Lecture 2 Introduction & General Features of Nervous System

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INTRODUCTION & GENERALFEATURES OF NERVOUS SYSTEM

2nd Lecture, 18th October, 2008

. .Azeem

Department of Physiology,Ummal Qura University,Makkah, Saudi Arabia

Updated October, 2008


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Electrotonic Potential

It is the passive flow of current without the

generation of Action Potential. It decays with distance traveled by the

potential along the membrane.


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PASSIVECURRENT

It decays with

to the

resistances

(longitudinal &transverse) in

the membrane


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Ionic Conductance & Potential

Movement of ions across the neuronal

membrane is ionic conductance.

neuronal membrane is ionic current.

Conductance of ions represented by gis responsible for membrane potential E.

For example K conductance is gk andpotential it will produce is Ek.


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Single Ion Conductance & Potential


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MembranePermeability

Versus

em rane

Potential


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Ionic conductance and ionic current

across neuronal membrane


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Generation of Action Potential


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Phases in Action Potential Generation

Resting Membrane Potential (In-active Membrane)

Stimulation (Must be Threshold Stimulus)

Threshold Level (-60mV)

Opening of Fast Sodium ChannelsVolta e Or Li and Gated Channels

Tremendous Sodium influx (Inward Current)

Rapid Depolarization (Reversal & Spike Potential)

Potassium Eflux (Outward Current)

Rapid Repolarization

(Restoration towards Resting Membrane Potential)

Hyperpolarization (+ve After Potential)

Restoration of Resting Membrane Potential (Na+/K+ Pump)


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Threshold

= Change

P = Permeability= o ass um

V= Voltage

Na = Sodium

Negative feed backprevent generation of

Action potential

Positive feed back

generates Action

potential

Threshold


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Propagation of Action Potential

Cyclic regeneration of Action potential along thewhole length of axonal or nerve fiber membrane.

This process resembles the burning of cigarette. The membrane potential travels a distance and

decay due to resistances in the membrane.

During decay the potential initiates ionicpermeability.

This permeability generates potential that againtravels a distance.

And this cycle continues till the Action Potentialreach the end of axon terminal.


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Cyclic Generation Of Action Potential

A

V1 -B

- P-C

- V2

A

V3 -B

- P-C

- V4

Repeatedly

Continues

Fig. 2.4 The three components of action potential porpagation. A depolarization V at one point of the fibre

results, through local current flow (A), in a smaller depolarization V1 some way down the axon. This triggersoff (B) a permeability change P in the membrane which in turn (C) produces a voltage V2 which is larger than

V1. The whole sequence is repeated indefinitely V3, V4, etc.The process A and C are common to all cells.But the conversion of voltage to permeability changes is found only in the nerve & muscle.

This diagram belongs to the book Neurophysiology by Carpenter.


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SYNAPSE

1. Electron microscopic

structural view

2. Model with Channels

for ionic movements & current

@from neurophysiology by Carpenter


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SYNAPSE & Phases of Synaptic

Transmission Ajunction between two neurons.

Arrival ofimpulse

Opening ofCa++ Channels at presynaptic

membrane

Vesicularexocytosis

Transmitter release in synaptic cleft.

Destroyed by choline-estrase, diffused invicinity

Interact with postsynaptic membrane receptors


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Why Chemical transmission is required?Where Electrical transmission can occur?

What are Voltage gated & ligand gated

Channels?


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Synaptic Transmitters

More than 40 important transmitter substances have

been discovered thus far.

Some of the best known are:

Acetylcholine,

Norepinephrine,

Epinephrine,

Histamine,

Gamma-aminobutyric acid (GABA),

Glycine ,

Serotonin, and

Glutamate.


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Central Nervous System Synapses &

synaptic Functions Succession of neurons, one after another in CNS

transmits impulses.

However, in addition, each impulse;

(1) may be blocked in its transmission from one neuron tothe next

(2) may be changed from a single impulse into repetitive

impulses

(3) may be integrated with impulses from other neurons to

cause highly intricate patterns of impulses in successive

neurons.

All these functions can be classified as synaptic

functions of neurons.


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Excitatory Post Synaptic Potentials

(EPSP) Release of excitatory neurotransmitter

Responsible for depolarization & repolarization of postsynaptic membrane, due to ionic flux of Sodium followedby potassium.

Excitatory Post Synaptic Potential may occur either of thefollowing three ways.

Opening of sodium channels

Depressed conduction through chlorideor potassium channels, or both.

Various changes in the internalmetabolism of the postsynaptic neuron to excite cell activity

to increase the number of excitatory membrane receptors

decrease the number of inhibitory membrane receptors.


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EPSP


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Inhibitory Post Synaptic Potentials(IPSP)

Release of inhibitory neurotransmitter

Hyper-polarization of the membrane due to ionic

flux of K and then restoration at resting potential.

