Peripheral auditory mechanisms Domina Petric, MD

Auditory function Auditory system transduces sound waves into distinct patterns of neural activity that are integrated with other sensations. Sound waves are collected and amplified by physical structures in the external and middle ear for transfer to neural elements in the inner ear. Biomechanichal properties of the inner ear decompose complex sound waves into sinusoidal components.

Auditory function Frequency, amplitude and phase are encoded in the firing of the receptor cell. Tonotopy is the systematic representation of sound frequencies. Tonotopy is preserved in the inner ear and throughout central processing stations. In the brainstem the auditory information is first processed and divided into several parallel pathways.

Auditory function Brainstem centers relay the information to the midbrain (inferior colliculus). Midbrain projects to the auditory thalamus (medial geniculate complex). Auditory cortex recieves inputs from the thalamus and processes more complex aspects of sounds (for example speech).

Amplitude is the intensity of sound. Frequency is the pitch of the tone (sound).

External ear (concha, pinna) is collecting sound frequencies from the environment. Sound is collected in the external auditory canal. Stapes is connected with the oval window. Sound energy is 200 fold amplificated from the point of tympanic membrane via the middle ear ossicles (malleus, incus, stapes). Pinterest.com

Biomechanical function of cochlea is to decompose complex sounds into their component frequencies. The neural function of the cochlea is to transduce the mechanical energy into neural signals. Vestibular part Auditory part https://owlcation.com

Outer hair cells have biomechanical function: they act as motor units  that amplify the movement of the basilar membrane in response to a stimulus and some of this added energy is transmitted back through the middle ear, where it can be recorded as an otoacoustic emission. Cilia of the inner hair cells are in contact with tectorial membrane. Sensory cells (inner hair cells) are between the basilar membrane and tectorial (roof) membrane. Cell bodies of the cochlear nerve are located in the spiral ganglion. http://www.neuroscientificallychallenged.com

Inner ear Stapes makes contact with the oval window. Deformation of the oval window (vibration) is transduced into the fluids in the Scala vestibuli and Scala tympani. The pressure on the oval window is relaxed with the outward bulge of the round window. Fluids then cause the vibration of the hair cells cilia.

Inner ear Base of the basilar membrane is tuned for high frequencies. Apex of the basilar membrane is tuned for low frequencies. Tip of the cochla is Helicotrema. Movement of the basilar membrane against the tectorial membrane causes the inner hair cell cilia to move

Inner ear Movement of hair cells stereocilia causes opening of the ion channels, ion influx and generation of the graded potential in the inner hair cells. In the Scala vestibuli and Scala tympani there is perilymph: relatively low concentration of potassium. Scala media is filled with endolymph: high concentration of potassium (+80 mV). Endolymph is rich with potassium because of the activity of the cells in Stria vascularis: highly vascularised structure with high metabolic activity.

Inner ear The cells of the Stria vascularis secrete potassium ions. On the tips of inner cells stereocilia are potassium channels. Protein called TIP LINK connects potassium channels of one cilia to potassium channels of other cilia. When the stereoicilia move in the direction from the smallest stereocilia towards the largest one, tip link protein is streched and the potassium channels are opened. When potassium ions influx into the inner hair cell, there is depolarisation.

Inner ear Graded potential of inner hair cell opens the voltage gated calcium channels on the lateral sides of the inner hair cell. Calcium rushes in through the basal lateral aspects of the inner hair cell. Then occurs calcium dependent exocytosis of synaptic vesicles. Neurotransmitter is released from the vesicles and makes contact with receptors at the peripheral end of the spiral ganglion processes.

Stereocilia move in the direction from the smallest towards the largest. Tip link protein streches and opens the potassium channels: influx of potassium ions causes the depolarisation of the inner hair cell. Calcium channels open: calcium ions influx causes the exocytosis of vesicles. Released neurotransmitter generates the action potential in the axons of the cochlear nerve.

That causes hyperpolarisation of the inner hair cell (-45 mV). Inner ear When stereocilias then move back in the direction from the largest to the smallest stereocilia, tip link protein relaxes and then the potassium channels close. That causes hyperpolarisation of the inner hair cell (-45 mV).

Auditory nerve Human ear hears from 20 Hz to 20 kHz. Helicotrema: 20 Hz. Base of the basilar membrane: 20 kHz. Different axon of the auditory nerve has its own BEST frequency: at that specific frequency has the low treshold for action potential. Tonotopic map from the basilar membrane is preserved as the tonotopic map in the auditory nerve.

Literature https://www.coursera.org/learn/medical- neuroscience: Leonard E. White, PhD, Duke University Pinterest.com https://owlcation.com http://www.neuroscientificallychallenged.com