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Sensory Receptors and the Peripheral Nervous System
The ongoing activity and output of the central nervous system (CNS) are greatly influenced, and sometimes more or less determined, by incoming sensory information. An example is our constant awareness of the position of our limbs in space and the use of this awareness in guiding movements. The basis of this incoming sensory information is an array of sensory receptors, a
a Receptor is a term with two meanings in neurobiology. A sensory receptor, as described in this chapter, is a specialized cell that conveys to the nervous system information about some stimulus. Neurotransmitter receptors, as described in Chapter 8 , are molecules in postsynaptic membranes. To make matters worse, some sensory receptors receive feedback synapses and so have neurotransmitter receptors in their membranes.
cells that detect various stimuli and produce receptor potentials in response, often with astonishing effectiveness. Rod photoreceptors, for example, can produce measurable responses to single photons (see Chapter 17 ), and olfactory receptors may be able to respond to single odorant molecules (see Chapter 13 ); auditory and vestibular receptors can respond to almost unimaginably small mechanical deflections (see Chapter 14 ). The physiological processes employed by sensory receptors turn out to be gratifyingly similar to those found in synapses.
This chapter considers some general principles of the anatomy and physiology of sensory receptors. The emphasis is on the general receptors of the body—the principal purveyors of sensory information to the spinal cord (see also Chapter 10 ). Specialized receptors, such as those of the eye, ear, mouth, and nose, are described in more detail in later chapters.
Receptors Encode the Nature, Location, Intensity, and Duration of Stimuli
There are many types of receptors on and within the human body and several different systems for classifying them. One system subdivides receptors according to the traditional five senses of vision, hearing, touch, smell, and taste. This system is too restrictive in that it does not recognize sensations such as balance, position, or pain, or sensory information from internal organs that usually does not reach consciousness. Another system distinguishes interoceptors, proprioceptors, and exteroceptors. Interoceptors monitor events within the body, such as distention of the stomach or changes in the pH of blood. Proprioceptors respond to changes in the position of the body or its parts; examples are the receptors in muscles and joint capsules. Vestibular receptors of the inner ear are commonly classified as proprioceptors because they signal movement and changes in the orientation of the head in space. Exteroceptors respond to stimuli that arise outside the body, such as the receptors involved in touch, hearing, and vision. Interoceptor-proprioceptor-exteroceptor terminology is used less commonly than in the past, partly because some receptors do not fit neatly and uniquely into one of these categories. For example, the visual system is very much involved in perception of motion and body position (i.e., proprioception), but visual receptors are exteroceptors.
Each Sensory Receptor Has an Adequate Stimulus, Allowing It to Encode the Nature of a Stimulus
A more commonly used and straightforward classification system subdivides receptors on the basis of the type of stimulus to which they are most sensitive (called the adequate stimulus ). Chemoreceptors include those for smell, taste, and many internal stimuli such as pH and metabolite concentrations. Photoreceptors are the visual receptors of the retina. Thermoreceptors respond to temperature and its changes. Mechanoreceptors, the most varied group, respond to physical deformation. These include cutaneous receptors for touch, receptors that monitor muscle length and tension, auditory and vestibular receptors, and others. Pain receptors are a bit difficult to classify in this system because different pain receptors have varying degrees of sensitivity to mechanical, thermal, and chemical stimuli. This problem is commonly finessed by classifying pain receptors separately as nociceptors (from the Latin verb nocere, meaning “to hurt,” as in noxious or obnoxious). Virtually all animals have these kinds of receptors, but this is not an exhaustive list. There are other kinds of energy, and various species have developed ways to sample them, such as magnetoreceptors used by migratory birds to help find their way, infrared receptors used by some snakes to detect warm animals nearby, and electroreceptors used by some fish to navigate.
To a first approximation, the type of receptor that is stimulated defines the nature, or modality, of the sensation that is experienced: you experience touch if something actually touches you or if a peripheral nerve attached to a touch receptor is stimulated electrically. Each sensory modality has a series of associated submodalities, or qualities. Stimuli delivered to the skin, for example, can feel like light touch, pressure, a tickle, or vibration. This roughly corresponds to the presence in the skin of multiple receptor types whose separate outputs are combined by the CNS to produce sensations that are richer and more complex: for example, sensations such as the texture of objects.
