Lecture 5 - Electrical Properties Of Cells - Action Potentials PDF

Electrical Properties of Cells: M S D C 14 0 1 : EN VIRONMENT & Action Potentials H E A LT H FA R I D F Y O U S S E F Definition An action potential is transient and regenerative c...

Electrical Properties of Cells: M S D C 14 0 1 : EN VIRONMENT & Action Potentials H E A LT H FA R I D F Y O U S S E F Definition An action potential is transient and regenerative change in the membrane potential that transmits electrical signals along nerve fibres This allows communication within the body Local Potentials are small changes in the resting membrane potential that are not propagated i.e. they are localized  LOCAL POTENTIALS ACTION POTENTIALS Graded Response All or nothing response Can be summated Cannot be summated Both depolarizing and Only depolarizing hyperpolarizing Conducted with decrement Conducted without decrement (are (decrease in size as the move from regenerative) their point of orgin) An action potential has four phases: Phase 0 – corresponds to the resting membrane potential and initial phase of slow depolarization below threshold Phase 1 – the rising phase, also known as the depolarization phase Phase 2 – the descending phase, also known as repolarization phase Phase 3 – the overshoot or hyperpolarization An action potential is initiated when stimulation is strong enough to cause the rmp to reach threshold (about -50mV) At threshold a positive feedback cycle is established Phase 1: End Properties of Voltage Gated Na Channels Properties of Voltage Gated Na Channels They open rapidly with little delay. They stay open for about 1msec and then automatically close (inactive). They stay in an inactivated state for a fixed period of time and cannot be depolorized again Several thousand channels need to open to trigger an action potential  Properties of K Channels Therefore at the peak of the action potential Na channels are closing and K channels are opening initiating the repolarization phase The hyperpolarization phase is a result of the duration of time the K channels stay open. N.B. they do not inactivate but rather deactivate once membrane potential returns to normal 10 I K+ 0 K: slow activation & no inactivation -10 Activates at potentials far from Vm Vm = -75 mV Vm = -30 mV Vrest= -90 mV, voltage clamp Refractory Period Conduction of Action Potentials Conduction of Action Potentials Conduction of Action Potentials Speed of Conduction Length Constant – distance over which response decays to 37% of regional size Action Potentials travel faster with greater length constant What does myelin do to the length constant? Saltatory Conduction Electrical Properties of Cells: M S D C 14 0 1 : EN VIRONMENT & Action Potentials H E A LT H FA R I D F Y O U S S E F