lect 5 nerve impulse part

8/3/2019 Lect 5 Nerve Impulse Part

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The nerve

impulse

SMS1084

Dr. Mohanad R. Alwan



stimulus

The passage of an impulse



stimulus


+ + + + + -

+ + + + + -

- - - - - +

- - - - - +



stimulus


+ + + + + -

+ + + + + -

- - - - - +

- - - - - +

Na+

Na+



stimulus


+ + + + + -

+ + + + + -

- - - - - +

- - - - - +

Na+

Na+

local electricalcircuit



The all or nothing law

An AP can only be generated if the stimulusreaches a certain threshold intensity.

Below this threshold, no AP can be created Once the threshold level is reached, the size

of an impulse is independent of the stimulus

So, a greater stimulus does not give a greateraction potential.



successive stimuli

increasingintensity of

stimulation



successive stimuli


stimulation

threshold intensity



successive stimuli


stimulation

threshold intensity

below threshold intensity:no action potentials



successive stimuli


stimulation


threshold intensity



successive stimuli


stimulation


threshold intensity

action potentials generated



Speed of transmission

In myelinated neuronesspeed of transmissionis up to 100 metres per

millisecond. In unmyelinated

neurones it is muchslower at about

2 m ms-1.



Speed of transmission

Myelin speeds up the speedof the impulse by insulatingthe axon.

Myelin is fatty and does not

allow Na+

or K+

to passthrough it.

So depolarisation (and APs)can only occur at the nodesof Ranvier.

So the AP „jumps‟ from onenode to the next.

This is known as salatoryconduction.



Advantages Increase speed of

transmission 100 fold.

Conserve energy as

sodium-potassiumpump only has tooperate at the nodesand fewer ions have to

be transported

Nerve fibres growing through

cylindrical Schwann cell formation .

Salatory conduction



axon

myelin sheath



axon

myelin sheathdirection of impulse



axon


+ -

+ -

+

+

-

-



axon


+ -

+ -

+

+

-

-

polarised depolarised



axon


+ -

+ -

+

+

-

-

polarised depolarisedlocal circuit



Cont.

Any thing that affects the rate of respiration,such as temperature, will affect thetransmission rate in a nerve.

This is because the restoration of the restingpotential is an energy-requiring processrelying upon ATP



Axon diameter

The thicker the axon,the faster the rate oftransmission.

Probably due to thegreater surface area ofthe membrane over

which ion exchangecan occur



Axon diameter

Giant axons found insome invertebrates(earthworms, marine

annelids) are thought tobe associated withrapid escape responses



The Threshold

–55mV represents the threshold potential

Beyond this we get a full action potential The membrane potential rises to +35mV

this is the peak of the action potential

The cells are almost at the equilibrium(balance) for Na+ ions.



Three states of a neuron

Resting potential The state during which no nerve impulse is being

conducted although the neuron is capable of doingso

Action potential The state during which the neuron is actively involved

in conducting a nerve impulse

Recovery/Refractory potential

The state during which the neuron is unable toconduct a nerve impulse since the neuron must“recover” following the last nerve impulse



1. Resting Potential

The state of the neuron when no nerve impulse is beingconducted

During resting potential there is an ion displacement betweenthe inside and the outside of the neuron (i.e. on either side ofthe neuron cell membrane) as follows:

There are more Na+ ions on the outside than on the inside

There are more K+ ions on the inside than on the outside

There are many large organic anions (-ve charged ions)locked inside since they are too big to pass through theneuron‟s cell membrane



Due to this difference in ion displacement there is a NETCHARGE difference across the cell membrane = MembranePotential.

This membrane potential when the neuron is at rest is calledthe Resting Potential=-70mv.

This difference in ion displacement and thus the restingpotential is largely maintained by a protein channel called theNa+/K+ PUMP



Na+/K+ PUMP

Powered by ATP this pump actively “pumps” Na+ ions out of the cell and K+ ions into the cell.

As a result of this active transport, the cytoplasm of the neuron

contains more K+ ions and fewer Na+ ions than thesurrounding medium.

The cell membrane also has 2 other separate protein channels,one that “leaks” K+ ions and one that “leaks” Na+ ions down

their electrochemical gradient (combo of concentration andelectrical).



Na+/K+ PUMP

There are more K+ channels than Na+ channels whichmeans more K+ ions leak out of the cell as opposed toNa+ leaking into the cell.

As a result, K+ ions leak out of the cell to produce a

negative charge on the inside of the membrane.

This charge difference is known as the resting potential ofthe neuron.



At rest, the inside of a

neuron's membrane hasa negative charge.

