U2L4: Action Potential and Synapses

Question 1

What is Resting Membrane Potential?

Answer 1

A neuron that is not transmitting a signal is said to be at its resting potential (-70mV) - this is called a polarized cell

Question 2

Explain all the steps and the process of an Action Potential

Answer 2

1. When a large stimulus reaches the membrane, an action potential results (@70mV); Begins as stimulus causes +ve charges from outside neuron to flow inward making inside of neuron less -ve
2. all-or-none response (action potential only produced if stimulus is strong enough to cause depolarization)
3. Membrane Po reaches -55 mV due to large stimulus
4. Na channels open, Na flows down gradient, cell depolarizes, more Na channels open (+ve feedback)
5. Past 0 mV, Na channels begin to close slowly, K+ channels begin opening, K+ flows out down gradient
6. K+ channels fully open at +30mV, Na channels fully closed
7. Repolarisation – the inside of the neuron starts to become more negative due to K+ ions flowing out of the neuron
8. Overshoot – K+ channels slow to close, potential goes below Po (refractory period)
9. Na-K pump working to re-establish Resting potential

Question 3

What happens during DEPOLARIZATION?

Answer 3

Na+ channels open and Na+ ions flow IN. K+ channels closed.

Question 4

What happens during REPOLARIZATION?

Answer 4

K+ channels open and K+ ions flow OUT. Na+ channels closed.

Question 5

How does Action Potential work
in a unmyelinated fibre?

Answer 5

The action potential will begin in one spot. Membrane at that location will undergo depolarization. The depolarizing membrane will cause adjacent channels in the membrane to open.

Question 6

How does Action Potential work
in a myelinated fibre (jawed vertebrates)?

Answer 6

Saltatory conduction due to myelin sheath
Charge jumps from one node to the next

Question 7

What is Synapse?

Answer 7

The site where a neuron makes a connection with either another neuron or effector (muscle fibre or gland)

Question 8

What is Electrical Synapse?

Answer 8

Direct connection between the cytoplasm of the two adjacent cells through gap junctions. Rapid transmission - common in invertebrates

Question 9

What is Chemical Synapse?

Answer 9

A chemical messenger called neurotransmitter released.
Action potential will initiate a sequence of events in the plasma membrane of the synaptic knob

Question 10

How does a chemical synapse work?

Answer 10

1. Action potential reaches axon terminal of presynaptic neuron
2. Triggers cause Ca channels to open, causing synaptic vesicles to fuse with the plasma membrane of the presynaptic cell.
3. Synaptic vesicles fuse with the membrane releasing their neurotransmitter into synaptic cleft
4. neuroT diffuse across the cleft and bind to receptor (R) located on postsynaptic membrane
5. R-neuroT complex initiates the opening of chemical sensitive ion channels in the post synaptic membrane allowing the influx of ions (if Na channels - depolarization, If K channels – hyperpolarisation)
6. If postsynaptic neuron becomes depolarized, it will generate new action potential that travels along axon to reach synapse with next neuron or effector.
7. neuroT in the cleft are broken down by neurotransmitter enzymes. Once the neurotransmitters are removed, the chemical sensitive ion gates close.