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Chapter 7 The Other Sensory Systems. Audition. Our senses have evolved to allow us to detect and interpret biologically useful information from our environment . However, we do not detect all sensory information in the world. Some sensory information lies beyond our ability to detect it.

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Chapter 7 The Other Sensory Systems

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  • Our senses have evolved to allow us to detect and interpret biologically useful information from our environment .

  • However, we do not detect all sensory information in the world.

  • Some sensory information lies beyond our ability to detect it.

  • We also tend to focus on information that is important or relevant.

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  • Audition refers to our sense of hearing.

  • Audition depends upon our ability to detect sound waves.

  • Sound waves are periodic compressions of air, water or other media.

  • Sound waves are “transduced” into action potentials sent to the brain.

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  • The amplitude refers to the height and subsequent intensity of the sound wave.

  • Loudness refers to the perception of the sound wave.

    • Amplitude is one factor.

  • Frequency refers to the number of compressions per second and is measured in hertz.

    • Related to the pitch (high to low) of a sound.

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  • Anatomist distinguish between:

    • The outer ear

    • The middle ear

    • The inner ear

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  • The outer ear includes the pinna and is responsible for:

    • Altering the reflection of sound waves into the middle ear from the outer ear.

    • Helping to locate the source of a sound.

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  • The middle ear contains the tympanic membrane which vibrates at the same rate when struck by sound waves.

  • Three tiny bones (malleus, incus, & stapes) transmit information to the oval window or a membrane in the middle ear.

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  • The inner ear contains a snail shaped structure called the cochlea which contains three fluid-filled tunnels (scala vestibuli, scala media, and the scala tympani).

  • Hair cells are auditory receptors that excite the cells of the auditory nerve when displaced by vibrations in the fluid of the cochlea.

    • Lie between the basilar membrane and the tectorial membrane in the cochlea.

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  • Pitch perception can be explained by the following theories:

  • Frequency theory - the basilar membrane vibrates in synchrony with the sound and causes auditory nerve axons to produce action potentials at the same frequency.

  • Place theory - each area along the basilar membrane is tuned to a specific frequency of sound wave.

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  • The current pitch theory combines modified versions of both the place theory and frequency theory:

    • Low frequency sounds best explained by the frequency theory.

    • High frequency sounds best explained by place theory.

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  • Volley principle states that the auditory nerve can have volleys of impulses (up to 4000 per second) even though no individual axon approaches that frequency by itself.

    • provides justification for the place theory and

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  • The primary auditory cortex is the ultimate destination of information from the auditory system.

    • Located in the superior temporal cortex.

  • Each hemisphere receives most of its information from the opposite ear.

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  • The superior temporal cortex contains area MT which allows for the detection of the location of sound.

  • Area A1 of the brain is important for auditory imagery.

  • The auditory cortex requires experience to develop properly.

    • Auditory axons develop less in those who are deaf from birth.

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  • The cortex is necessary for the advanced processing of hearing.

    • Damage to A1 does not necessarily cause deafness unless damage extends to the subcortical areas.

  • The auditory cortex provides a tonotopic map in which cells in the primary auditory cortex are more responsive to preferred tones.

    • Some cells respond better to complex sounds than pure tones.

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  • Areas around the primary auditory cortex exist in which cells respond more to changes in sound.

  • Cells outside A1 respond to auditory “objects” (animal cries, machinery noise, music, etc.).

    • Because initial response is slow, most likely responsible for interpreting the meaning of sounds.

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  • About 99% of hearing impaired people have at least some response to loud noises.

  • Two categories of hearing impairment include:

    • Conductive or middle ear deafness.

    • Nerve deafness.

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  • Conductive or middle ear deafness occurs if bones of the middle ear fail to transmit sound waves properly to the cochlea.

  • Caused by disease, infections, or tumerous bone growth near the middle ear.

  • Can be corrected by surgery or hearing aids that amplify the stimulus.

  • Normal cochlea and normal auditory nerve allows people to hear their own voice clearly.

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  • Nerve or inner-ear deafness results from damage to the cochlea, the hair cells or the auditory nerve.

  • Can be confined to one part of the cochlea.

    • people can hear only certain frequencies.

