Sensory Physiology General Properties of Sensory Systems 10 Receptors Are Sensitive to Particular Forms of Energy Sensory Transduction Converts Stimuli into Graded Potentials A Sensory Neuron Has a Receptive Field The CNS Integrates Sensory Information Coding and Processing Distinguish Stimulus Properties Somatic Senses Pathways for Somatic Perception Project to the Cortex and Cerebellum Touch Receptors Respond to Many Diff erent Stimuli Temperature Receptors Are Free Nerve Endings Nociceptors Initiate Protective Responses Pain and Itching Are Mediated by Nociceptors Chemoreception: Smell and Taste Olfaction Is One of the Oldest Senses Taste Is a Combination of Five Basic Sensations Taste Transduction Uses Receptors and Channels The Ear: Hearing Hearing Is Our Perception of Sound Sound Transduction Is a Multistep Process The Cochlea Is Filled with Fluid Sounds Are Processed First in the Cochlea Auditory Pathways Project to the Auditory Cortex Hearing Loss May Result from Mechanical or Neural Damage Nature does not The Ear: Equilibrium communicate with The Vestibular Apparatus Provides Information about Movement man by sending and Position encoded messages. The Semicircular Canals Sense Rotational Acceleration — Oscar Hechter, in Biology The Otolith Organs Sense Linear Acceleration and Head Position and Medicine into the Equilibrium Pathways Project Primarily to the Cerebellum 21st Century, 1991 The Eye and Vision The Skull Protects the Eye Background Basics Light Enters the Eye through the Pupil The Lens Focuses Light on the Retina Summation Phototransduction Occurs at the Retina Second messenger systems Photoreceptors Transduce Light into Electrical Signals Threshold Signal Processing Begins in the Retina G proteins Plasticity Tonic control Membrane potential Graded potentials Neurotransmitter Vestibular release hair cells From Chapter 10 of Human Physiology: An Integrated Approach, Sixth Edition. Dee Unglaub Silverthorn. Copyright © 2013 by Pearson Education, Inc. All rights reserved. 343 Sensory Physiology magine floating in the dark in an indoor tank of buoyant Information Processing by the Table salt water: there is no sound, no light, and no breeze. Th e air 10.1 and water are the same temperature as your body. You are Sensory Division I in a sensory deprivation chamber, and the only sensations you Stimulus Processing Usually Conscious are aware of come from your own body. Your limbs are weight- less, your breath moves in and out eff ortlessly, and you feel your Special Senses Somatic Senses heart beating. In the absence of external stimuli, you turn your Vision Touch awareness inward to hear what your body has to say. In decades past, fl otation tanks for sensory deprivation were Hearing Temperature a popular way to counter the stress of a busy world. Th ese facili- ties are hard to fi nd now, but they illustrate the role of the aff er- Taste Pain ent division of the nervous system: to provide us with information about the environment outside and inside our bodies. Sometimes Smell Itch we perceive sensory signals when they reach a level of conscious Equilibrium Proprioception awareness, but other times they are processed completely at the subconscious level ( Tbl. 10.1 ). Stimuli that usually do not reach Stimulus Processing Usually Subconscious conscious awareness include changes in muscle stretch and ten- Somatic Stimuli Visceral Stimuli sion as well as a variety of internal parameters that the body moni- tors to maintain homeostasis, such as blood pressure and pH. Th e Muscle length and Blood pressure responses to these stimuli constitute many of the subconscious re- tension fl exes of the body, and you will encounter them as we explore the processes that maintain physiological homeostasis. Proprioception Distension of gastrointestinal tract In this chapter we are concerned primarily with sensory stimuli whose processing reaches the conscious level of perception. Blood glucose concentration Th ese stimuli are those associated with the special senses of vision, Internal body temperature RUNNING PROBLEM Osmolarity of body fluids Lung inflation Ménière’s Disease On December 23, 1888, Vincent Van Gogh, the legendary pH of cerebrospinal fluid French painter, returned to his room in a boardinghouse in Arles, France, picked up a knife, and cut off his own ear. A pH and oxygen content of blood local physician, Dr. Felix Ray, examined Van Gogh that night and wrote that the painter had been “assailed by auditory hallucinations” and in an eff ort to relieve them, “mutilated hearing, taste, smell, and equilibrium, and the somatic senses of himself by cutting off his ear.” A few months later, Van Gogh touch, temperature, pain, itch, and proprioception. Propriocep- committed himself to a lunatic asylum. By 1890, Van