DR.ZAHOOR ALI SHAIKH LECTURE GRADED POTENTIAL & ACTION POTENTIAL

Different Stimuli Cause Electrical Signals 2 Electrical Signals are produced due to changes in ions movement across the membrane. Electrical Signals produce: 1. Graded Potential or Local Potential 2. Action Potential – It sends signal over long distance.

GRADED POTENTIAL 3

4 Graded Potential is local change in the membrane potential. E.g. RMP changes from -70 mv to -60 mv (a 10mv change). Graded Potential is due to Na+ entry at the small region of plasma membrane. It is localized change in the membrane. Graded Potential can be summated by giving stronger stimulus.

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GRADED POTENTIAL ‘Important Points’ 6 It is localized. It can be summated. Longer the stimulus – longer the duration of graded potential. Graded Potential die down over short distance. Example of Graded Potential: - Receptor Potential, Pace-maker Potential

ACTION POTENTIAL 7

8 Action Potential is brief, rapid, large, about 100 mV change in Membrane potential in which inside of excitable cell becomes more positive than outside, for short time. Action Potential are conducted or propagated.

ACTION POTENTIAL (cont) 9  How action potential occurs ? When stimulus is applied, RMP which is -70mV changes to Threshold potential which is -50 or -55 Mv. At threshold potential, rapid depolarization takes place and inside membrane becomes more positive.

ACTION POTENTIAL (cont) 10  What is the cause of Depolarization? Depolarization is due to Na+ influx (going inside). Peak potential is usually at +30 mV. After depolarization, membrane Repolarizes quickly, dropping back to Resting Membrane Potential -70 mV.

ACTION POTENTIAL (cont) 11  What is the cause of Repolarization ? Repolarization is due to k+ efflux (going outside). If more K+ goes out, it causes more Negativity inside e.g. -80 mV and it is called HYPERPOLARIZATION. Time for AP in neuron is 1 msec.

ACTION POTENTIAL (cont) 12 AP is referred as Spike potential because it appearance looks like spike. When excitable membrane produces AP, it is said it is Firing. Therefore Action potential, Spike, Firing all refer to same thing.

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ACTION POTENTIAL ‘Important Points’ 16 If initial stimulus (triggered depolarization) does not reach threshold stimulus – NO AP takes place. Therefore Threshold level is ALL or NONE point.

ACTION POTENTIAL (cont) 17 Action Potential follows All or None Law. It means excitable membrane either responds to a stimulus with a maximal action potential or it does not respond with an action potential.

Refractory period during AP 18 Refractory period is that period,during which no new action potential can be initiated. Refractory Period – Two Types: 1– Absolute Refractory period 2– Relative Refractory

Refractory Period 19  Absolute Refractory Period It is that period of action potential during which no new action potential can be initiated even by strong stimulus.  Relative Refractory period It is that period during which second action potential can be produced by very strong stimulus.

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Comparison of Graded Potentials and Action Potentials Graded Potential Action Potential Stimulus does not reach threshold level. 2. Stimulus causes local change in membrane potential e.g. -70 to -60mv 3. It dies down over short distance. 4. Can be summated. 5. Does not obey all or none law. 1. Stimulus reaches threshold level therefore causes AP. 2. Stimulus causes depolarization. 3. It is propagated. 4. Can not be summated. 5. Obeys all or none law.

What You Should Know From This Lecture 22 Graded potential ( local potential ) and its characteristics Threshold level Action potential (AP) -- Depolarization, Repolarization, Hyperpolarization. Absolute refractory period during AP Relative refractory period AP Difference between Graded potential and Action potential

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