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.
Free nerve endings. Each skin nerve, in addition to the large
myelinated afferents, contains a large number (over 50% of the fibres) of smaller
myelinated and unmyelinated (Aδ and C) axons. Some of the C fibres
are, of course, efferent postganglionic sympathetic fibres. However, a large
number of the remaining fibres are afferents that terminate in free nerve
endings and not in corpuscular structures (Fig. 55a). Many of these are thermoreceptors
or nociceptors.
Thermoreceptors. Thermoreceptors mediate the sensations of cold
and warmth. In the skin of humans, there are specific cold and
warm points at which only the sensation of cold or warmth can be elicited.
These are specific cold and warmth receptors; however, they share the following
characteristics: (i) maintain discharge at constant skin temperature,
with the discharge rate proportional to the skin temperature (static response);
(ii) have small receptive fields (1 mm2 or less), each afferent fibre
supplying only one or a few warm or cold points; and (iii) serve not only as sensors
for the conscious sensation of temperature, but also participate (together
with temperature sensors in the hypothalamus and spinal cord) in the thermoregulation
of the body.
Nociceptors and pain. Pain differs from the other sensory
modalities with regard to the kind of information it conveys. It informs us of
a threat to our bodies when it is activated by noxious (tissue-damaging)
stimuli. Nociception is defined as the reception, conduction and
central processing of noxious signals. This term is used to make a clear distinction between these ‘objective’
neuronal processes and the ‘subjective’ sensation of pain, which is
defined as an unpleasant sensory and emotional experience associated with
actual or potential damage, or described in terms of such damage.
Nociceptors are found in the skin, visceral organs and
muscle (cardiac and skeletal), and are associated with blood vessels. The qualities
of pain are divided into somatic and visceral. If somatic
pain is derived from the skin, it is called superficial pain, and, if
from muscle, bone joints or connective tissue, it is called deep pain.
If superficial pain is produced by piercing the skin with a needle, the subject
feels a sharp pain; this easily localized sensation fades away rapidly when the
needle is removed. This sharp, localized initial pain (also called first
or fast pain) is often followed, particularly at high stimulus
intensities, by delayed pain (also called second or slow pain),
which has a dull (or burning) character with a delay of about 1 s. This delayed
pain is more diffuse spatially, dies out slowly and is not so easily localized.
Deep pain is dull in nature, poorly localized and has a tendency to
radiate into the surroundings.
The responses of the body in terms
of both the distress and suffering and the autonomic and motor responses to
pain depend on the quality of pain (Fig. 55c). Delayed pain and deep
pain are accompanied by a feeling of unpleasantness, and often
elicit autonomic reflexes of nausea, heavy sweating and lowered
blood pressure. Initial pain gives rise, by contrast, to protective
reflexes, i.e. flexor withdrawal reflex. Visceral pain (pain from
organs such as the kidney, stomach and gallbladder) tends to be dull and
diffuse in character and resembles deep pain.
Histologically, the nociceptors are
free nerve endings attached to either Aδ fibres or C fibres.
It has been proposed that, in the case of superficial pain, the transmission of
initial (fast) pain is via Aδ fibres, whereas delayed (slow)
pain is signalled by the smaller C fibres. The time difference between
initial (fast) pain and delayed (slow) pain appears to be explained by the
difference in the conduction velocities of the fibres concerned.
Inhibitory influences. Like all other sensory inputs, the
nociceptive afferent influx is exposed to inhibitory influences at the
receptor, on its way to and through the spinal cord and in the higher levels of
the central nervous system. Many of the modern treatments elicit or enhance
these inhibitory processes, pharmacologically using drugs, physically using
cold or warm wrappings, short-wave radiation, massage and exercise, and by the
electrical stimulation of certain structures, including peripheral nerves. Acupuncture
and transcutaneous electrical nerve stimulation (TENS) may
possibly depend on the activation and maintenance of inhibitory processes.
Naturally occurring endorphins, enkephalins and dynorphins are
thought to contribute to these processes. These are endogenous,
pain-controlling opiates produced by the body that attach to the specific
opiate receptors, so as to inhibit the sensation of pain without affecting the
other sensory modalities.