As the figure shows, aNa+ / K+ pump in the cell

membrane pumpssodium out of the celland potassium into it.

As a result, the inside ofthe membrane builds upa net negative chargerelative to the outside.



-70

-55

0

+35

Threshold

mV

Time

Resting potential Action potential

Na+ channels close

and K+ channels

open, K+ floods out

of neurone

Resting potential



Potassium takes over

After Na+ moves in passively until the Na+ channels start to close

At the same time K+ permeability increases as

voltage-gated K

+

channels open – they are abit slower to respond to the depolarisation thanthe Na+ channels

The K+ ions move out

This makes the cell negative inside with respectto outside again

The membrane potential falls



Action Potential An action potential occurs when a neuron is conducting a

nerve impulse In order for an action potential to occur, the neuron must

receive sufficient stimulation to open enough Na gates toreach the threshold level.

If sufficient Na gates are opened to reach the threshold level,other Na and K gates will be stimulated to open.

This results in a self-propagating wave of action potentialsand Na and K gates opening along the entire length of theneuron and an action potential and nerve impulse occur

Since an action potential will only occur if the membranethreshold level is reached, an action potential can also be

described as an all or none response.

Action potential can be divided into 2 phases:depolarization & repolarization



Depolarization (upswing) If a neuron received sufficient stimulation to reach the

membrane threshold, successive Na gates along the entireneuron membrane will open

The opening of the Na gates allows Na ions to move into the

neuron

The movement of Na ions into the neruon causes themembrane potential to change from -70mV to +40mV

As the membrane potential becomes more positive, Na

gates begin to close.

At the end of depolarization, the Na gates are all closed.



Repolarization (Down-swing)

At the end of the depolarization phase, K gates begin toopen, allowing K to leave the neuron.

These K gates are activated at the +ve membrane potentialvalue of about +40mV.

The movement of K ions out of the neuron produces achange in membrane potential such that the potentialbecomes more –ve.

Following repolarization, the K gates close slowly



During the conduction of a nerve impulse, each

successive section of a neuron’s membrane willundergo an action potential consisting of

depolarization followed by repolarization

Thus the nerve impulse is the movement of the

action potential along the neuron cell

membrane

R /R f t P t ti l



Recovery/Refractory Potential

Immediately following an action potential, a neuronis unable to conduct a nerve impulse until it has

recovered because its Na gates won’t open

A neuron which is undergoing recovery is said to be

refractory since it cannot conduct a nerve impulse.



During the recovery phase the following events are

occurring:1. The K gates are closing

2. The Na/K pump is returning the Na ions to the outside and Kions to the inside of the neuron

3. The membrane potential is returning to its resting value of -70mV

Once the recovery phase is complete, the neuron is nolonger in its refractory period and is ready to conduct another

nerve impulse.



Action Potential Stages: Overview

Figure 8-9: The action potential



The refractory period

The membrane starts to recover and thepotassium channels open

Even before it is completely repolarised anAP can occur if the stimulus is more intensethan the normal threshold level

This period is known as the relative

refractory period and lasts about 5 ms.




The refractory periodmeans that impulsescan only travel one way

down the axon as theregion behind theimpulse can not bedepolarised.




It also limits thefrequency at whichsuccessive impulses

can pass along theaxon




Following the passage of an AP, there is atime delay before the next one can pass

This is called the refractory period

During this time sodium channels in themembrane are closed, preventing the inwardmovement of Na+ ions

This is known as the absolute refractoryperiod (about 1 ms).



n e u r o n e

e x c

i t a b i l i t y

0 1 2 3 4 5 6 7 8time / ms



n e u r o n e

e x c

i t a b i l i t y

0 1 2 3 4 5 6 7 8time / ms

restingexcitability



n e u r o n e

e x c

i t a b i l i t y

0 1 2 3 4 5 6 7 8time / ms

restingexcitability

stimulus



n e u r o n e

e x c

i t a b i l i t y

0 1 2 3 4 5 6 7 8time / ms

restingexcitability

stimulus

absoluterefractoryperiod



n e u r o n e

e x c

i t a b i l i t y

0 1 2 3 4 5 6 7 8time / ms

restingexcitability

stimulus




n e u r o n e

e x c

i t a b i l i t y

0 1 2 3 4 5 6 7 8time / ms

restingexcitability

stimulus


normalrestingexcitability



n e u r o n e

e x c

i t a b i l i t y

0 1 2 3 4 5 6 7 8time / ms

restingexcitability

stimulus


relative refractory periodnormalrestingexcitability

refractory period

lect 5 nerve impulse part

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