Propagation of an Action Potential Learning Outcomes: 1)Suggest why an action potential only propagate in one direction. 2)Explain how an action potential propagates along an axon 3)Distinguish between propagation of an unmyelinated axon and a myelinated axon. Exam Question: Explain how an action potential is generated in an axon (8 max)  The resting potential of -65mV is depolarised to +40mV.  When a stimulus induces an action potential, voltage gated Na channels open and allow an influx of Na+ ions into the axon.  This causes the opening of more Na channels and even more Na+ ions influx.  This causes the potential difference to increase to +40mV.  At this p.d., the voltage gated Na+ channels close and the voltage gated K channels open. K+ ions now efflux out of the axon.  This initiates even more k+ channels opening and even more K+ ions move out by diffusion.  This reduces the p.d. (called repolarisation) back towards resting potential.  The repolrsiation overshoots and reaches -75mV. This is called hyperpolarisation. At this point the voltage gated K channels close once more.  During hyperpolarisation, the sodium potassium pumps re-establishes the resting potential, ready for a new action potential. During hyperpolarisation, the membrane is unable to depolarise until resting potential is re-established.

How does it pass? The Mexican Wave Learning Outcomes: 1)Suggest why an action potential only propagate in one direction. 2)Explain how an action potential propagates along an axon 3)Distinguish between propagation of an unmyelinated axon and a myelinated axon.

How does it pass? The Mexican Wave Learning Outcomes: 1)Suggest why an action potential only propagate in one direction. 2)Explain how an action potential propagates along an axon 3)Distinguish between propagation of an unmyelinated axon and a myelinated axon. 1) The first part of the axon depolarises and an action potential is initiated by the opening of Na channels and then the opening voltage gated Na channels. 2) The Na influx begins to spread to the second part of the axon, the increase in Na concentration here, causes the opening of Voltage gated Na channels in this part of the axon and then this part depolarises. The first part of the axon begins to repolarise by voltage gated K channels opening.

Why does it pass in one direction only? The refractory Period – the time it takes for the previous section of an axon to return to resting potential before it be depolarised. So why does an action potential only spread in one direction? The previous part of the action cannot be depolarised straight away, only the next part which has not yet been depolarised. Na ions will only cause depolarisation there. The refractory period also ensures that there are only discrete impulses. (impulses are separated from each other not summated) and makes sure the axon can fire only a limited number of APs. Learning Outcomes: 1)Suggest why an action potential only propagate in one direction. 2)Explain how an action potential propagates along an axon 3)Distinguish between propagation of an unmyelinated axon and a myelinated axon.

Passage along a myelinated axon The myelin sheath in most axons actually insulates the axon and stops depolarisation occurring on the membrane. Because of this, the membrane can only depolarise……? Learning Outcomes: 1)Suggest why an action potential only propagate in one direction. 2)Explain how an action potential propagates along an axon 3)Distinguish between propagation of an unmyelinated axon and a myelinated axon.

Saltatory Conduction An action potential covers more distance each time it propagates if on a myelinated axon compared o an unmyelinated axon. What does this suggest? Increase speed of impulse from 30ms -1 to 90ms -1 This faster speed is called saltatory conduction. Learning Outcomes: 1)Suggest why an action potential only propagate in one direction. 2)Explain how an action potential propagates along an axon 3)Distinguish between propagation of an unmyelinated axon and a myelinated axon.

Other factors that effect the speed of a nerve impulse. 1) Myelin sheath – Saltatory conduction 2) The diameter of the axon ◦ Larger diameter – less ion leakage – faster propagation 3) Temperature: ◦ Increased rate of diffusion of ions ◦ Higher rate of respiration (enzymes) – increased function of Na-K pump to restore resting potential ◦ Recued after 37 degrees (enzymes in respiration and pump) Learning Outcomes: 1)Suggest why an action potential only propagate in one direction. 2)Explain how an action potential propagates along an axon 3)Distinguish between propagation of an unmyelinated axon and a myelinated axon.

Threshold: All or Nothing When a stimulus causes the initial opening of Na channels, a certain number of Na channels have to open before the Voltage gated channels open and depolarisation occurs. This is called Threshold. If stimulus is weak, what might you expect? If stimulus is strong, what might you expect? Learning Outcomes: 1)Suggest why an action potential only propagate in one direction. 2)Explain how an action potential propagates along an axon 3)Distinguish between propagation of an unmyelinated axon and a myelinated axon.