5.3.5 Transmission of Nerve Impulses

Question 1

How is the action potential transmitted along an axon?

Answer 1

An action potential triggered in the neurone causes depolarisation of that section of the axon

The current causes the opening of sodium ion channels a little further up the axon

This causes an influx of sodium ions in this section of the axon generating an action potential in this direction

The previous section of the axon is in the repolarisation stage (the sodium channels are closed and potassium channels are open) and is unresponsive

This makes the action potentials discrete events and means the impulse can only travel in one direction

Question 2

What is the all or nothing principle?

Answer 2

When receptors (such as chemoreceptors) are stimulated, they are depolarised

If the stimulus is very weak or below a certain threshold, the receptor cells won’t be sufficiently depolarised and the sensory neurone will not be activated to send impulses

If the stimulus is strong enough to increase the receptor potential above the threshold potential then the receptor will stimulate the sensory neurone to send impulses

This is an example of the all-or-nothing principle

An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential

Rather than staying constant, threshold levels in receptors often increase with continued stimulation, so that a greater stimulus is required before impulses are sent along sensory neurones