passage of action potentials along an non-myelinated axon

• Sodium ions diffuse into the axon during depolarisation
• Diffusion of sodium ions sideways within the axon down their electrochemical gradient is called a local current (or local circuit)
• Positive charge accumulates under an adjacent patch of membrane, taking it to threshold potential
• Voltage-gated sodium ion channel proteins open in the next patch of membrane, allowing sodium ions to flood into the axon by diffusion, generating new local currents
• Local currents flow both backwards and forwards within an axon, but the action potential can only go forwards due to the refractory period
• The refractory period occurs while the membrane potential repolarises from +40 mV back to resting membrane potential
• During the refractory period, the membrane is unable to generate a new action potential as voltage-gated sodium ion channel proteins are closed and cannot be opened
• Once resting potential is restored, a new action potential can be generated
• The problem with non-myelinated axons is that the passage of an action potential is rather slow.

  This is because the whole length of the cell-surface membrane needs to be progressively depolarised to pass an impulse from one end of an axon to another.