Sensory Receptors
The sensory receptor is a
specialized cell. In mammals, receptors fall into five groups: mechanoreceptors, thermoreceptors, nociceptors,
chemoreceptors (Chapter 56) and photoreceptors (Chapter 57).
There is further specialization within these groups. Each receptor responds to
one stimulus type; this property is called the specificity of the receptor.
The stimulus that is effective in eliciting a response is called the adequate
stimulus.
Transduction processes. Some receptors consist of a nerve fibre alone
(e.g. free nerve endings), others consist of a specialized accessory structure
(e.g. olfactory receptors, Pacinian corpuscles), and others are more complex
and consist of a specialized receptor cell which synapses with a neurone, in
other words a secondary sensory cell (e.g. gustatory receptors and Merkel’s
discs).
Mechanoreceptors. These are found all over the body. Those in
the skin have three main qualities: pressure, touch and vibration (or
acceleration) (Fig. 55a,b). When the responses to constant stimuli are studied
in the various receptors, the receptors can be divided into three types on the
basis of their adaptive properties: slowly adapting receptors that
continue to fire action potentials even when the pressure is maintained for a
long period (e.g. Ruffini’s endings, tactile discs, Merkel’s
discs); moderately rapidly adapting receptors that fire for about
50–500 ms after the onset of the stimulus, even when the pressure is maintained
(e.g. hair follicle receptors, Meissner’s corpuscles); and very
rapidly adapting receptors that fire only one or two impulses (e.g. Pacinian
corpuscles) (Fig. 55b). These three types of receptor are examples of
receptors in the skin that detect intensity, velocity and vibration
(or acceleration), respectively.
