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Introduction to Peripheral Organization of Somatosensory Systems

Peripheral Somatosensory Neurons. The cell bodies of the first-order (1°) somatosensory afferent neurons² are located in posterior root or cranial root ganglia (i.e., are part of the peripheral nervous system, Figure 2.1). The 1° afferents are pseudounipolar cells. The cell body gives rise to a single process that divides to form a peripheral axon and a central axon. The peripheral axon travels to and ends in the skin, muscle, tendon or joint and the central axon travels to and ends in the central nervous system.

Somatosensory Receptor Organ. The receptors of most sensory systems are located in specialized sensory receptor organs (e.g., the photoreceptors in the eye and the auditory and vestibular hair cells in the inner ear) or within a restricted part of the body (e.g., the taste buds in the mouth and the olfactory receptors in the olfactory mucosa of the nose). For the tactile component of the somatosensory system, the skin covering the entire body, head and face functions as the touch receptor organ, whereas joint tissues, muscles and tendons act as the proprioception receptor organs. These sensory receptor organs "house" the somatosensory receptors and deliver the somatosensory stimuli to the receptors.

Sensory Receptors. Specialized sensory receptor cells (e.g., the photoreceptors of the eye) are located in specialized receptor organs, produce receptor potentials, contain synaptic specializations, and release neural transmitters (Figure 2.2). Specialized sensory receptors may be modified neurons (e.g., the photoreceptors and olfactory receptors) or modified epithelial cells (e.g., taste receptors and the auditory and vestibular hair cells).

There is only one type of sensory receptor cell in the somatosensory system, the Merkel cells, and they are found only in skin. The vast majority of somatosensory receptors are not specialized receptor cells. That is, they are formed by the endings of the somatosensory 1° afferent peripheral axon and adjacent tissue (Figure 2.3). There is no synaptic specialization or neurotransmitter within the adjacent tissue. The adjacent tissue also does not generate receptor potentials.

Instead of ending on specialized receptors, most peripheral axons of somatosensory 1° afferents travel to skin, muscle or joint, branch near their terminal sites, and end in the skin (Figure 2.4), muscle, tendon or joint tissue.

All the peripheral terminal branches of a 1° somatosensory axon end in a specific type of tissue (e.g., skin) and not in multiple types of tissue (i.e., not in skin and muscle). All the peripheral terminal branches of a 1° axon form only one type of somatosensory receptor.

Many of the 1° somatosensory afferent terminals are enveloped in a connective tissue capsule along with surrounding muscle, tendon or cutaneous cells, or end on hair follicles. The hair follicles and the encapsulated tissue adjacent to the 1° afferent terminals (i.e., skin, muscle, tendon, and joint tissues) contain no synaptic specializations and do not generate receptor potentials or release neural transmitters. The complex of encapsulated tissue and afferent endings and the complex of hair follicle and afferent endings play a role in the receptor transduction process, and each complex is considered to form a "somatosensory receptor". Many other 1° somatosensory axons branch and terminate in skin, muscle, or joint as free nerve endings. These endings are bare of myelin, are not encapsulated and are not associated with a specific type of tissue.

The sensitivity of the receptors to specific stimuli (e.g., touch verses muscle stretch) is determined by the location of the receptor and by the non-neural tissue surrounding the 1° afferent terminal (Figure 2.6).