1. Describe the parts of the ear and the auditory pathways.
2. Describe the detection of pitch, timbre, and the location of the source of a sound.
3. Describe the structures and functions of the vestibular system.
4. Describe the cutaneous senses and their response to touch, temperature, and pain.
5. Describe the somatosensory pathways and the perception of pain.
6. Describe the four taste qualities, the anatomy of the taste buds and how they detect taste, and the gustatory pathway and neural coding of taste.
7. Describe the major structures of the olfactory system, explain how odors are detected, and describe the patterns of neural activity produced by these stimuli.

• Pitch—A perceptual dimension of sound; corresponds to the fundamental frequency.
• Hertz(Hz)—Cycles per second.
• Loudness—A perceptual dimension of sound; corresponds to intensity.
• Timbre (tim ber or tamm ber)-- A perceptual dimension of sound; corresponds to complexity.
• Tympanic membrane—The eardrum.
• Ossicle (ahss I kul)—One of the three bones of the middle ear.
• Malleus—The "hammer"; the first of the three ossicles.

• Incus—The "anvil"; the second of three ossicles.
• Stapes (stay peez)—The "stirrup"; the last of the three ossicles.
• Cochlea (cock lee uh)—The snail shaped structure of the inner ear that contains the inner ear that contains the auditory transducing mechanisms.
• Oval window—An opening in the bone surrounding the cochlea that reveals a membrane, against which the baseplate of the stapes presses, transmitting sound vibrations into the fluid within the cochlea.
• Organ of Corti—The sensory organ on the basilar membrane that contains the auditory hair cells.
• Hair cell—The receptive cell of the auditory apparatus.
• Basilar membrane (bazz I ler)—A membrane in the cochlea of the inner ear; contains the organ of Corti.
• Tectorial membrane (tek torr ee ul)—A membrane located above the basilar membrane; serves as a shelf against which the cilia of the auditory hair cells move.
• Round window—An opening in the bone surrounding the cochlea of the inner ear that permits vibrations to be transmitted, via the oval window, into the fluid in the cochlea.

• Cilium—A hairlike appendage of a cell involved in movement or in transducing sensory information; found on the receptors in the auditory and vestibular system.

• Cochlear nerve—The branch of the auditory nerve that transmits auditory information from the cochlea to the brain.
• Cochlear nucleus—One of a group of nuclei in the medulla that receive auditory information from the cochlea.
• Superior olivary complex—A group of nuclei in the medulla; involved with auditory functions, including localization of the source of sounds.
• Lateral limniscus—A band of fibers running rostrally through the medulla and pons; carries fibers of the auditory system.

• Tonotopic representation (tonn oh top ik)—A topographically organized mapping of different frequencies of sound that are represented in a particular region of the brain.
• Place code—The system by which information about different frequencies is coded by different locations on the basilar membrane.
• Cochlear implant—An electronic device surgically implanted in the inner ear that can enable some deaf people to hear.

• Rate code—The system by which information about different frequencies is coded by the rate of firing of neurons in the auditory system.
• Fundamental frequency—The lowest, and usually most intense, frequency of a complex sound; most often perceived as the sound’s basic pitch.
• Overtone—The frequency of complex tones that occurs at multiples of the fundamental frequency.

• Phase difference—The difference in arrival times of sound waves at each of the eardrums.

The respective organ for audition is the organ of Corti, located on the basilar membrane. When sound strikes the tympanic membrane, it sets the ossicles into motion, and the baseplate of the stapes pushes against the membrane behind the oval window. Pressure changes thus applied to the fluid within the cochlea cause a portion of the basilar membrane to flex, causing the basilar membrane to move laterally with respect to the tectorial membrane that overhangs it. This movement pulls directly on the cilia of the outer hair cells and causes movements in the fluid within the cochlea, which, in turn, causes the cilia of the inner hair cells to wave back and forth. These mechanical forces open ion channels in the tips of the hair cells and thus produce receptor potentials.

The hair cells form synapses with the dendrites of the bipolar neurons whose axons give rise to the cochlear branch of the eighth cranial nerve. The central auditory system involves several brain stem nuclei, including the cochlear nuclei, superior olivary complexes, and inferior colliculli. The medial geniculate nucleus relays auditory information to the primary auditory cortex on the medial surface of the temporal lobe.

