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GENESIS OF RESTING

MEMBRANE POTENTIAL

WHAT IS RESTING MEMBRANE POTENTIAL (RMP)In a resting cell/neuron, there is difference in electrical charge on

either side of the membrane ( i.e Resting Mem. Potential )

+ +

+ +

+ +

+ +

+ +

+ + outside (ECF)

inside

(ICF)

Normal value

- 70 mV (Nerve)

- 90 mV (Muscle)

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FACTORS CONTROLLING RMP

1. Unequal distribution of ions or nonelectrolytes (proteins)

. Main ions in ECF Na

+

, CL

-

, HCO

-

3

. Main ions in ICF K+, HPO

-

4

. Main nonelectrolyte in ICF proteins

[Net positive charge outside and net negative charge inside]

2. Selective permeability of plasma membrane to ions & nonelectrolytes

. K+ permeability is 100 times greater than to Na+ ions

. Not permeable to protein, organic phosphate and organic ions. (i.e more

negative charge inside)

3. Na+- K

+pump mechanism

. 3 Na+

out of the cell for every 2 K+

it pumps in leading to net loss of + charge

inside the cell

MAGNITUDE OF THE RMP IS DETERMINED BY

1. Mainly distribution of Na+, K

+,CL and Proteins

-

2. Permeability of the memb. To Na+, K

+,CL and Proteins

3. The activity of Na

+

- K

+

pump

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IONIC BASIC OF LOCAL RESPONSE

(Non-propagated local potential)

. In a resting neuron, the membrane is more permeable

to K+

than Na+

. When a neuron is stimulated with subthreshold

stimulus, the permeability of the membrane to Na+

increased at the point of stimulation and there is

influx of Na+

into the cell (depolarization) leading to

decreased RMP (more positive, less negative)

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IONIC BASIC OF ACTION POTENTIAL

( Propagated Potential )

. Threshold stimulus

. The permeability of the membrane to Na+

( i.e Na+

conductance increased as a result of opening of voltage

gated Na+

channel )

. Na+

influx into the cell ( depolarization)

( opening of voltage gated Na+

channel is short-lived and soon

closed. Therefore Na+

conductance decreased. )

. At the same time, the permeability of the membrane to K+ asa result of opening of voltage gated K

+channels. It leads to K

+

efflux ( repolarization ). ( The opening of K+

channel is slower

and more prolonged then the opening of Na+ channels.)

. Na+- K

+pump restores the concentration gradient of Na

+and

K

+

to its original.

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ACTION POTENTIAL

+ 40

+ 20

0

- 20

- 40

- 60

- 70

- 80

13

2 4

A B

1 2

A = Depolarization

B = repolarization

1 = Absolute refractory period

2 = Relative refractory period

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CHARACTERISTICS OF LOCAL NON-PROPAGATED POTENTIAL. Does not obey all or non law

. Can not transmit impulse

. Can summate ( add up ) to reach the threshold

. Has no refractory period

CHARACTERISTICS OF ACTION POTENTIAL ( AP )

2 types of refractory period

1. Absolute refractory period ( i.e period of time during which a second AP can

not be initiated even with a very strong stimulus )

2. Relative refractory period ( i.e period of time during which a second AP can

be initiated but only by a stronger than normal stimulus - superthreshold

stimulus )

. Threshold stimuli can only generate AP

. Obey all or non principle

( i.e Superthreshold stimuli amplitude of AP remained the same )

. Has refractory period ( I.e period of time during which the neuron can not

generate another AP )

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ACTION POTENTIAL

occurs in living organisms to produce physiological

effects such as :

1. Transmission of nerve impulse along nerve fibres

2. Release of chemical transmitters in synapse

3. Contraction of muscle

4. Activation or inhibition of glandular secretion