the nerve impulse part 2

The nerve impulse

Part 2

Progress of an impulse

When an impulse reaches any point on the axon an action potential (AP) is generated

Small local currents occur at the leading edge of the AP

Sodium ions move across the membrane towards negatively charged regions.

This excites the next part of the axon so the AP progresses along its length

The local currents change the potential of the membrane, creating a new action potential ahead of the impulse.

stimulus

The passage of an impulse

stimulus

The passage of an impulse

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stimulus

The passage of an impulse

+ + + + + -

+ + + + + -

- - - - - +

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Na+

Na+

stimulus

The passage of an impulse

+ + + + + -

+ + + + + -

- - - - - +

- - - - - +

Na+

Na+

local electrical circuit

The all or nothing law

An AP can only be generated if the stimulus reaches 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 greater

action potential.

successive stimuli

successive stimuli

increasing intensity of stimulation

successive stimuli

increasing intensity of stimulation

threshold intensity

successive stimuli

increasing intensity of stimulation

threshold intensity

below threshold intensity: no action potentials

successive stimuli

increasing intensity of stimulation

below threshold intensity: no action potentials

threshold intensity

successive stimuli

increasing intensity of stimulation

below threshold intensity: no action potentials

threshold intensity

action potentials generated

The all or nothing law

The difference between a weak and a strong stimuli is due to the frequency of the APs

A weak stimulus gives few APs A strong stimulus gives more APs ….(and is also likely to result in APs in more

neurones)

The refractory period

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

This is called the refractory period During this time sodium channels in the

membrane are closed, preventing the inward movement of Na+ ions

This is known as the absolute refractory period (about 1 ms)

neur

one

exci

tabi

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

neur

one

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

resting excitability

neur

one

exci

tabi

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

resting excitability

stimulus

neur

one

exci

tabi

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

resting excitability

stimulus

neur

one

exci

tabi

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

resting excitability

stimulus

neur

one

exci

tabi

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

resting excitability

stimulus

absolute refractory period

neur

one

exci

tabi

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

resting excitability

stimulus

absolute refractory period

neur

one

exci

tabi

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

resting excitability

stimulus

absolute refractory period

normal resting excitability

neur

one

exci

tabi

lity

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

resting excitability

stimulus

absolute refractory period

relative refractory periodnormal resting excitability

neur

one

exci

tabi

lity

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

resting excitability

stimulus

absolute refractory period

relative refractory periodnormal resting excitability

refractory period

The refractory period

The membrane starts to recover and the potassium channels open

Even before it is completely repolarised an AP can occur if the stimulus is more intense than the normal threshold level

This period is known as the relative refractory period and lasts about 5 ms.

The refractory period

The refractory period means that impulses can only travel one way down the axon as the region behind the impulse can not be depolarised.

The refractory period

It also limits the frequency at which successive impulses can pass along the axon

Speed of transmission

In myelinated neurones speed of transmission is up to 100 metres per millisecond.

In unmyelinated neurones it is much slower at about

2 m ms-1.

Speed of transmission Myelin speeds up the speed

of the impulse by insulating the axon.

Myelin is fatty and does not allow Na+ or K+ to pass through it.

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

So the AP ‘jumps’ from one node to the next.

This is known as salatory conduction.

Salatory conduction

Advantages Increase speed of

transmission 100 fold. Conserve energy as

sodium-potassium pump only has to operate at the nodes and fewer ions have to be transported

Nerve fibres growing through cylindrical Schwann cell formation.

axon

myelin sheath

axon

myelin sheathdirection of impulse

axon

myelin sheathdirection of impulse

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+ -

+

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axon

myelin sheathdirection of impulse

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+ -

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polarised depolarised

axon

myelin sheathdirection of impulse

+ -

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polarised depolarisedlocal circuit

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

This is because the restoration of the resting potential is an energy-requiring process relying upon ATP

Axon diameter

The thicker the axon, the faster the rate of transmission.

Probably due to the greater surface area of the membrane over which ion exchange can occur

Axon diameter

Giant axons found in some invertebrates (earthworms, marine annelids) are thought to be associated with rapid escape responses