Lecture 8 VIII Nerve

8/3/2019 Lecture 8 VIII Nerve

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The VIII Cranial Nerve

(also known the Vestibulocochlear Nerve)-Made up of the Auditory branch from the

cochlea and the Vestibular branch from

the SSCs and Otolith organs



STRUCTURE



Nerve vs. Neuron

A neuron is one single cell

A nerve is a collection of neurons

Neuron

Nerve



VIII nerve

Links peripheral (outer, middle and inner ear)and central auditory system (brain and spinalcord)

Tonotopic (by frequency) arrangement of fibers from hair cells in the cochlea

Two types of afferent (ascending) fibers ± Type I: 85-95 % - connected to IHC- many fibers

connect to one IHC (many-to-one)

± Type II: 5-15 % - connected to OHC ± one fiber connects to many OHC (one-to-many)

Between 40,000-50,000 neurons



Types of Neurons Type I Radial Afferent

neurons

± Fewer IHC¶s than OHC¶s (3

rows vs. 1 row)

Type I neurons, however, are themost common in the auditory

system

± Many Type I afferent neurons

attach to 1 IHC (10-20 on each

IHC)

Type II Spiral Afferent

neurons

± Connect one-to-many to OHC¶s

from Donald Sinex USU

Spiral Ganglion



Afferent Pathway

Type I and Type II Neurons Type I Radial±many neuronsconnect to one IHC

± 85-95% of afferentneurons are radial

Type II Spiral ±one fiber connectsto many OHCs ± 5-15% of afferents

are spiral



Tonotopicity of Auditory Nerve

Auditory nerve ismade up of manyneurons comingfrom the hair cells

Neurons comingfrom the base of the cochlea (HF)surround theoutside of the

auditory nerve Neurons from the

apex (LF) traveldown the center of the auditory nerve



A nerve is a bundle of individual

neurons

myelin

telodendria



Neurons review

Neurons - building blocks of CNS

± Communicate information from neuron to neuron

(interneuron), organ, gland, muscle fiber (motor

neuron), or sensory receptor (sensory neuron)

Components of a neuron: dendrites, cell body,

axon, myelin, nodes of ranvier, telodendria

The dendrite is separated from the transmitting

axon by a narrow gap called a synapse Afferent neuron ± Ascending to the brain

Efferent neuron ± Descending from the brain



Neurons Review

Information is received from another cellthrough a synapse at the dendrites

Depolarization of the cell occurs

± If enough of a polarity change, impulse istransmitted along the axon to the telodendria

± Neurotransmitters are released into thesynapse and picked up by another neuron,muscle fiber, sensory organ, etc.

Input can be excitatory (passed on) or inhibitory (stopping)





Synapse



Axon and Myelin Sheath

Myelin ± a fatty substance that insulates

the axon and prevents the diffusion of ions

through the wall of the axon

Nodes of Ranvier ± electrical current

jumps from node to node rather than

diffusing along the entire length of the

membrane



Myelin, Nodes, Axon and

Bundle

A nerve is a bundle of axons

Nodes of Ranvier



Axon Na+ positively charged sodium (pumped into the cell at

rest) K+ positively charged potassium (constantly being

pushed out of the cell at rest)

Cl- chlorine with extra electron, negatively charged

Cell Wall ± some ions flow through, others cannot fit

± Always want to reach equilibrium ± Resting potential -50mV (inside the cell)

3 K+ out for every 2 Na+ ions in = overall negative charge



Depolarization



A nerve impulse travels along the axon by a combination of

electrical and chemical conduction



Nuclei

Soma - one cell body

Spiral Ganglion - a cluster of cell bodies

(collection of cell bodies)

Groups of neural cell bodies

in the nervous system are

called Spiral Ganglia

Soma

Spiral Ganglion



Efferent pathway (or

Olivocochlear S

ystem) Descendingpathway that carriesinformation from thebrainstem to thecochlea

Uncrossed pathwayfrom brainstem toipsilateral (same

side) ear Crossed pathway to

contralateral(opposite side) ear

Located in the

Superior

Olivary

Complex



FUNCTION



Afferent (Ascending) Pathway

The pathway that carries

infor mation from the

cochlea to the brain



Efferent (Descending) Pathway

Has inhibitory (opposite of excitatory) function

± Can stop neurotransmitters from HC¶s beingtransmitted to nerves

Originate in the superior olive and endprimarily on the HC¶s to modulate function

Not studied as much ± difficult to isolatedescending pathway in alive mammals

H

ypotheses of Function: ± 1) Helps with selective attention ± allows brain toinhibit/suppress/turn off certain backgroundnoises while attending to signal

± 2) Protects the ear from damage caused by

intense sounds



Tuning Curves

Frequency tuning curves of six differentfibers in the auditory nerve.

Each graph plots, across all frequencies

to which the fiber responds, i.e., the

minimum sound level required to increase

the fiber's firing rate above its

spontaneous firing level

The lowest point in the plot is the

weakest sound intensity to which the

neuron will respond

The frequency at this point is called the

neuronµs characteristic frequency Tips of afferent tuning curves

become narrower with increasing

characteristic frequency



Phase Locking Remember, the phase of a waveform

depends on the point in the cyclebetween max displacement in onedirection and max displacement inanother

Neurons as a group tend to fire at

specific points on the waveform(i.e. at a specific phase/degree) ± Only true of low frequency sounds

Why not high frequency? ± Neurons can¶t keep up

± Once neurons fire, there is a refractory period where they need to rest beforefiring again

± With high frequency sounds, the nextcycle occurs before the refractory periodis over and cannot fire precisely at the

same point in the cycle



Phase Locking

Temporal response patterns of a low-frequency axon in the auditory nerve.

The stimulus waveform is indicated beneath the histograms, which show the

phase-locked responses to a 50-ms tone pulse of 260 Hz.

Note that the spikes are all timed to the same phase of the sinusoidalstimulus.



Phase Locking Poststimulus-time (PST) histograms of auditory nervefibers

At low frequencies, neurons have enough time to rest andfire again to the next cycle

Not able to at high frequencies

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Lecture 8 VIII Nerve

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