Nerve Impulses Flashcards
List features of a myelinated motor neurone:
-dendrite
-cell body
- nucleus
- myelin sheath
- node of ranvier
- Schwann cell
- axon terminal
What is the resting potential mV?
-70mV
What is an action potential?
The potential difference across the membrane reverse and the neurones voltage increases beyond the resting potential to +40mV. the membrane is described as depolarised. It’s more positive in the axon than outside.
What causes depolarisation? And what is it?
The permeability of the membrane to Na+ changes in response to a stimulus by a receptor (generator potential). Na+ voltage gated channels open and Na+ rush into the axon down their electrochemical gradient. K+ voltage-gated channels close, so K+ movement out of the axon stops. The potential difference across the membrane is reversed, the membrane is more positive on the inside than the outside and is described as depolarised with a potential difference of +40mV.
What do dendrites do?
Carry action potentials to surrounding cells
What are Schwann cells for?
They wrap around the axon to form the myelin sheaths which are lipids and therefore don’t allow for charged ions to pass through it, hence there are gaps between (nodes of ranvier)
What is a resting potential?
They wrap around the axon to form the myelin sheaths which are lipids and therefore don’t allow for charged ions to pass through it, hence there are gaps between (nodes of ranvier)
How is resting potential maintained?
By a NA-K pump involving active transport (and ATP). The pump moves 2 K+ ions in and 3 Na+ ions out. Maintaining an electrochemical gradient and results in K+ diffusing out by facilitated diffusion through K+ voltage gated channels, and Na+ diffusing in by facilitated diffusion through Na+ voltage gated channels, however the membrane is more permeable to K+ ( as there’s more channels and more open) so more are moved out hence the -70mV.
Describe the all-or-nothing principle
If the depolarisation doesn’t exceed -55mV then an action potentials and the impulse aren’t produced.(nothing)
Any stimulus that does trigger depolarisation to -55mV will always peak at the same maximum voltage. Bigger stimuli instead increase the frequency of action potentials.(all)
This makes sure animals only respond to large enough stimuli, rather than responding to every slight change in the environment
Describe the all-or-nothing principle
If the depolarisation doesn’t exceed -55mV then an action potentials and the impulse aren’t produced.(nothing)
Any stimulus that does trigger depolarisation to -55mV will always peak at the same maximum voltage. Bigger stimuli instead increase the frequency of action potentials.(all)
This makes sure animals only respond to large enough stimuli, rather than responding to every slight change in the environment
Why is a refractory period important?
After an action potential has been generated, the membrane enters a refractory period when it can’t be stimulated because sodium ion channels are recovering and can’t be opened. (-90mV)
This is important because:
-it ensures discrete impulses are produced, meaning an action potential can’t be immediately generated after another to make sure each impulse is separate.
-ensures action potentials travel in one direction, which stops the action potential from spreading out in two directions which could prevent a response
-it limits the number of impulse transmission to prevent over reaction to a stimulus and overwhelming the senses.
Describe what a resting potential is caused by
—>electrochemical gradients - there is a higher concentration of potassium ions inside the axon than outside it in the tissue fluid, there is a higher concentration of sodium ions outside the axon in the tissue fluid.
—> movement of sodium and potassium ions - K+ move out the axon down the electrochemical gradient through open K+ channels and sodium ions don’t move into the axon down the electrochemical gradient as the Na+ channels are closed. The movements are by facilitated diffusion.
—> differential membrane permeability- at rest the membrane of a nerve cell is relatively impermeable to Na+ but permeable to K+. The membranes differing ionic permeability to Na+ and K+ is caused by the number of open ion channels, at rest the K+ channels are open and Na+ channels are closed.
Describe repolarisation
-about 0.5 milliseconds after the sodium voltage gated channels have opened the close again.
-permeability to K+ ions increases as the potassium voltage-gated channels open and K+ move quickly out of the axon down their electrochemical gradient.
-this causes repolarisation on the axon returning to the potential difference to -70mV
What would happen to the resting potential when a metabolic inhibitor is added to a neurone and why? (4 marks)
Respiration stops and so does ATP production so there’s no ATP for active transport in the Na-K ion pump to pump Na+ into the axon and K+ out again so the electrochemical gradients decrease and facilitated diffusion would cause K+ to move towards equilibrium, and the potential difference would move closer to 0mV.
Describe hyperpolarisation
An overshoot of -90mV. It occurs due to the outward Na+ movement as the channels are slow to close. This is stopped as once again when the outside is more positive than the inside and further Na+ movement is repelled by like charges.
Describe a refractory period
The sodium/potassium pump moves sodium ions out of the axon by active transport and potassium ions back in. It takes a few milliseconds for the ions to be moved back to their original positions and restore the electrochemical gradients i.e. more sodium ions outside the axon than in and more potassium ions inside the axon than out. In this time no new action potential can pass.
D