Gogh tion , which is defi ned as the awareness of body movement and po- was dead by his own hand. Historians have postulated sition in space, is mediated by muscle and joint sensory receptors that Van Gogh suff ered from epilepsy, but some American called proprioceptors and may be either unconscious or conscious. neurologists disagree. They concluded that the painter’s If you close your eyes and raise your arm above your head, you are strange attacks of dizziness, nausea, and overwhelming aware of its position because of the activation of proprioceptors. tinnitus (ringing or other sounds in the ears), which he described in desperate letters to his relatives, are more We fi rst consider general properties of sensory pathways. consistent with Ménière’s disease, a condition that aff ects the We then look at the unique receptors and pathways that distin- inner ear. Today, Anant, a 20-year-old college student, will be guish the diff erent sensory systems from one another. examined by an otolaryngologist (ear-nose-throat specialist) to see if his periodic attacks of severe dizziness and nausea are caused by the same condition that might have driven Van General Properties Gogh to suicide. of Sensory Systems All sensory pathways have certain elements in common. They begin with a stimulus, in the form of physical energy that acts on a sensory receptor. Th e receptor is a transducer 344 Sensory Physiology that converts the stimulus into an intracellular signal, usu- million associated parts, and the human eye has about 126 mil- ally a change in membrane potential. If the stimulus is above lion sensory receptors. threshold, action potentials pass along a sensory neuron to the central nervous system, where incoming signals are in- tegrated. Some stimuli pass upward to the cerebral cortex, Receptors Are Sensitive to Particular where they reach conscious perception, but others are acted on subconsciously, without our awareness. At each synapse Forms of Energy along the pathway, the nervous system can modulate and Receptors in the sensory system vary widely in complexity, shape the sensory information. ranging from the branched endings of a single sensory neuron Sensory systems in the human body vary widely in com- to complex, highly organized cells such as photoreceptors. Th e plexity. Th e simplest systems are single sensory neurons with simplest receptors consist of a neuron with naked (“free”) nerve branched dendrites that function as receptors, such as pain and endings ( Fig. 10.1a). In more complex receptors, the nerve itch receptors. Th e most complex systems include multicellular endings are encased in connective tissue capsules ( Fig. 10.1 b). sense organs , such as the ear and the eye. The cochlea of the Th e axons of both simple and complex receptors may be myelin- ear contains about 16,000 sensory receptors and more than a ated or unmyelinated. 10 SENSORY RECEPTORS (a) Simple receptors are neurons (b) Complex neural receptors have nerve (c) Most special senses receptors are cells with free nerve endings. They endings enclosed in connective tissue capsules. that release neurotransmitter onto sensory may have myelinated or This illustration shows a Pacinian corpuscle, neurons, initiating an action potential. The unmyelinated axons. which senses touch. cell illustrated is a hair cell, found in the ear. Stimulus Stimulus Stimulus Free nerve endings Enclosed nerve Specialized receptor ending cell (hair cell) Layers of connective tissue Synaptic vesicles Synapse Unmyelinated axon Myelinated axon Myelinated axon Cell body Cell body Cell body of sensory neuron Fig. 10.1 345 Sensory Physiology Th e special senses have the most highly specialized recep- tors. The receptors for smell are neurons, but the other four Table 10.2 special senses use non-neural receptor cells that synapse onto Types of Sensory Receptors sensory neurons. Th e hair cell of the ear, shown in Figure 10.1c, is an example of a non-neural receptor. When activated, the hair Type of Receptor Examples of Stimuli cell releases a neurotransmitter that initiates an action potential Chemoreceptors Oxygen, pH, various in the associated sensory neuron. Both neural and non-neural organic molecules such as receptors develop from the same embryonic tissue. glucose Non-neural accessory structures are critical to the opera- tion of many sensory systems. For example, the lens and cornea Mechanoreceptors Pressure (baroreceptors), cell stretch of the eye help focus incoming light onto photoreceptors. Th e (osmoreceptors), vibration, hairs on our arms help somatosensory receptors sense move- acceleration, sound ment in the air millimeters above the skin surface. Accessory structures oft en enhance the information-gathering capability Photoreceptors Photons of light of the