1. Nerve Action potential L 21

2. Learning objectives Genesis of nerve action potential
Potential changes, phases of AP
Ionic basis
Role of voltage gated channels
Differences between AP & electrotonic potentials
Properties of Nerve action potential
Propagation of action potentials in nerve fibers
All or none law
Refractory period and significance

3. NERVE ACTION POTENTIAL
Defined as Rapid, transient & large changes in membrane potential resulting in reversal of polarity, which is conducted.
Develops only at the threshold potential.
Follows a ‘triggering event’(stimulus).
Consists of rapid depolarization followed by rapid repolarization of membrane .

4. DEFINITIONS Polarized state Electrotonic potentials Depolarization
Repolarization
Hyperpolarization
Threshold potential (Firing level)
Spike potential

5. Polarized state ECF ICF ECF AXON +++++++++++++++++++++
AXON
ICF
Polarized state

6. mV -60 -70 -80 Threshold potential DEPOLARIZATION REPOLARIZATION RMP
HYPERPOLARIZATION

7. Electrotonic potentials
catelectrotonic
anelectrotonic

8. Response of a Nerve fibre to subthreshold stimuli of various strengths
Volt mv
- 55 mv
Threshold
-70mv
RMP
Time in msecs

9. Local or Electrotonic potentials (GRADED)
Names depending on the site: eg, receptor potential, synaptic potential, end plate potential
-Not well propagated
-Magnitude directly proportional to stimulus strength (GRADED)
-Depolarozing (cathode) or hyperpolarizing(anode)
-Summation possible
-No threshold
- No latent period
-No refractory period

11. Configuration of monophasic AP from a single nerve fiber

12. Phases of an Action Potential
Stimulus artifact
Latent period
A short phase of slow depolarization
At a particular level of membrane potential (threshold level), there appears a sharp upstroke-the rate of depolarization is markedly increased called as Rapid depolarization.

13. The membrane potential reaches about +35 mv.
There is a rapid down stroke- i.e rapid repolarization.
The sharp upstroke and down stroke constitute a spike potential
A brief period of “after-hyperpolarization” commonly follows the phase of repolarization i.e. the membrane potential falls even below the RMP
Finally RMP is restored

15. Latent period
It is the period between the stimulus artifact and beginning of depolarization
Duration of Latent period determined by:
1.Distance between the stimulating electrodes and the recording electrodes
2. Rate of conduction of nerve impulse (velocity of conduction or speed of nerve)

16. Depolarization
Depolarization up to 7mv by stimulus opens few Na+ channels.
Depolarization of cell membrane up to 15 mV – i.e at memb pot. of -55 mV, the voltage -gated Na channels open fully
The membrane potential at which Na+ channels fully open (-55 mV) is known as “firing level”
Positively charged Na+ enter the cell, along electrical & chemical gradient.
When inside is +35mv, influx of Na+ stops. Actual equilibrium potential for Na+ is +60 mv, reason why it doesn’t reach that level is, Na+ conductance is short lived due channels attaining inactivated state. And the gradient for net movement also reversed.

17. Ionic basis of nerve action potential
Rapid depolarization:
Opening of Voltage Gated Na+ channels
Explosive increase in permeability at threshold level
Na+ influx along electrochemical gradient Ra

18. (decreased membrane potential) POSITIVE FEEDBACK CYCLE
Triggering event
Depolarization
(decreased membrane potential)
Na+ influx
Opening of voltage
gated channels
POSITIVE FEEDBACK CYCLE

22. Repolarization
The repolarization is caused by opening of voltage-gated K+ channels
K+ open slowly and close slowly
The net movement of completes the process of repolarization (inside becomes negative)
As the closure of K+ is slow process, repolarization goes further down, is followed by a small period of after-hyperpolarization.
Now the Na+-K+ pump starts operating and bring Na+ and K+ back to the pre-action potential position

23. Characteristics of Action potential
1.Action potential follows all or none law.
The height of an action potential is always same, irrespective of the strength of stimulus. (all)
No action potential is produced if the stimulus is subthreshold (none)
The voltage-gated Na+ channels open to the same extent if the stimulus is threshold or suprathreshold
2. Once generated, AP is conducted in the tissue

24. Criteria
Graded Potential
Action Potential
Size
2.Polarity
3.Propagation
4. Summation
5. Threshold
6. Refractory Period
Graded, depends on stimulus streg
Depol or Hyper
Not well
propagated
Possible
Nil
All or none
Depolarization
Well propagated
Not possible
15 mV Depol
Present

25. Initiation of action potentials in body
In a sensory neuron, at the beginning of nerve fiber (at the first node of Ranvier if myelinated) by stimulation of sensory receptors.
Triggering event is the ‘GENERATOR potential’.
‘GENERATOR potential’ is an example of graded potential.

26. Initiation of action potentials in a sensory neuron
First node of Ranvier (ACTION POTENTIAL) To
CNS
Sensory receptor
(GENERATOR POTENTIAL)
Initiation of action potentials in a sensory neuron

27. Initiation of action potentials in body
In a motor neuron (or in an interneuron), at the ‘initial segment’ by the synaptic activity at the dendrites.
Triggering event is the Synaptic activity at the dendrites (or at the soma) that generates ‘Excitatory Post Synaptic Potentials’.
‘Excitatory Post Synaptic Potentials’ are examples of graded potentials.

28. Initiation of action potentials in a motor neuron
Initial segment (ACTION POTENTIAL) To
muscle
Dendritic zone
(EPSP)
Initiation of action potentials in a motor neuron

29. Properties of nerve action potential
Self propagation (non-decremental)- conduction types-point to point (in non myelinated) & saltatory (in myelinated nerve)
All-or-None law
Refractory period

30. Self propagation of action potential
Action potential is propagated in an unmyelinated axon fiber by successive electrotonic depolarization to the threshold, of the membrane ahead of the action potential
In a myelinated axon successive depolarization occurs at the nodes of Ranvier (Saltatory conduction).

31. Propagation of action potential (impulse) in non myelinated axon (top) myelinated axon (bottom)

32. Conduction What are the meanings of these terms ? Bidirectional
Orthodromic conduction
Antidromic conduction

33. All or none phenomenon- response to varying stimuli strength

34. Refractory period
Period of non-responsiveness of membrane to another stimulus
Two components
Absolute refractory period
Relative refractory period
Ensures one-way propagation of action potential
Also limits frequency of action potentials

35. Absolute refractory period
interval during which second AP cannot be initiated, regardless the strength of stimulus/li.
Occurs from firing level to 1/3 of repolarization phase.
Due to inactivation of Na+ channel.
Na+ channel will not open until membrane repolarizes.

36. Relative refractory period
interval during which second AP may not be initiated, but still possible to be generated.
Occurs from starting of 1/3 of repolarization phase to starting point of hyperpolarization phase.
Due to delay in closing of voltage-gated K+ channel leads to more K+ efflux from cell - membrane potential lower than RMP.

37. Changes in excitability during an AP

38. So the excitability of the axon varies during Action potential
Other factors influencing excitability are:
Ions: calcium, potassium, Sodium
Temperature
Timing of stimulus
Local anesthesia

39. Learning outcomes Describe phases of an Action Potential
Describe the characteristics of an AP
Differentiate Action potential & electrotonic potentials
Describe propagation of AP along a myelinated & un myelinated nerve fiber
Describe the properties of Action potential
Describe changes in excitability during
different phases of AP.