Auditory Nerve

Two parts (1) Cochlear portion (also called auditory portion)

Pars Inferior (includes Saccule)(2) Vestibular portion

Pars Superior

Nerve supply to inner ear - VIIIth

Nerve supply to inner ear - VIIIth

The inner ear: nerve supply

The brain has both ascending (afferent pathway) and descending (efferent pathway) tracts

The inner ear is innervated by the VIIIth cranial nerve (the acoustic nerve)

The VIIIth nerve has two major divisions, one vestibular and the other auditory (both are afferent)

There is also a small efferent bundle

THE NEURON

• A nerve cell with all of its processes• Components of neurons – Cell body and neurites:

– Cell body• Contains the nucleus• The “brain” of the cell

– Neurites: Dendritic surface• Analogous to a sensory surface – many branches• The “input” side of a neuron

– Neurites: Axonal surface• Analogous to a cable carrying information (chemical

or electrical) away from the cell body• The “output” side of the neuron

Construction of Neuron

Construction of Neuron

Construction of Neurons

Types of Neurons

Pyramidal Cell

Pyramidal Cell “Forest”

Myelin

Dendritic Surface

Axon

Neurons in Hippocampus

Cell Body

Nerve cellsGlial (support) cells

THE NEURON

• The Cell Body – contents and function– Nucleus: Stores genes (in mature neurons the

chromosomes do not replicate)– Cytoplasm – Contains the axon hillock, an important

component to the generation of action potentials• Golgi complex – maintenance of intracellular

reactions and cell membrane• Mitochondria – possess enzymes for metabolic activity

that generates energy for the cell• Microfilaments and microtubules – transport of

enzymes and nutrients throughout the cell

THE NEURON

• The Cell Membrane (Plasma Membrane)– The cell wall (or external cell boundary)– Maintains separation between intra- and extra-cellular

fluids – Lined with the cell coat, which facilitates ion transport– Semipermeable membrane that allows some ions to pass

while restricting the flow of others– While at rest, membrane allows potassium (K+) pass

through (permeate) the membrane– Restricts the flow of (Na+)– Prepares the neuron for an action potential

THE NEURON

• The Cell Processes (neurites)– Dendrites

• Profuse branching• Short processes• Dendritic spines attached to process• Considered extensions of cell body (increase surface

area• Conduct impulses TOWARD the cell body

THE NEURON

• The Cell Processes (neurites)– Axon

• Longest cell process (from 0.1 mm to 3 m)• Usually arises from cell body at axon hillock• May have some collateral branching• Distal ends (axon terminals)• Diameter variable – relation to transmission speed• Encased in axolemma• Usually conducts impulses AWAY from the cell body

THE NEURON

• The Cell Processes (neurites)– Initial segment (axon hillock)

• Most excitable part of the axon• Initiates action potential

• Synapse– Allows for functional interneuronal

communication– Unidirectional “communication”– Chemical or electrical (chemical most common)

The Synapse

• Types– Axodendritic is most common– Axosomatic– Axoaxonic

• Cleft– Space between cells that allows for transport of

neurotransmitter/neuromediators– Each side of the cleft consists of a presynaptic

or postsynaptic membrane

Synaptic Cleft

http://www.youtube.com/watch?v=HXx9qlJetSU

Synaptic Cleft

THE NEURON

• The Resting potentials– Neurons carry negative intracellular potentials when

they are at rest– The resting potential is approximately -80mV– This means that the electrical charge inside the cell is

far more negative than the surrounding (or extracellular) electrical charge

– When a structure has an electrical charge that is 0 V it is said to be neutral

– In the CNS, extracellular fluid is either near neutral or positive

THE NEURON

• Cellular potentials– A voltage is a potential to do work– The greater the difference between positive and negative

poles, or charges, the more powerful the flow of ions– Think of batteries with pos. and neg. poles– When voltage changes TOWARD zero, it is

depolarization (excitatory)– When voltage changes AWAY FROM zero,

hyperpolarization (inhibitory)– V (extracellular) – V (intracellular) = Battery Voltage

THE NEURON – potentials

• EPSP and IPSP– Excitatory or inhibitory post-synaptic potential– Release of Neurotransmitter across synapse

produces excitatory (depolarizing effect)• Main agent is ACh (Acetylcholine) which facilitates

many muscle movements

– Or inhibitory (hyperpolarizing effect)• Main agent is gamma-amniobutyric acid (GABA)

which is important in sensory systems and for fine muscle control

Membrane PotentialsEndolymph:

Positively-chargedFluid (Approx. +80mV)

Hair cell: Negatively-

charged Space(Approx. -70 mV)

The cell bodies of the afferent auditory neurons (nerve fibers) constitute the spiral ganglia

Reaching the rim of the osseous spiral lamina, the nerve fibers pass in bundles through a small opening called habenula perforataThe osseous spiral lamina is not solid; it is hollowed out to accommodate the nerve fibers

Sulcus = grooveLamina = thin plate or layerOsseous = boneHabenula = neighboring group of nerve cells

Organ of Corti

Limbus = a distinctive border, or edge

The inner ear: afferent nerve supply

About 30,000 afferent fibers Approx. 95% of the fibers innervate IHCs One peripheral axon runs to a single IHC with which it forms a single synapseOne IHC communicates with multiple neurons (up to 30)

The inner ear: afferent nerve supply

• Remaining 5% of the fibers synapse onto OHCs

• Each axon spiral basally after entering the organ of Corti and branch to connect about ten OHCs, generally in the same row.

