Central auditory pathways

Question 1

Describe the connection of auditory nerve afferents with the inner hair cells

Answer 1

Inner hair cells are located o along the basilar membrane in the cochlear at different frequency sensitive locations that are synapses on to AN afferents (one inner hair cells makes multiple connections with many synapses but AN afferents only relieve input from a single inner hair cell), when a sound wave enters the inner ear it will to vibration of small hair like structures found on top of inner hair cell known as outer hair cell which lead to stereocillia tilt this release’s neurotransmitter glutamate which innovates the AN afferents this will be converted to electrical impulses sent to auditory cortex for sound processing/ interpretation in the brain.

Question 2

How well does the basement membrane code the frequency of sound

Answer 2

The louder a sound is will activate more AN afferents and a specific frequency within the sound will activate a set of inner hair cells and AN afferents.

At low frequency the BM can resolve/capture individual harmonics and at high frequency the harmonics blur into each other as it’s not as distinguishable

Question 3

What’s the difference between spectrogram and neurogram

Answer 3

Neurogram represents firing pattern of AN afferents and conveys info about the frequency components within the sound.

Spectrogram is used to analyse speech and the characteristics of formants

Question 4

How are natural sounds distinguishable by their time and frequency domains

Answer 4

In time domain natural sounds like a bird chirping or a dog barking have distinct duration and rhythm that have different characteristics within its waveform. On the other hand natural sounds contain info in the frequency domain such that we’re able to distinguish between a range of frequencies ie high pitch to low pitch sounds depending on their intensities difference

Question 5

Explain how auditory coding involves a place code and temporal code (phase locking)

Answer 5

Place coding is when inner hair cells are positioned along the basilar membrane in the cochlear that are sensitive to different frequencies.
High frequencies found at the base.
Low frequencies found at the apex. Temporal coding refers to the periodicity or temporal structure of the sound. They have a repeating pattern of peaks and troughs in their waveform. When these sounds are presented to the auditory system the firing of the ana synchronises with the peaks and troughs of the soundwave.
This synchronisation is known as phase locking conveying infomrson about the precise timing of the sounds periodicity. Provides perception of the rhythm and timing cues, this occurs only up to a certain frequency as the brain cannot phase lock fast enough at high frequencies. Only receive signals of the frequency range rather than precise timing pattern, as ana are not able to follow the peak and trough synchronation fast enough.

Question 6

Outline the ascending auditory brain stem network

Answer 6

Cochlea located in the inner ear converts sound vibration into electrical signals that can be processed by the brain.

Cochlear nucleus (located in brainstem), this is where the electrical signals generated in the cochlea travel to the cochlear nucleus which is the first relay station in the brain stem and analyses frequency and intensity of sound before transferring them.

Superior olivary complex, this is responsible got comparing and integrating sounds from both ears allowing for localisation and detecting direction of sound sources.

Inferior colliculus, from the SOC, the auditory signals are relayed to the IC and integrates and processes the sound information.

Medial geniculate body/nucleus, this is located in the thalamus and helps filter and selects relevant auditory information before transmitting it further.

Auditory cortex is located in the temporal lobe of the brain and process and transmits auditory info to higher brain regions helping us understand sound coming from the environment

Question 7

describe the models for the coding of sound locations

Answer 7

Interaural time different model focuses on itd as a primary cue for sound lolcalisation. It suggests that neurons in the brain stem and auditory cortex compare the time differences between sound arrival at each ear estimating the location of the sound source.

Interaural intensity difference model.
This detects the difference in intensity of sound from both ears.
Mechanism -
Stronger stimulus to left ear excites the LSO.
This stimulus also inhibits the right LSO via MNTB inter neurons.
Excitation from light side is greater than inhibition from right side resulting in next excitation to higher centres.
Inhibition from left side is greater than excitation from right side resulting in net inhibition on the right and no signal to higher centres.