Action Potentials

Question 1

How do neuron’s function and send signals?

Answer 1

Neurons send signals by changing their membrane potential in two main ways:

1. Action potentials
2. Graded/local/postsynaptic potentials

Question 2

Hyperpolarisation

Answer 2

Inside becomes more negative
-Usually due to K+ leaving the neuron

Question 3

Depolarisation

Answer 3

Inside of membrane becomes more positive
- Usually due to Na+ coming into the neuron

Question 4

Repolarisation

Answer 4

return to resting membrane potential from being depolarised

Question 5

What are graded/local potentials in neurons?

Answer 5

Graded or local potentials cause the initial change in membrane potential

• in dendrites or cell body
• variable in size (not all or nothing)

Question 6

What is an action potential and how does it function?

Answer 6

An action potential is a rapid, brief change in membrane potential that signals over long distances: Does not diminish in size or strength over distance.

Triggered if a graded potential depolarizes the membrane to -50 mV.

• Initiated in the axon hillock.
• “All or nothing” -> once they are triggered in the axon

Question 7

What mediates the ion flow during action potentials?

Answer 7

Voltage-gated sodium and potassium channels mediate the ion flow during action potentials.

Question 8

How do voltage-gated Na+ channels function?

Answer 8

Have two gates and alternate between three states:

Closed: At resting state, no Na+ enters the cell.

Opened: Depolarization opens the channels, allowing Na+ to enter.

Inactivated: Channels are blocked by inactivation gates soon after opening.

Question 9

Voltage gated K+ channels

Answer 9

Have one gate and two states

-Closed:Resting state, so no K+ exits the cell through them

-Opened: Depolarisation, allowing K+ to exit the cell

Question 10

How do local anaesthetics work?

Answer 10

Local anesthetics block Na+ channels, keeping them inactive, thus preventing depolarization and halting action potential initiation.

Question 11

How do action potentials propagate in unmyelinated fibres?

Answer 11

Depolarization triggers Na+ channel opening, leading to adjacent axon depolarization and the initiation of another action potential, propagating along the axon

Question 12

What are the refractory periods of an action potential?

Answer 12

Absolute Refractory Period: No new action potentials; Na+ channels open then inactivate.

Relative Refractory Period: New potentials possible with strong stimulus; some Na+ channels closed.
Limits firing frequency; ensures unidirectional propagation.

Question 13

How do refractory periods ensure action potentials only travel in one direction?

Answer 13

Action potentials travel in one direction, from the cell body to the axon terminal, because after a patch of membrane fires an action potential, it enters a “resting” phase and cannot fire another one immediately.

Question 14

Factors affecting the speed of action potential propagation

Answer 14

Axon Diameter: Thicker axons propagate faster due to less resistance.

Temperature: Faster reactions at higher temperatures.

Degree of Myelination: Insulates the membrane, enabling “saltatory conduction.”

Question 15

Saltatory conduction in myelinated fibres

Answer 15

fast transmission of action potentials as they leap between nodes of Ranvier along the axon, boosting signal speed compared to non-myelinated fibers.

Question 16

Multiple Sclerosis

Answer 16

Autoimmune attack on myelin slows and stops impulse conduction, leading to symptoms like pain, numbness, tingling, difficulty walking, fatigue, and muscle spasms.

Question 17

How action potentials convey information

Answer 17

Firing Frequency: Action potentials signal information through their frequency.

All-or-Nothing: frequency indicates stimulus strength.

High Frequency: Indicates a strong stimulus.
Low Frequency: Indicates a weaker stimulus.

Question 18

Membrane potential phases

Answer 18

Resting Potential (Phase 1):
* Neuron at rest, stable at -70 mV.
* Na⁺ and K⁺ voltage-gated channels closed.

Depolarizing Trigger (Phase 2):
* Event makes membrane less negative, hits threshold.

Rising Phase (Phase 3):
* Na⁺ channels open, rapid depolarization.

Peak (Phase 4):
* Na⁺ channels start inactivating, K⁺ channels open.

Falling Phase (Phase 5):
* Repolarization, K⁺ flows out.

Hyperpolarization (Phase 6):
* K⁺ channels overstay, slight negativity.

Return to Rest (Phases 7 & 8):
* Membrane stabilizes back to -70 mV.
* Na⁺ and K⁺ channels closed.

Neuron is ready for another action potential