Physiology of Hearing PDF

💡 Physiology of Hearing Human ear’s range is 20 - 20,000 Hz Sound of >130 dB causes pain A is the record of a pure tone. B has a greater amplitude and is louder...

💡 Physiology of Hearing Human ear’s range is 20 - 20,000 Hz Sound of >130 dB causes pain A is the record of a pure tone. B has a greater amplitude and is louder than A. C has the same amplitude as A but a greater frequency, and its pitch is higher. D is a complex wave form that is regularly repeated. Such patterns are perceived as musical sounds, whereas waves like that shown in E, which have no regular pattern, are perceived as noise. The ear pinna collects the sound waves and directs them to the external auditory meatus. The external auditory meatus conducts sound waves. The wax acts as lubricant and the hairs are important for trapping foreign particles. Physiology of Hearing 1  The Eustachian tube opens during: coughing, snoring Functions: Equalize pressure Helps drain the fluid from the middle ear Middle & Inner Ear Function Middle Ear Osicles Impedance matching function The small surface area of the oval window together with the mechanical advantage of the lever system, produces a 20–30 dB amplification of the sound pressure level Middle Ear Muscles Damping function [attenuation reflex] or [tympanic reflex] Simultaneous contraction of the tensor tympani and stapedius muscles reduces the transmission of sound. Functions of Attenuation reflex Protection of the inner ear from high-intensity sounds. Mask low-frequency sounds in loud environments Physiology of Hearing 2  Decrease a person's hearing sensitivity to his or her own speech. Inner Ear (Cochlea) Perilymph & Endolymph Perilymph is the fluid present in Vestibular and tympanic ducts. It is similar in its composition to plasma. Endolymph is the fluid present in the Cochlear duct. It is similar to Physiology of Hearing 3  intracellular fluid {rich in K+ and low in Na+} Intermediate cells are vascular cells that secrete endolymph. Organ of Corti It is the sensory organ for the perception of sound. It lies on the basilar membrane. It is formed of two sets of hair cells, inner and outer (auditory receptors) that rest on supporting cells. Bases and sides of the hair cells synapse with the nerve endings (cochlear nerve)- Spiral ganglion- Cochlear nerve. There are 3500 inner hair cells arranged in one row and 20000 outer hair cells arranged in 3 rows. A thin viscous elastic membrane [tectorial membrane] covers the rows of the hair cells, and the tips of the hairs are embedded into it. > 90% of auditory impulses are transmitted through inner hair cells. > Outer hair cells act as “Cochlear Amplifier” that sharpen the sounds and refine sounds of close frequencies. Electric Response in Cochlea Physiology of Hearing 4  Glutamate is the most important afferent neurotransmitter within the inner ear. Efferent cochlear neurotransmitters include dopamine, gamma aminobutyric acid (GABA), acetylcholine (ACH) and serotonin. Hearing Process Overview a. Vibration of Eardum b. Vibration of middle ear bones c. Vibration of oval window d. Movement of fluid in cochlea e. Vibration of basilar membrane f. Receptor hair cells bend g. Influx of K+ into the hair cells h. Action potential generated i. Auditory nerve → brain Basilar Membrane Frequency Coding Physiology of Hearing 5  The basilar membrane is narrow and stiff at the base of the cochlea, becoming wider and less stiff at the apex, and in consequence the place where it vibrates maximally is frequency dependent. At the basal end it is tuned to high frequencies, while its apical end resonates in response to low frequencies. This mechanism allows frequency coding of sounds. Sound Localisation Time of the arrival f the stimulus at 2 ears The sound is louder on the side closest to the surface Clinical Importance 1. Deafness Deafness is reduction in the hearing ability. It may be partial or complete. There are two main types of deafness: a. Conductive deafness Results from interference with sound conduction from the air to the cochlea which can be caused by: a)Obstruction of the external ear canal [wax, foreign bodies, otitis externa] b)Damage or perforation of tympanic membrane c)Infection of the middle ear d)Otosclerosis e)Blockage of the Eustachian tube b. Sensorineural deafness (Permanent) Physiology of Hearing 6  This results from interference with the transmission of nerve impulses from the cochlea to the auditory cortex. Main causes: i. Damage to the basilar membrane or the organ of Corti [as with prolonged use of antibiotic streptomycin] ii. Damage to the cochlear nerve [severe head injuries or certain brain tumours, such as acoustic neuroma]. iii. 4. Meniere's syndrome [increase of the pressure of the endolymph] iv. Extensive lesions of the auditory nervous pathway especially if the lesion is bilateral [severe head injuries or certain brain tumours] 2. Presbycusis Presbycusis is an age-related hearing loss that is caused by the loss of outer hair cells and a cochlear amplifier. Characterized by Loss of hearing of high-frequency sounds. 