.

Each of the four regions of the spinal cord contains multiple segments corresponding approximately to the different vertebrae in each region; there are 8 cervical segments, 12 thoracic segments, 5 lumbar segments, and 5 sacral segments. The actual substance of the mature spinal cord does not look segmented, but the segments of the four spinal regions are nonetheless defined by the number and location of the dorsal and ventral roots that enter or exit the spinal cord. The spinal cord varies in size and shape along its rostrocaudal axis because of two organizational features.

First, relatively few sensory axons enter the cord at the sacral level. The number of sensory axons entering the cord increases at progressively higher levels (lumbar, thoracic, and cervical). Conversely, most descending axons from the brain terminate at cervical levels, with progressively fewer descending to lower levels of the spinal cord. Thus, the number of fibers in the white matter is highest at cervical levels (where there are the highest numbers of both ascending and descending fibers) and lowest at sacral levels. As a result, sacral levels of the spinal cord have much less white matter than gray matter, whereas the cervical cord has more white matter than gray matter.

The second organizational feature is variation in the size of the ventral and dorsal horns. The ventral horn is larger at the levels where the motor nerves innervate the arms and legs. The number of ventral motor neurons dedicated to a body region roughly parallels the dexterity of movements of that region. Thus, a larger number of motor neurons is needed to innervate the greater number of muscles and to regulate the greater complexity of movement in the limbs as compared with the trunk. Likewise, the dorsal horn is larger where sensory nerves from the limbs enter the cord. Limbs have a greater density of sensory receptors to mediate finer tactile discrimination and thus send more sensory fibers to the cord. These regions of the cord are known as the lumbosacral and cervical enlargements.