Propagation of action potential

Generation & Propagation of action potential

1. DILSHANA FATHIMA M.Sc. BIOCHEMISTRY PROPAGATION OF ACTION POTENTIAL

2. Dendrite conducts signal from a sensory cell or neighbouring neuron towards the cell body. Axon conducts signal away from cell body to another neuron or effector cell. Axon ending relays signal to next neuron or effector cell. INTRODUCTION

3. Difference in voltage b/w the inside & outside of the cell as measured across the cell membrane. When a neuron is not being stimulated, it maintains a resting potential Ranges from -40mV to -90mV Average about -70mV RESTING POTENTIAL

4. Is the entire series of charges which contribute towards the changes in membrane potential. Occurs in response to a threshold stimulus Either ocurs completely/it does not occur at all(all/none principle) Has 2 main phases: Depolarising phase: - ve memberane potential becomes less – ve reaches zero & then becomes + ve Repolarising phase: the memberane potential is restored to the resting state of -70mV ACTION POTENTIAL

5. Following the repolarising phase, there may be an after hyperpolarising phase, during which the memberane potential temporarily becomes more – ve than the resting level Caused by volage gated ion channels Voltage gated Na+ channels Activation & inactivation phase At rest, activation gate closed, inactivation gate open Transient influx of Na+ causes the membrane to depolarize

6. Voltage gated K+ channels Single activation gate i.e , closed in the resting state K+ channels opens slowly Efflux of K+ repolarizes the memberane The after hyperpolarizing phase occurs when the voltage gated K+ channels remain open after the repolarizing phase ends Action potential occurs in the memberane of the axon when depolarisation reaches a certain level – Threshold ( abt -55mV in the neuron)

7. Threshold in a particular neuron is usually constant An action potential will occur in response to a threshold stimulus & not to a subthreshold stimulus Several action potentials will form in response to a supra threshold stimulus

8. When a stimulus causes the memberane of the axon to depolarise to threshold, voltage gated Na+ channels open rapidly rapid influx of Na+ ions into the cell inside of the cell membrane become more + ve than outside This change is called depolarisation Membrane potential changes from -55mV to +30mV DEPOLARISING PHASE

9. Each voltage gated Na+ channels have 2 separate gates, an activation gate (AG) & an inactivation gate (IAG) In resting state, IAG is open AG is closed Na+ cannot move into the cell In activated state, Both AG & IAG are open Inflow of Na+

10. As more channels open Na+ inflow increases Membrane depolarises further More Na+ channels open This is an example for + ve feedback mechanism

11. After the AG of the voltage gated channels open, the IAG closes & it is in an inactivated state In addition to opening voltage gated Na+ channels, a threshold level depolarisation also opens voltage gated K+ channels Opening of voltage gated K+ channels occurs at about the same time the voltage gated Na+ channels closes The slower opening of voltage gated K+ channels & closing of previously open Na+ channels produce the repolarising phase REPOLARISING PHASE

12. Slowing of Na+ inflow & acceleration of K+ outflow causes the memberane potential to change from +30mV to -70mV Repolarisation allows inactivated Na+ channels to revert to resting state

13. While the voltage gated K+ channels are open, outflow of K+ may be large enough to cause an after hyperpolarising phase of an action potential During this phase, the voltage gated K+ channels remain open & the memberane potential becomes even more – ve (-90mV) As the voltage gated K+ channels close, the memberane potential returns to the resting level of -70mV AFTER HYPERPOLARISIG PHASE

14. The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold level During an absolute refractory period, even a strong stimulus cannot initiate a 2 nd action potential This period coincides with the period of Na+ channel activation & inactivation Large diameter axons have a larger surface area & have a brief absolute refractory period of about 0.4mSec REFRACTORY PERIOD

15. Small diameter axons have absolute refractory periods as long as 4mSec The relative refractory period is the period of time during which a 2 nd action potential can be initiated, but only by a larger than normal stimulus It coincides with the period when the voltage gated K+ channels are still open after inactivated Na+ channels have returned to their resting state

16. To communicate information, action potentials must travel from where they arise at the trigger zone of the axon to the axon terminals It is not decremental Keeps its strength as it spreads along the membrane This mode of conduction is called propagation PROPAGATION OF ACTION POTENTIAL

17. Each action potential in its rising phase, reflects a reversal in membrane polarity + ve charges due to influx of Na+ can depolarise the adjacent region to threshold So the next region produces its own action potential The previous region repolarises back to the resting membrane potential Signal does not go back towards the cell body

18. 2 ways to increase velocity of conduction Axon has a larger diameter Axon is myelinated There are 2 types of conduction Continuous conduction Saltatory conduction

19. Involves step by step depolarisation & repolarisation Ions flow through their voltage gated channels Occurs in unmyelinated axons & in muscle fibres Action potential propagates only a relatively short distance in a few milliseconds CONTINUOUS CONDUCTION

20. Occurs along myelinated axons Occurs because of uneven distribution of voltage gated channels Few voltage gated channels are present in regions where a myelin sheath covers the axolemma At the Nodes of Ranvier , the axolemma has many voltage gated channels SALTATORY CONDUCTION

21. Current carried by Na+ & K+ flows across the membrane mainly at the nodes When an action potential propagates along a myelinated axon, an electric current flows through the extracellular fluid surrounding the myelin sheath & through the cytosol from 1 node to the next Action potential at the 1 st node generates ionic currents in the cytosol & extracellular fluid that depolarize the membrane to threshold opening Na+ channels at the 2 nd node

22. The resulting ion flow through the opened channels constitutes an action potential at the 2 nd node Then the action potential at the 2 nd node generates an ionic current that opens voltage gated Na+ channels at the 3 rd node & so on Each node repolarizes after it depolarises

23. Amount of myelination Action potentials propagate more rapidly along myelinated axons than along unmyelinated axons. Axon diameter Large diameter axons propagate action potentials faster than smaller ones due to their large surface area. Temperature Axons propagate action potentials at lower speed when cooled. FACTORS THAT AFFECT THE SPEED OF PROPAGATION

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