5.3.5 Transmission of Nerve Impulses

• Once the action potential has been initiated in a neurone, the depolarisation moves, in one direction along the entire length of an axon like a wave.

Transmission along an axon

• 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

How an impulse is transmitted in one direction along the axon of a neurone

The all or nothing principle

• 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

The receptor potential increases as the strength of the stimulus increases. As the strength of stimulus increases beyond the threshold, the frequency (not amplitude) of impulses increases.