Inhibitory Post Snaptic Potential may occur either.

Opening of chloride ion channels through the

postsynaptic neuronal membrane.

Increase in conductance of potassium ions out of

the neuron.

Activation of receptor enzymes

increase the number of inhibitory synaptic receptors or

decrease the number of excitatory receptors.


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IPSP


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"Second Messenger" System in thePostsynaptic Neuron The process of memory-require prolonged changes in neurons

for seconds to months after the initial transmitter substance isgone.

The ion channels are not suitable for causing prolonged

postsynaptic neuronal changes because these channels closewithin milliseconds after the transmitter substance is no lon erpresent.

However, in many instances, prolonged postsynaptic neuronalexcitation or inhibition is achieved by activating a "secondmessenger" chemical system inside the postsynaptic neuronalcell itself, and then it is the second messenger that causes the

prolonged effect. There are several types of second messengersystems.

One of the most common types uses a group of proteins calledG-proteins.


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"Second messenger" system by which a transmitter substance from an initial neuron canactivate a second neuron by first releasing a "G-protein" into the second neuron'scytoplasm. Four subsequent possible effects of the G-protein are shown, including 1,

opening an ion channel in the membrane of the second neuron; 2, activating an enzymesystem in the neuron's membrane; 3, activating an intracellular enzyme system; and/or 4,causing gene transcription in the second neuron


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Feed back Inhibition

Action

Discharge--!!!!!!!

Excitation

EfferentDischarge

--!!!!!!!

Excitation

Inhibition

Inhibition


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Presynaptic inhibition is caused by release of an inhibitorysubstance at the presynaptic nerve fibrils.

In most instances, the inhibitory transmitter substance is GABA(gamma-aminobutyric acid).

Large numbers of chloride ions diffuse into the terminal fibril.

The synaptic transmission inhibits because they cancel much ofthe excitatory effect of the positively charged sodium ions that

PRESYNAPTIC INHIBITION

also enter the terminal fibrils when an action potential arrives.

Presynaptic inhibition occurs in many of the sensory pathwaysin the nervous system.

Presynaptic Terminal

Inhibition at

Post Synaptic Neuron

Small Presynaptic Inhibitory terminal

Gaba

Na+

Cl-

Release of Inhibitory transmitter

Excitation of

Presynaptic Terminal

Sodium influx in post synaptic neuron

Cl- influx in

Post synaptic neuron


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POST SYNAPTIC POTENTIALS

Time course and summation of postsynaptic

potentials

Spatial summation in neurons TheThreshold for firing

Temporal Summation Simultaneous summation of Inhibitory andExcitatory Postsynaptic Potentials

Facilitation of Neurons


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Activation of multiple presynaptic terminals for the

Summation of post synaptic potentials on widely

spaced areas of the neuronal membrane is called

Spatial summation.

Spatial Summation

Temporal Summation

Activation of a single presynaptic terminal for successive

& rapid discharges that "summate." to excite a post

synaptic neuron is called Temporal summation.

Often the summated postsynaptic potential is excitatory

but has not risen high enough to reach the threshold for

firing by the postsynaptic neuron.

When this happens, the neuron is said to be Facilitated.

Facilitation


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Relation of State of Excitation of theNeuron to the Rate of Firing

Excitatory State

What is the mechanism for translatin increasedexcitatory state into increased firing rate of aneuron?

The rate of firing of a neuron is determined byhow much its excitatory state is abovethreshold


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SOME SPECIAL CHARACTERISTICS OFSYNAPTIC TRANSMISSION

Fatigue of synaptic transmission

Post tetanic facilitationEffect of acidosis and alkalosis

Effect of hypoxia

Effect of drugs

Synaptic delay


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CHEMICAL SUBSTANCES THATFUNCTION AS SYNAPTIC

TRANSMITTERS

Small-molecule, rapidly acting transmittersRecycling of the small-molecule types of vesicles.

Characteristics of some of the more important small-moleculetransmitters

Neuro-peptides

Usually only a single type of small molecule transmitter isreleased by each type of neuron

After a transmitter substance is released at a synapse, itmust be removed

Electrical Events During Neuronal excitation


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ThanksTHREE ADVICES FROM YOUR TEACHER:1. ALWAYS READ BOOKS

2. COMBINE THE CONCEPT YOU GET FROM LECTURE &

BOOKS FOR A TOPIC

3. WRITE THIS CONCEPT IN YOUR OWN WORDS

BOOKS RECOMMENDED:

1. TEXT BOOK OF MEDICAL PHYSIOLOGY BY GUYTON

2. MEDICAL PHYSIOLOGY BY GANONG

3. Neurophysiology by Carpenter

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Lecture 2 Introduction & General Features of Nervous System