AUDITION auditory pathway endococlear potential
- 1. • These tip links are attached with mechanosensitive cation channels • They get open up when the shorter cilia moves towards the stereocilia. • When stereocilia is pushed towards the higher neighboring ones the ion channel get open up causing entry of K+ (mainly) and Ca2+
- 2. • Upward displacement bends the stereocilia toward the tallest cilium, which leads to K+ influx through K+ channels and depolarization of the hair cells; • downward deflection bends the stereocilia in the opposite direction, which closes K+ channels and hyperpolarization of the hair cell
- 4. • Very fine processes called tip links tie the tip of each stereocilium to the side of its higher neighbor, and mechanically sensitive cation channels are at the junction in the taller process. • When the shorter stereocilia are pushed toward the taller ones, the channel open time is increased. • K+, the most abundant cation in endolymph, and Ca2+ enter via the channel and induce depolarization. • A myosin-based molecular motor in the taller neighbor then moves the channel toward the base, releasing tension in the tip link. • This causes the channel to close and restores the resting state. • Depolarization of hair cells causes them to release a neurotransmitter, probably glutamate, which initiates depolarization of neighboring afferent neurons.
- 5. • When the organ of corti moves up, tectorial membrane moves forwards relative to basilar membrane. • This bends stereocilia towards highest cilia--- produces depolarization. • Bending of stereocilia away from highest cilia produces hyper polarization.
- 6. • The K+ that enters hair cells are recycled • It enters supporting cells and then passes on to other supporting cells by way of gap junctions. • In the cochlea, it eventually reaches the stria vascularis and is secreted back into the endolymph, completing the cycle. What happens to K+?
- 8. • Receptors – hair cells • Innervated by the auditory portion of 8th cranial nerve whose cell bodies are present in the spiral ganglion located in the modiolus.
- 9. Spiral ganglion → Cochlear division of VIII nerve ↓ Dorsal & ventral cochlear nuclei ↓ Superior olivary nucleus (contralateral) in pons ↓ Lateral leminiscus ↓ Inferior colliculus in midbrain ↓ Medial geniculate body in thalamus ↓ Auditory radiations → Primary auditory cortex
- 11. • Axons of spiral ganglion of internal ear (that innervate the hair cells) form cochlear (auditory) division of VIII nerve. • Auditory nerve enters the medulla, ends in ventral & dorsal cochlear nuclei. First synapse occurs here. • Second order neurons which arise in cochlear nuclei end variously in superior olivary nucleus, nucleus of lateral leminiscus & the inferior colliculi.
- 12. • Third order neurons arise in these nuclei & end in medial geniculate body(thalamus). • Some fibres also send collaterals to reticular formation of both sides.
- 13. • From medial geniculate body another set of neurons start & their processes via auditory radiation go to the cerebral cortex Superior temporal gyrus (area 41,42). • Here nerve impulses perceived as sound. • Auditory information such as loudness,pitch, source & direction are perceived.
- 14. Features of auditory pathway • Bilateral representation • Descending pathway: olivo-cochlear bundle-outer hair cells • Role in general arousal-reticular formation • Spatial organization/Tonotopic organization- from cochlea to auditory cortex the fibers are tone/pitch specific
- 15. • Air/ Ossicular conduction • Main pathway • Via TM→ auditory ossicles→fluid in inner ear • Bone conduction : • Transmission of vibration of bones of skull → fluid in inner ear Transmission of sound
- 16. • Because the inner ear, the cochlea, is embedded in a bony cavity in the temporal bone, called the bony labyrinth, vibrations of the entire skull can cause fluid vibrations in the cochlea. • Therefore, under appropriate conditions, a tuning fork or an electronic vibrator placed on any bony protuberance of the skull, but especially on the mastoid process near the ear, causes the person to hear the sound. • However, the energy available even in loud sound in the air is not sufficient to cause hearing via bone conduction unless a special electromechanical sound-amplifying device is applied to the bone.
- 17. • Sound entering to the inner ear through oval window spread along the scala vestibuli as a travelling wave. • Most of the sound energy passes from scala vestibuli to endolymph. • This wave causes vibration of basilar membrane. • This causes vibration of organ of corti. Vibration of basilar membrane
- 18. Cochlear microphonics • The sum of receptor potentials of the hair cells(outer hair cells) recorded extracellularly. • Are similar to generator potential. • This can be recorded by placing and electrode in the scala media and the other electrode in scala tympani. • It forms an oscillatory wave pattern. • The microphonic potentials recorded are having same polarity of acoustic stimulus(cochlear microphonics)
- 19. Cochlear microphonics • Cochlea can function as a microphone. • Keep an active electrode on the cochlea and an indifferent electrode on some other part of the ear and connect these electrodes to an audio amplifier. • The cochlear microphonic potentials recorded have the same frequency and intensity of the sound fed into the ears during recording. • The source of the cochlear microphonic potentials is the Outer hair cells.