Many Sensory Receptors Have a Receptive Field, Allowing Them to Encode the Location of a Stimulus
Specific wiring patterns in ascending sensory pathways and in the cerebral cortex preserve information about the nature of a stimulus. In some sensory systems, individual receptors convey information not only about the nature of a stimulus but also about its location. That is, in addition to adequate stimuli, individual receptors may have receptive fields, particular areas in the periphery where application of an adequate stimulus causes them to respond. The receptive field of a cutaneous receptor, for example, is an area of skin where its receptive endings reside ( Fig. 9.1 ). The receptive field of a retinal photoreceptor correlates to some small location in the outside world whose image falls on a particular spot on the retina where that photoreceptor is located. This preservation of spatial information in the CNS is part of the distorted maps of visual sensory stimuli (see Fig. 3.31 ). Even arrays of receptors with no obvious spatial domain to map may have systematic representations of some other parameter, such as the mapping of sound frequencies in the cochlea and in auditory cortex (see Fig. 14.19 ).
Neurons in successive levels of sensory pathways—second-order neurons, thalamic and cortical neurons—also have receptive fields, although they may be considerably more elaborate than those of the receptors. Neurons in visual cortex, for example, typically respond not to spots of light but rather to edges with particular orientations.
Receptor Potentials Encode the Intensity and Duration of Stimuli
The nature and location of a stimulus are indicated by the identities of the receptors that respond. To a great extent the intensity and duration of a stimulus are indicated by the size and duration of the receptor potentials produced—more intense stimuli produce larger receptor potentials, and longer stimuli cause longer receptor potentials ( Fig. 9.2 ). There is a bit more to the intensity-duration story than this, however. Some sensory systems include more sensitive and less sensitive receptors (e.g., the rods and cones of the retina), so increasing intensity may be signaled in part by the identities of the active receptors. In addition, as discussed in the next section, some sensory receptors produce only brief receptor potentials, even in response to maintained stimuli.
Most Sensory Receptors Adapt to Maintained Stimuli, Some More Rapidly Than Others
Nearly all receptors show some adaptation, which means they become less sensitive during the course of a maintained stimulus. b
b Nociceptors are a prominent exception. Many do not adapt to maintained stimuli, and, as discussed later in this chapter, repeated or prolonged noxious stimuli can make some nociceptors even more sensitive.
Those that adapt relatively little are called slowly adapting and are suitable receptors for such things as static position. Those that adapt a great deal typically do so quickly and are called rapidly adapting; they are better suited to indicate change and movement of stimuli ( Fig. 9.3 ). Some rapidly adapting receptors (e.g., the Pacinian corpuscles shown in Fig. 9.8 ) do so completely and signal only the beginning and end of a stimulus; others continue to respond, but at a diminished level, throughout a stimulus. Adaptation is usually a property of one or more parts of the receptor's membrane: Ca 2+ ions entering through transduction channels, for example, set in motion biochemical processes that decrease the sensitivity of some receptors. In addition, various accessory structures may modify the physical stimulus before it reaches the sensory ending, as in the case of the multilayered capsules of Pacinian corpuscles.
Adaptation is a receptor-level process, but the CNS also has ways to regulate the sensitivity of receptors on a moment-to-moment basis. One such mechanism is the control of other structures related to receptors—for example, controlling the amount of light reaching the retina by regulating pupil size, or controlling the muscle fibers in stretch receptors (see Fig. 9.15 ). Another mechanism used is efferent projections from the CNS that synapse on receptor cells themselves; these are prominent in the inner ear (see Chapter 14 ) but occur in some other sensory systems as well.
