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ACTION POTENTIAL:

ACTION POTENTIAL:. Dr. Ayisha Qureshi Assistant Professor MBBS, MPhil. DEFINITIONS:. Stimulus: A stimulus is an external force or event which when applied to an excitable tissue produces a characteristic response. Subthreshold stimulus:

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ACTION POTENTIAL:

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  1. ACTION POTENTIAL: Dr. AyishaQureshi Assistant Professor MBBS, MPhil

  2. DEFINITIONS: Stimulus: A stimulus is an external force or event which when applied to an excitable tissue produces a characteristic response. Subthreshold stimulus: A stimulus which is too weak to produce a response is called a Subthreshold stimulus. Threshold stimulus: The minimum strength of stimulus that can produce excitation is called a Threshold stimulus. Suprathreshold stimulus: Stimuli having strengths higher than threshold stimulus are called Suprathreshold stimuli.

  3. REMEMBER: IMPORTANT: • Sodium voltage-gated channels:are fast channels & have 2 gates: - An outer Activation gate(closed in resting state) - An Inner Inactivation gate(open in resting state) • Potassium channels are slow channels & have only ONE gate. • These channels are different from Sodium & Potassium leak channels. • The Sodium-Potassium PUMP is present separately.

  4. Sodium & Potassium voltage-gated channels:

  5. Action potential:

  6. Action Potential: Definition: An Action Potential is a self-propagating wave of electro-negativity that passes along the surface of the axolemma of the nerve fibers.

  7. We know that the inside of the nerve membrane is negative with respect to the outside (RMP=—90 mv) • When an effective stimulus(threshold or suprathreshold) is applied, the electrical charge on the membrane is reversed: at the active part of the nerve fibre the outside becomes negative as compared to the corresponding region in the interior. This is called DEPOLARIZATION and forms the Action Potential.

  8. PHASES OF AN ACTION POTENTIAL: Phase 1: Depolarization Phase 2: Repolarization Phase 3: Hyperpolarization

  9. IONIC BASIS OF AN ACTION POTENTIAL: • DEPOLARIZATION: Sodium (Na) Influx • REPOLARIZATION: Potassium (K) Efflux • HYPERPOLARIZATION: Leakage of excess Potassium (K) ions through the slow closing K channels. • RETURN OF THE AP TO THE RMP FROM HYPERPOLARIZATION: Sodium-Potassium Pump

  10. Why does the depolarization not reach the Nernst potential of +66mv for sodium? There are 2 main reasons. At +35 mv: • Sodium Influx stops because Inactivation gates of Sodium channels close although the activation gates are open & thus no sodium can enter • Potassium Efflux starts because slow Potassium channel gates open and potassium moves out.

  11. State of SODIUM channel gates: • Resting state: - Inactivation gates: OPEN - Activation gates: CLOSED • Depolarization: - Activation gates: OPEN - Inactivation gates: OPEN • Peak: - Inactivation gates: CLOSED - Activation gates: OPEN • Repolarization: - Inactivation gates: OPEN - Activation gates: CLOSED

  12. VIVA QUESTIONS: • AFTER-DEPOLARIZATION: The descending limb of the action potential does not reach the baseline abruptly, but it shows a delay of several milliseconds. This is due to decreased rate of K efflux at this time. The excitability & conductivity of the fibre are increased during this phase. • AFTER-HYPERPOLARIZATION: Same as Hyperpolarization....

  13. DEFINITIONS: LATENT PERIOD: It is the time period between the application of a stimulus and the start of the response (Action Potential) DEPOLARIZATION: When during the transit changes in the action potential, the Potential difference between the inside of the membrane (-90mv) and outside (0mv) decreases it is called depolarization. ( the tracing will move upwards in the AP diagram) REPOLARIZATION: A return to the resting membrane potential from either direction (i.e. de- or hyper-polarization) is called repolarization. HYPERPOLARIZATION: When during the transit changes in the action potential, the Potential difference between the inside of the membrane (-90mv) and the outside (0mv) increases it is called Hyperpolarization.

  14. PROPAGATION OF AN ACTION POTENTIAL: Conduction of an Action Potential in an Unmyelinated nerve fibre:

  15. Question: • Why and how does the action potential spread in the forward direction only? • Why does NOT the action potential spread in the reverse direction?

