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.