Sensory Receptors All Share Some Organizational Features
Although their morphologies vary widely, all receptors have three general parts: a receptive area, an area rich in mitochondria (near the receptive area), and a synaptic area, where the receptor's message is passed toward or into the CNS ( Fig. 9.4 ). The receptive area may have specializations suited to the adequate stimulus, as in the case of photoreceptors, which have an elaborately folded array of photopigment-bearing membrane; in other cases, there are no obvious specializations in this area. The area rich in mitochondria is either immediately adjacent to the receptive membrane or nearby and is presumed to supply the energy needs of the transduction process. In some receptors the synaptic area may be far removed from the other two, as in the case of a cutaneous mechanoreceptor with its receptive endings in the skin and its synaptic terminals in the spinal cord or brainstem.
Sensory Receptors Use Ionotropic and Metabotropic Mechanisms to Produce Receptor Potentials
Sensory receptors transduce (Latin for “lead across”) some physical stimulus into an electrical signal—a receptor potential—that the nervous system can understand. Receptor potentials, like other electrical signals across neuronal membranes, are produced by the opening or closing of ion channels. (The only known exception in primates is the taste receptors that detect saltiness, as discussed in Chapter 13 .) In many ways, most sensory receptors can be thought of as analogous to postsynaptic membranes, and their adequate stimuli as analogous to neurotransmitters ( Fig. 9.5 ). Just as postsynaptic membranes use ionotropic and metabotropic mechanisms to produce postsynaptic potentials, almost all known transduction mechanisms involve ion channels whose conductance is affected either directly by a stimulus (as in transmitter-gated ion channels) or indirectly by way of a G protein–coupled mechanism. As in the case of synapses, some sensory receptors (e.g., somatosensory mechanoreceptors) produce depolarizing receptor potentials when stimulated, and others (e.g., photoreceptors) produce hyperpolarizing receptor potentials. Auditory and vestibular receptors can produce either, depending on the details of the stimulus.
Receptors with directly gated ion channels include the somatosensory receptors discussed in this chapter, auditory and vestibular receptors, some taste receptors, and some visceral receptors. Some have channels that are directly sensitive to mechanical distortion, and others have channels directly gated by some molecule or ion or by temperature changes. Receptors with a G protein–coupled transduction mechanism include photoreceptors, olfactory receptors, some taste receptors, and some visceral receptors.
All Sensory Receptors Produce Receptor Potentials, but Some Do Not Produce Action Potentials
Receptor potentials, like postsynaptic graded potentials, are focally produced events that spread electrotonically. If a particular sensory receptor is physically small relative to its length constant—that is, if it contacts the next cell in its neuronal pathway close to the site of transduction—the receptor potential can adequately modulate the rate of transmitter release at the synaptic terminal. This in turn causes a postsynaptic potential and typically a change in action potential frequency, in the second cell ( Fig. 9.6A ). This is the case for many receptors, prominently including photoreceptors and auditory and vestibular receptors, all of which produce receptor potentials but no action potentials. Some receptors, however, must convey information over long distances (e.g., from a big toe to the spinal cord), even though the receptor potential dies out within a few millimeters. In such cases, most of the receptor, beginning near the site of transduction, is capable of propagating action potentials. The action potential frequency is then modulated by the receptor potential (see Fig. 9.6B ). Receptor potentials that directly cause changes in action potential frequency are also called generator potentials. All somatosensory receptors of the body operate in this manner, as do olfactory receptors and many visceral receptors. In vertebrates, all produce depolarizing receptor potentials, and an increased frequency of action potentials, in response to stimulation.
Somatosensory Receptors Detect Mechanical, Chemical, or Thermal Changes
Somatosensory receptors include an assortment of mechanoreceptors, thermoreceptors, and nociceptors. All are pseudounipolar neurons with cell bodies in a dorsal root or cranial nerve ganglion, a central process that terminates in the spinal cord or brainstem, and a peripheral receptive ending in someplace such as the skin, a muscle, or a joint (see Fig. 9.1 ). A great deal is understood about the moment-to-moment responses of many of these receptors, mostly from animal studies, but also in large part because it is possible to record from the axons of single receptors of human volunteers, using a process called microneurography (see Fig. 9.14C , insets).
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