1. Action Potential
(L4)

2. Electrical signals in neurons
Neurons are electrically excitable due to the voltage difference across their membrane
Communicate with 2 types of electric signals
graded potentials that are local membrane change
action potentials that can travel long distances

3. Graded Potentials
They are either excitatory (Post Synaptic Potential; EPSP) or Inhibitory Post Synaptic Potential (IPSP)
They are localized. occur most often in the dendrites and cell body of a neuron and don’t travel to axons.
The signals vary in amplitude (size), depending on the strength of the stimulus.
They can summate in one of the two ways:
Spatial summation of effects of neurotransmitters released from several end bulbs onto one neuron.
Temporal summation neurotransmitters released from 2 or more firings of the same end bulb in rapid succession onto a second neuron

4. Action Potentials
How signals travel
from here to here?

5. Signal transmission at the chemical synapse
Action potential reaches end bulb and voltage-gated Ca2+ channels open
Ca2+ flows inward triggering release of neurotransmitter
Neurotransmitter crosses synaptic cleft & binding to ligand-gated receptors
the more neurotransmitter released the greater the change in potential of the postsynaptic cell
Synaptic delay is 0.5 msec
One-way information transfer

6. LENGTH CONSTANT

7. Three Possible Responses
Small EPSP occurs
potential reaches -56 mV only
An impulse is generated
threshold was reached
membrane potential of at least -55 mV
IPSP occurs
membrane hyperpolarized
potential drops below -70 mV
Copyright 2009 John Wiley & Sons, Inc.

8. Generation of Action Potentials
An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization).
During an action potential, voltage-gated Na+ and K+ channels open in sequence
According to the all-or-none principle, if a stimulus reaches threshold, the action potential is always the same.
A stronger stimulus will not cause a larger impulse.
Copyright 2009 John Wiley & Sons, Inc.

9. VOLTAGE-GATED SODIUM CHANNELS

10. VOLTAGE-GATED POTASSIUM CHANNELS

11. Action Potentials

12. Generation of Action Potentials

13. IONIC MOVEMENTS RESPONSIBLE FOR ACTION POTENTIAL

14. Refractory period
Period of time during which neuron can not generate another action potential.
Absolute refractory period
even very strong stimulus will not begin another AP
inactivated Na+ channels must return to the resting state before they can be reopened
large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible.
Relative refractory period
a suprathreshold stimulus will be able to start an AP
K+ channels are still open, but Na+ channels have closed

15. Copyright 2009 John Wiley & Sons, Inc.
Propagation of an Action Potential in a neuron after it arises at the trigger zone
Copyright 2009 John Wiley & Sons, Inc.

16. 1. Conduction of action potential in
non-myelinated axons

17. 2. Conduction of action potential in
myelinated axons

18. Continuous versus Saltatory Conduction
Continuous conduction (unmyelinated fibers)
step-by-step depolarization of each portion of the length of the axolemma
Saltatory conduction
depolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channels
current carried by ions flows through extracellular fluid from node to node

19. Factors that affect the speed of propagation
Amount of myelination
Axon diameter
Temperature
Copyright 2009 John Wiley & Sons, Inc.

20. Speed of impulse propagation
The propagation speed of a nerve impulse is not related to stimulus strength.
larger, myelinated fibers conduct impulses faster due to size & saltatory conduction
Fiber types
A fibers largest (5-20 microns & 130 m/sec)
myelinated somatic sensory & motor to skeletal muscle
B fibers medium (2-3 microns & 15 m/sec)
myelinated visceral sensory & autonomic preganglionic
C fibers smallest ( microns & 2 m/sec)
unmyelinated sensory & autonomic motor

21. Compound Action Potential
Each peak was named: alpha –
the first to appear; beta - the next,
and so on.
The first signal to arrive at a
distant recording site has travelled the
fastest!
So each peak represents a set
of axons with similar conduction velocity
velocity is calculated from
the distance between R1 and R3 and the
time taken to traverse that
distance (distance/time) =
velocity (ranges from 0.5 to ~100 ms-1)

22. Compound Action Potential
In a mixed nerve, action potential appears with multiple peaks and is known as a compound action potential.
This action potential results from the summation of action potentials of all fibers in the nerve.
Its shape is due to the fact that a mixed nerve is made up of a different types of family of fibers with varying speed of conduction.
Therefore when all fibers are stimulated, the activity in fast conducting fibers arrives at the recording electrode sooner than the activity in slower fibers. The number and size of the peaks vary with the type of fibers in the particular nerve being studied. The mammalian nerve fibers are divided into A, B, and C groups with further subdividing the A group into ,, and  fibers.
A fibers have the thickest covering of myelin and are larger, therefore they conduct faster as fast as 100m/sec. C fibers are small and unmyelinated and so its conduction velocity is much smaller ( m/S).

23. Encoding of stimulus intensity
If action potential is all-or-none phenomenon how do we detect the
Stimuli of different intensity ?
Why does a light touch feels different than a firmer pressure?
1. The Frequency of firing of action potential
2. The number of sensory neurons recruited

24. Functions of action potentials
Information delivery to CNS
carriage of all sensory input to CNS. Block APs in sensory nerves by local anaesthetics. This usually produces analgesia without paralysis. WHY? Because LAs more effective against small diameter (large surface area to volume ratio) C fibers than a-motorneurones.
Information encoding
The frequency of APs encodes information (remember amplitude cannot change) -
Rapid transmission over distance (nerve cell APs)
Note: speed of transmission depends on fiber size and whether it is myelinated. Information of lesser importance carried by slowly conducting unmyelinated fibers.
In non-nervous tissue APs are the initiators of a range of cellular responses
muscle contraction
secretion (eg. Adrenalin from chromaffin cells of medulla)