HEARING

In the case of hearing, detection of sounds is not carried out directly by neuronal receptors, but is first transformed. The sound vibrations enter the ear canal (called external auditory meatus) and bounce on the tympanic membrane, causing it to itself vibrate. These vibrations are in turn passed thru three very small bones (ossicles) connected in chain. The first ossicle is in direct contact with the tympanic membrane, and is called the malleus, it is in turn coupled to the incus in such a way that the resulting movement pushes in and out the third ossicle, the stapes. The stapes is connected in turn to a membrane called the oval window of the cochlea.



The cochlea is a bony canal made into a spiral like a garden snail and filled with liquid. This canal has three longitudinal compartments. The first compartment is the scala vestibuli, and it is at this compartment that the stapes connects. The second compartment is the scala tympani and has communication with the scala vestibuli at the end of the cochlea, in a point called helicotrema. This configuration forms a U shaped continued physical tube. Between both these compartments there is a narrower tube, called the scala media. The membrane that divides the scala media from the scala tympani is called basilar membrane, and along this membrane rests the organ of Corti, which is the sensory transduction organ.


For sound to be transduced into neural patterns, the pushing and pulling of the stapes at the oval window causes pressure to rise and lower inside the scala vestibuli. But since it is connected at the heliocotrema with the scala tympani, the pressure change carries back thru this compartment up to the beginning of the U shape at a flexible membrane just below the oval window, called the round window. This membrane bulges and sucks in with the differences in pressure, letting the fluid circulate. These changes in pressure deform the membranes of the scala media, which can then activate or not the organ of Corti. The organ of Corti has receptor cells that have cilia (hairlike protrusions) showing into the scala media called hairy cells. On top of the cilia of hairy cells is lying a gelatinous material called tectorial membrane, which makes contact at its base with the membrane separating the scala media from the vestibuli. When the vibrations travel thru the scalas there is a difference in sincronicity between them, which causes the basilar membrane holding the organ of Corti to oscillate with respect to the tectorial membrane, stimulating the cilia of hairy cells. This mechanical movement produces a depolarization of these cells which can then transmit the signal.


Along the cochlea, but outside it, runs a chain of neurons that form the spiral ganglion, these neurons project their dendrites to the hairy cells, and their axon away from the cochlea and into the cranium to form part of the vestibulocochlear nerve. The auditory fibers then synapse in the dorsal and ventral choclear nuclei of the brain stem. The connections from this point can go directly to the contralateral inferior colliculus, or relay in the superior olivary nucleus and from there project efferents to both inferior colliculi. The auditory neurons of the inferior colliculus project their axons to the ipsilateral medial geniculate nucleus of the thalamus, which in turn relay the information to the primary auditory cortex, just below the superior temporal gyrus in the temporal lobe.


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