15.2- THE NERVE IMPULSE Flashcards
What can a nerve impulse be defined as?
self-propagating wave of electrical activity that travels along the axon membrane
What is a nerve impulse the temporary reversal of?
temporary reversal of electrical pd across axon membrane
Between what states id the temporary reversal of electrical pd between? (nerve impulse)
resting potential and action potential
How is the movement of ions i.e. sodium ions + potassium ions across the axon membrane controlled? #1 (hint- bilayer)
phospholipid bilayer of axon plasma membrane prevents sodium + potassium ions diffusing across it
How is the movement of ions i.e. sodium ions + potassium ions across the axon membrane controlled? #2 (hint- channel proteins)
channel proteins span phospholipid bilayer
channel proteins have ion channels which pass through them
some of these have ‘gates’ which can be opened/closed so sodium/potassium ions can move through them by facilitated diffusion at any one time, but not on other occasions
How is the movement of ions i.e. sodium ions + potassium ions across the axon membrane controlled? #3 (hint- carrier proteins)
some carrier proteins actively transport potassium ions into axon + sodium ions out of axon
mechanism can be called sodium-potassium pump
What is the charge of the inside of the axon like relative to the outside?
negatively charged
Range of values for resting potential and what is it usually like in humans?
50 to 90 millivolts (mV), but usually around 65mV in humans
What is the axon said to be when it is at 65mV?
polarised
What is the establishment of the pd due to- where are Na+ ions transported and by what?
sodium ions actively transported out of axon by sodium-potassium pumps
What is the establishment of the pd due to- where are K+ ions transported and by what?
potassium ions actively transported into axon by sodium-potassium pumps
What is the establishment of the pd due to- is the active transport of sodium or potassium greater and what is the ratio like?
active transport of sodium ions greater than that of potassium ions, so 3 sodium ions move out for every 2 potassium ions
What is the establishment of the pd due to- how is an electrochemical gradient created?
although sodium + potassium ions both positive, outward movement of sodium ions greater than inward movement of potassium ions
so there’re more sodium ions in tissue fluid surrounding axon than in cytoplasm, + more potassium ions in cytoplasm than in tissue fluid
What is the establishment of the pd due to- where do sodium ions begin to diffuse back into?
sodium ions begin to diffuse back naturally into axon while potassium ions begin to diffuse back out of axon
What is the establishment of the pd due to- what are the gates for potassium ions and sodium ions like?
most of the gates in channels that allow potassium ions to move through open, while most of gates in channels that allow sodium ions to move are closed
What does a stimulus of sufficient size detected by a receptor in the nervous system cause?
its energy causes temporary reversal of charges either side of this part of axon membrane
What happens to charge inside the membrane if the stimulus is great enough?
negative charge of -65mV inside membrane becomes positive charge of around +40mV
When inside the membrane becomes a positive charge of around +40 mV, what is this called?
action potential
When inside the membrane is around +40mV, what is this part of the axon membrane said to be?
depolarised
Why does the depolarisation occur?
as channels in axon membrane change shape, + hence open/close, depending on voltage across membrane
What are channels in the axon membrane that change shape and hence open/close depending on the voltage across the membrane called?
voltage-gated channels
1 At resting potential are the potassium voltage-gated channels and sodium voltage-gated channels open?
some potassium voltage-gated channels open but sodium voltage-gated channels closed
2 What does the energy of the stimulus cause the sodium voltage-gated channels to do?
some sodium voltage-gated channels in axon membrane opened + so sodium ions diffuse into axon through channels along their electrochemical gradient
being positively charged, they trigger reversal in pd across membrane
3 What happens as the sodium ions diffuse into the axon?
more sodium channels open, causing even greater influx of sodium ions by diffusion
4 What happens once the action potential is around +40mV?
voltage gates on sodium ion channels close + voltage gates on potassium ion channels begin to open
5 With some potassium voltage-gated channels now open, what happens?
electrical gradient that was preventing further outward movement of potassium ions now reversed, causing more potassium ion channels to open
means more potassium ions diffuse out, starting repolarisation of axon
6 What does the outward diffusion of these potassium ions cause?
temporary overshoot of electrical gradient, with inside of axon being more negative than usual
closable gates on potassium ion channels now close + activities of sodium-potassium pumps once again cause sodium ions to be pumped out + potassium ions in
resting potential of -65mV re-established + axon said to be repolarised
What is the movement of sodium ions inwards during the action potential called and what kind of process is it?
diffusion- passive process
What process maintains the resting potential and what kind of process is it?
active transport- active process