  • Can be inherited or caused by prenatal problems or early childhood disorders (rubella, syphilis, inadequate oxygen to the brain during birth, repeated exposure to loud noises, etc).

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  • Tinnitus is a frequent or constant ringing in the ears.

    • experienced by many people with nerve deafness.

  • Sometimes occurs after damage to the cochlea.

    • axons representing other part of the body invade parts of the brain previously responsive to sound.

    • Similar to the mechanisms of phantom limb.

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  • Sound localization depends upon comparing the responses of the two ears.

  • Humans localize low frequency sound by phase difference and high frequency sound by loudness difference.

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  • Three mechanisms:

    • High-frequency sounds (2000 to 3000Hz) create a “sound shadow”, making the sound louder for the closer ear.

    • The difference in the time of arrival at the two ears is most useful for localizing sounds with sudden onset.

    • Phase difference between the ears provides cues to sound location for localizing sounds with frequencies up to 1500 Hz.

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The Mechanical Senses

  • The mechanical senses include:

    • The vestibular sensation

    • Touch

    • Pain

    • Other body sensations

  • The mechanical senses respond to pressure, bending, or other distortions of a receptor.

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The Mechanical Senses

  • The vestibular sense refers to the system that detects the position and the movement of the head.

    • Directs compensatory movements of the eye and helps to maintain balance.

  • The vestibular organ is in the ear and is adjacent to the cochlea.

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The Mechanical Senses

  • The vestibular organ consists of two otolith organs (the saccule and untricle) and three semicircular canals.

  • The otolith organs have calcium carbonate particles (otoliths) that activate hair cells when the head tilts.

  • The 3 semicircular canals are oriented in three different planes and filled with a jellylike substance that activates hair cells when the head moves.

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The Mechanical Senses

  • The vestibular sense is integrated with other sensations by the angular gyrus.

    • Angular gyrus is an area at the border between the parietal and temporal cortex.

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The Mechanical Senses

  • The somatosensory system refers to the sensation of the body and its movements.

    • Includes discriminative touch, deep pressure, cold warmth, pain, itch, tickle and the position and movement of the joints.

  • Touch receptors may be simple bare neurons, an elaborated neuron ending, or a bare ending surrounded by non-neural cells that modify its function.

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The Mechanical Senses

  • The pacinian corpuscle is a type of touch receptor that detects sudden displacement or high-frequency vibrations on the skin.

  • Mechanical pressure bend the membrane.

    • increases the flow of sodium ions and triggers an action potential.

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The Mechanical Senses

  • Information from touch receptors in the head enters the CNS through the cranial nerves.

  • Information from receptors below the head enter the spinal cord and travel through the 31 spinal nerves to the brain.

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The Mechanical Senses

  • Each spinal nerve has a sensory component and a motor component and connects to a limited area of the body.

  • A dermatome refers to the skin area connected to a single sensory spinal nerve.

  • Sensory information entering the spinal cord travel in well-defined and distinct pathways.

    • Example: touch pathway is distinct from pain pathway.

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The Mechanical Senses

  • Various aspects of body sensations remain partly separate all the way to the cortex.

  • Various areas of the thalamus send impulses to different areas of the somatosensory cortex located in the parietal lobe.

  • Different sub areas of the somatosensory cortex respond to different areas of the body.

  • Damage to the somatosensory cortex can result in the impairment of body perceptions.

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The Mechanical Senses

  • Pain depends on several types of axons, several neurotransmitters, and several brain areas.

  • Mild pain triggers the release of glutamate while stronger pain triggers the release of glutamate and substance P.

    • Substance P results in the increased intensity of pain.

    • Morphine and opiates block pain by blocking these neurotransmitters.

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The Mechanical Senses

  • Opioid mechanisms are systems that are sensitive to opioid drugs and similar chemicals.

  • Activating opiate receptors blocks the release of substance P in the spinal chord and in the periaqueductal grey of the midbrain.

  • Enkephalins refer to opiate-type chemical in the brain.

  • Endorphins- group of chemicals that attach to the same brain receptors as morphine.

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The Mechanical Senses

  • Discrepancy in pain perception can partially be explained by genetic differences in receptors.

