(S1W1) Neurophysiology part 4

Question 1

What is the threshold usually for stimulating an action potential?

Answer 1

about -55 mV

Question 2

Which part of a neuron does a signal first arrive at?

Answer 2

the dendrites

Question 3

Are the voltage sensitive sodium channels in the membrane open or closed at rest? And why?

Answer 3

closed so that sodium ions cannot just diffuse in and out of the membrane

Question 4

What activates/opens the sodium channels in the membrane?

Answer 4

a change in voltage of about 15m mV so the electrical charge passes the thresholf of -55 mV

Question 5

What happens when the sodium channels in the membrane are opened by the change in voltage?

Answer 5

sodium ions rush in

Question 6

What is meant by membrane potential?

Answer 6

the difference in electric potential between the inside and outside of a membrane

Question 7

How does an increase in voltage above the threshold cause depolarisation?

Answer 7

• an increase in voltage above the threshold of -55 mV opens the sodium channels within the membrane
• this allows sodium ions to rush in
• meaning you have lots of positive ions rushing in, making the difference less negative which is known as depolarisation

Question 8

What is meant by the propagation of an action potential?

Answer 8

• once the sodium channels open and positive sodium ions rush in
• the sodium ions then diffuse outwards within the membrane
• which then causes teh potential to reach the threshold further along the membrane
• causing sodium channels to open further along the membrane
• allowing more sodium ions to rush in
• which then diffuse down the membrane and cause more sodium channels to open and so on

Question 9

What is meant by the ‘all or nothing phenomenon’ in terms of actional potentials?

Answer 9

• there is or isn’t an action potential - no such thing as a bigger or smaller action potential
• so any stimulus that reaches the threshold will trigger an action potential no matter how big or small and it doesn’t matter if the stimulus reaches way over the threshold there is no such thing as a bigger actional potential

Question 10

What happens if the stimulus doesn’t reach the threshold?

Answer 10

The sodium potassium pump will pump out 3 sodium ions for every 2 potassium ions it pumps in, maintaining and restoring the resting membrane potential of about -70 mV