Pitch is encoded by two means. High-frequency sounds cause the base of the basilar membrane (near the oval window) to flex; low-frequency sounds cause the apex (opposite end) to flex. Because high and low frequencies thus stimulate different groups of auditory hair cells, frequency is encoded automatically. The lowest frequencies cause the apex of the basilar membrane to flex back and forth in time with the acoustic vibrations. The outer hair cells act as motive elements rather than as sensory transducers, contracting in response to activity of the afferent axons and modifying the mechanical properties of the basilar membrane.

The auditory system is analytical in its operation. That is, it can discriminate between sounds with different timbres by detecting the individual overtones that constitute the sounds and producing unique patterns of neural firing in the auditory system.

Left-right localization is performed by analyzing binaural differences in arrival time, in phase relations, and in intensity. The location of sources of brief sounds (such as clicks) and sounds of frequencies below approximately 3000 Hz is detected by neurons in the medial superior olivary complex, which respond most vigorously when one ear receives the click first or when the phase of a sine wave received by one ear leads that received by the other. The location of sources of high frequency sounds is detected by neurons in the lateral superior olivary complex, which respond most vigorously when one organ of Corti is stimulated mere intensely than the other.

To recognize the source of sounds, the auditory system must recognize the constantly changing patterns of activity received from the axons in the cochlear nerve. Studies have found neurons in the auditory cortex that respond to complex stimuli, such as ascending or descending pitches, series of tones, combinations of two or more tones, or even species-specific vocalizations. Bilateral lesions of the auditory cortex of monkeys produce severe impairments in hearing, and lesions of the left auditory cortex impair the ability to discriminate the vocalizations of other monkeys.

• Vestibular sac—One of a set of two receptor organs in each inner ear that detect changes in the tilt of the head.
• Semicircular canal—One of the three ringlike structures of the vestibular apparatus that detect changes in head rotation.

• Figure 7.12 shows the labyrinths of the inner ear, which include the cochlea, the semicircular canals, and the two vestibular sacs: the utricle ("little pouch") and the saccule ("little sack"). (See Figure 7.12) The semicircular canals approximate the three major planes of the head: sagittal, transverse, and horizontal. Receptors in each canal are activated by changes in rotation of the head in one plane. Figure 7.13 shows cross sections through one semicircular canal. The semicircular canals consists of a membranous canal floating within a bony one; the membranous canal contains a fluid called endolymph and floats within a fluid called perilymph. An enlargement called the ampulla contains the organ in which the sensory receptors reside. The sensory receptors are hair cells similar to those found in the cochlea. Their cilia are embedded in a gelatinous mass called the cupula, which blocks part of the ampulla. When the head suddenly turns, inertia causes the endolymph to move relative to the cupula, and the bending of the cupula exerts a shearing force on the cilia of the hair cells. (See Figure 7.13.)
• The vestibular sacs (the utricle and saccule) work very differently. These organs are roughly circular, and each contains a patch of receptive tissue. The receptive tissue is located on the "floor" of the utricle and on the "wall" of the saccule when the head is in and upright position. The receptive tissue, like that of a semicircular canals and cochlea, contains hair cells. The cilia of these receptors are embedded in an overlying gelatinous mass, which contains something rather unusual: otoconia, which are small crystals of calcium carbonate. The weight of the crystals causes the gelatinous mass to shift in position as the orientation of the head changes. Thus, movement produces a shearing force on the cilia of the receptive hair cells.

• Utricle (you trih kul)—One of the vestibular sacs.
• Saccule (sak yule)—One of the vestibular sacs.
• Ampulla (am pull uh)—An enlargement in a semicircular canal; contains the cupula and the crista.
• Cupula (kew pew uh)—A gelatinous mass found in the ampulla of the semicircular canals; moves in response to the flow of the fluid in the canals.

The semicircular canals are filled with fluid. When the head begins to rotating or comes to rest after rotation, inertia causes the fluid to push the cupula to one side or the other. This movement exerts a shearing force on the cupula, the organ containing the vestibular hair cells. The vestibular sacs contain a patch of receptive tissue that contains hair cells whose cilia are embedded in a gelatinous mass. The weight of the otoconia in the gelatinous mass shifts when the head tilts, causing a shearing force on some of the cilia of the hair cells.

Each hair cell contains one long cilium and several shorter ones. These cells form synapses with dendrites of bipolar neurons whose axons travel through the vestibular nerve. The receptors also receive efferent terminal buttons from neurons located in the cerebellum and medulla, but the function of these connections is not known. Vestibular information is received by the vestibular nuclei in the medulla, which relay it on to the cerebellum, spinal cord, medulla, pons, and temporal cortex. These pathways are responsible for control of posture , head movements, eye movements, and the puzzling phenomenon of motion sickness.

