Psyc 362 Chapter 7 text Audition the body senses and the chemical senses Audition The Stimulus Sounds are produced by objects that vibrate and set molecules of air into motion the molecules condense and pull apart producing traveling waves Waves causing vibrations between 30 and 20 000 times per second will stimulate receptor cells in human ears to be perceived as sounds Sound varies in loudness pitch and timbre Pitch perception is determined by the frequency of vibration as measured in Hz cycles per second Loudness is a function of intensity the degree to which the condensations and rarefactions waves being pulled apart of air differ from each other Timbre provides info about the nature of the particular sound The particular complex mixture of a stimuli determines the sound s timbre Anatomy of the ear Sound is funneled via the pinna external ear through the ear canal to the tympanic membrane eardrum which vibrates with the sound the middle ear is a hollow region behind the tympanic membrane which contains ossicles bones of the middle ear which are set into vibration by the tympanic membrane The malleus hammer connects with the tympanic membrane and transmits vibrations via the incus anvil and stapes stirrup to the cochlea snail shaped the cochlea is the structure containing the receptors for the auditory system Behind the oval window the opening in the bony process surrounding the cochlea the baseplate of the stapes presses against the membrane Note malleus incus and stapes are each ossicles in appropriate order of their actions in the middle ear the inner ear includes the cochlea which is filled with fluid sounds transmitted through the air must be transferred into this liquid medium This is effective with the chain of ossicles serving a means of energy transmission by providing a mechanical advantage The Cochlea is divided longitudinally into 3 sections scala vestibuli scala media and scala tympani organ of corti is the receptive organ consisting of the basilar membrane the hair cells the auditory receptor cells they pass through the reticular membrane and the tectorial membrane The ends of some hair cells attach to this fairly rigid membrane which projects overhead like a shelf The hair cells are anchored to the basilar membrane via dieters s cells Sound waves cause the basilar membrane to move relative to the tectorial membrane which bends the cilia of hair cells producing receptor potentials the round window is a membrane covered opening that allows the fluid inside the cochlea to move back and forth When the baseplate of the stapes pushes in the membrane behind the round window bulges out fenestration window making is a surgical procedure used to help those suffering from their bone in the middle ear growing over the round window so their basilar membrane cannot easily flex back and forth the procedure involves drilling a tiny hole in the bone where the round window should be Auditory hair cells and the transduction of auditory information cilia are fine like appendages on hair cells that are arranged in rows according to height The hair cells form synapses with dendrites of afferent bipolar neurons flexing movements of the basilar and tectorial membranes bend cilia in one direction or the other The tips of the cilia of outer hair cells are attached directly to the tectorial membrane while those of inner hair cells don t touch the overlying membrane Cilia are stiff and rigid because they contain a core of actin filament surrounded by myosin filaments Cilia are linked to each other by stretchy tip links which are attached to the end of one cilium and to the side of an adjacent cilium Points of attachment are called insertional plaques each which vontain a single ion channel that opens and closes according to streching Movement of the bundle of cilia in the direction of the tallest of them stretches these linking fibers causing tension however tension is reduced with movement in the opposite direction Bending of a bundle of cilia will produce a receptor potential The Auditory pathway The cochlear nerve a branch of the auditory nerve with thick and myelinated axons sends information from the organ of Corti inner hair cells to the brain The outer hair cells send much less info through the cochlear nerve with thin and unmyelinated sensory axons much less important plus they are effector cells involved in altering the mechanical characteristics of the basilar membrane to influence the effects of sound vibrations on the inner hair cells The Cochlear nucleus in the medulla receives auditory information from the cochlea The nucleus then has axons that send to the superior olivary complex in the medulla which then axons from their pass through the lateral lemniscus a large fiber bundle to the inferior colliculus in the dorsal midbrain Neurons there send axons to the medial geniculate nucleus of the thalamus which send their axons to the auditory cortex of the temporal lobe Auditory info is also relayed to the cerebellum and and reticular formation Tonotopic representation Different parts of the basilar membrane respond best to different frequencies of sound the core region is the primary auditory cortex located on a gyrus on the dorsal surface of the temporal lobe It transmits auditory information to the first level of the belt region auditory association cortex that surrounds the core region Then information is sent to the parabelt region of the auditory association cortex The auditory association cortex is arranged into two streams 1 anterior stream beginning in anterior parabelt region and is involved with analysis of complex sounds WHAT 2 posterior stream beginning in the posterior parabelt region and is involved with sound location WHERE Perception of Pitch by detection of frequency and the cochlea detects frequency by two means place coding allows for the detection of moderate to high frequencies This is possible because stimuli of different frequencies cause different parts of the basilar membrane to flex back and forth a code represents a means by which neurons can represent information The frequency of the sound is coded by the particular neurons that are active that is because neurons at one end of the basilar membrane are excited by higher frequencies and those at the other end are excited by lower frequencies cochlear implants are used to restore hearing in people with deafness caused by damage to the hair cells in order to restore a person s ability to understand