Study guidelines
- 1.
Define sensory unit, sensory overlap, receptive field, and receptor adaptation.
- 2.
State locations and properties of the three kinds of encapsulated receptor.
- 3.
Sketch a hair follicle with its nerve palisade and rings.
- 4.
Name two kinds of mechanoreceptors 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 at the base of the dermis. Within the plexus, individual nerve fibres divide and overlap extensively with others before terminating at higher levels of the skin. Because of the overlap, the area of anaesthesia resulting from injury to a cutaneous nerve (e.g. superficial radial, saphenous) is smaller than its anatomic territory.
Sensory units
A given stem fibre forms the same kind of nerve ending at all of its terminals. In physiologic recordings the stem fibre 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 cm 2 on the upper arm, 1 cm 2 at the wrist, and 5 mm 2 on the finger pads.
Sensory units interdigitate so that different modalities of sensation can be perceived from a given patch of skin.
Sensory units
A given stem fibre forms the same kind of nerve ending at all of its terminals. In physiologic recordings the stem fibre 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 cm 2 on the upper arm, 1 cm 2 at the wrist, and 5 mm 2 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 towards the skin surface, many sensory fibres 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) .
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 fibres are finely myelinated (Aδ). The other kind comprises polymodal nociceptors; these are C-fibre units able to transduce mechanical deformation, intense heat (some also intense cold), and irritant chemicals.
C-fibre units are responsible for the axon reflex ( Clinical Panel 11.1 ).
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