HUMAN PHYSIOLOGY AN INTEGRATED APPROACH FIFTH EDITION Dee Vnglaub Silverthorn, Ph.D. University of Texas, Austin WITH CONTRIBUTIONS BY Bruce R. Johnson, Ph.D. Cornell University AND William C. Ober, M.D. Illustration Coordinator Claire W. Garrison, R.N. Illustrator Andrew C. Silverthorn, M.D. Clinical Consultant .~ ~ "'~oo" " "•'. _4t\ll\'I!t.~ PEARSON B(,l\jamill ----­ Cummings San Francisco Boston New York Cape Town Hong Kong London Madrid Mexico City Montreal Munich Paris Singapore Sydney Tokyo Toronto 336 Chapter 10 Sensory Physiology Each sensory receptor h as an adequate stimulus, a partic­ Types of Sensory Receptors ular form of energy to which it is most responsive. For exam­ ple, thermoreceptors are more sensitive to temperature changes TYPE OF RECEPTOR EXAMPLES OF STIMULI than to pressure, and mechanoreceptors respond preferentially to stimuli that deform the cell membrane. Although receptors Oxygen, pH, various organic Chemoreceptors are specific for one form of energy, they can respond to most molecules such as glucose other forms if the intensity is high enough. Photoreceptors of Mechanoreceptors Pressure (baroreceptorsl. cell the eye respond most readily to light, for instance, but a blow stretch (osmoreceptors), vibra­ tion, acceleration, sound to the eye may cause us to "see stars," an example of mechan­ ical energy of sufficient force to stimulate the photoreceptors. Photo receptors Photons of light Sensory receptors can be incredibly sensitive to their pre­ Thermoreceptors Varying degrees of heat ferred form of stimulus. For example, a single photon of light stimulates certain photoreceptors, and a single odorant molecule may activate the chemoreceptors involved in the sense of smell. The minimum stimulus required to activate a receptor is known potential in the associated sensory neuron. Both neural and as the threshold, just as the minimum depolarization required non-neural receptors develop from the same embryonic tissue. to trigger an action poten tial is called the threshold [ ~ p. 2."7J. Non-neural are critical to the operation accessory structures How is a physical or chemical stimulus converted into a of many sensory systems. For example, the lens and cornea of change in membrane potential? The stimulus opens or closes the eye help focus incoming light onto photoreceptors. The ion channels in the receptor membrane, either directly or indi­ hairs on our arms help somatosensory receptors sense move­ rectly (through a second messenger). In most cases, channel ment in the air millimeters above the skin surface. Accessory opening results in net influx of Na + or other cations into the structures often enhance the information-gathering capability receptor, depolarizing the membrane. In a few cases, the re­ of the sensory system. sponse to the stimulus is hyperpolariza tion when K+ leaves the Receptors are divided into four major groups, based on the cell. In the case of vision, the stimulus (light) closes cation type of stimulus to which they are most sensitive (Tbl. e ). 10-2 channels to hyperpolarize the receptor. Chemoreceptors respond to chemical ligands that bind to the The change in sensory receptor membrane PQtential is a receptor (taste and smell, for example). Mechanoreceptors re­ graded potential [ ~ p )-"Hl called a receptor potential. In spond to various forms of mechanical energy, including pres­ some cells, the receptor potential initiates an action potential sure, vibration, gravity, acceleration, and sound (hearing, for that travels along the sensory fiber to the CNS. In other cells, example). Thermoreceptors respond to temperature, and receptor potentials influence neurotransmitter secretion by the photoreceptors for vision respond to light. receptor cell, which in turn alters electrical activity in an asso­ ciated sensory neuron. CONCEPT CHECK A Sensory Neuron Has a Receptive Field 1. What advantage do myelinated axons provide? Somatic sensory and visual neurons are activated by stimuli that 2. What accessory role does the outer ear (the pinna) play in the auditory system? fall within a specific physical area known as the neuron's 3. For each of the somatic and visceral stimuli listed in Table 10-1, receptive field. For example, a touch-sensitive neuron in the skin which of the following receptor types is the appropriate trans­ responds to pressure that falls within its receptive field. In the ducer: mechano-, chemo-, photo-, or thermoreceptors? simplest case, one receptive field is associated with one sensory Answers : p. 383 neuron (the primary sensory neuron in the pathway), which in turn synapses on one CNS neuron (the secondary sensory Sensory Transduction Converts Stimuli neuron). However, receptive fields frequently overlap with into Graded Potentials neighboring receptive fields. (Primary and secondary sensory How do receptors convert diverse physical stimuli, such as light neurons are also known as first-order and second-order neurons .) or heat, into electrical signals? The first step is transduction, In addition, sensory neurons of neighboring receptive the conversion of stimulus energy into information