Action potential Types of stimulus Threshold stimulus The

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Action potential

Action potential

ØTypes of stimulus: ØThreshold stimulus: The stimulus with the intensity equal to threshold ØSubthreshold

ØTypes of stimulus: ØThreshold stimulus: The stimulus with the intensity equal to threshold ØSubthreshold stimulus: The stimulus with the intensity weaker than the threshold ØSuprathreshold stimulus: The stimulus with the intensity greater than the threshold. 2

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Recording of action potential in a nerve fiber

Recording of action potential in a nerve fiber

 • Sudden change in the membrane potential from normal RMP to a positive

• Sudden change in the membrane potential from normal RMP to a positive potential and coming back to Resting stage. Phases of AP • Resting stage • Depolarization stage • Repolarization stage

stages of action potential RESTING STAGE Membrane is polarized at rest RMP = -90

stages of action potential RESTING STAGE Membrane is polarized at rest RMP = -90 mv DEPOLARIZATION STAGE • Permeable to Na +ions • Mp becomes positive • opening of voltage gated Na + Channels

Depolarization

Depolarization

Repolarisation stage • Permeable to K+ • MP becomes negative • Opening of voltage

Repolarisation stage • Permeable to K+ • MP becomes negative • Opening of voltage gated K+ channel

Repolarization

Repolarization

Phase of Repolarization • Rapid falling phase • After-depolarization Phase of After-hyperpolarization • More

Phase of Repolarization • Rapid falling phase • After-depolarization Phase of After-hyperpolarization • More negative than RMP • MP returns to resting level

Ionic basis of action potential Stimulus artifact • Irregular deflection of baseline due to

Ionic basis of action potential Stimulus artifact • Irregular deflection of baseline due to current leakage from stimulating electrode to recording electrode Latent period • Time taken for the impulse to travel from stimulating electrode to recording electrode.

Depolarization • Reduction of membrane potential from negative to zero Firing level • Initial

Depolarization • Reduction of membrane potential from negative to zero Firing level • Initial 15 mv of depolarization • Opening of voltage gated Na +channels

Action potentials: Rapid depolarization • When partial depolarization reaches the activation threshold, voltage-gated sodium

Action potentials: Rapid depolarization • When partial depolarization reaches the activation threshold, voltage-gated sodium ion channels open. • Sodium ions rush in. • The membrane potential changes from -70 m. V to +40 m. V. + - - Na+ Na+ +

Zero potential • Overshoots zero potential to +35 mv • Repolarisation • K+ efflux

Zero potential • Overshoots zero potential to +35 mv • Repolarisation • K+ efflux due to opening of voltage gated K+ channels

Action potentials: Repolarization • Sodium ion channels close and become refractory. • Depolarization triggers

Action potentials: Repolarization • Sodium ion channels close and become refractory. • Depolarization triggers opening of voltage-gated potassium ion channels. • K+ ions rush out of the cell, repolarizing and then hyperpolarizing the membrane. Na+ K+ K+ + -

After depolarization • Slow k+ efflux After hyper polarization • Na+-k+ pump is activated

After depolarization • Slow k+ efflux After hyper polarization • Na+-k+ pump is activated to achieve the ionic composition

Voltage-gated + Na channels • These channels have two voltage sensitive gates. • At

Voltage-gated + Na channels • These channels have two voltage sensitive gates. • At resting Em, one gate is closed and the other is open. • When the membrane becomes depolarized enough, the second gate will open. • After a short time, the second gate will then shut.

Voltage-gated • Voltage-gated K+ channels have only one gate. • This gate is also

Voltage-gated • Voltage-gated K+ channels have only one gate. • This gate is also activated by depolarization. • However, this gate is much slower to respond to the depolarization. + K channels

Andrew Fielding Huxley “for their discoveries concerning the ionic mechanisms involved in excitation and

Andrew Fielding Huxley “for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane” The Nobel Prize in Physiology or Medicine (1963) Alan Lloyd Hodgkin 22

Mechanisms of action of local and general anesthetics & venoms: Local anesthetics attach to

Mechanisms of action of local and general anesthetics & venoms: Local anesthetics attach to Na+ channels, preventing Na+ inflow General anesthetics (ether, chloroform) Open K+ channels: clamp potential Scorpion Venom: Keeps Na+ channels open and K+ channels closed Tetrodotoxin (TTX, from puffer fish) blocks Na+ channels Cyanide blocks ATP-dependent Na+-K+ pump

Properties of action potential: • Threshold potential • All or none law • Propagative

Properties of action potential: • Threshold potential • All or none law • Propagative • Cannot be summated