Keywords
primary afferents, receptive field, neuronal adapting, nociceptors, proprioception, blood-nerve barrier
Chapter Outline
Receptors Encode the Nature, Location, Intensity, and Duration of Stimuli, 50
Each Sensory Receptor Has an Adequate Stimulus, Allowing It to Encode the Nature of a Stimulus, 50
Receptor Potentials Encode the Intensity and Duration of Stimuli, 50
Sensory Receptors All Share Some Organizational Features, 51
Somatosensory Receptors Detect Mechanical, Chemical, or Thermal Changes, 52
Nociceptors Have Both Afferent and Efferent Functions, 52
Receptors in Muscles and Joints Detect Muscle Status and Limb Position, 53
Visceral Structures Contain a Variety of Receptive Endings, 53
Peripheral Nerves Convey Information to and From the CNS, 53
The Diameter of a Nerve Fiber Is Correlated With Its Function, 54
Neural traffic to and from the CNS travels in peripheral nerves . The afferent fibers in these peripheral nerves either have endings that respond to physical stimuli (making them primary afferents that are also sensory receptors ) or carry information from separate sensory receptor cells in the periphery. The efferent fibers end on muscle fibers, autonomic ganglia, or glands.
Receptors Encode the Nature, Location, Intensity, and Duration of Stimuli
The job of sensory receptors collectively is to produce electrical signals that represent all relevant aspects of stimuli—what kind of stimulus, where it is, how intense, when it starts and stops. Sometimes a single receptor can do all of this, but often one or more populations of receptors are required.
Each Sensory Receptor Has an Adequate Stimulus, Allowing It to Encode the Nature of a Stimulus
Sensory receptors transduce (“lead across”) some aspect of the external or internal environment into a graded electrical signal (a receptor potential ). Each receptor is more sensitive to one kind of stimulus, called its adequate stimulus , than to others. Hence, there are chemoreceptors , photoreceptors , thermoreceptors , and mechanoreceptors , and the identity of the particular receptors responding to a stimulus provides some initial information about the nature of that stimulus. Individual types of receptors within these broad classes are usually more finely tuned to particular aspects of a stimulus category, providing even more information about the nature of a stimulus. For example, although all the mechanoreceptors of the inner ear are very similar to each other, some are set up to respond best to sound vibrations, others to the position of the head (see Chapter 14 ).
Many Sensory Receptors Have a Receptive Field, Allowing Them to Encode the Location of a Stimulus.
Some receptors are tuned not only to their adequate stimulus but also to the location of the stimulus. All receptors obviously can only respond to stimuli that reach them, but some receptors and their central connections are specialized to preserve information about location. For example, single cutaneous receptors respond only to stimuli that affect areas of skin containing their endings, and the CNS keeps track of which individual receptors respond to determine the location of a touch or pinch. An area of the body or outside world in which stimuli cause electrical changes in a receptor is called the receptive field of that receptor, and, because this spatial information is preserved in sensory pathways, neurons located more centrally in these pathways also have receptive fields.
For some other receptors, receptive fields either are less relevant or the concept just does not apply. Examples are many visceral receptors keeping track of things like blood pressure or glucose concentration, or vestibular receptors monitoring head position.
Receptor Potentials Encode the Intensity and Duration of Stimuli
To a first approximation, receptors encode the intensity and duration of stimuli by the size and duration of the receptor potentials they produce ( Fig. 9.1 ). There’s actually a little more to it than this, though, because in some systems increasing intensity is signaled by recruiting additional, less sensitive, receptors (e.g., rods for dim light and cones for bright light). In addition, some receptors signal only the beginning and end of a stimulus and do not respond to maintained stimuli.
Most Sensory Receptors Adapt to Maintain Stimuli, Some More Rapidly Than Others.
Some receptors produce a maintained response to a constant stimulus, and so are called slowly adapting . The response of others declines and may disappear entirely during a constant stimulus; these are called rapidly adapting . Rapidly adapting receptors can therefore act like miniature differentiators, producing a constant response to a steadily changing stimulus. The classic example of a rapidly adapting receptor is the Pacinian corpuscle , which responds only briefly at the beginning and end of a constant mechanical stimulus but responds continuously to vibration.
Most receptors, unlike the two shown in Fig. 9.2 , are actually somewhere between the extremes of slowly and rapidly adapting. The response may be exaggerated at the beginning (or end) of a stimulus but maintained to some extent throughout the stimulus (see Fig. 9.1 ).
Sensory Receptors All Share Some Organizational Features
Sensory receptors, like neurons in general, have parts specialized for receiving stimuli (in this case sensory rather than synaptic stimuli) and parts specialized for transmitting information to other neurons. They also typically have numerous mitochondria near the receptive area, presumably to supply energy for transduction processes.
Sensory Receptors Use Ionotropic and Metabotropic Mechanisms to Produce Receptor Potentials.
The transduction mechanisms used by sensory receptors are gratifyingly similar to the mechanisms used in the production of postsynaptic potentials ( Fig. 9.3 ). Some are depolarizing, others hyperpolarizing; some use direct alteration of ion channels, others use G protein–coupled mechanisms ( Table 9.1 ). Many of the receptor molecules used by sensory receptors are actually closely related to postsynaptic receptor molecules, but are simply set up so that they respond to a stimulus rather than to a neurotransmitter.
| Stimulus-Gated Channels | G Protein–Coupled Mechanisms |
|---|---|
| Most somatosensory receptors | Some pain receptors Photoreceptors |
| Hair cells (cranial nerve VIII) | Olfactory receptors |
| Some taste receptors | Some taste receptors |
| Some visceral receptors | Some visceral receptors |
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