Action Potential

ACTION POTENTIAL Dr. Niranjan Murthy HL Asst Prof of Physiology

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-80 mV

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-80 mV

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[K+] = 2.5 [Na+] = 125 [Cl-] = 130 A+

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-80 mV

[K+] = 135 [Na+] = 7 [Cl-] = 11 A-

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Resting membrane potential • Difference in the electrical potential between inside and outside of cell at rest • RMP of excitable tissues: (i) Skeletal muscle: -90 mv (ii) Smooth muscle: -40 to -60 mv (iii) Cardiac muscle: -70 mv (iv) Nerve fiber: -70 to -90 mv

Distribution of ions across cell membrane

Values in mmol/L

Ionic channels of concern • Leaky cationic & anionic channels • Voltage gated Na+ channel • Voltage gated K+

Donnan Effect

Equilibrium potential ofsodium = +61 mv Potassium = -94 mv

Goldman-Hodgkin-Katz Equation

Measurement of membrane potential

Action potential • A brief & propagative change in the membrane potential after excitation

Phases of Action Potential 1. Resting phase: Polarized state; -70 mv 2. Depolarization phase: membrane becomes highly permeable to Na+ ions; membrane potential shoots above zero 3. Repolarization phase: Na+ channels close & K+ channels open; potential falls back to resting levels 4. After depolarization phase: final 1/3rd of repolarization; slow; 4 msec 5. After hyperpolarization phase: 1-2 mv hyperpolarization; 35-40 msec

Other terminologies • Latent period • Stimulus artifact • Threshold or firing level

Ionic basis of AP 1. Depolarization phase: It is due to opening of voltage-gated Na+ channels and pouring in of Na+ ions 2. Repolarization phase: due to closure of Na+ channels and opening of voltage-gated K+ channels & efflux of K+ ions 3. After depolarization: due to slow efflux of K+ ions 4. After hyperpolarization: due to delayed closure of voltage-gated K+

Voltage gated channels

Changes in Na+ & K+ conductance during an AP

Cathode ray oscilloscope

Voltage clamp techniques

Biphasic Action Potential

Compound Action Potential

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