9 Sensory Receptors and the Peripheral Nervous System
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).
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 Maintained Stimuli, Some More Rapidly Than Others
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 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