• Cutaneous sense (kew tane ee us)—One of the somatosenses; includes sensitivity to stimuli that involve the skin.
• Kinesthesia—Perception of the body’s own movements.
• Organic sense—A sense modality that arises from receptors located within the inner organs of the body.
• Glabrous skin (glab russ)—Skin that does not contain hair; found on the palms and soles of the feet.
• Ruffini corpuscle—A vibration sensitive organ located in hairy skin.
• Pacinian corpuscle (pa chin ee un)—A specialized, encapsulated somatosensory nerve ending that detects mechanical stimuli, especially vibrations.
• Meissner’s corpuscle—the touch-sensitive end organs located in the papillae, small elevations of the dermis that project up into the epidermis.
• Merkel’s disk—The touch-sensitive end organs found at the base of the epidermis, adjacent to sweat ducts.

• Prostaglandin—A member of a family of fatty acid derivatives that serve as hormones; first discovered in the prostate gland; involved in many physiological processes, including pain perception.

• Phantom limb—Sensations that appear to originate in a limb that has been amputated.
• Nucleus raphe magnus—A nucleus of the raphe that contains serotonin-secreting neurons that project to the dorsal gray matter of the spinal cord and is involved in analgesia produced by opiates.

• Cutaneous sensory information is provided by specialized receptors in the skin. Pacinian corpuscles provide information about vibration. Ruffini corpuscles, similar to Pacinian corpuscles but considerably smaller, respond to low-frequency vibration, usually referred to as "flutter." Meissner’s corpuscles, found in papillae and innervated by several axons, respond to mechanical stimuli. Merkel’s disks, also found in papillae, consist of single, flattened dendritic endings next to specialized epithelial cells. These receptors respond to mechanical stimulation. Painful stimuli are detected primarily by free nerve endings.
• Our somatosensory system is most sensitive to changes in mechanical stimuli. Unless the skin is moving, we do not detect nonpainful stimuli, because the receptors adapt to constant mechanical pressure. Temperature receptors also adapt; moderate changes in skin temperature are soon perceived as "neutral," and deviations above or below this temperature are perceived as warmth or coolness.
• Precise, well-localized somatosensory information is conveyed by a pathway through the dorsal columns and their nuclei and the medial lemniscus, connecting the dorsal column nuclei with the ventral posterior nuclei of the thalamus. His nucleus projects to the primary somatosensory cortex. Information about pain and temperature ascends the spinal cord through the spinothalamic system. Organic sensibility reaches the central nervous system by means of axons that travel through nerves of the autonomic nervous systems.
• Pain perception is not a simple function of stimulation of pain receptors; it is a complex phenomenon that can be modified by experience and the immediate environment. The phantom limb phenomenon, which often is accompanied by phantom pain, appears to be inherent in the organization of the parietal lobe.
• Just as we have mechanisms to perceive pain, we have mechanisms to reduce it- to produce analgesia. Under the appropriate circumstances, neurons in the periaqueductal gray matter are stimulated through synaptic connections with the frontal cortex, amygdala, and hypothalamus. In addition, some neurosecretory cells in the brain release enkephalins, a class of endogenous opioids. These neuromodulators activate receptors on neurons in the periaqueductal gray matter and provide additional stimulation of neurons in this region. Connections from the periaqueductal gray matter to the nucleus raphe magnus of the medulla activate serotonergic neurons located there. These neurons send axons to the dorsal horn of the spinal cord gray matter, where they inhibit the transmission of pain information to the brain. In humans, chronic pain is sometimes treated by implanting electrodes in the periaqueductal gray matter or the thalamus and permitting the patients to stimulate the brain through these electrodes when the pain becomes severe.

