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).

Clinical Panel 11.1 Neurogenic inflammation: the axon reflex

When sensitive skin is stroked with a sharp object, a red line appears in seconds, owing to capillary dilatation in direct response to the injury. A few minutes later, a red flare spreads into the surrounding skin, owing to arteriolar dilatation, followed by a white wheal, owing to exudation of plasma from the capillaries. These phenomena constitute the triple response. The wheal and flare responses are produced by axon reflexes in the local sensory cutaneous nerves. The sequence of events follows the numbers in Figure CP 11.1.1.

1 The noxious stimulus is transduced (converted to nerve impulses) by polymodal nociceptors.

2 As well as transmitting impulses to the central nervous system in the normal, orthodromic direction, the axons send impulses in an antidromic direction from points of bifurcation into the neighboring skin. The nociceptive endings respond to antidromic stimulation by releasing one or more peptide substances, notably substance P.

3 Substance P binds with receptors on the walls of arterioles, leading to arteriolar dilatation—the flare response.

4 Substance P also binds with receptors on the surface of mast cells, stimulating them to release histamine. The histamine increases capillary permeability and leads to local accumulation of tissue fluid—the wheal response.

Figure CP 11.1.1 The axon reflex.

Follicular nerve endings (Figure 11.1A, D)

Just below the level of the sebaceous glands, myelinated fibers apply a palisade of naked terminals along the outer root sheath epithelium of the hair follicles. Outside this is a circumferential set of terminals.

Each follicular unit supplies many follicles, and there is much territorial overlap. Follicular units are rapidly adapting: they fire when the hairs are being bent, but not when the bent position is held. Rapid adaptation accounts for our being largely unaware of our clothing except when donning or offing.

Merkel cell–neurite complexes (Figure 11.1A, C)

Expanded nerve terminals are applied to Merkel cells (tactile menisci) in the basal epithelium of epidermal pegs and ridges. These Merkel cell–neurite complexes are slowly adapting. They discharge continuously in response to sustained pressure, for example while we hold a pen or wear spectacles, and they are markedly sensitive to the edges of objects held in the hand.

Encapsulated nerve endings

The capsules of the three nerve endings to be described comprise an outer coat of connective tissue, a middle coat of perineural epithelium, and an inner coat of modified Schwann cells (teloglia). All three are mechanoreceptors, transducing mechanical stimuli.

• Meissner’s corpuscles are most numerous in the finger pads, where they lie beside the intermediate ridges of the epidermis (Figure 11.2A–C). In these ovoid receptors, several axons zigzag among stacks of teloglial lamellae. Meissner’s corpuscles are rapidly adapting. Together with the slowly adapting Merkel cell–neurite complexes, they provide the tools for delicate detective work on textured surfaces such as cloth or wood, or on embossed surfaces such as Braille text. Elevations as little as 5 µm in height can be detected!

• Ruffini endings are found in both hairy and glabrous skin (Figures 11.1A and 11.2D). They respond to drag (shearing stress) and are slowly adapting. Their struc-ture resembles that of Golgi tendon organs, having a collagenous core in which several axons branch liberally.

• Pacinian corpuscles (Figure 11.2B, E) are the size of rice grains. They number about 300 in the hand. They are subcutaneous, close to the underlying periosteum, and numerous along the sides of the fingers and in the palm. Inside a thin connective sheath are onion-like layers of perineural epithelium containing some blood capillaries. Innermost are several teloglial lamellae surrounding a single central axon that has shed its myelin sheath at point of entry. Pacinian corpuscles are rapidly adapting and are especially responsive to vibration—particularly to bone vibration. In the limbs, many corpuscles are embedded in the periosteum of the long bones.

Figure 11.2 Innervation of glabrous skin. (A) Finger pad showing distribution of two types of sensory nerve ending. (B) Tissue block from (A) showing positions of four types of sensory nerve endings. (C) Meissner’s corpuscle. (D) Nerve ending of Ruffini. (E) Pacinian corpuscle.

Pacinian corpuscles discharge one or two impulses when compressed, and again when released. In the hands, they seem to function in group mode: when an object such as an orange is picked up, as many as 120 or more corpuscles are activated momentarily, with a momentary repetition when the object is released. For this reason, they have been called ‘event detectors’ during object manipulation.

The digital receptors are classified as follows by sensory physiologists.

• Merkel cell–neurite complexes = SA I

• Meissner’s corpuscles = RA I

• Ruffini endings = SA II

• Pacinian corpuscles = RA II

When three-dimensional objects are being manipulated out of sight, significant contributions to perceptual evaluation are made by muscle afferents (especially from muscle spindles) and articular afferents from joint capsules. The cutaneous, muscular, and articular afferents relay information independently to the contralateral somatic sensory cortex. The three kinds of information serve the function of tactile discrimination. They are integrated (brought together at cellular level) in the posterior part of the contralateral parietal lobe, which is specialized for spatial sense, both tactile and visual. Spatial tactile sense is called stereognosis. In the clinic, stereognosis is tested by asking the patient to identify an object such as a key without looking at it.

Cutaneous sensory effects of peripheral neuropathies are described in Chapter 12. Clinical Panel 11.2 gives a short account of leprosy.

Clinical Panel 11.2 Leprosy

The leprosy bacillus enters the skin through minor abrasions. It travels proximally within the perineurium of the cutaneous nerves and kills off the Schwann cells. Loss of myelin segments (’segmental demyelination’) blocks impulse conduction in the larger nerve fibers. Later, the inflammatory response to the bacillus compresses all the axons, leading to Wallerian degeneration of entire nerves and gross thickening of the connective tissue sheaths. Patches of anesthetic skin develop on the fingers, toes, nose, and ears. The protective function of skin sensation is lost, and the affected parts suffer injury and loss of tissue. Motor paralyses occur later on as a consequence of invasion of mixed nerve trunks proximal to the points of origin of their cutaneous branches.

Core Information

Cutaneous nerves branch to form a dermal nerve plexus where individual afferent fibers branch and overlap. Each stem fiber and its terminal receptors constitute a sensory unit. The territory of a stem fiber is its receptive field.

Sensory units with free nerve endings include thermoreceptors and both mechanical and thermal nociceptors. Follicular units are rapidly adapting touch receptors, active only when hairs are in motion. Merkel cell–neurite complexes are slowly adapting edge detectors.

The encapsulated endings are mechanoreceptors. Meissner’s corpuscles lie beside intermediate ridges in glabrous skin and are rapidly adapting. Ruffini endings lie near hair follicles and fingernails; they are slowly adapting drag receptors. Pacinian corpuscles are subcutaneous, rapidly adapting event detectors and vibration receptors.

Coded information from skin, muscles, and joints is integrated at the level of the posterior parietal lobe of the brain, yielding the faculties of tactile discrimination and stereognosis.

Lauria G, Lombardi R. Skin biopsy: a new tool for diagnosing peripheral neuropathy. BMJ. 2007:1159-1162.

Meftah E-M, Chapman CE. Tactile perception of roughness. Exp Brain Res. 2009;3:235-244.

Schepers RJ, Ringkamp M. Thermoreceptors and thermoceciptive afferents. Neurosci Behav Rev. 2009;33:205-212.

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