11 Innervation of skin
Sensory Units
A given stem fiber forms the same kind of nerve ending at all of its terminals. In physiologic recordings, the stem fiber and its family of endings constitute a sensory unit. Together with its parent unipolar nerve cell, the sensory unit is analogous to the motor unit described in Chapter 10.
Nerve Endings
Free nerve endings (Figure 11.1A, B)

Figure 11.1 Innervation of hairy skin. (A) Three morphologic types of sensory nerve ending in hairy skin. (B) Free nerve ending in the basal layer of the epidermis. (C) Merkel cell–neurite complex. (D) Palisade and circumferential nerve endings on the surface of the outer root sheath of a hair follicle.
Functions
C-fiber units are responsible for the axon reflex (Clinical Panel 11.1