3. Cochlear Implants Able to restore limited but useful sound sensation in patients with deafness due to cochlear damage, provided that there is still a functioning auditory nerve. Physiology of Hearing 7  4. Hearing Tests Weber’s test: Place the base of the tuning fork on the forehead Normally the sound will be heard equal on both ears If the sound is higher in the affected ear means it is conductive deafness If the sound is lower in the affected ear it is nerve deafness. 5. Audiometry and Audiogram Physiology of Hearing 8  Equilibrium Function Maintenance of balance in relation to gravity and body posture Vestibular System Physiology of Hearing 9  The vestibular apparatus is the sensory organ for detecting sensations of equilibrium. It is encased in a system of bony tubes and chambers located in the petrous portion of the temporal bone, called the bony labyrinth. Within this system are membranous tubes and chambers called the membranous labyrinth. The membranous labyrinth is the functional part of the vestibular apparatus. Macular Structure & Activity Physiology of Hearing 10  Directional Activity of Hair Cells When the stereocilia and kinocilium bend in the direction of the kinocilium, it opens several hundred fluid channels in the neuronal cell membrane around the bases of the stereocilia, and these channels are capable of conducting large numbers of positive ions… causing In each macula, each of the hair depolarization cells is oriented in a different Conversely, bending the pile of direction, so that some of the hair stereocilia in the opposite direction cells are stimulated when the head closes the ion channels, causing bends forward, some are hyperpolarization stimulated when it bends backwards, others are stimulated when it bends to one side, and so forth Therefore, a different pattern of excitation occurs in the macular nerve fibres for each orientation of the head in the gravitational field. It is this “pattern” that apprises the brain of the head’s orientation in space. Semi-Circular Canals Physiology of Hearing 11  Anterior, Posterior and lateral Perpendicular to each other Each detects a rotational change in equilibrium in one of the three planes in space Each has an expanded end – Ampulla contains receptor structure – crista ampullaris The three semicircular ducts in each vestibular apparatus, known as the superior (anterior), posterior, and lateral (horizontal) semicircular ducts, are arranged at right angles to one another so that they represent all three planes in space. When the head is bent forward about 30 degrees, the lateral semicircular ducts are approximately horizontal with respect to the surface of the earth; the anterior ducts are in vertical planes that project forward and 45 degrees outward, whereas the posterior ducts are in vertical planes that project backward and 45 degrees outward. Crista Ampullaris Physiology of Hearing 12  In each ampulla is a small crest called crista ampullaris. On top of this crista is a loose gelatinous tissue mass, the cupula. The direction of fluid movement is opposite to the direction of rotation due to inertia. Summary Physiology of Hearing 13  Vestibular Pathway Vestibular nerve fibres have connections with: Cerebellum Higher centres Reticular nuclei and brain stem Spinal cord via vestibulospinal and reticulospinal pathways –The signals to the spinal cord control the interplay between facilitation and inhibition of many anti-gravity muscles There are also connections with extra-ocular muscles which help to fixate gaze on a point while moving the head (vestibulo-ocular reflex). Vestibular-Ocular Reflex Physiology of Hearing 14  This reflex occurs when attempting to fixate gaze on a point while moving the head. So, the eyes must move in the opposite direction from the head. For example, if you ask a patient to continue looking at your nose while turning the head to the right, the patient’s eyes will have to move to the patient’s left to maintain fixation. This reflex is mediated by communication between the vestibular nuclei and the abducens nucleus on the left so as to coordinate left-eye abduction (via CN 6) and right-eye adduction. Applied Aspects Disorders in the vestibular system will be presented by: vertigo and nystagmus Vertigo is the sense of rotation in the absence of actual rotation Nystagmus, the jerky movement of the eye seen at the start and end of a period of rotation. Resting nystagmus indicates pathology. Physiology of Hearing 15