Summary of Nerve impulses

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  • Created by: Hindleyc
  • Created on: 09-04-19 16:22
What is the speed of conduction of impulse not affected/ influenced by
Strength of stimulus
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What is depolarisation
Stimulating electrode cause the membrane potential to change a little and the voltage gated ion channels can detect this change and when the potential reaches -55 the sodium channels open for 0.5 ms
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What does this cause
Na+ to rush in making inside of cell more +ve
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what is this phase referred to as
a depolarisation since the normal voltage polarity (-ve inside) is reversed (becomes +ve inside)
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What is repolarisation
When the membrane potential reaches +35 the K channels open for 0.5ms causing K ions to rush out making the inside -ve again
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What is this called since it restores original polarity
repolarisation
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When an impulse travels along an axon what happens
Cell membrane changes permeability to Na+
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What does it occur in response to
a stimulus in a sensory neurone or the arrival of a NT chemical in a relay/motor neurone
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When a N is stimulated, the axon membrane shows a dramatic inc in permeability to
Na+
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What do specific Na+ channels do
Open up allowing more Na+ to diffuse rapidly down their conc and electrochemical gradient (into cell)
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As a result what happens
PD across the membrane is briefly reversed (+40mv) with the cell now becoming +ve on the inside (depolarisation) and it only lasts about 1 millisecond
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What is this depolarisation known as
Action potential
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At the end of this brief depolarisation what happens
the Na+ channels close again and excess Na+ are rapidly pumped out by the sodium pump using ATP
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What happens to The permeability of K+
Temporarily increased as voltage dependent K+ channels open as a result of depolarisation
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Then what happens
K+ diffuse out of the axon down their conc and electrochemical gradient attracted by the -ve charge on the outside of the membrane
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As the Na+ and K+ leave the cell what happens
The inside becomes -ve relative to the outside once again
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What does this take
A few milliseconds before the RP is restored and the axon is ready to carry another impulse
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What does hyperpolarization do
Delay in closing K+ channels too many K+ out so Na+/k+ ATPase pump restores
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During depolarisation what are the channels
Na+ open so Na moves in and K closed but leak out
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During repolarisation What are the channels
Na closed but leak out and K open so moves out
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What is the inside relative to outside during depolarisation
positive
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What is inside relative to outside during repolarisation
Negative
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For a short period after repolarisation what are both the Na+ and K+ gated channels
closed
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What happens during this period
the membrane can't be depolarised and no impulse can pass
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What is this called
The refractory period
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Define refractory period
Period in which another AP can't be fired- limit on freq of AP
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What is fast conduction essential/ideal for
Conducting information vital for survival
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Why is the refractory period important
Each AP is kept discrete with no overlapping, ensures the AP pass in 1 direction only,
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What does the membrane then do
Settle down and resting potential is achieved again
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Why must it be restored (resting potential)
Before another action potential can occur
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How long does whole event take
3ms
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What is resting potential determined mainly by and what is the action potential mainly by
K+ and Na+
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What does the AP pass along the axon as
Wave of depolarisation
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At the site of the action potential what is the outside of the axon
-ve charged
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In theory what can the AP be transmitted over, why
An infinite distance, don't lose strength and the Ap is self generating
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What can't the axon do during depolarisation and repolarisation
Fire another action potential
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What is the resting potential (normal resting state)
Axon at normal resting excitability and allows AP to be generated
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After AP generated what can't be produced in region
Another impulse
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Regardless of
strength of stimulus
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What is this called
Absolute refractory period (1ms)
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What is it not in
First ms when AP initially fired
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In the relative refractory period what happens
AP can only be initiated if the stimulus is more intense than the normal threshold value
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How long
5ms
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If a weak/ medium size stimulus
Cant fire
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If intense stimulus
Another AP can be fired
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After the refractory period what does the axon do
returns to resting potential
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What usually happens
Wait until now to fire another weak/ medium stimulus- can now possibly generate further AP
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What is. a stimulus
Change in the environment and this is what is needed to start a nerve impulse
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What do animals detect stimuli using
sensory receptors
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What does the body need
A way of distinguishing between weak and strong stimuli
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The bigger the stimulus
the greater the number of AP started
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What is it that varies
NUMBER (freq) of AP which vary, not their size
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What does each type of receptor have
A different type of gated channel for Na eg in eyes gated channels opened by light energy
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What does the stimulus make receptor cells
More permeable to Na+ which then diffuses through the receptor cell membrane depolarising it slightly
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If depolarisation is big enough what happens
a series of action potentials is triggered
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What is depolarisation in sensory receptor called
The receptor potential
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If it exceeds threshold level what occurs
AP
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What is the Threshold stimulus intensity
Level at which a stimulus has to be to generate an AP
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If stimulus exceeds this threshold level what happens
The gated Na+ channels in the membrane open
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If stimulus is below threshold level
no depolarisation occurs
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Stronger the stimulus
greater number Action potential passing per second- high frequency
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What is the size of each AP
Not affected by the strength of the stimulus
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What does it always do
rise from -70 to +35
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or
Doesn't happen at all (either have AP or don't)
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What is this called
All or nothing law
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What does the tightly packed myelin sheath do
electrically insulate cells
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What do Nodes of ranvier do
Tiny gap between Schwann cells where axon exposed
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Why insulate?
Small electrical current would otherwise get lost, electrical current would spread to other axons and it helps impulse move at faster speed (jump on gaps)
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What was thought
Every part of axon depolarises then repolarise in wave of depolarisation
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In reality
This is not what happens
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What does happen
Jump from node to node, not whole axon depolarised if myelinated - would otherwise be slow as every little bit would have to be depolarised
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What is the speed of Nerve impulse affected by
Myelination, axon diameter, Temperature
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Why is the system of myelinated axon more efficient
Portion of exposed axon membrane so AP can jump
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What is this called
saltatory conduction - much quicker action potential than the step by step conduction that occurs in non-myelinated neurone
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What does axon diameter do
A greater diameter means faster impulse travels
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Why
Small SA:VOL less ion leak out so easy for action potential
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What does temperature do
Higher the temperature the faster the speed of the impulse
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why
temperature affects rate of diffusion of ions across the axon
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and
Nodes between the myelin segments contain high concentrations of sodium channels.
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Other cards in this set

Card 2

Front

What is depolarisation

Back

Stimulating electrode cause the membrane potential to change a little and the voltage gated ion channels can detect this change and when the potential reaches -55 the sodium channels open for 0.5 ms

Card 3

Front

What does this cause

Back

Preview of the front of card 3

Card 4

Front

what is this phase referred to as

Back

Preview of the front of card 4

Card 5

Front

What is repolarisation

Back

Preview of the front of card 5
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