PHYSIO

PHYSIOLOGY OF HEARING
Hey guys, since this would be a doctor-patient explanation, it is best to keep it short and simple for the patient. However, it could be discussed in greater depth with the rest of the PCL group after the role play (with relevant anatomical terminology).
Very briefly, we would be touching with much reference to the anatomy of the ear. Terms we must be familiar with would be the auricle (pinna), external ear, tympanic membrane (ear drum), ossicles (malleus, incus, stapes), cochlea, vestibular, scala vestibuli, scala tympani, scala media, organ of corti, stria vascularis, tectorial membrane, round and oval window.
Impedance matching (force and amplitude, ¾ in stapes in comparison to malleus)
We can start of by saying that the fluid in the scala vestibuli and tympani is the perilymph and the scala media contains the endolymph. The difference between these two fluids are that the perilymph is more similar to the CSF and is almost the same in fluid electrolyte levels since the scala tympani and scala vestibuli communicates directly with the subarachnoid space around the brain. Conversely the endolymph that fills the scala media is an entirely different fluid secreted by the stria vascularis, a highly vascular area on the outer wall of the scala media. Endolymph contains a high concentration of potassium and a low concentration of sodium in contrast to the perilymph. An electrical potential of about +80mV exist all the time between the perilymph and endolymph, with positivity inside the scala media. This is called endocochlear potential and it is generated by continual secretion of the positive potassium ions into the scala media by the stria vascularis.
The importance is that the tops of the hair cells project through the reticular lamina and are bathed by the endolymph of the scala media, whereas perilymph bathes the lower bodies of the hair cells. Furthermore, hair cells have a negative intracellular potential of about -70mV with respect to the perilymph and -150mV to the endolymph. This high electrical potential at the tips of the stereocilia (hair cells) sensitizes the cell an extra amount and increasing the ability to respond to the slightest sound.
So very briefly, when sound is present, the pinna acts as a funnel shape to direct the sound waves into the external ear canal. This sound wave then hits the tympanic membrane. The handle of the malleus is attached to the membrane and when the tympanic membrane vibrates (tensor tympani muscle keeps this membrane taut), the malleus starts to move. The malleus and incus are only movable in a certain plane due to the reinforcements of ligaments. The malleus moves, incus moves and the stapes moves too. The stapes is held with loose annular ligaments to the oval window. When it moves, the stapes would hit the oval window and causes vibrations into the perilymph in the scala vestibuli. This would then cause fluid conduction (scala media and vestibuli are considered as a single chamber) and move the basiliar membrane where the organ of corti with all the hair cells are located.
The hair cells are hyperpolarised when moved in one direction and depolarized in the opposite direction. All these signals are generated and sent to the cochlear nerve. The conduction of sound would then move to the scala tympani and exit the cochlea.
However not only this path could conduct sound. 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 itself. A tuning fork or an electronic vibrator placed on any bony protuberance of the skull especially on the mastoid process causes the person to hear the sound.
Interesting facts
1) Attenuation of loud sounds by contraction of the stapedius muscle and the tensor tympani muscle.
2) Auditory signals are mainly transmitted by inner hair cells, outer hair cells control sensitivity at different pitches (tuning) but loss of outer cells would actually lose a large amount of hearing
3) Alternating hair cell potential, opening of 200-300 cationic channels and glutamate being a possible neurotransmitter
4) Loudness is determined by: 1. Sound louder, amplitude vibration of basiliar increases, hair cells excited more rapidly, 2. Amplitude increase, more hair cells on fringes stimulated, spatial summation of impulses, 3. Outer hair cells do not become stimulated significantly until reaches high intensity vibration of basiliar membrane.
5) 10 fold increase of sound = 1 bel
6) Nerve fibers from spiral ganglion of corti enter the dorsal and ventral cochlear nuclei located in the upper part of the medulla. At this point, all fibers synapse and second order neurons decussate to the opposite side of the brain stem to terminate in the superior olivary nucleus. A few second order fibers also pass to the superior olivary nucleus on the same side. From the superior olivary nucleus, they pass upward through the lateral lemniscus, some terminate at the nucleus of the lateral lemniscus but many bypass this nucleus and travel on to the inferior colliculus, where all or almost all of the auditory fibers synapse. Finally by way of auditory radiation (from medial geniculate nucleus) to the auditory cortex located mainly in the superior gyrus of the temporal lobe.
7) 6 tonotopic maps of auditory cortex found in brain. Removal of the primary auditory cortex do not actually cause deafness but can greatly reduce or aboish ability to discriminate different sound pitches and patterns of sound.
8) Lesions to the associated auditory areas render the person unable to comprehend of interpret sound, Wernicke area.
9) Direction of sound and distance: 1. Time lag between enty of sound into one ear and opposite ear, 2. Different between intensities of sounds in the two ears. Lateral superior olivary nucleus determines difference in intensities of sound to detect direction. Medial superior olivary nucleus determines the time lag to detect direction.
Pathophysiology
2 types of deafness: 1. Nerve deafness, 2. Conduction deafness.
Nerve: nerve damage (cochlea or auditory nerves)
Conduction: physical dysfunction