Cochlear/Auditory Neuronstwo groups

Type Imyelinated fibersconnect to IHCsmake up 90-95% of auditory neuronslarge in diameter

Type II unmyelinated fibersconnect to OHCsmake up 5-10% of auditory neuronssmall in diameter

Auditory Nerve Fibers from cochlea to cochlear nucleus (first brainstem nucleus)

The inner ear: efferent nerve supply

About 1,500-3,000 fibers

Innervation is provided primarily at the base of the cell (at OHCs) and the afferent nerve ending (at IHCs)

The inner ear: efferent nerve supply

OHC

IHC

Roughly 70% innervate OHCs

w/ an excitatory effect (electromotility)

The remaining 30% innervate IHCs w/ an inhibitory effectSynapse directly onto afferent nerve endings

• Membrane potential = the voltage difference (or electrical potential difference) between the interior and exterior of a cell

• depolarization = a decrease in the cell’s membrane potential (typically occurs during stimulation)

• Receptor potential = an initial response of a receptor cell to a stimulus, consisting of a change in voltage across the receptor membrane related to the stimulus strength

• In other words a receptor potential is a depolarizing event resulting from current flow into the cell

• A stimulus causes the cell potential to move toward 0 mV, i.e., the depolarization

• A stimulus of an adequate magnitude is needed for transduction

Summary of HC transduction

Depolarization causes the release of neurotransmitter (glutamate) which depolarizes the afferent terminal of the AN fibers.

Hyperpolarization of the IHC reduces the spontaneous release of neurotransmitter.

Mechano-electric transduction of IHC

The bending of cilia allows K+ to flow into the IHC

The IHC is depolarized

Voltage-gated Ca2+ channels are open

Neurotransmitter (glutamate) is released

OHC contraction enhances the motion, IHC stereocilia are bent

A bioelectric signal is sent via the type I afferent nerve fiber

Membrane Potentialshttp://www.youtube.com/watch?v=32aeRcWkLS8

http://www.youtube.com/watch?v=90cj4NX87Yk&NR=1

THE NEURON

• Action potential– Linked to cellular excitation– Neuron may be stimulated electrically, mechanically, or

chemically– The important change involves an increase in the cell’s

permeability to Na+ ions– Influx of Na+ ions depolarizes (excites) neuron– Wave of permeability to Na+ spreads across cell

membrane and neurites from its origin at the axon hillock

– Quickly passes, followed by reduced permeability

Action Potential

1

2

3

4

time

Intr

acel

lula

r P

oten

tial (

mV

)

-80mV

0mV

Trigger Potential of neuron (1)All-or-none response (2)

Absolute refractory period (3)Relative refractory period (4)

Action Potential

• Trigger Potential– Slight variability across neurons– Approx. -40mV

• Amplitude (all or none)– Change in voltage caused by passing of ions through

‘gates’ or ‘channels’ in the cell walls– When unstimulated (at resting potential), K gates more

permeable– When stimulated/depolarized, K gates small, Na gates

large (briefly)– Switchover occurs during absolute refractory period

• Refractory Periods

http://www.blackwellpublishing.com/matthews/channel.html

Action Potential: http://www.youtube.com/watch?v=90cj4NX87Yk&NR=1

Saltatory conduction

http://www.blackwellpublishing.com/matthews/actionp.html

http://www.brainviews.com/abFiles/AniSalt.htm

All-or-none lawIf an action potential occurs:

it occurs at the full capacity of the neuron and the size of the spike is always the same;

it travels the full length of the axon;

the speed the spike travels is always the same.

Examples of cochlear potentialsWhen an acoustic stimulus is presented, it evokes a change in the electric current flowing through the hair cells. There are three sound-evoked cochlear potentials that can be measured within or near the cochlea (e.g., RW, promontory):Cochlear Microphonic (CM); the Summating Potential (SP); the Compound Action Potential (CAP) that derives from synchrony of afferent responses at the beginning of a stimulus.

Bar Graph of a Neuron’s Firing Rate (PST Histogram)

P/CNS Structures

• Nerves/tracts/nuclei/ganglia– Ascending and descending pathways– Afferent vs. efferent (sensory vs. motor)– Central vs. peripheral

• Brainstem– Medulla– Pons– Midbrain– Cranial Nerves and nuclei

CNS Structures

• Cerebellum– Cerebellar hemispheres– Vernix

• Cerebrum – Divisions– Telencephalon (hemispheres)– Diencephalon (“in-between brain”)

PNS Structures

• Nerves – Construction– Fibers Endoneurium Perineurium Epineurium

– Collection of Axons in PNS– Cranial Nerves – 12 pairs that exit brain through

openings in the skull (foramen)• May be sensory, motor, or mixed• Several are of relevance in CDIS (you should know AT LEAST

V, VII, VIII, IX, X, XII)

– Spinal Nerves – 31 pairs that exit the spinal cord through openings in the vertebra

• Mixed nerves with a sensory branch and a motor branch

PNS Structures

• Ganglia– Collection of cell bodies in the PNS– Generally consist of unipolar neurons

surrounded by the epineurium and perineurium of peripheral nerves

– Appear as swellings, on a nerve’s course

CNS Structures

• Nucleus– Collection of cell bodies in the CNS– “Relay stations” or processing centers in

which neural activity is summed, inhibited, etc.

– Many CNS structures we know by name (thalamus, for example) are nuclei

– May also integrate neural activity from across several sources, or modalities

CNS Structures

• Tract– Collection of axons in the CNS– Pipelines, or conduits through with neural

activity migrates from one place to another– Connect different areas of brain that may be

associated with different functions