Auditory Nerve and Central Pathway

auditory nerve connects to the

hair cells inside the cochlea

nerve

bundle of axons

axon (nerve fiber)

part of a neuron that conducts action potentials

neuron

specialized cell in the nervous system. produce a train of electrical spikes called action potentials and pass information to others in one direction only.

neurons respond to

neurotransmitters

dendrites bring

information into the cell body

a typical neuron has many

dendrites.

neurons usually lack

myelin insulation

axons carry information

away from the cell body

usually, each neuron has only one

axon

axons tend to branch

farther away from the cell body

myelinated nerve fibers are wrapped in an electrical-insulating myelin sheath that allow for

significantly faster and more efficient nerve impusle conduction

auditory neurons use both dendrites and axons to

transfer information from the hair cells to higher centers

neurotransmitters are released at

synapses.

in the auditory system, neurotransmitters

pass information from the hair cells to the auditory nerve fibers, which then fire spikes that carry the signal forward.

two main types of fibers connect to eh organ of Corti

afferent and efferent

afferent fibers

ascending. they carry sensory information from the cochlea to the braina

fferent fibers include

inner radial fibers and outer spiral fibers

efferent fibers

descending. they carry commands from the auditory central nervous system back down to the hair cells

efferent fibers include

outer radial fibers and inner spiral fibers

dual communication between afferent and efferent fibers ensures that

the brain receives sound information and cab also modulate the ear’s response

afferent cell bodies are locate din the

spiral ganglion, inside the cochlea

afferent neurons can be divided into two groups

type I neurons (inner raidal fibers) and type II neurons (outer spiral fibers)

type I nerons or inner radial fibers are

large, myelinated, and they connect to inner hair cells

type II neurons or outer spiral fibers are

small, unmyelinated, and they connect to outer hair cells

auditory nerve has

~30000 afferent neurons

on average, each inner hair cell is connected by

8 to 10 radial fibers

a single outer spiral fiber may contact

from 10 to 100 outer hair cells

multiple afferent fibers contacting one inner hair cells provides

specificity and allows us to distinguish between different sounds with high accuracy

one outer spiral fiber connecting to many outer hair cells supports

sensitivity and makes the system better at detecting low-level (soft) sounds

efferent neuron pathways originate in the

superior olivary complex, located in the brainstem

through the olivocochlear bundle, efferent neurons send

signals to either inner or outer hair cells

efferent neuron feedback system helps

regulate and fine-tune hearing

efferent neurons are divided into two groups

medial olivocochoear fibers (outer radial fibers) and lateral olivocohclear fibers (inner spiral fibers)

medial olivocochlear fibers (outer radial fibers)

terminate directly on the outer hair cells and arise from the medial nucleus of the superior olivary complex

lateral olivocochlear fibers (inner spiral fibers)

terminate on the afferent neurons that contact the inner hair cells

auditory nerve codes for intensity or loudness by

changing the rate of discharge (the number of spikes per second increases as sound gets louder)

most neurons saturate / stop increasing their firing rate

about 20 to 60 decibles above their threshold

tonotopy

along the basilar membrane, different locations response maximally to different frequencies

tuning curve

recording the discharge rate of an auditory nerve fiber at different sound frequencies

each fiber has a characteristic frequency

the frequency at which it responds at the lowst sound level

place coding ensures that

frequency is represented by where the neural spikes originate along the cochlea

phase locking

for frequencies below about 4 kilohertz, auditory neurons can synchronize their firing to a particular phase of the sound wave

volley theory

groups of neighboring neurons work together to overcome single neuron limitation to encode timing information and fires at a particular phase of the waveform, but together they represent the overall frequency.

volley theory strategy allows

auditory system to encode frequencies higher than what individual neurons could represent on their own

sound causes movement of the

stereocilia

movement of the stereocilia triggers

neurotransmitter release at the base of the hair cells

neurotransmitter release at the base of the hair cells generates

a graded potential in the auditory nerve fiber

graded potential in the auditory nerve fiber triggers

all-or-none action potential

action potential travels

along the axon and is passed on to the next neuron

inner hair cells provide

specificity

outer hair cells provide

sensitivity

sound intensity is encoded by

recruiting fibers with different thresholds

sound frequency is encoded both by

timing between spikes and the characteristic frequency of the fibers

central auditory system relies on

neural networks

neural networks

groups of neurons working together to process sound

higher up in the central auditory system, neurons show

increased specialization, responding to more complex sound features

plasticity

capable of reorganizing and adapting based on experience and learning

plasticity is critical for

speech and language development

central nervous system contrains several auditory nuclei

cochlear nucleus, superior olivary complex, lateral lemniscus, inferior colliculus, and medial geniculate nucleus.

nucleus

a collection of neuron cell bodies connected by projections

ascending pathway

cochlea, cochlear nuclear, superior olivary complex, lateral lemniscus, inferior colliculus, medial geniculate body in the thalamus, and finally the auditory cortex

brainstem contains several important auditory n uclei

superior olivary complex, lateral lemniscus, and inferior colliculus

important feature of the ascending central auditory system

follows both contralateral and ipsilateral pathways (sound information from each ear goes to both sides of the brain)

the first nucleus to receive binaural input

superior olivary complex

binaural input is critical for

sound localization

binaural input integration ensures

both ears contribute to our sense of hearing space

ascending auditory pathway includes several key nuclei

cochlear nucleus, superior olivary complex, inferior colliculus, and the medial geniculate

the cochlear nucleus

the first place where sound information is delivered bilaterally

inferior colliculus

information integration center

medial geniculate body

primary sensory input to auditory cortex

auditory perception

speech understanding and sound recognition