30 Hair cell transduction by ion channel opening Source: Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Siegel GJ, Agranoff BW, Albers RW, et al., editors; Downloaded from
31 Sensory pathway for hearing Hair cells in specific area of basilar membrane become stimulatedSensory neuron axons (cell bodies in spiral ganglion) make up cochlear branch of vestibulocochlear nerve (VIII)Sensory neuron axons synapse onto neurons in cochlear nucleus (medulla oblongata)Information ascends bilaterally (often synapsing on the way) to inferior colliculus (midbrain)Inferior colliculus neurons synapse at medial geniculate nucleus (thalamus)Projection fibers from thalamus reach primary auditory cortex (temporal lobe)
33 Tonotopic organization Different frequency sounds excite different basilar membrane regions (apex: low frequencies; base: high frequencies)Cochlear nucleus (first auditory area in CNS) has a “map” of basilar membrane, i.e. frequency map: tonotopic mapTonotopic map seen in successive higher centers, up to & including primary auditory cortex
34 Tonotopic organization of primary auditory cortex Source: Lynch, downloaded
35 Localizing soundsMost auditory information crosses over but some doesn’t, so brainstem and cortical areas get inputs from both earsRight versus left arrival time differenceRight versus left intensity differenceBoth are used to localize sounds
36 Conduction deafnessSound energy is not conducted from outside world to the receptors, i.e. doesn’t make it to inner earCauses include:Water or excess cerumen in external earScarring or perforation of tympanic membraneImmobility of ear ossicles (fluid, pus, tumor; otosclerosis)Otosclerosis: abnormal bony growth around stapes footplate prevents normal stapes movement.
37 Sensorineural (nerve) deafness Most common cause of permanent deafnessDamage to hair cell receptorsNormal (young) range: 20–20,000 Hz; hearing loss later, high frequencies go firstLoud noise, infection, some drugsDamage to nerve or to central auditory pathways