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This chapter explores the auditory system, including the transduction of sound waves into neural impulses, pitch perception, detection of loudness and sound complexity, sound localization, and the role of the auditory cortex in sound recognition.
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Chapter 7 Hearing, Balance, and the Cutaneous and Chemical Senses
The Auditory System: • Sound - Vibrations in a material medium, such as air, water, or metal. • Sound waves vary along three dimensions: • Frequency refers to the number of vibrations per second and is measured in hertz (Hz). We perceive the frequency of a sound as pitch. • Amplitude refers to the loudness of a sound wave and is measured in decibels (dB). • Timbre refers to the combination of multiple frequencies that make up complex sounds and give them their characteristic qualities.
The Human Ear
The Transduction of Sound Waves into Neural Impulses • The inner hair cells have a resting potential of -60 mV. • When cilia bend in the direction of the longest cilium the membrane depolarizes. • This leads to a rapid influx of Ca2+ ions into the hair cells, which results in the release of glutamate.
Pitch Perception: Early Research • Place Theory of Pitch Perception • The view that different sounds activate nerve fibers at different locations on the basilar membrane. • High-pitched sounds activate the nerve fibers at the base of the membrane near the oval window • Low-pitched sounds stimulate nerve fibers at the opposite end of the basilar membrane.
Pitch Perception: Early Research • Frequency Theory of Pitch Perception • The view that the firing rate in the auditory nerve matches the frequency of the sound. That is, the basilar membrane vibrates in synchrony with the sound wave. • We now know that the firing rate matches the frequency because of the volley principle: • while one group of neurons in the auditory nerve is firing, another group is recovering from its previous activity • the end result being that the combined firing of all the groups matches the frequency of the sound.
Pitch Perception: Current Theory • The current theory of pitch perception uses a combination of the previous theories: • From 20 Hz to 400 Hz, frequency theory accounts for pitch perception (the firing rate of individual neurons in the auditory nerve directly matches the frequency of the sound). • From 400 Hz to 4 kHz, volley principle takes over. • Beyond 4 kHz, place theory comes into play (the place of maximal vibration on the basilar membrane determines the pitch that we perceive). • Additionally, both place theory and the volley principle work for sounds from about 1 kHz to 4 kHz (may explain our greater sensitivity to pitches within this range).
Detection of Loudness • The nervous system has two mechanisms for determining the intensity of a stimulus: • The rate of firing of individual neurons • The number of neurons firing • The higher the firing rate, or the greater the number of neurons firing, the more intense the stimulus.
Detection of Sound Complexity • Pure tones are sounds of only one frequency; complex sounds have two or more frequencies. • Combination of frequencies produces what we perceive as the timbre of a particular sound. • According to the place theory, because each sound frequency activates a specific part of the basilar membrane, a complex sound produces a unique pattern of neural activity.
Sound Localization • For both low-pitched sounds and high-pitched sounds, the cues to sound localization are based on differential time of arrival at the two ears. • As long as the sound does not come from the median plane, the sound will arrive at one ear slightly before it gets to the other ear, which allows us to locate the direction from which a sound comes from.
The Role of the Auditory Cortex in Sound Recognition • Auditory receptors encode sound: • Frequency • Intensity • Timbre • Receptors send this information to the primary auditory cortex. • In auditory cortex, some neurons respond selectively to specific aspects of sounds; others react to more complex aspects of the sound stimulus.
The Role of the Auditory Cortex in Sound Recognition • Sound is identified as the neural information moves from the primary auditory cortex to the anterior part of the lateral surface of the superior temporal gyrus • Sound is localized as it moves to the posterior part of the superior temporal gyrus and then to the parietal cortex
The Vestibular Sense • The sense responsible for maintaining balance. • Enables us to walk on two feet, keep our head upright, and adjust our eye movements to compensate for our head movements. Phillippe Petit
Components of the Vestibular System • Vestibular sacs - Provide information about the position of the head relative to the body. • Utricle and saccule -The two vestibular sacs containing the vestibular receptor cells, or hair cells. • Semicircular canals - Fluid-filled canals that provide information related to head movements or rotations. • Ampulla, crista, cupula
Vestibular Pathways • Vestibular hair cells • convert information about passive head movement and active head rotation into an increase or decrease in neurotransmitter release • synapse with bipolar neurons • Cell bodies of bipolar neurons form: • vestibular ganglia (receive input from vestibular hair cells) • axons of the vestibular ganglia become the vestibular nerve (combine with cochlear nerve fibers to form the auditory nerve)
Motion Sickness • Feelings of dizziness and nausea; occur when the body is moved passively without motor activity and corresponding feedback to the brain. • Two types of motion sickness: • Detects movements but motor actions that could have produced the movement have not occurred • Detects movement inconsistent with the information about movement sensed by the eyes
The Somatosenses • The skin sensations of touch, pain, temperature, and proprioception. • Proprioception -The somatosense that monitors body position and movement, acts to maintain body position, and ensures the accuracy of intended movements • located in the muscles, tendons, and joints • essential to the control of movement.
