Neuron Physiology

Nerve

Parts Of A Neuron

Cell body (soma) Axon Dendrites

Neurons

Axon of anotherneuron

Axon of anotherneuron

Cell BodyCell BodyDendritesDendrites

AxonAxon

MyelinSheathMyelinSheath

Dendrites of another neuron

Dendrites of another neuron

Cell Body

Soma or prikariyon Nucleus and cytoplamic organelles Nissels granules = RER Cytoskeletal proteins like

neurofilaments ,microtubules , actin microfilaments

Nucleus contain one or two nucleolus ,and no centriole

Terminally differentiated cells

Contd

Soma extend into dendrites on one end and axon on the other

Membrane called plasmalemma

Dendrites

Numerous short extensions from cell body

Increasing surface area Dendritic spines Receive incoming signals and

transmits them Helps in protein synthesis Helps in generating ACTION

POTENTIAL

Axon

Axon fibers form the nerve Long cylindrical process extends away from

cell body Transmits output signals (Action Potential ) Contains organelles , cytoskeleton, axoplasm Axon hillock - initial segment portion of

neuron Axon terminal – axon divides into several

processes and has synaptic knobs Buttons contains neurotransmitters

Myelination

It’s a protective insulator covering of the axon

Formed by schwann cells Double layer membrane of a single schwann

cell wraps itself several times around axon 1 mm in length , 8 -10 micrometer in

thickness Layers stick to each other due to protein p0 Schwann cell nucleus lies in the outermost

layer

Contd

Nodes of Ranvier Internodal distance is 1-2mm and each

node is 0.5-1 micrometer

Functions Of Myelination

Increases speed of conduction It reduces energy expenditure by the

cell Provides protective covering of the

axon

Types Of Axon

Myleniated Axons Unmyelinated Axons

Timing Of Myelination

Different neurons in body shows diff time of myelination

Starts early as 4th month of fetal life and completes by 2 years

Axoplasmic Transport

Transfer of substances between cell body and axon terminal

For the development of neuron Bi-directional transport

Types Of Axoplasmic Transport

Anterograde transport Retrograde transport Transneuronal transport

Anterograde Transport

From ell body to axon terminal Mainly for neurotransmitters Two types – fast and slow Fast – 400mm/day by kinesin Slow- 0.5 – 2 mm/day Slow transport helps in nerve repair

Retrograde Transport

From axon terminal to cell body 200mm /day done by dynin Keeps soma informed about synaptic

environment Transports viruses such as varicella

Zoster , rabies Transport toxins Transports nerve growth factor

Neuronal Ion Channels

Voltage gated Na channels are mainly concentrated on nodes of ranvier

Voltage gated Ca channels are mainly present at the axon terminal for neurotransmitter secretion

Classification Of Nerve Fibers

Based on function Myelination Erlanger and gasser classification

Erlanger And Gasser Classification

Applied Aspects

Demyelinating diseases 1) multiple

sclerosis 2) Gullian barre

syndrome

Nerve Injury

Wallerian degeneration

Grades Of Nerve Injury

5 grades 1st-transient loss of function 2nd- severe injury due to prolonged

pressure, intact endoneurium complete recovery but delayed

3rd- severe damage to endoneurial sheath

4th- disruption of nerve fasciculi 5th- complete transection

Wallerian Degenration

Factors Promoting Nerve Growth

Neurotrophins Other growth factors ( FGF,IGF-1,

PDGF , CNTF,GDNF)

Neurotrophins

Nerve growth factor Brain derived neurotrophic factor Neurotrophin-3 Neurotrophin 4/5