Lec 3 Neurobio of Neuron

7/29/2019 Lec 3 Neurobio of Neuron

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Atlantic University School of MedicineNeuroscience

Neurobiology of the Neuron


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Characteristics of Neurons

Neurons are excitable cellsspecialized to transmit stimulivia nerve impulses.

They vary considerably in size

and structure.

Neurons consist of a cell

body, neurites (which extendfrom the cell body) and anaxon (a long tubular neurite).

Neurites responsible for

receiving information and

conducting it TOWARD thecell body are called dendrites.

Dendrites and axons are

often referred to as nervefibers.


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Types of Neurons


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Types of Neurons

Unipolar neurons: cell bodyhas a single neurite thatdivides a short distance fromthe cell body into two

branches.

One usually goes to aperipheral structure theother to the CNS.

This type of neuron is found inthe posterior (dorsal) rootganglion.


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Types of Neurons

Bipolar neurons: have

an elongated cell body

from which two long

dendrites extend from

each end.

This type of neuron is

found among retinalbipolar cells and the

cells of the cochlearand vestibular ganglia.


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Types of Neurons

Multipolar neurons:neurons that have anumber of neuritesarising from the cell

body. Aside from the axon, the

rest of the neurites aredendrites.

These are thepredominate type of cellin the brain and spinalcord.


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Golgi type I Neurons

Silver-stained Purkinje cells of the cerebellar cortex

Classification by Size


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Golgi type II Neurons

Silver stained section of the cerebral cortex.


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Golgi type I neurons: have a long axonthat may be 1 meter or more in length.

The axons form long fiber tracts of the

brain and spinal cord and nerve fibers ofthe peripheral nerves.

Examples ofGolgi type I neurons include

pyramidal cells of the cerebral cortex,Purkinje cells of the cerebellar cortexand motor cells of the spinal cord.


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Golgi type II neurons : have short axons

that terminate in the region of the cell ofthe neighboring cell body or is totallyabsent.

They greatly outnumber Golgi type Ineurons.

They usually have a star-shapedappearance.

These are found in the cerebral andcerebellar cortex and are ofteninhibitory in function.


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Different Types of Neurons


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Structure of the NeuronNucleus

nuclear envelope

nuclear poresCytoplasm

Nissl substance (rough ER-protein synthesis)

chromatolysis

Golgi complex

Mitochondria

Neurofibrilsneurofilaments

Microfilaments

Microtubules

Lysosomes

Centrioles

Lipofuscin (pigment material)Melanin granules

Plasma Membrane & Excitation of the Plasma Membrane


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Anterior gray column of the spinal cord


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nuclear envelope



chromatolysis

Golgi complex

Mitochondria


Microfilaments

Microtubules

Lysosomes

Centrioles




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nuclear envelope



chromatolysis

Golgi complex

Mitochondria


Microfilaments

Microtubules

Lysosomes

Centrioles




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Posterior root ganglion lipofuscin granules within sensory neurons


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Longitudinal Section Transverse section


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nuclear envelope

nuclear pores

Cytoplasm


chromatolysis

Golgi complex

Mitochondria


Microfilaments

Microtubules

Lysosomes

Centrioles




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Plasma Membrane

About 8nm thick

Composed of an inner and outer layer separatedby a middle layer of lipid (phospholipidbilayer).

Certain protein molecules lie within thephospholipid layer and span the entire width ofthe lipid layer(channels).

Carbohydrate molecules attached to the outside

of the membrane form the cell coat orglycocalyx.


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Resting potential

In the resting state (unstimulated) K+ ionsdiffuse through the plasma membrane from the

cell cytoplasm to the tissue fluid.

K+permeability is much greater than to Na+ ions

so that passive efflux of K+ is greater than theinflux of Na+.

This results in a steady potential difference of

about -80mV (inside is more negative relative

to outside).


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Action Potential

Stimulation of a nerve cell via electrical,chemical or mechanical stimulation results ina rapid change in membrane permeability toNa+ ions.

Na+ ions diffuse through the plasma membrane

into the cell cytoplasm from the tissue fluid. Results in the membrane becomingdepolarized.

This sudden influx of Na+ ions followed by the

altered polarityproduces the action potential(+40mV).

Na+ permeability ceases whereas K+increases returning the cell back to the restingstate.


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Refractory Period

Once a nerve impulse has spread over the

plasma membrane another action

potential cannot be elicited

immediately.

The duration of this non-excitable state is

called the refractory period.

S i d I hibi i


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Summation and Inhibition

The greater the strength of the initial stimulus, the largerthe initial depolarization and the greater the spread into thesurrounding areas of the plasma membrane.

