Auditory Nerves
Auditory Nerve
-- Overview: The auditory nerve is a crucial component involved in the process of hearing, transmitting sound signals from the cochlea to the brain for interpretation.
Sound Processing
Mechanics of Sound:
The tympanic membrane is set into vibration by sound waves.
The stapes footplate moves in-and-out of the oval window, creating pressure changes in the cochlea.
The basilar membrane moves up and down in response to these pressure changes.
Inner hair cells transduce the vibrations of the basilar membrane into neural signals sent to Type I fibers.
The auditory nerve then sends this coded information to the brain.
Auditory Nerve Structure
Formation:
Auditory nerve fibers (ANFs) from various parts of the cochlea converge in the modiolus to form the cochlear portion of the 8th cranial nerve, also known as the auditory nerve.
The fibers from across the cochlea create the auditory nerve.
Inner Hair Cell Connections:
Each inner hair cell has approximately 20 afferent fibers that connect to it, enabling a detailed transmission of auditory information.
ANF Responses
Measurement:
The activity of individual auditory nerve fibers can be assessed by placing a recording electrode into the auditory nerve.
Action Potentials:
ANFs produce all-or-none electrical discharges known as action potentials or “spikes.”
They exhibit a spontaneous firing rate, discharging at a low rate even when not stimulated.
When stimulated by sound, the discharge rate increases above this spontaneous firing rate.
Graphs of ANF Response:
When no stimulus is present, spontaneous discharges occur.
Under stimulation, ANFs show a higher discharge rate dependent on the intensity of the stimulus:
Low Intensity Stimulus: Higher discharge rate during stimulus compared to the spontaneous rate.
More Intense Stimulus: Even higher discharge rate observed at increased stimulus intensities.
Post-Stimulus Time (PST) Histogram
Function:
The PST histogram illustrates the auditory nerve fiber's response during sound stimuli presentation.
It indicates that actual ANF responses are complex and follow a sequence:
Spontaneous Activity prior to onset of a tone.
Initial Onset Response when the stimulus begins.
Adaptation occurs leading to a steady-state response reduction.
Offset Response that is lower than the spontaneous rate after the stimulus stops.
Recovery Phase returning to the original spontaneous rate.
Response Magnitude:
Evaluated during the steady-state period; increases with higher stimulus levels until a saturation point is reached, after which the discharge rate no longer increases.
Threshold & Dynamic Range
Definitions:
Threshold: This is defined as the intensity level at which the auditory nerve fiber (ANF) first responds to sound.
Saturation: The intensity level at which the firing rate of the ANF no longer increases with increasing stimulus levels.
Dynamic Range (DR):
Defined as the difference between the intensity level at saturation and the threshold:
( ext{saturation level}) - ( ext{threshold level})(60 dB - 20 dB = 40 dB)$$ \n
Characteristics of ANFs:
Most ANFs have dynamic ranges that vary from 20 dB to 50 dB.
The staggered thresholds across different ANFs allow for effective coding of sound intensities across the full range of human hearing.
Dynamic Range of ANFs
Distribution:
Individual nerve fibers have a dynamic range estimated between 20 dB and 50 dB.
Staggered thresholds facilitate detection of loudness variations over a larger range (from 0 dB to 120 dB SPL).
Coding of Frequency
Coded Information:
Frequency perception is primarily coded by the place of stimulation on the basilar membrane.
This place coding is complemented by a secondary mechanism that also influences pitch perception, tied to the individual nerve response patterns.
Phase Locking:
ANF responses code frequency in their temporal firing patterns, termed phase locking.
In phase locking, nerve fibers fire preferentially during the excitatory phase of the stimulus.
Phase Locking Period Histogram:
This shows the timing of neural firing during the presence of a pure tone.
Phase locking is noted in ANF responses up to frequencies of 3-4 kHz. At frequencies exceeding this range, temporal jitter inhibits accurate phase locking.
Auditory Nerve Fibers: Summary
Intensity and Frequency Coding by the Auditory Nerve:
Intensity is coded through the discharge rate of the nerve fibers.
Frequency is coded through:
Place stimulation along the basilar membrane for all frequencies.
Temporal coding (phase locking) for frequencies less than 3-4 kHz.
Practice Problems
Identify the Graph:
A graph that shows the discharge rate of an auditory nerve fiber during a sound stimulus is referred to as a (c) PST histogram.
Without Stimulus:
In the absence of sound, an auditory fiber typically (c) produces spikes at the “spontaneous firing rate.”
Central Auditory Pathways
Brain Structure Overview:
The central auditory pathways encompass several key structures of the brain:
Auditory Cortex: Higher-level processing of sound.
Medial Geniculate Body (Thalamus): Relay station for auditory information.
Inferior Colliculus (Midbrain): Important for sound localization and integration.
Superior Olivary Complex (Pons): Involved in binaural hearing.
Cochlear Nucleus (Medulla): Entry point for auditory nerve signals.
Key Features:
These auditory pathways are crossed, meaning input from both ears is integrated.
The superior olivary complex and higher-level nuclei receive binaural input, crucial for sound localization.
Tonotopic maps are maintained throughout the pathway, systematically organizing frequencies by location.
Auditory Processing in the Cortex
Organization in Auditory Cortex:
By the time signals reach the auditory cortex, they have been processed along several dimensions, such as determining if sounds are speech-like or non-speech.
Some higher processes, such as the startle reflex, originate from the inferior colliculus, indicating early processing of sound features.