Ganglion cells in the somatosensory system have axons that divide into peripheral and central processes. The peripheral processes merge to form peripheral nerves. Sensory nerves consist solely of axons from ganglion cells, while mixed nerves contain both sensory and motor fibers for muscle innervation. At the target organ, the peripheral process of a sensory axon branches multiple times, with each terminal branch acting as a sensory receptor. In some cases, the nerve ending is encapsulated by accessory cells, forming the receptor as a whole.

The central axonal process of a ganglion cell enters the spinal cord through a dorsal root or the brainstem via a cranial nerve. The central process typically gives rise to multiple branches that may synapse with various cell types, including second-order neurons in somatosensory pathways. These central branches can terminate at or near the segmental level of entry or project to brainstem nuclei.

Second-order neurons involved in somatosensory perception pathway project to specific thalamic nuclei, where third-order neurons are located. These neurons then project to the primary somatosensory cortex (S-I). Within the cortex, somatosensory information is processed in S-I and several higher-order cortical areas. Some second-order neurons also transmit somatosensory information to the cerebellum for motor coordination.

The organization of the somatosensory system differs from other senses. While other sensory systems have receptors concentrated in a single organ, such as the eye for vision, somatosensory receptors are distributed throughout the body and head.

The somatosensory system has three main divisions based on receptor distribution. The exteroceptive division provides information about skin contact with external objects, relying on various cutaneous receptors for mechanoception, nociception (pain), and temperature. This chapter will mainly focus on understanding this division. The proprioceptive component provides information about body and limb position and movement, primarily through receptors in joints, muscles, and tendons. The ascending central pathways originating from these receptors play a role in conveying proprioceptive information.
Each ganglion cell gives off an axon that after a short distance divides into a peripheral process and a central process. The peripheral processes of the ganglion cells coalesce to form peripheral nerves. A purely sensory nerve will have only axons from such ganglion cells; however, mixed nerves, which innervate muscles, will contain both afferent (sensory) fibers and efferent (motor) fibers. At the target organ the peripheral process of an afferent axon divides repeatedly, with each terminal branch ending as a sensory receptor. In most cases the free nerve ending by itself forms a functional receptor, but in some the nerve ending is encapsulated by accessory cells and the entire structure (axon terminal plus accessory cells) forms the receptor.
The central axonal process of the ganglion cell either enters the spinal cord via a dorsal root or enters the brainstem via a cranial nerve. A central process typically gives rise to numerous branches that may synapse with a variety of cell types, including second-order neurons of the somatosensory pathways. The terminal location of these central branches varies depending on the type of information being transmitted. Some terminate at or near the segmental level of entry, whereas others project to brainstem nuclei.
Second-order neurons that are part of the pathway for the perception of somatosensory information project to specific thalamic nuclei where the third-order neurons reside. These neurons in turn project to the primary somatosensory cortex (S-I). Within the cortex, somatosensory information is processed in S-I and in numerous higher-order cortical areas. Somatosensory information is also transmitted by other second-order neurons to the cerebellum for use in its motor coordination function.
The organization of the somatosensory system is quite distinct from that of the other senses, which has both experimental and clinical implications. In particular, other sensory systems have their receptors localized to a single organ, where they are present at high density (e.g., the eye for the visual system). In contrast, somatosensory receptors are distributed throughout the body (and head).
Subdivisions of the Somatosensory System
The somatosensory system receives three broad categories of information based on the distribution of its receptors. Its exteroceptive division is responsible for providing information about contact of the skin with objects in the external world, and a variety of cutaneous mechanoceptive, nociceptive (pain), and thermal receptors are used for this purpose. Understanding this division will be the main focus of this chapter. The proprioceptive component provides information about body and limb position and movement and relies primarily on receptors found in joints, muscles, and tendons. The ascending central pathways that originate
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