Lecture 8 - Action potential.pdf
- 2. Action Potential (1 hr) • Initiation of AP • Behavior of ionic channels in AP • Stimulus parameters • All -or- none principle & refractoriness • Propagation of AP 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 2
- 3. • Action potential is developed when an excitable cell (nerve or muscle) get excited. • Excitability: the ability of a tissue to respond to a stimulus. • Stimulus: a change of environment (electrical or chemical) which elicit or tends to elicit a response. • Threshold stimulus: when the strength of the stimulus is just adequate to elicit a response. – When a stimulus is applied whose strength is threshold or above the tissue responds by developing an action potential. Action Potential 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 3
- 4. • Polarized state – is resting condition, when cell maintains a potential difference across the membrane, the interior being negative as compared to exterior. • Depolarization – when potential difference across the membrane is reduced, i.e. interior becomes less negative. • Repolarization – after depolarization, the membrane returns to its polarized state. • Hyperpolarization – when the interior of the cell becomes more negative. • Cells become more excitable (get excited by sub threshold stimulus) during depolarization. • Cells become less excitable (can be excited by only supra threshold stimulus) during hyperpolarization. 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 4
- 5. Initiation of the Action Potential • Positive-feedback vicious cycle opens more sodium channels • First, as long as the membrane of the nerve fibre remains undisturbed, no action potential occurs. • Any event causes enough initial rise in membrane potential (from –90 millivolts toward zero level), causes beginning of opening many voltage-gated sodium channels. – This allows rapid inflow of sodium ions, which causes a further rise in the membrane potential, thus opening still more voltage- gated sodium channels and allowing more streaming of sodium ions to the interior 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 5
- 6. 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 6 Depolarization Opening of more Na channels – Positive feedback Opening of K channels during repolarization & Closure (Negative feedback) returns to Polarized state
- 7. Threshold for Initiation of the Action Potential • This occurs when the number of Na+ ions entering the fiber becomes greater than the number of K+ ions leaving the fibre. • A sudden rise in membrane potential of 15 to 30 millivolts (firing level ) usually is required. • Sudden increase in the membrane potential in a large nerve fibre from –90 millivolts up to about –65 millivolts usually causes the explosive development of an action potential. 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 7
- 8. Action Potential • Action potential is temporary reversal in the membrane potential that is transmitted along the nerve / muscle (membrane and tubule). • Action potentials are obtained when a nerve or a muscle gets excited by electrical, mechanical, thermal or chemical stimulus. • The strength of stimulus must be threshold level or above (so that the depolarization reaches firing level) to set off an action potential. 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 8
- 9. 1. RMP 2. Depolarizing stimulus. 3. Depolarizes to firing level. 4. Rapid Na+ entry cause rapid depolarization. 5. Na+ channels close, K+ channels open. 6. Repolarization K+ comes out 7. Hyperpolarization: Additional K+ leaves the cell 8. K+ channels close 9. RMP established. 1 2 3 4 5 6 7 8 9 30 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 9
- 10. Action Potential 29/7/2021 • Stimulus depolarizes membrane potential to firing level. • Opening of all voltage gated (activation gates) Na+ channels (Fast channels). – Na+ rush inside the cell towards the concentration and electrical gradient. – Rapid depolarizing phase of action potential is due to rapid entry of Na+ and membrane potential comes to + 30 mV. • At this point (+ 30 mV): – Na+ Channels close. Prevent further entry of Na+ halts depolarization. – K+ channels open, K+ start coming out of cell causing repolarization. Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 10
- 11. Action Potential 29/7/2021 • After repolarization, reaches RMP, the ionic gates of Na+ channels are restored. • But K+ channels are slow to close. Additional amount of K+ leave cell causing hyperpolarization. • Eventually the slow K+ channels close and membrane potential comes back to resting level. Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 11
- 12. 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 12
- 13. All or none law All or none law • When a stimulus is applied to an excitable tissue either it will not respond at all or it will respond maximally. • This means if the strength of stimulus is less than that of threshold there will be no response (no action potential). • If the strength of stimulus is at threshold level, the tissue responds maximally . • Further increase in strength of stimulus (no matter how much) beyond threshold does not increase the response. i.e., amplitude of action potential does not increase. 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 13
- 14. All- or- none law Action potential in an average neurone • The amplitude and duration (3 milli seconds ) remain same in spite of wide change of intensity of the stimulus 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 14
- 15. Refractory Period • After an excitable tissue is excited, for a brief period of time it does not respond to a second stimulus. This period is called refractory period. It is divided into two parts : • Absolute refractory period : the tissue remain completely unexcitable • Relative refractory period: the tissue responds to a strong (supra threshold) stimulus. 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 15
- 16. •In an excitable tissue, a new action potential can only be formed when the preceding action potential is over and potential has returned to the original RMP. •T he reason is the Na+ channel remain closed till most of the repolarization has occurred. • During the hyperpolarized state (relative refractory period) the membrane is less excitable and therefore needs strong stimulus for excitation. Refractory Period 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 16
- 17. Propagation of the Action Potential • Action potential elicited at any one point on an excitable membrane usually excites adjacent portions of membrane, resulting in propagation of the action potential along membrane. • Nerve or muscle impulse : Transmission of depolarization along a nerve or muscle fibre • When nerve fibre has been excited, – permeability to sodium increases “local circuit” of current flow from depolarized areas of membrane to adjacent resting membrane areas. – i.e., positive electrical charges are carried by inward-diffusing sodium ions through the depolarized membrane, for several millimeters in both directions along the core of the axon. 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 17
- 18. Conduction of action potential in both directions along the conducting nerve 29/7/2021 Prof Dr K M Padmavathy MBBS, MD Physiology, Wollega University 18