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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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Cutaneous and Subcutaneous Somatic Sensory Receptors

The specialized sensory receptors in the cutaneous and subcutaneous tissues are dauntingly diverse (Table 9.1). They include free nerve endings in the skin, nerve endings associated with specializations that act as amplifiers or filters, and sensory terminals associated with specialized transducing cells that influence the ending by virtue of synapse-like contacts. Based on function, this variety of receptors can be divided into three groups: mechanoreceptors, nociceptors, and thermoceptors. On the basis of their morphology, the receptors near the body surface can also be divided into free and encapsulated types. Nociceptor and thermoceptor specializations are referred to as free nerve endings because the unmyelinated terminal branches of these neurons ramify widely in the upper regions of the dermis and epidermis; their role in pain and temperature sensation is discussed in Chapter 10. Most other cutaneous receptors show some degree of encapsulation, which helps determine the nature of the stimuli to which they respond.

Table 9.1. The Major Classes of Somatic Sensory Receptors.

Table 9.1

The Major Classes of Somatic Sensory Receptors.

Despite their variety, all somatic sensory receptors work in fundamentally the same way: Stimuli applied to the skin deform or otherwise change the nerve endings, which in turn affects the ionic permeability of the receptor membrane. Changes in permeability generate a depolarizing current in the nerve ending, thus producing a receptor (or generator) potential that triggers action potentials, as described in Chapters 2 and 3. This overall process, in which the energy of a stimulus is converted into an electrical signal in the sensory neuron, is called sensory transduction and is the critical first step in all sensory processing.

The quality of a mechanosensory (or any other) stimulus (i.e., what it represents and where it is) is determined by the properties of the relevant receptors and the location of their central targets (Figure 9.1). The quantity or strength of the stimulus is conveyed by the rate of action potential discharge triggered by the receptor potential (although this relationship is nonlinear and often quite complex). Some receptors fire rapidly when a stimulus is first presented and then fall silent in the presence of continued stimulation (which is to say they “adapt” to the stimulus), whereas others generate a sustained discharge in the presence of an ongoing stimulus (Figure 9.2). The usefulness of having some receptors that adapt quickly and others that do not is to provide information about both the dynamic and static qualities of a stimulus. Receptors that initially fire in the presence of a stimulus and then become quiescent are particularly effective in conveying information about changes in the information the receptor reports; conversely, receptors that continue to fire convey information about the persistence of a stimulus. Accordingly, somatic sensory receptors and the neurons that give rise to them are usually classified into rapidly or slowly adapting types (see Table 9.1). Rapidly adapting, or phasic, receptors respond maximally but briefly to stimuli; their response decreases if the stimulus is maintained. Conversely, slowly adapting, or tonic, receptors keep firing as long as the stimulus is present.

Figure 9.1. General organization of the somatic sensory system.

Figure 9.1

General organization of the somatic sensory system. (A) Mechanosensory information about the body reaches the brain by way of a threeneuron relay (shown in red). The first synapse is made by the terminals of the centrally projecting axons of dorsal root (more...)

Figure 9.2. Slowly adapting mechanoreceptors continue responding to a stimulus, whereas rapidly adapting receptors respond only at the onset (and often the offset) of stimulation.

Figure 9.2

Slowly adapting mechanoreceptors continue responding to a stimulus, whereas rapidly adapting receptors respond only at the onset (and often the offset) of stimulation. These functional differences allow the mechanoreceptors to provide information about (more...)

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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK11162

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