Journey from hair cells to the brain

Step 1: Neurotransmitter binding

• the neurotransmitter material is released from the hair cell base to receptors on the auditory nerve fibers

Step 2: action potentials (nerve impulses)

• neurotransmitters trigger an electrical signal in auditory nerve fibers

• the signal travels along the length of the nerve fibers in the form of an action potential

If an action potential is being triggered…

• that means neurotransmitters are released and the stereocillia is bending to the tallest tip link

When the stereocilia is bending towards the tallest tip link (excitation, depolarization)

• the basilar membrane moves up

When stereocilia is bending towards the shortest tip link (inhibition, polarization)

• the basilar membrane moves down

If enough neuro material is released

• action potentials are also released

Low frequencies are closer

to the apex

Auditory nerve fibers are

• tonotopically arranged

• and connected to the basilar membrane

Electrical impulses represent

encoded auditory information and are transmitted to the brain

Threshold for activation

• in order for an action potential to be generated, a threshold must be reached

• this threshold is a critical level of depolarization (increase in positive charge inside the cell) that must be achieved through the opening of voltage-gated ion channels

Increased number of “spikes”

during depolarization

Decreased number of “spikes”

during hyperpolarization

What are action potentials?

• electrical signals that travel along the auditory nerve to the brain

Each auditory-nerve fibre

• responds only to a narrow range of frequencies

If signal level is lower on the graph

• the threshold is lower (you are more sensitive to it)

Noise exposure

• when the narrow v shape curve becomes more flat, it starts to become difficult to hear lower sounds

• you have frequency smearing

Synaptic transmission to auditory fibers

• the action potentials generated in the auditory nerve fibers travel towards the brain through the auditory pathways

• these pathways relay auditory information from the cochlea to various regions of the brain, including the auditory cortex, where sound perception and interpretation occur

What is the stage of the afferent auditory pathway immediately following the cochlear nucleus?

• superior olivary complex

auditory processing

• once the electrical signals reach the auditory cortex in the brain, they are processed and interpreted

• the brain analyzes various aspects of the sound, such as frequency, intensity, and spatial location, to create our perception of the sound

Tonotopicity

• it starts in the basilar membrane, where sound waves are deconstructed into their frequencies

• different frequencies activate different regions along its length

auditory nerve coding

• each auditory nerve fiber is most responsive to a specific frequency range

• tonotopic organization continues through the auditory pathway

temporal coding

• the timing of action potentials (nerve impulses) in response to sound plays a role in distinguishing

• the brain can detect the timing of neural responses to different frequencies

spatial processing

• the brain processes auditory information from both ears to determine the direction and location of sound sources, aiding in the separation of different sound sources in space

selective attention

• our ability to selectively attend to specific sounds or frequencies enables us to focus on one source of interest while filtering out others