slide on membrane potential

Membrane potential
n Electrical potential exist across cell membrane

n Generates electrochemical impulses
Membrane potential caused by diffusion
n K concentration is very great inside the cell
n K concentration is very low outside the cell
n Assume that the membrane is permeable only to k
n Concentration gradients dictates K to diffuse out
n K carry + charges resulting to electropositvity outside and electronegativity inside
n Thus concentration difference of ions across a permeable membrane cause the membrane potential
Diffusion potential and concenetration difference
n Nearnst potential –potential level across the membrane that prevents diffusion of the ion in either direction

- measured by the ratio of ion concentration
from two sides of the membrane
n Nearnst potential

n EMF(millivolts) = +61 log conc. I
______
conc. O
n Calculated potential is the potential inside the cell
n When the membrane is permeable to other ions
the diffusion potential that develops depend
on the following:

1. polarity of the electrical charge
2. permeability of the membrane
3. concentration of the respective ions in
and out of the membrane
n Goldman hogkin katz equation
Resting membrane potential of a nerve
n membrane potential of a large nerve = -90m
n Origin of membrane potential
1. from the K diffusion potential = -94mv
2.from the Na diffusion potential = +61mv
considering both with goldman equation
= -86mv
3.from Na and K pump = -4mv
Nerve action potential
n Rapid changes in the membrane potential
n Conduction of nerve signal
n Stages of action potential

1.resting stage- membrane is polarized
- negative membrane potential

2. depolarization stage – membrane becomes
permeable to Na
- polarized state is lost

3. repolarization stage – Na channel closes
- K channel opens
Voltage gated sodium and potassium channel
n Role in causing action potential
n Voltage gated Na channel has 2 gates
1. activation gate
2. inactivation gate
- at resting state(-90mv) activation gate is closed
and inactivation gate is open
- when membrane potential becomes less negative
(-70 to -50mv) conformational change in
activation gate open Na flow toward inside
- this is the activated state
n Inactivation of Na channel

n Closure of the inactivation gate follows a few 10,000th of a second after the activation gate is open
Role of other ions during action potential
n 1. impermeant negatively charged ions inside the axon

-protein molecules, organic and phosphate compounds

-deficit in positive ions inside the membrane leave an
excess negatively charged ion
n Increase permeability of Na channels when there is deficit of calcium ions

-Na channels are activated by very little increase of
membrane potential above normal level – highly
excitable state tetany


- calcium ions binds with exterior surface of Na channel
increasing the requirement to open the gate
n 3.chloride ions
- leak channel
- passive role
Initiation of action potential
n Any rise in membrane potential

n Opening of the voltage gated Na channel
Threshold for action potential
n Sudden rise of membrane potential by
15 to 30mv
n Threshold for stimulation is -65mv
Accommodation of the membrane
n Failure to fire despite rising voltage
n Very slow rise in membrane potential
n Inactivating gates will have time to close at the
same time that the activating gates are opening
n Ineffective Na flow
Propagation of action potential
n Excitation of adjacent area
n Transmission of depolarization along nerve
or muscle IMPULSE
n Direction of depolarization all branches
of nerve fiber
all or nothing principle
n Once an action potential has been elicited at any
point on the membrane, the depolarization
process will travel over entire membrane if
the conditions are right or it might not travel
at all if conditions are not right

n Safety factor for propagation
Initiation and excitation of action potential
n Any factor causing Na diffusion
n Mechanical
n Chemical
n Electrical
n Threshold for initiation of action potential

- rise in membrane potential of 15-30mv

- -65mv ---- threshold for stimulation

Plateau in some action potential
n Membrane does not repolarize immediately after
depolarization--- heart muscle
n causes: 2 types of channel
1.fast channel – v gated Na channel
2.slow channel – allows diffusion mostly calcium but
also Na
--activation of fast channel causes spike portion
-- slow but prolonged activation of slow channel is responsible for plateau
-- v-gated K channel are slowly activated sometimes – delays the return
of membrane potential

rhythmicity of action potential
n Repetitive discharge
n Heart muscle, smooth muscle, neurons of CNS

n Reexcitation process – resting membrane must already be
permeable enough to Na ions
n Resting membrane potential of -60 to -70mv not enough to
keep the Na channel and Ca channel closed

special aspect of action potential
n Myelinated and unmyelinated fibers
n Myelin sheath is deposited around axon by Schwann cell

n Sphyngomyelin – excellent insulator
n Node of Ranvier – uninsulated areas
n Saltatory conduction in myelinated fibers

n Value of saltatory conduction
1. increases velocity of nerve conduction
2. conserve energy
3. allows repolarization process to occur with very
little transfer of ions
excitation
n Any factor that can cause Na ion influx
n Maybe mechanical, chemical or electrical factor
n Decrease in electrical voltage across membrane

n Threshold for excitation and acute local potentials
-a weak stimulus may not be able to excite a fiber
- progressive increased in stimulus will reach a point when
excitation does take place
inhibition of excitability
n Stabilizers and local anesthesia
n Decrease membrane excitability
n Increase extracellular calcium
n Decrease extracellular calcium level
n Local anesthesia makes activation gate difficult to open