  • Gate theory suggests that the spinal cord areas that receive messages from pain receptors also receive input from other skin receptors and from axons descending from the brain.

    • These other areas that provide input can close the “gates” and decrease pain perception.

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The Mechanical Senses

  • Special heat receptors account for the pain associated with a burn.

  • Heat receptors can also be activated by acids.

  • Capsaicin is a chemical found in hot peppers that directly stimulates these receptors and also triggers an increase in the release of substance P.

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The Mechanical Senses

  • Pain activates the hypothalamus, amygdala, and cingulate cortex.

    • results in an emotional component to pain.

  • A placebo is a drug or other procedure with no pharmacalogical effect.

  • Placebo’s decrease pain perception by decreasing the brains emotional response to pain perception and not the sensation itself.

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The Mechanical Senses

  • Mechanisms of the body to increase sensitivity to pain include:

    • Damaged or inflamed tissue releases histamine, nerve growth factor, and other chemicals that increase the number of sodium gates in nearby pain receptors.

      • Pain responses are thus magnified.

    • Certain receptors become potentiated after an intense barrage of painful stimuli.

      • leads to increased sensitivity or chronic pain later.

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The Mechanical Senses

  • Pain is best controlled by preventing the brain from being bombarded with pain messages.

  • Bombarding the brain with pain messages results in the increased sensitivity of the pain nerves and their receptors.

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The Mechanical Senses

  • Morphine and other opiates are the primary drugs for controlling serious pain.

  • Morphine inhibits slow, dull pain carried by thin unmyelinated axons.

    • Sharp pain is conveyed by thicker myelinated axons.

    • Not influenced by morphine and endorphins.

  • Ibuprofen, an anti-inflammatory drug, controls pain by reducing the release of chemicals from damaged tissues.

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The Mechanical Senses

  • The release of histamines by the skin produce itching sensations.

  • The release of histamine by the skin activates a distinct pathway in the spinal cord to the brain.

  • Impulses travel slowly along this pathway (half a meter per second).

  • Pain and itch have an inhibitory relationship.

    • Opiates increase itch while antihistamines decrease itch.

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The Chemical Senses

  • Coding in the sensory system could theoretically follow:

    • The labeled-line principle in which each receptor responds to a limited range of stimuli and sends a direct line to the brain.

    • Across-fiber pattern in which each receptor responds to a wider range of stimuli and contributes to the perception of each of them.

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The Chemical Senses

  • Vertebrate sensory systems probably have no pure label-lined codes.

  • The brain gets better information from a combination of responses.

    • Example: auditory perception and color perception both rely on label-lined codes.

  • Taste and smell stimuli activate several neurons and the meaning of the response of a single neuron depends on the context of responses by other neurons.

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The Chemical Senses

  • Taste refers to the stimulation of the taste buds.

  • Our perception of flavor is the combination of both taste and smell.

    • Taste and smell axons converge in the endopiriform cortex.

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The Chemical Senses

  • Receptors for taste are modified skin cells.

  • Taste receptors have excitable membranes that release neurotransmitters to excite neighboring neurons.

  • Taste receptors are replaced every 10 to 14 days.

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The Chemical Senses

  • Papillae are structures on the surface of the tongue that contain the taste buds.

  • Each papillae may contain zero to ten taste buds.

  • Each taste bud contains approximately 50 receptors.

  • Most taste buds are located along the outside of the tongue in humans.

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The Chemical Senses

  • Procedures that alter one receptor but not others can be used to identify taste receptors.

  • Adaptation refers to reduced perception of a stimuli due to the fatigue of receptors.

  • Cross-adaptation refers to reduced response to one stimuli after exposure to another.

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The Chemical Senses

  • Western societies have traditionally described sweet, sour, salty and bitter tastes as the “primary” tastes and four types of receptors.

  • Evidence suggests a fifth type of glutamate receptor.

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The Chemical Senses

  • The saltiness receptor permits sodium ions to cross the membrane.

    • results in an action potential.

  • Sourness receptors close potassium channels when acid binds to receptors.

    • results in depolarization of the membrane.

  • Sweetness, bitterness, and umami receptors activate a G protein that releases a second messenger in the cell when a molecule binds to a receptor.

