1. Lecture 4- Action Potential propagation and synaptic transmission ©Dr Bill Phillips 2002, Dept of Physiology Continuous Propagation of action potentials Saltatory Propagation Electrical and chemical synapses Reasons for chemical synapses

2. Continuous propagation The action potential normally starts at the axon hillock where the density of voltage- gated Na + channels is high In un-myelinated nerves the action potential propagates continously along the axons by sequentially activating populations of Na + channels in adjoining segments of axon

3. Continuous Propagation of the AP in un-myelinated nerve fibres

4. The membrane has electrical properties that affect the spread of changes in membrane potential but allow leakage of local circuit currents out of the membrane impedes the depolarising current down the membrane stored charge must be discharged

5. Electrical components of the resting membrane

6. Saltatory Propagation Adaptation to permit faster propagation Myelin internodes formed by glial (Schwann) cells wrapped around internode regions of axons (~0.2mm long) Inward current during the rising phase of the action potential creates “local circuits” Local circuits depolarise neigbouring “Node of Ranvier” stimulating regeneration of the action potential

8. In saltatory propagation: Voltage-gated Na + channels are concentrated at the axon hillock and Nodes of Ranvier The Hodgkin Cycle is triggered at one Node after another. This amplifies the signal. The signal travels passively as an electrical current between Nodes. The thick myelin insulation of the Internode allows the local circuit current to spread much further and faster than in un-myelinated fibres

9. Synaptic transmission

10. Synaptic connections between cells Electrical synapses: local circuit currents flow through gap-junctions between adjacent cells Chemical Synapses: chemical neurotransmitter substance is released by the presynaptic terminal onto the postsynaptic cell to activate its receptor proteins

11. What is a chemical synapse Nerve terminal releases chemical neurotransmitter when action potential depolarises the terminal membrane Chemical transmitter diffuses across synaptic cleft and binds to a receptor protein Receptor protein on postsynaptic membrane initiates changes in the postsynaptic cell

12. Reasons for having chemical synapses A mplification I nhibition M odification P lasticity

13. A mplification Depolarising current Small < Large

14. I nhibition Presynaptic Depolarisation Postsynaptic Hyperpolarisation

15. M odification

16. P lasticity