AP - Overview

1. AP - Overview (Click here for animation of the gates)

2. The passage and speed of an action potential

3. The Refractory Period • There is a time after depolarisation where no new AP can start – called the refractory period. • Time is needed to restore the proteins of voltage sensitive ion channels to their original resting conditions. • Na+ channels cannot be opened, as it can’t be depolarised again. WHY? • AP travel in one direction only. • Produces discrete impulses. • Limits the frequency of impulses.

4. After the action Potential • During the action potential, the membrane is depolarised. • Following the impulse K+ ions move out of the membrane, this is repolarisation • The membrane briefly becomes hyperpolarised (more negative on the inside than usual) • The Na+ / K+ channels close

5. The refractory period • With the Na+ / K+ channels closed, the cation pumps can now begin to restore the balance between the ions • Na+ is pumped out and K+ pumped in. • During this time a new action potential can not be set up until resting potential is achieved.

6. Purpose of refractory period • Ensures action potential move in one direction (from receptor to effector) • To distinguish between one action potential and the next (the greater the stimulus, the higher the frequency).

7. Waves of Depolarisation • After an action potential, some of the sodium diffuse sideways. • Causing sodium ion channels in the next region of the neurone to open. • Causes impulse to propagate along the neurone.

8. AP – All or nothing • AP only happens if the stimulus reaches a threshold value. • Stimulus is strong enough to cause an AP • It is an ‘all or nothing event’ because once it starts, it travels to the synapse. • AP is always the same size • An AP is the same size all the way along the axon. • The transmission of the AP along the axon is the nerve impulse. • Bigger stimulus will cause more frequent action potentials.

9. Myelination

10. Unmyelinated Neurones • Localised electrical currents are set up and the action potential is propagated along the neurone. • The wave travels the whole length of the neurone.

11. Na+ Na+ Na+ Sodium channel Nodes of Ranvier Myelinated Neurones • The axons of many neurones are encased in a fatty myelin sheath (Schwann cells). • Where the sheath of one Schwann cell meets the next, the axon is unprotected. • The voltage-gated sodium channels of myelinated neurons are confined to these spots (called nodes of Ranvier).

12. Na+ Na+ Na+ Sodium channel Nodes of Ranvier Myelinated Neurones • The in rush of sodium ions at one node creates just enough depolarisation to reach the threshold of the next. • In this way, the action potential jumps from one node to the next (1-3mm) – called saltatory propagation • Results in much faster propagation of the nerve impulse than is possible in unmyelinated neurons.

13. Factors Affecting the Speed of an AP 1. Myelin sheath – electrical insulator – the AP jumps from one Node of Ranvier to another = SALTATORY CONDUCTION. • Myelinated = 90ms-1 • Unmyelinated = 30ms-1

14. Factors Affecting the Speed of an AP 2. Diameter of the axon – greater diameter = faster conductance (due to less leakage).

15. Factors Affecting the Speed of an AP 3. Temperature – higher temp = faster nerve impulse (rate of diffusion is faster, enzyme activity is faster e.g. ATPase.

16. How do we detect the size of a stimulus? • The number of impulses in a given time – the larger the stimulus, the more impulses generated. • By having neurones with different threshold values – the brain interprets the number and type of neurones and therby determines its size.