• The Stimuli
• Anatomy of the Taste Buds and Gustatory Cells
• Detection of Gustatory Information
• Umami. The taste sensation produced by glutamate.
• Chorda tympani. A branch of the facial nerve that passes beneath the eardrum; conveys taste information from the anterior part of the tongue and controls the secretion of some salivary glands.
• The Gustatory Pathway

• Taste receptors detect only four sensory qualities: bitterness, sourness, sweetness, and saltiness. Bitter foods often contain plant alkaloids, many of which are poisonous. Sour foods have usually undergone bacterial fermentation, which can produce toxins. On the other hand, sweet foods (such as fruits) are usually nutritious and safe to eat, and salty foods contain an essential cation, sodium. The fact that people in affluent cultures today tend to ingest excessive amounts of sweet and salty foods suggests that these taste qualities are naturally reinforcing.
• Saltiness receptors appear to be simple sodium channels. Sourness receptors appear to detect the presence of hydrogen ions, which closes potassium channels located on the cilia and depolarizes the membrane of the cell. The structure of molecules that taste bitter appears to include a hydrophobic residue, and some also have a region with a positive charge. Most molecules that taste sweet have a hydrogen ion situated 0.3 nm from a site that will accept a hydrogen ion. The taste of glutamate receptor (mGluR4). Some animals may also be able to taste complex carbohydrates.
• Gustatory information from the anterior part of the tongue travels through the chorda tympani, a branch of the facial nerve that passes beneath the eardrum on its way to the brain. The posterior part of the tongue sends gustatory information through the glossopharyngeal nerve, and the palate and epiglottis send gustatory information through the vagus nerve. Gustatory information is received by the nucleus of the solitary tract (located in the medulla) and is relayed by the ventral posteromedial thalamus to the primary gustatory cortex in the opercular and insural regions of the frontal lobes. The caudolateral orbitofrontal cortex contains the secondary gustatory cortex. Gustatory information is also sent to the amygdala, hypothalamus, and basal forebrain.

• The Stimulus
• Anatomy of the Olfactory Apparatus

• Olfactory epithelium. The epithelial tissue of the nasal sinus that covers the cribiform plate; contains the cilia of the olfactory receptors.
• Olfactory bulb. The protrusion at the end of the olfactory tract; receives input from the olfactory receptors.
• Mitral cell. A neuron located in the olfactory bulb that receives information from olfactory receptors; axons of mitral cells bring information to the rest of the brain.
• Olfactory glomerulus. A bundle of dendrites of mitral cells and the associated terminal buttons of the axons of olfactory receptors.
• Transduction of Olfactory Information
• Detection of Specific Odors

• The olfactory receptors consist of bipolar neurons located in the olfactory epithelium that lines the roof of the nasal sinuses, on the bone that underlies the frontal lobes. The receptors send processes toward the surface of the mucosa, which divide into cilia. The membranes of these cilia contain receptors that detect aromatic molecules dissolved in the air that sweeps past the olfactory mucosa. The axons of the olfactory receptors pass through the perforations of the cribriform plate into the olfactory bulbs, where they form synapses in the glomeruli with the dendrites of the mitral cells. These neurons send axons through the olfactory tracts to the brain, principally to the amygdala, the pyriform cortex, and the entorhinal cortex. The hippocampus, hypothalamus, and orbitofrontal cortex receive olfactory information indirectly.
• Aromatic molecules produce membrane potentials by interacting with a newly discovered family of receptors molecules, which may number up to 1000. These receptors are coupled to a special protein, G olf. This protein activates an enzyme that opens sodium channels and depolarizes the membrane. Each glomerulus receives information from only one type of olfactory receptor. This means that the task of detecting different odors is a spatial one; the brain recognizes odors by means of the patterns of activity created in the glomeruli.

• Audition

1. The bones of the middle ear transmit sound vibration from the eardrum to the cochlea, which contains the auditory receptors the hair cells.
2. The hair cells send information through the eighth cranial nerve to nuclei in the brain stem; it is then relayed to the medial geniculate nucleus and finally to the primary auditory cortex.
3. The ear is analytical; it detects individual frequencies by means of place coding and rate coding. Left-right localization is also accomplished by two means: arrival time (phase differences) and binaural differences in intensity.

• Vestibular System

1. The vestibular system helps us to maintain our balance and makes compensatory eye movements to help us maintain fixation when our head moves. The semicircular canals detect head rotations and the vestibular sacs detect changes in the tilt of the head.

• Somatosenses

1. Cutaneous receptors in the skin provide information about touch, pressure, vibration, changes in temperature, and stimuli that cause tissue damage.
2. Pain perception helps protect us from harmful stimuli. Sensitivity to pain is modulated by the release of the endogenous opiates by cells in the brain.

• Gustation

1. Taste receptors on the tongue respond to bitterness, sourness, sweetness, and saltiness and, together with olfactory information, provide us with information about complex flavors.

• Olfaction

1. The olfactory system detects the presence of aromatic molecules. The discovery of a family of receptors coupled to a special G protein (G olf) suggests that several hundred different receptors may be involved in olfactory discrimination.