Somatic nervous system

In humans beings the somatic nervous system consists of 31 pairs of spinal nerves, each pair occurring from a single segment of the spinal cord, and 12 pairs of the cranial nerves that come from the brain. Afferent axons in the spinal and cranial nerves entering the CNS carry sensory information from muscles, skin, joints, and viscera. The majority is wired to mechanoreceptors that inform about mechanical forces, the others are nociceptors that signal tissue damage, and some (limited to skin) are connected to temperature-sensitive thermoreceptors. Efferent fibers leaving the CNS are axons of motor neurons supplying the skeletal muscles.

Approximately all the spinal nerves are mixed, having both sensory and motor fibers. The exceptions are C1 that is motor and Cx 1 that is a sensory. Out of the cranial nerves only 4 are mixed.

Each of the spinal nerve is formed from a dorsal root housing sensory axons and a ventral root carrying motor axons. The cell bodies of the main afferent neurons lie in the dorsal root ganglia (DRG) just outside the spinal cord. There is pair of DRG for each and every spinal segment. The Efferent neuron cell bodies lie within the spinal cord.

                                                             Figure: Origin of a spinal nerve from a spinal cord segment.