Auditory pathways
- 1. Auditory Pathways and Perception Dr. Karishma R Pandey Assistant Professor Dept. of Basic And Clinical Physiology BPKIHS
- 2. Presentation Plan……. 1) Sound transmission from source to the receptor 2) Auditory pathway 3) Cortical presentation of sound 4) Attributes of sound 5) Tympanic reflex and Masking 6) Deafness and Tests to determine the functionality of ear
- 4. The waves are transformed by the eardrum and auditory ossicles into movements of the foot plate of the stapes. These movements set up waves in the fluid of the inner ear. Basilar membrane is not under tension readily depressed into the scala tympani by the peaks of waves in the scala vestibuli Sound Transmission
- 5. When the stapes moves, both membranes move in the same direction, but they are hinged on different axes, so a shearing motion bends the hairs. The hairs of the inner hair cells are bent by fluid moving between the tectorial membrane and the underlying hair cells.
- 6. The action of the waves on the organ of Corti generates action potentials in the nerve fibers. The ear converts sound waves in the external environment into action potentials in the auditory nerves. The wave is dissipated at the round window.
- 7. The primary auditory cortex : Brodmann's area 41 Low tones: anterolaterally and High tones : posteromedially in the auditory cortex. Auditory Pathway
- 9. The auditory association areas adjacent to the primary auditory receiving areas are widespread. Brodmann's area 22: processing of auditory signals related to speech. Planum temporale more active on the left side than on the right side. Area 22 on the right side : melody, pitch, and sound intensity. The auditory pathways are also very plastic, and, like the visual and somasthetic pathways, they are modified by experience Auditory Cortical Presentation
- 10. Attributesof Sound 1) Frequency 2) Intensity 3) Direction 4) Pattern
- 11. Pitch : frequency (number of waves per unit of time). Sound waves Repeating patterns musical sounds Aperiodic nonrepeating vibrations noise Pitch discrimination : about 2000 Best: 1000- to 3000-Hz range Poor: at high and low pitches 16-20,000 Hz audible range Pitch in conversation avg male voice :120 Hz avg female voice: 250 Hz
- 12. Movements of the foot plate of the stapes set up a series of traveling waves in the perilymph of the scala vestibuli. Height increases to a maximum and then drops off rapidly. Distance between this point and the stapes is inversely related to the pitch of the sound High-pitched: maximum height near the base low-pitched: peak near the apex
- 13. Traveling wave set up by a tone produces peak depression of the basilar membrane, and consequently maximal receptor stimulation, at one point. Major determinant of the pitch perceived
- 14. 10 db= 1 bel 0 db= auditory threshold 20 db= whisper 60 db= normal conversation 80 db= shouting 120 db = uncomfortable 140 db= damages cochlear receptors loudness : amplitude of a sound wave Sound intensity in bel = Log 10 intensity of the sound heard _________________________ intensity of the standard sound
- 15. Direction Horizontal plane: detecting the difference in time between the arrival of the stimulus in the two ears sound is louder on the side closest to the source. Front/ Back: pinna is turned slightly forward. Vertical plane: reflections of the sound waves from the pinnal surface change as sounds move up or down, and thus changes in the sound waves
- 16. Pattern : Sequence in which different component of sound appear
- 17. Loud sounds initiate a reflex contraction of the middle ear muscles (tensor tympani and stapedius). They pull the manubrium of the malleus inward and the footplate of the stapes outward. This decreases sound transmission. Function: protective, Prevent strong sound waves from causing excessive stimulation of the auditory receptors. Tympanic reflex/ Acoustic reflex
- 18. Masking The hearing threshold for a given sound increases in the presence of background sound/ noise.
