Innervation of skin

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11 Innervation of skin

Study Guidelines

1 Learn to define sensory unit, sensory overlap, receptive field, receptor adaptation.

2 Be able to state locations and properties of three kinds of encapsulated receptor.

3 Be able to sketch a hair follicle with its nerve palisade and rings.

4 Try to appreciate the huge numbers of mechanoreceptors that are deployed in the hands. Name two kinds of receptors used to discriminate textures, for example to read Braille.

5 Consider a quick preview of the Clinical Panel on peripheral neuropathies in Chapter 12.

From the cutaneous branches of the spinal nerves, innumerable fine twigs enter a dermal nerve plexus located in the base of the dermis. Within the plexus, individual nerve fibers divide and overlap extensively with others before terminating at higher levels of the skin. Because of overlap, the area of anesthesia resulting from injury to a cutaneous nerve (e.g. superficial radial, saphenous) is smaller than its anatomic territory.

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.

The territory from which a sensory unit can be excited is its receptive field. There is an inverse relationship between the size of receptive fields and sensory acuity; for example, fields measure about 2 cm² on the upper arm, 1 cm² at the wrist, and 5 mm² on the finger pads.

Sensory units interdigitate so that different modalities of sensation can be perceived from a given patch of skin.

Nerve Endings

Free nerve endings (Figure 11.1A, B)

As they run toward the skin surface, many sensory fibers shed their perineural sheaths and then their myelin sheaths (if any) before branching further in a subepidermal network. The Schwann cell sheaths open to permit naked axons to terminate between collagen bundles (dermal nerve endings) or within the epidermis (epidermal nerve endings).

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

Some sensory units with free nerve endings are thermoreceptors. They supply either ‘warm spots’ or ‘cold spots’ scattered over the skin. Two kinds of nociceptors (pain-transducing) units with free endings are also found. One kind responds to severe mechanical deformation of the skin, for example pinching with a forceps. The parent fibers are finely myelinated (Aδ). The other kind comprises polymodal nociceptors; these are C-fiber units able to transduce mechanical deformation, intense heat (some also intense cold), and irritant chemicals.

C-fiber units are responsible for the axon reflex (Clinical Panel 11.1

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