Action Potentials DR QAZI. OBJECTIVES 1.Define the action potential. 2.Describe the changes during...
36
Action Potential DR QAZI
Transcript of Action Potentials DR QAZI. OBJECTIVES 1.Define the action potential. 2.Describe the changes during...
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….
:
:
:
SUMMARY
