Origin and propagation of action potential

Question 1

What is the resting membrane potential value, and how is this influenced by potassium and sodium ion permeability/conductance?

Answer 1

-70mV

potassium ion permeability/conductance is high and sodium ion permeability/conductance is low. so resting membrane potential is closer to potassium equilibrium potential

Question 2

How is the threshold voltage for excitation to generate an action potential reached?

Answer 2

An inward current caused by small ionic changes, external potential change, Excitatory Post Synaptic Potentials eg. raises membrane potential to threshold -55mV

Question 3

After the threshold voltage for excitation is reached, what happens to the voltage-gated sodium channels?

Answer 3

Sodium ion conductance/permeability rapidly increases due activation/opening of more voltage-gated sodium channels after threshold voltage is reached

Question 4

What causes depolarisation of the cell membrane, and what is the change in voltage of the cell?

Answer 4

Voltage-gated sodium channels open and there is large, rapid sodium ion influx, so inner cell becomes less negative (depolarisation)

Membrane potential increases from -70mV to +30mV

Question 5

What are the 2 mechanisms used for cell membrane repolarisation, and how does this result in repolarisation?

Answer 5

Closing voltage-gated sodium channels which also terminates the inward current

Voltage-gated potassium channels open so that potassium permeability/conductance is greater than at rest

Greater outcurrent of potassium ions causes inner cell to be more negatively charged than outside so that it is near resting membrane potential again, which is repolarisation

Question 6

How does hyperpolarisation occur after an action potential is generated?

Answer 6

so many potassium ions have left cell that the membrane potential is now more negative than the resting membrane potential

Question 7

What is the refractory period, and why does it occur after an action potential is generated?

Answer 7

Duration before another action potential can be generated

Caused when hyperpolarised cell membrane causes voltage-gated channels to become temporarily inactivated

Question 8

What allows propagation of action potentials to occur?

Answer 8

Passive spread of depolarisation

Question 9

Explain how passive spread of depolarisation occurs to allow action potential propagation?

Answer 9

Sodium ions that flow into one area of axon membrane cause depolarisation of adjacent area, this area reaches threshold voltage of excitation and generates another action potential

Question 10

What direction are action potentials propagated in, and why?

Answer 10

Unidirectionally downstream axon

upstream voltage-gated ion channels are inactivated due to refractory period so action potential can’t be propagated in direction towards initial action potential

Question 11

What way can action potentials be propagated faster?

Answer 11

Improved conduction velocity

Question 12

How does a larger axon diameter cause faster action potential propagation?

Answer 12

Larger axon diameter allows greater current flow, so less time is needed to change the electrical charge and depolarise the adjacent area of the axon membrane, reaches threshold voltage for excitation at faster conduction velocity rate

Question 13

What is myelin sheath composed of, and what is it produced by?

Answer 13

Composed of protein and fatty substances, produced by schwann cells

Question 14

How does axon myelination cause faster action potential propagation?

Answer 14

Myelin sheath insulates axon so that current dissipates less across axon membrane, so action potential needs to be regenerated less frequently

Question 15

In myelinated axons, where are action potentials regenerated?

Answer 15

At every Node of Ranvier

Question 16

How can nodes of Ranvier regenerate action potentials in myelinated axons?

Answer 16

They are non-myelinated areas that contain high densities of sodium and potassium voltage-gated channels

Question 17

How do nodes of Ranvier cause passive spread of depolarisation in myelinated axons?

Answer 17

when a node becomes depolarised, sodium ion channels open and generate AP, depolarisation spreads to next downstream node

Question 18

What is saltatory conduction?

Answer 18

action potential travels very fast through myelinated areas due to current not dissipating, then action potential slows at nodes of ranvier, which makes it seem as if action potential is jumping from node-node