- 20. • These potentials are not action potentials. • The cochlear microphonic potential is resistant to ischemia and anesthesia and it shows no latency or refractory period and does not obey all-or-none law. • Thus these potentials are similar to the generator potentials in the hair cells. • Degeneration of organ of Corti abolishes this potential.
- 21. Theories of hearing /pitch discrimination • Travelling wave theory • /Place Theory/Von Bekesy • Volley theory • Telephone theory • Resonance theory
- 22. • Sound → distortion of Basilar mem. • Site of max distortion depends on frequency • High pitch → near base • Low pitch → near apex Travelling wave theory
- 23. • High-frequency sound wave travels only a short distance along the basilar membrane before it reaches its resonant point and dies • Medium-frequency sound wave travels about halfway and then dies • Very low-frequency sound wave travels the entire distance along the membrane.
- 24. • Another feature of the traveling wave is that it travels fast along the initial portion of the basilar membrane but becomes progressively slower as it goes farther into the cochlea. • The cause of this difference is the high coefficient of elasticity of the basilar fibers near the oval window and a progressively decreasing coefficient farther along the membrane. • This rapid initial transmission of the wave allows the high-frequency sounds to travel far enough into the cochlea to spread out and separate from one another on the basilar membrane. • Without this rapid initial transmission, all the high-frequency waves would be bunched together within the first millimeter or so of the basilar membrane, and their frequencies could not be discriminated
- 25. DETERMINATION OF SOUND FREQUENCY—THE “PLACE” PRINCIPLE • low-frequency sounds cause maximal activation of the basilar membrane near the apex of the cochlea, and high-frequency sounds activate the basilar membrane near the base of the cochlea. Intermediate-frequency sounds activate the membrane at intermediate distances between the two extremes. • There is spatial organization of the nerve fibers in the cochlear pathway, all the way from the cochlea to the cerebral cortex. • Recording of signals in the auditory tracts of the brain stem and in the auditory receptive fields of the cerebral cortex shows that specific brain neurons are activated by specific sound frequencies. • Therefore, the major method used by the nervous system to detect different sound frequencies is to determine the positions along the basilar membrane that are stimulated the most, called the place principle for the determination of sound frequency.
- 26. • Therefore, the major method used by the nervous system to detect different sound frequencies is to determine the positions along the basilar membrane that are stimulated the most, called the place principle for the determination of sound frequency
- 27. • The distal end of the basilar membrane at the helicotrema is stimulated by all sound frequencies below 200 cycles/sec. • Therefore, it has been difficult to understand from the place principle how one can differentiate between low sound frequencies in the range of 200 down to 20 cycles/sec. These low frequencies have been postulated to be discriminated mainly by the so-called volley or frequency principle. That is, low-frequency sounds, from 20 to 1500 to 2000 cycles/ sec, can cause volleys of nerve impulses synchronized at the same frequencies, and these volleys are transmitted by the cochlear nerve into the cochlear nuclei of the brain. • It is further suggested that the cochlear nuclei can distinguish the different frequencies of the volleys. In fact, destruction of the entire apical half of the cochlea, which destroys the basilar membrane where all lower frequency sounds are normally detected, does not totally eliminate discrimination of the lower frequency sounds.
- 29. Deafness • Clinical deafness may be due to impaired sound transmission in the external or middle ear (conduction deafness) • Or to damage to the hair cells or neural pathways (nerve deafness). Degree of hearing loss Hearing threshold 0 not significant 0-25 dB 1 mild 26-40 dB 2 moderate 41-55 dB 3 Moderately severe 56-70 dB 4 severe 71-91 dB 5 profound Above 91 dB
- 30. Conductive hearing loss • Any disease process which interfere with the conduction of sound from external ear to cochlea causes conduction hearing loss. • Causes- ❖ External ear: obstruction in the ear canal Eg: foreign body, wax, tumors ❖ Tympanic membrane –perforation ❖ Middle ear cavity- fluid in middle ear(otitis media) ❖ Otosclerosis ❖ Eustachian tube obstruction.