Action PotentialsDR QAZI

OBJECTIVES

1. Define the action potential.

2. Describe the changes during action potential.

3. Discuss conduction (propagation) of action potential

4. Describe recording of monophasic action potential.

Action Potential

-Rapid, large alterations in the membrane potential during which time the membrane potential may change 100 mV,( -70 to +30), and then repolarize to its RMP

1. Nerve2. Muscle cells 3. Endocrine4. Immune5. Reproductive cells

FUNCTIONS OF AP

1. Relay a neuron’s message over a relatively long

distance, leading to NT release.

2. Relay the activation signal over the surface of a muscle

cell.

3. “Motivate” neuroendocrine cells to release hormones.

4. Spread an activation response over the membrane

surface of immune cells.

5. Relay the message of fertilization over the surface of

an egg.

Stimulus:sudden change of (internal or external) environmental condition - cell.

5

5 types

1.Submiminal

2. Miminal

3.Submaximal

4. Maximal

5. Supra maximal

6

Excitatory

Excitatory

Inhibitory

Time

Mem

bra

ne P

ote

nti

al (

mV

)

Temporal & Spatial Summation

Temporal Summation

a

b

c

d

a

b

c

d

Local Response1. It s a graded potential2. Its propagation is

electronic conduction3. Subthreshold stimulus4. It can be summed by 2

ways1. Spatial summation2. Temporal summation

1. All-or-none principle. 2. Amplitude- same.

CODING OF INFORMATION

1.

2.

3.

Weak stimulus

Moderate stimulus

Strong stimulus

Pattern = Intensity of stimulus frequency of APs

Place = type of stimulus Visual, auditory, pain, etc.

Brain area that receives signal Doctrine of Specific Nerve Energies

IS

Stages of AP 1. NORMAL, UNPOLARIZED, EQULIBRIUM 2. POLARIZED RMP 3. STIMULUS

4. ARTIFACT 5. FIRING LEVEL 6. ELECTROTONIC POTENTIAL GP 7. THRESHOLD

8. UPSTROKE / ASCENDING WAVE /DEPOLARIZATION 9. OVERSHOOT10. ZERO –LEVEL ISOPOTENTIAL PEAK

11. DOWN STROKE/DESCEDINGWAVE/ REPOLARIZATION12. SPIKE POTENTIAL 13. -ve AFTER DEPOLARISATION

14 +ve AFTERDEPOLARISATION / HYPERPOLARIZATION/

UNDER SHOOT

RMP= -65mV

12

-65

Electrotonic potentials & local response

10-20 mv < -ve than the RMP . trigger AP

2 types ;-

1. cat –electrotonic potentials= is a depolarising---- allows the Na+ ion to move in

2. an–electrotonic potentials = is a hyperpolarising current allows the k+ ion to move out

-70

-55

0

+30

-80

Time

Depolarization

Na+ influx

= 100 mVAmplitude

- 70 mV to +30 mV

Rapid depolarization

1. When partial depolarization reaches the activation threshold, voltage-gated sodium ion channels open.

2. Sodium ions rush in.

3. The membrane potential changes from -70mV to +40mV.

Na+

Na+

Na+

-

+

+

-

Depolarization

-70

-60

0

+35

-80

Time

Repolarization

K+ efflux

Repolarization

1. Na++ ion channels close and become refractory.

2. Depolarization triggers opening of voltage-gated K+ ion channels.

3. K+ ions rush out of the cell, repolarizing and then hyperpolarizing the membrane.

K+ K+

K+Na+

Na+

Na+

+

-

Repolarization

-70

-60

0

+35

-80

Time

After- hyperpolarization

Refractory Period

◦Absolute Lasts 1 msec Complete insensitivity exists to

another stimulus From beginning of action potential

until near end of repolarization. No matter how large the stimulus, a

second action potential cannot be produced.

Has consequences for function of muscle

◦Relative A stronger-than-threshold stimulus

can initiate another action potential

1. M gate= activation gate on Na channel; opens quickly when membrane is depolarized

2. H gate- inactivation gate on Na channel; Closes slowly after membrane is depolarized

Voltage-Gated Na++Channel

1. SINGLE GATE (N) that stays open as long as Vm is depolarized.

2. slowly this allows the Vm to depolarize due to Na influx;

3. Na and K currents do not offset each other right away

Gate on the Delayed Rectifier K+Channel

Hogkin’s cycle Positive feedback loop

Na+ enters(depolarization

V-gate Na+ channels open

Ionic Events UnderlyingAction Potentials

Action Potential Propagation:

1. Local Currents depolarize adjacent channels causingdepolarization and opening of adjacent Na channels

2. Question: Why doesn’t the action potential travel backward?

Transmission of a signal1. Think dominoes!

1. start the signal

knock down line of dominoes by tipping 1st one

trigger the signal

2. propagate the signal

do dominoes move down the line?

no, just a wave through them!

3. re-set the system

before you can do it again, have to set up dominoes again

reset the axon

Saltatory Conduction

Saltatory ConductionImpulse Conduction in Myelinated Neurons

11-34

Most Na+ channels concentrated at nodes. No myelin present.

Leakage of ions from one node to another destabilize the second leading to another action potential in the second node. And so on….

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SUMMARY