that can be fields may exhibit convergence [ ~ p. 282], in which multiple processed by the nervous system [ ~ [1. lK2]. In many recep­ presynaptic neurons provide input to a smaller number of post­ tors, the opening or closing of ion channels converts mechan­ synaptic neurons (Fig. 10-2 e ). Convergence allows multiple ical, chemical, thermal, or light energy directly into a change simultaneous subthreshold stimuli to sum at the postsynaptic in membrane potential. Some sensory transduction mecha­ (secondary) neuron. When multiple primary sensory neurons nisms include signal transduction and second messenger systems converge on a single secondary sensory neuron, their individ­ [ ~ p. I H..J.l that initiate the change in membrane potential. ual receptive fields merge into a single, large secondary receptive field, as in Figure 10-2. General Properties of Sensory Systems 337 Primary sensory The primary sensory neurons Information from the neurons converge on one secondary secondary receptive sensory neuron. ~ .. - field goes to the brain. g ~ o The receptive fields of three primary sensory neurons overlap to form one large secondary receptive field. SECTION THROUGH SPINAL CORD • FIGURE 10-2 Receptive fields of sensory neurons. Convergence of pri­ mary sensory neurons allows simultaneous subthreshold stimuli to sum at the second­ ary sensory neuron and initiate an action potential. (In this illustration, only part of the secondary sensory neuron is shown.) 10 The size of secondary receptive fields determines how sen­ brain as a single pinprick. In these areas, many primary neu­ sitive a given area is to a stimulus. For example, sensitivity to rons converge on a single secondary neuron, so the secondary touch is demonstrated by a two-point discrimination test. In receptive field is very large (Fig. 1O-3a e ). In contrast, more some regions 'of skin, such as that on the arms and legs, two sensitive areas of skin, such as the fingertips, have smaller re­ pins placed within 20 mm of each other are interpreted by the ceptive fields, with as little as a 1:1 relationship between (a) Convergence of many primary (b) When fewer neurons converge, sensory neurons creates a very secondary receptive fields are Compass with points ~ large secondary receptive field. much smaller. The two stimuli separated by 20 mm Two simuli that fall within the activate separate pathways and same secondary receptive field are perceived as distinct stimuli. are perceived as a single point, because only one signal goes to the brain. Primary ( sensory neurons Secondary sensory .­ neurons One signal goes to the brain . Two signals go to the brain. • FIGURE 10-3 Two-point discrimination varies with the size of the sec­ ondary receptive field. (a) Less-sensitive regions of the skin are found on the arms and legs. In these regions, two stimuli 20 mm apart cannot be felt separately. (b) In more sensitive regions, such as the fingertips, two stimuli separated by as little as 2 mm will be perceived as two distinct stimuli. 338 Chapter 10 Sensory Physiology ~r-=--:"""",,~-.=------- Primary somatic Gustatory cortex ----~ sensory cortex Olfactory cortex Olfactory bulb Auditory cortex Visual cortex .!III... Olfactory pathways from III, the nose project through the olfactory bulb to the olfactory cortex . .. !II. Most sensory pathways project .. ~ to the thalamus. The thalamus modifies and relays information to cortical centers. 4) Equilibrium pathways project primarily to the cerebellum. FIGURE QUESTION Which sensory pathways shown do not synapse in the thalamus? e FIGURE 10-4 Sensory pathways in the brain. Most pathways except the olfactory pathway pass through the thalamus on their way to the cerebral cortex. primary and secondary sensory neurons. In these regions, two Only olfactory [olfacere, to snim information is not routed pins separated by as little as 2 mm can be perceived as two sep­ through the thalamus. The sense of smell, a type of chemorecep­ arate touches (Fig. lO-3b). tion, is considered to be one of the oldest senses, and even the most primitive vertebrate brains have well-developed regions The eNS Integrates Sensory Information for processing olfactory information. Information about odors Sensory information from much of the body enters the spinal travels from the nose through the first cranial nerve l ~ p. J LO] cord and travels through ascending pathways to the brain. and olfactory bulb to the olfactory cortex in the cerebrum. Some sensory information goes directly into the brain stem via Perhaps it is because of this direct input to the cerebrum that the cranial nerves [ ~ p.{IIl J. Sensory information that initi­ odors are so closely linked to memory and emotion. Most people ates visceral reflexes is integrated in the brain stem or spinal have experienced encountering a smell that suddenly brings cord and usually does not reach conscious perception. An ex­ back a flood of memories of places or people from the past. ample of an unconscious visceral reflex is the control of blood One interesting aspect of eNS processing of sensory infor­ pressure by centers in the brain stem.