If multiple excitatory stimuli are applied to the surface of aneuron then the effect can be summated.

Inhibition, orhyperpolarization, is produced by an influx ofCl- ions through the plasma membrane into the neuron.


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Ion Channels


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Ion Channels

Ion channels exist in at least two conformational states: open andclosed.

Gating involves the twisting or distortion of the various sub-

units of a channel protein producing a wider or a more narrowlumen.

Gating occurs in response to voltage change, presence of a

ligand, stretch or pressure.

I Ch l


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Ion Channels


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Nerve Cell: Axon and Dendrites

Dendrites: the short processes of the cellbody.

In various neurons finer branches have

large numbers of small projections calleddendritic spines.


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Dendritic spines on pyramidal neurons of the cerebral cortex.


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Nerve Cell: Axon and Dendrites

Axon: the name given to the longest

process of the cell body.

It arises from a small conical elevation on

the cell body, devoid of Nissle granules

called the axon hillock.

Distal ends of the terminal branches of the

axons that are often large are calledterminals.

Th A


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The Axon

The plasma membrane bounding the axon

is the axolemma. The cytoplasm of the axon is termed the

axoplasm.

Axoplasm does not possess Nisslgranules or Golgi complex.

The initial segment of the axon (first 50 to100um) after it leaves the axon hi l lockisthe most excitable part of the axon andis the site at which an action potentialalways originates.

Axon Hillock


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Axon Hillock

Longitudinal section

of a neuron from thecerebral cortex.


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Axon Transport

Anterograde transport: materials that aretransported from the cell body to theaxon terminals.

Fast anterograde transport (100 to 400mm

per day) refers to the transport ofproteinsand transmitter substances or theirprecursors.

Slow anterograde transport (0.1 to 3mm per

day) refers to the transport ofaxoplasm andmicrofilaments and microtubules.

A T t


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Axon Transport

Retrogradetransport: materials

that are transported

from the terminals to

the cell body.


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Synapses

Where two neurons come into closeproximity and functional interneuronal

communication occurs is called a

synapse. Most neurons make synaptic connections

to 1000 or more neurons and may

receive up to 10,000 connections from

other neurons.


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Synapses

Communication at a

Synapse takes place in

one direction only.

Synapses occur in a

number of forms:

AxodendriticAxosomatic

Axoaxonic

f


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Types of Synapses


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Synapses

There are two types of synapses:

chemical and electrical.

Most synapses are chemical and utilize

a neurotransmitter which passes across

the narrow space between the cells and

attaches to a protein molecule called a

receptor.


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Neurotransmitter Action


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Transmitter is released

Receptor on postsynaptic cell bind transmitter ligand

Produce an EPSP or IPSP

Ligand Gated (fast, i.e. nicotinic acetylcholine, glutamate) or

G-protein linked (slow, Dopamine)

Synapses


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Synapses

The apposedsurfaces of theterminal axon andanother neuron arecalled thepresynaptic andpostsynapticmembranes and areseparated by thesynaptic cleft (20-30nm wide).

Chemical Synapses


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Chemical Synapses

Presynapticvesicles,

mitochondria, and

occasional lysosomes

are present in the

cytoplasm close to

the presynaptic

membrane.


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Chemical Synapses

The presynaptic

terminal contains

many small

presynaptic vesicles

that contain

molecules of various

neurotransmitters or

one specificneurotransmitter.


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Chemical Synapses

Once released the

vesicles fuse with the

presynaptic

membrane and

discharge the

neurotransmitter into

the synaptic cleft by a

process calledexocytosis.


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Depending on the summation of variousinputs into the primary cell, the cell can beexcited orDEPOLARIZED, and an action

potential will be initiated at the initialsegment and travel down the axon.

If the overall effect results in aHYPERPOLARIZED cell, the neuron will

be inhibited and no nerve impulse willarise.


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I ti ti f N t itt


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Inactivation of Neurotransmitter

N d l t


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Neuromodulators

Neuromodulators are neurotransmitters either

co-released with the primary transmitter orpackaged separately in other vesicles andreleased.

They are capable ofmodulating and modifying

the activity of the postsynaptic neuron. They may enhance, prolong, inhibit or limit

the effect of the principle neurotransmitter onthe postsynaptic membrane.

Neuromodulators act through a secondmessenger system (G-proteins).

Electrical Synapses


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Electrical Synapses

Electrical synapses are gap junctions.

There is no chemical transmitter.

They are fast.

Electrical Synapses


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Electrical Synapses

The rapid spread of activity from one neuron toanother ensures that a group of neuronsperforming an identical function act together.

Electrical synapses are bidirectional, chemical

synapses are not.


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Lec 3 Neurobio of Neuron