  16. Unmyelinated Nerve fiber • Once an action potential is initiated at the axon hillock, no further triggering event is necessary to activate the remainder of the nerve fiber. The impulse is automatically conducted throughout the neuron. • For the action potential to spread from the active to the inactive areas, the inactive areas must somehow be depolarized to threshold. This depolarization is accomplished by local current flow between the area already undergoing an action potential and the adjacent inactive area • This depolarizing effect quickly brings the involved inactive area to threshold, at which time the voltage-gated Na channels in this region of the membrane are all thrown open, leading to an action potential in this previously inactive area. Meanwhile, the original active area returns to resting potential as a result of K efflux.

  17. VIVA Question: • Does the action potential become weak (decremental) as it travels down the nerve fiber? • NO, the action potential does NOT become weak as it travels down the nerve fiber. In fact, the AP does NOT travel down the nerve fiber but triggers a new AP in every new part of the membrane. It is like a “wave” at a stadium. Each section of spectators stands up (the rising phase of an action potential), then sits down (the falling phase) in sequence one after another as the wave moves around thestadium. The wave, not individual spectators, travels around the stadium.

  18. Thus, the last action potential at the end of the axon is identical to the original one, no matter how long the axon is. In this way, action potentials can serve as long-distance signals without becoming weak or distorted or decremental.

  19. VIVA Question: • Why does NOT the action potential spread in the reverse direction? • If AP were to spread in both directions, which is forward and backward, it would be chaos, with the numerous AP’s bouncing back & forth along the axon until the axon eventually fatigued. This does not happen due to the Refractory period. During and after the generation of an AP, the changing status of the voltage-gated Na and K channels prevents the AP from being generated in these areas again.

  20. Conduction of ap in a Myelinated nerve fiber:

  21. Continuous Conduction • Occurs in unmyelinated axons. • In this situation, the wave of de- and repolarization simply travels from one patch of membrane to the next adjacent patch. • APs moved in this fashion along the sarcolemma of a muscle fiber as well. • Analogous to dominoes falling.

  22. In a Myelinated Nerve Fibre an Action Potential travels by SALTATORY Conduction, which is in a jumping manner from one Node of Ranvier to the next Node of Ranvier, While in an Unmyelinated Nerve Fibre an Action Potential travels from POINT TO POINT. At the nodes of ranvier, there are an increased number of Sodium channels present.

  23. Which do you think has a faster rate of AP conduction – myelinated or unmyelinated axons?

  24. The answer is a myelinated axon. • If you can’t see why, then answer this question: Could you move 100ft faster if you walked heel to toe or if you bounded in a way that there were 3ft in between your feet with each step?

  25. Which do you think would conduct an AP faster – an axon with a large diameter or an axon with a small diameter?

  26. The answer is an axon with a large diameter. • If you can’t see why, then answer this question: Could you move faster if you walked through a hallway that was 6ft wide or if you walked through a hallway that was 1ft wide?

  27. Name the events & ions responsible for: • Depolarization • Repolarization • Hyperpolarization OR Undershoot • Return of the AP from the Overshoot to the RMP

  28. Properties of A nerve fibre:

  29. 1. ALL OR NONE LAW (also called the All or Nothing Law) On application of a stimulus, an excitable membrane either responds with a maximal or full-fledged action potential that spreads along the nerve fiber, or it does not respond with an action potential at all. This property is called the all-or-none law. (This is in direction proportion to the strength of the stimulus applied.) e.g: This is similar to firing a gun. Either the trigger is NOT pulled sufficiently to fire the gun (subthreshold stimulus) OR it is pulled hard enough to fire the gun (threshold is reached). Squeezing the trigger harder does not produce a greater explosion, just as pulling the trigger halfway does not cause the gun to fire halfway.

  30. Some Action Potential Questions • What does it mean when we say an AP is “all or none?” • Can you ever have ½ an AP? • How does the concept of threshold relate to the “all or none” notion? • Will one AP ever be bigger than another? • Why or why not?

  31. 2. Refractory period: - Absolute refractory period- relative Refractory period

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