Depolarisation

Question 1

Gist

Answer 1

The rapid influx of sodium ions increasing the overall charge of the inside of the membrane (depolarisation)

Requires a threshold level to be surpassed for gated ion channels to open. The same number of channels open no matter how strong the stimulus is (identical action potential)

Question 2

How are strong stimuli distinguished from a weak stimuli?

Answer 2

2) Stronger stimuli cause a greater number of nerve fibres to depolarise than a weak stimulus

Thus,
The number of nervous impulses generated will be greater for a stronger stimulus

Question 3

Step 1)

Answer 3

1) A stimulus is received by the dendrites and the ligand gated channels on the pre-synaptic neuron are stimulated to open.

Question 4

Step 2)

Answer 4

2) This causes sodium ions to move into the cell and the membrane potential to rise (less negative) =slow depolarisation occurs

Question 5

Step 3)

Answer 5

3) If the stimulus is strong enough, causing the membrane potential to rise more than 15mV (to –55mV) the threshold has been met and an all-or-none response will occur.

Question 6

Step 4)

Answer 6

4) This means that the movement of sodium ions moving into the cell proceeds independently of the stimulus. The size of the response is not related to the strength of the stimulus.

Question 7

Step 5)

Answer 7

5) The sodium voltage gated channels are stimulated to open and sodium ions rush into the cell = rapid depolarisation occurs

Question 8

Step 6)

Answer 8

6) The membrane potential rises rapidly due to the influx of positive sodium ions

Question 9

Step 7)

Answer 9

7) When the membrane potential reaches +40mV the membrane is said to be depolarised and the sodium voltage gated channels are stimulated to become inactive and close.

Question 10

Step 8)

Answer 10

8) At max depolarisation the number of sodium ions entering the cell = the number of potassium ions leaving the cell = no net movement of ions.