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The Chemical Senses

  • Different chemicals also result in different temporal patterns of action potentials and activity in the brain.

  • Taste is a function of both the type of cell activity, as well as the rhythm of cell activity.

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The Chemical Senses

  • Bitter receptors are sensitive to a wide range of chemicals with varying degrees of toxicity.

    • Over 40 types of bitter receptors exist.

  • Most taste cells contain only a small number of these receptors.

  • We are sensitive to a wide range of harmful substances, but not highly sensitive to any single one.

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The Chemical Senses

  • Taste coding in the brain depends upon a pattern of responses across fibers in the brain.

  • The brain determines taste by comparing the responses of several types of taste neurons.

  • Receptors converge their input onto the next cells in the taste system.

  • Cells thus respond best to a particular taste but others as well.

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The Chemical Senses

  • Different nerves carry taste information to the brain from the anterior two-thirds of the tongue than from the posterior tongue and throat.

  • Taste nerves project to a structure in the medulla known as the nucleus of the tractus solitarius (NTS).

    • projects information to other parts of the brain.

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The Chemical Senses

  • Various areas of the brain are responsible for processing different taste information.

    • The somatosensory cortex responds to the touch aspect of taste.

    • The insula is the primary taste cortex.

  • Each hemisphere of the cortex is also responsive to the ipsilateral side of the tongue.

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The Chemical Senses

  • Genetic factors and hormones can account for some differences in taste sensitivity.

  • Variations in taste sensitivity are related to the number of fungiform papillae near the tip of the tongue.

  • Supertasters have higher sensitivity to all tastes and mouth sensations in general.

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The Chemical Senses

  • Olfaction is the sense of smell and refers to the detection and recognition of chemicals that contact the membranes inside the nose.

  • Olfaction is more subject to adaptation than our other senses.

  • Olfactory cells line the olfactory epithelium in the rear of the nasal passage and are the neurons responsible for smell.

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The Chemical Senses

  • Olfactory receptors are located on cilia which extend from the cell body into the mucous surface of the nasal passage.

  • Vertebrates have hundreds of olfactory receptors which are highly responsive to some related chemicals and unresponsive to others.

  • Olfaction processes a wide variety of airborne chemicals, hence the need for many different types of receptors.

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The Chemical Senses

  • Proteins in olfactory receptors respond to chemicals outside the cells and trigger changes in G protein inside the cell.

  • G protein then triggers chemical activities that lead to action potentials.

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The Chemical Senses

  • Axons from olfactory receptors carry information to the olfactory bulb in the brain.

  • Coding in the brain is determined by which part of the olfactory bulb is excited.

  • Chemicals excite limited parts of the olfactory bulb with similar chemicals exciting similar parts.

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The Chemical Senses

  • The olfactory bulb sends axons to the cerebral cortex where messages are coded by location.

  • The cortex connects to other areas that control feeding and reproduction.

    • Both behaviors are highly influenced by smell.

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  • Olfactory receptors are replaced approximately every month, but are subject to permanent impairment from massive damage.

  • Anosmia refers to a general lack of olfaction.

  • Specific anosmia refers to the inability to smell a single type of chemical.

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The Chemical Senses

  • Individual differences in olfaction exist regarding olfaction.

  • Women detect odor more readily than men and brain responses are stronger.

  • The ability to attend to a faint odor and become more sensitive to it is characteristic of young adult women and thus seems to be influenced by hormones.

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The Chemical Senses

  • The vomeronasal organ (VNO) is a set of receptors located near the olfactory receptors that are sensitive to pheromones.

  • Pheromones are chemicals released by an animal to affect the behavior of others of the same species.

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The Chemical Senses

  • The VNO and pheromones are important for most mammals, but less so for humans.

  • The VNO is tiny in human adults and has no receptors.

  • Humans unconsciously respond to some pheromones through receptors in the olfactory mucosa.

    • Example: synchronization of menstrual cycle’s in women.

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The Chemical Senses

  • Synesthesia is the experience of one sense in response to stimulation of a different sense.

    • Estimates suggest 1 in every 500 people (Day, 2005).

  • fMRI case studies show activity in both the auditory and visual cortex responsive to color when exposed to spoken language.

    • Suggests some axons from one area have branches to other cortical regions.