Friday, November 25, 2011

Free Nerve Endings

Free nerve endings are found in almost every tissue of the body. In the skin, they reach into the lower layers of the epidermis (stratum germinativum). At the end of their terminal branches, the axons send nodular or finger-like evaginations through gaps in the Schwann cell sheath. These evaginations are covered only by the basement membrane and represent the receptor segments of free nerve endings to which the sensations of pain and cold are attributed. Delicate nerve fibers encircle the hair follicles, and their terminal segments ascend or descend parallel to the hair shaft. The receptor terminals lose their myelin sheath and are enclosed between two Schwann cells (sandwich arrangement), which leave a cleft along the entire terminal segment. Through this cleft the axon terminal reaches the surface where it is covered only by the basement membrane. The nerve endings are radially arranged around the hair follicle in such a way that the sensory clefts face the follicle. Every movement of the hair causes mechanical stimulation of the nerve endings, which is perceived as touch.

Also associated with hair follicles are Merkel’s touch cells. These are large, clear epithelial cells that lie between the basal cells of the outer root sheath and send out finger-like processes into their surroundings. Deformation of these cells through movement of the hair results in stimulation of the associated nerve fiber. The nerve fiber loses its myelin sheath as it penetrates the basement membrane and sends branches to several tactile cells. The terminal segment widens into a tactile meniscus and forms synapse-like membrane contacts with the Merkelcell.