❤️ 3.6 - 3.6.2 - 3.6.2.1 Nerve impulses (A-level only) Flashcards
Neurone cell membranes are polarised at … .
rest.
What is a neurones resting state?
When the neurone is not being stimulated.
What happens in a neurones resting state?
The outside of the cell membrane is positively changed compared to the inside. This is because there are more positive ions outside the cell than inside.
The membrane is polarised during its … … .
resting state.
What does polarised mean?
That there is a difference in charge (called a potential difference or voltage) across it.
What is the voltage across the membrane when it is at rest (called the resting potential)?
-70 mV (millivolts).
What is the resting potential created and maintained by?
The sodium-potassium pumps and potassium ion channels in a neurone’s membrane.
What type of transport does the sodium-potassium pump use to move sodium ions out of the neurone for every two potassium ions moved in?
Is ATP needed?
Active transport.
ATP is needed.
What type of transport does the potassium ion channel use to diffuse potassium ions out of the neurone, down their concentration gradient?
Is ATP needed?
Facilitated diffusion.
ATP is not needed.
What do the sodium-potassium pumps move?
They move sodium ions out of the neurone, but the membrane is not permeable to sodium ions, so they can’t diffuse back in. This creates a sodium ion electrochemical gradient (a concentration gradient of ions) because there are more positive sodium ions outside the cell than inside.
They also move potassium ions in to the neurone, but the membrane is permeable to potassium ions so they diffuse back out through potassium ion channels.
This makes the outside of the cell positively charged compared to the inside.
The sodium-potassium pump, potassium ion channel and sodium ion channel are all types of transport what?
protein.
When do neurone cell membranes become depolarised?
When they are stimulated.
What does a stimulus trigger?
Other ion channels, called sodium ion channels, to open.
What happens if a stimulus is big enough?
It will trigger a rapid change in potential difference and a sequence of events known as an action potential will occur.
What are the 5 stages of an action potential?
1) Stimulus.
2) Depolaristation.
3) Repolarisation.
4) Hyperpolarisation.
5) Resting potential.
What happens to the potential difference during the stimulus of an action potential?
This excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative.
What happens to the potential difference during the depolarisation of an action potential?
If the potential difference reaches the threshold (around -55mV), more sodium ion channels open. More sodium ions diffuse rapidly into the neurone.
What happens to the potential difference during the repolarisation of an action potential?
At a potential difference of around +30mV the sodium ion channels close and potassium ion channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential.
For repolarisation to occur, the sodium ions channels have to close or the membrane will remain depolarised.
True or false?
True.
What happens to the potential difference during the hyperpolarisation of an action potential?
Potassium ion channels are slow to close so there is a slight overshoot where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (i.e. less than -70mV).
What happens to the potential difference during the resting potential of an action potential?
The ion channels are reset. The sodium-potassium pup returns the membrane to its resting potential and maintains it until the membranes excited by another stimulus.
Why after an action potential, can the cell membrane not be excited again?
The ion channels are recovering and they can’t be made to open - sodium ion channels are closed during depolarisation and potassium ion channels are closed during hyperpolarisation.
What is the period of recovery after an action potential called?
Refractory period.
Why does the action potential move along a neurone as?
A wave of depolarisation.
How is a wave of depolarisation caused?
1) When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways.
2) This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part.
= This causes a wave of depolarisation to travel along the neurone.
(The wave moves away from the parts of the membrane in the refractory period because these parts can’t fire an action potential).
What does the refractory period produce?
Discrete impulses.
What are the ion channels doing during the refractory period?
Recovering and can’t be opened.
What does the refractory period act as?
A time delay between one action potential land the next.
What are 3 important things about action potentials?
- DON’T OVERLAP but pass along as discrete (separate) impulses.
- There is a LIMIT to the frequency at which the nerve impulses can be transmitted.
- Action potentials are unidirectional (ONLY travel in ONE direction).
What nature do action potentials have?
All-or-nothing.
What does it mean that action potentials have an all or nothing nature?
One the threshold is reached, an AP will always fire with the same change in voltage, no matter how big the stimulus is. If the threshold is not reached, an AP will not fire.
A bigger stimulus will not cause a bigger action potential, but will cause them to fire more frequently.
True or false?
True.
What are the 3 factors which affect the speed of conduction of action potentials?
1) Myelination.
2) Axon diameter.
3) Temperature.
How is myelination a factor which affects the speed of conduction of action potentials?
The myelin sheath is an electrical insulator.
In the peripheral nervous system, the sheath is made of a type of cell called a Schwann cell.
Between the Schwann cells are tiny patches of bare membrane called the nodes of ranvier. S ion channels are concentrated at the nodes.
In a myelinated neurone, depolarisation only happens at the nodes of ranvier (where S ions can get through the membrane).
The neurones cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse jumps from node to node.
This is called SALTATORY CONDUCTION (really fast).
Why is the speed of conduction of action potentials slower in non-myelinated neurones than in saltatory conduction which happens in myelinated neurones?
In a non-myelinated neurone, the impulse travels as a wave along the whole length of the axon membrane (so you get depolarisation along the whole length of the membrane).
How is axon diameter a factor which affects the speed of conduction of action potentials?
Bigger diameters - Action potentials are conducted quicker along axons because there is less resistance to the flow of ions than in the cytoplasm of a smaller axon. With less resistance, depolarisation reaches other parts of the neurone cell membrane quicker.
How is temperature a factor which affects the speed of conduction of action potentials?
As the temperature increases, the speed of conduction increases too, because ions diffuse faster. The speed only increases up to around 40 decrees C though - after that the proteins begin to denature and the speed decreases.
