5.3.5 Transmission of Nerve Impulses

Question 1

How are nerve impulses transmitted?

Answer 1

• When the action potential is initiated, depolarisation occurs
• The depolarisation moves in one direction along the axon (like a wave)
• The current causes opening of sodium ion channels further up the axon
• Influx of sodium ions in that section of axon generates action potential in that direction
• Previous section of axon is repolarised (cannot be depolarised currently)
• So action potentials are discrete events, so impulse can only travel in 1 direction

Question 2

What is the all or nothing principle?

Answer 2

• When receptors are stimulated they are depolarised
• If stimulus is weak or below certain threshold, receptor cells won’t be sufficiently depolarised
• Sensory neurone will not be stimulated to send impulses
• If stimulus strong enough to increase electrical potential in receptor above threshold potential then receptor stimulates sensory neurone to send impulses
• So an impulse is only transmitted if initial stimulus is sufficient whereby the electrical potential rises above threshold potential