Propagating an action potential
An action potential generated at one point on an axon can cause a sequence of further action potentials along its length. Together, this movement of electrical activity along the axon forms a nerve impulse.
In a non-myelinated axon, successive sections of the axon membrane are depolarised.
- An action potential causes Na+ to enter one region of the axon, increasing the sodium ion concentration inside that region.
- Na+ diffuses along the axon into the neighbouring region.
- The movement of positive ions depolarises the adjacent membrane.
- If this depolarisation reaches the threshold potential, voltage-gated sodium ion channels in the neighbouring region open.
- More Na+ enters and a new action potential is generated there.
- This sequence repeats along the axon so that the nerve impulse advances.
Why transmission is one-way
The membrane immediately behind the advancing action potential is hyperpolarised. It therefore cannot immediately produce another action potential.
The membrane ahead can still be depolarised to threshold, so further action potentials are produced in the forward region rather than the region that has just been activated.
Exam Tip: Build a propagation answer as a chain: Na+ enters → Na+ diffuses along the axon → the neighbouring membrane depolarises → threshold is reached → voltage-gated Na+ channels open → another action potential forms.