- 19. 1) Ossicular conduction: Conduction of sound waves to the fluid of the inner ear via the tympanic membrane and the auditory ossicles; main pathway for normal hearing 2) Air conduction: Sound waves also initiate vibrations of the secondary tympanic membrane that closes the round window 3) Bone conduction: transmission of vibrations of the bones of the skull to the fluid of the inner ear; transmission of extremely loud sounds Conduction of Sound Waves
- 20. Most common sensory defect in humans. Presbycusis: Gradual hearing loss associated with aging, Due to gradual cumulative loss of hair cells and neurons. Hearing loss is a multifactorial disorder caused: both genetic and environmental factors. Deafness/Hearing Loss:
- 21. Conductive (or conduction) and sensorineural hearing loss. Conductive deafness Impaired sound transmission in the external or middle ear Impacts all sound frequencies. Causes : 1) Plugging of the external auditory canals with wax (cerumen) or foreign bodies, 2) Otitis externa (inflammation of the outer ear, "swimmer's ear") and 3) Otitis media (inflammation of the middle ear) causing fluid accumulation, 4) Perforation of the eardrum, and 5) Osteosclerosis Deafness
- 22. Sensorineural deafness 1) Most commonly the result of loss of cochlear hair cells 2) Prolonged exposure to noise damages the outer hair cells 3) Problems with the eighth cranial nerve (tumors) or 4) Problems within central auditory pathways (tumor of cerebellopontine angle and vascular damage in the medulla) Impairs the ability to hear certain pitches while others are unaffected. Aminoglycoside antibiotics (streptomycin and gentamicin) Obstruct the mechanosensitive channels in the stereocilia of hair cells and can cause the cells to degenerate
- 23. Weber Rinne Schwabach Method Base of vibrating tuning fork placed on vertex of skull. Base of vibrating tuning fork placed on mastoid process until subject no longer hears it, then held in air next to ear. Bone conduction of patient compared with that of normal subject. Normal Hears equally on both sides. Hears vibration in air after bone conduction is over. Conduction deafness (one ear) Sound louder in diseased ear because masking effect of environmental noise is absent on diseased side. Vibrations in air not heard after bone conduction is over. Bone conduction better than normal (conduction defect excludes masking noise). Sensorineural deafness (one ear) Sound louder in normal ear. Vibration heard in air after bone conduction is over, as long as nerve deafness is partial. Bone conduction worse than normal.
- 24. Auditory acuity : measured with an audiometer. Presents the subject with pure tones of various frequencies through earphones. At each frequency, the threshold intensity is determined and plotted on a graph as a percentage of normal hearing. This provides an objective measurement of the degree of deafness and a picture of the tonal range most affected. Audiometry
Editor's Notes
- #4: Sound transmision
- #15: 0 db=intensity of the standard sound
- #17: Tone BM beaks down sound into all its component frquencies fundamental
- #19: Masking The hearing threshold for a given sound increases in the presence of background sound/ noise. Since the background sound stimulates the auditory receptors and nerve fibers, so they will be in relative/ absolute refractory period when a given sound arrives. Thus the sensitivity of given sound decreases in the presence of another sound
- #22: According to the World Health Organization, over 270 million people worldwide have moderate to profound hearing loss, with one fourth of these cases beginning in childhood. Presbycusis, the gradual hearing loss associated with aging, affects more than one-third of those over 75 and is probably due to gradual cumulative loss of hair cells and neurons. In most cases, hearing loss is a multifactorial disorder caused by both genetic and environmental factors osteosclerosis in which bone is resorbed and replaced with sclerotic bone that grows over the oval window. Extensive damage to the auditory cortex of the dominant hemisphere (usually the left hemisphere), will lead to difficulties in the recognition of speech, while extensive damage to the auditory cortex of the minor (right) hemisphere affects recognition of timbre and the interpretation of temporal sequences of sound, both of which are important in music and speech.
- #23: Aminoglycoside antibiotics (streptomycin and gentamicin) obstruct the mechanosensitive channels in the stereocilia of hair cells and can cause the cells to degenerate, producing sensorineural hearing loss and abnormal vestibular function. Damage to the outer hair cells by prolonged exposure to noise is associated with hearing loss. Other causes include tumors of the eighth cranial nerve and cerebellopontine angle and vascular damage in the medulla