Skin Receptors • The functions of the skin include: • protecting the internal organs from injury • helping regulate body temperature by producing sweat, which cools the body when it becomes too hot • providing a first line of defense against invading microorganisms.
Somatosensory Pathways • The dorsal column-medial lemniscal system • begins in the spinal cord and transmits information about touch and proprioception to the primary somatosensory cortex. • The anterolateral system • begins in the spinal cord and transmits information about temperature and pain to the brain stem, reticular formation, and the primary and secondary somatosensory cortices. • The spinocerebellar system • begins in the spinal cord and transmits proprioceptive information to the cerebellum.
The Experience and Control of Pain • Pain has both negative and positive functions: • Chronic pain can be the bane of a person’s existence. • However, under ordinary circumstances, pain is extremely useful, warning us of potential injury and inducing us to seek appropriate treatment.
Gate-Control Theory of Pain • Melzack & Wall (1965) • Gate-control theory of pain - Input from pain receptors will produce the perception of pain only if the message first passes through a “gate” in the spinal cord and lower brain stem structures. • Theory emphasizes that messages from the brain can open or close the spinal cord gate, helping us to understand the psychological nature of pain - why our sensation of pain can be affected by our thoughts and feelings.
Neuromatrix Theory of Pain • Melzack (1999) • Neuromatrix theory of pain - A theory that accounts for types of pain unexplained by the gate-control theory of pain. • Severe, chronic pain existing in the absence of injury or disease.
The Chemical Senses • Chemical senses include the gustatory and olfactory systems. • Both are intermingled in our eating experiences, in that much of what we report as the taste of food actually comes from its odor.
Gustation • Gustatory sense -The sense of taste. • Tastes can be classified according to four primary sensations: • Sweet (stimulate sugar receptors) • Sour (stimulate H+ receptors) • Bitter (stimulate alkaloid compound receptors) • Salty (stimulate NaCl receptors)
Taste Receptors • Papilla - A small, visible bump on the tongue that contains taste bumps. • Taste bud - A cluster of taste receptors that lie either near or within a papilla. • Three kinds of papillae contain taste buds: • Foliate • Circumvallate • Fungiform
Genetics of Taste • People differ in their sensitivity to bitter and some sweet tastes. • These individual differences appear to be partly related to the number of taste buds on the tongue: • Supertasters (25% of people) have the most taste buds - about 425 per square cm on the tongue tip. • Medium tasters (50% of people) have about 184 taste buds per square cm. • Non-tasters (25% of people) have about 96 per square cm.
Mechanisms of Taste Reception • Mechanism differs for each of the four basic tastes: • Salty food activates a taste receptor by causing Na+ ions to move through Na+ ion channels in the cell membrane. • H+ ions in sour foods and sugar molecules in sweet foods close the K+ ion channels in receptor membranes, preventing K+ ions from leaving the cell. • In bitter foods, alkaloid compounds trigger the movement of Ca2+ ions into the cytoplasm from storage sites in the taste receptor, increasing the release of neurotransmitters.
Olfaction • The sense of smell. • Habituation - can occur quickly with smells. Whether pleasant or unpleasant, we rapidly “get used to” smells. • This sensory adaptation is caused by decreased responding by receptors when they are exposed to the same stimulus for a continuous period of time.
Olfactory Receptors • Olfactory epithelium - The mucous membrane in the top rear of the nasal passage; lined by olfactory receptors. • Humans have approximately 50 million olfactory receptors that detect smell • other species, such as dogs, may have up to 20 times as many, with each cell having more than 10 times as many cilia.
