Neurons conduct electrical signals along their axons to transmit information in the nervous system. At a synapse, an action potential causes neurotransmitter release from the presynaptic neuron. The neurotransmitters bind to receptors on the postsynaptic neuron, which may trigger a new action potential. Propagation of the action potential down the axon relies on voltage-gated ion channels and myelination of long axons to allow saltatory conduction and faster signal transmission.

1. NERVEIMPULSE
CONDUCTION
Dr. Faridullah Khan
Physiology Dept
RMU

2. TYPICALNEURON

3. Neuron
•
Dendrite - conducts “signal” toward the cell body -- [input zone]
–
–
often short, numerous & highly branched
signal comes from sensory cell or neighboring neuron
•
Axon - usually a single fiber -- [conducting zone]
– conducts signal away from cell body to another neuron or
effector cell
•
Axon Ending
a cluster of branches (100’s to 1000’s)
– each with a bulblike synaptic knob
– relays signal to next neuron / effector cell

4. Neuron
•
–
–
•
•

5. RESTING POTENTIAL
Resting potential may be defined as the difference in
voltage between the inside and outside of the cell as
measured across the cell membrane.
• When a neuron is not being stimulated, it maintains
a resting potential Ranges from –40 to –90 millivolts
(mV) Average about –70 mV

6. RESTING POTENTIAL
 Two major forces act on ions in establishing the
resting membrane potential
1. Electrical potential produced by unequal
distribution of charges
2. Concentration gradient produced by unequal
concentrations of molecules from one side of
the membrane to the other

7. RESTINGPOTENTIAL
Sodium–potassium pump creates significant concentration
gradient
• Concentration of K+is much higher inside the cell
• Membrane not permeable to negative ions
• Leads to build up of positive charges outside and negative
charges inside cell
• Attractive force to bring K+ back inside cell
• Equilibrium potential – balance between diffusional force
and electrical force
8

8. ACTION POTENTIAL
•
Action potential may be defined as the entire series of changes
which contribute towards the changes in membrane potential
Action potentials:-
– Result when depolarization reaches the threshold potential
(–55 mV)
– Depolarizations bring a neuron closer to the threshold
– Hyperpolarizations move the neuron further from the
threshold
– Caused by voltage-gated ion channels
• Voltage-gated Na+ channels
• Voltage-gated K+channels

9. ACTIONPOTENTIAL
• Voltage-gated Na+ channels
– Activation gate and inactivation gate
– At rest, activation gate closed, inactivation gate open
– Transient influx of Na+ causes the membrane to depolarize
• Voltage-gated K+ channels
– Single activation gate that is closed in the resting state
– K+channel opens slowly
– Efflux of K+ repolarizes the membrane

10. ACTIONPOTENTIAL
• The action potential has three phases
– Rising, falling, and undershoot
• Action potentials are always separate, all-or- none
events with the same amplitude
• Do not add up or interfere with each other
• Intensity of a stimulus is coded by the frequency, not
amplitude, of action potentials
11

11. 12

12. GENRATION OF ACTION POTENTIAL

13. PROPAGATION OF ACTION POTENTIAL
• Propagation of action potentials
– Each action potential, in its rising phase, reflects a reversal
in membrane polarity
– Positive charges due to influx of Na+can depolarize the
adjacent region to threshold And so the next region
produces its own action potential
– Meanwhile, the previous region repolarizes back to the resting
membrane potential
• Signal does not go back toward cell body

14. 15

15. PROPAGATION OF ACTION POTENTIAL
• Two ways to increase velocity of conduction
–Axon has a large diameter
• Less resistance to current flow
• Found primarily in invertebrates
–Axon is myelinated
• Action potential is only produced at the nodes of
Ranvier
• Impulse jumps from node to node
• Saltatory conduction 16

16. 17
SALTATORY CONDUCTION

17. NEUROTRANSMISSION
•
Electrical [no synapse]
– common in heart & digestive tract - maintains steady, rhythmic
contraction
– All cells in effector contain receptor proteins for
neurotransmitters
•
Chemical - skeletal muscles & CNS
– presence of gap (SYNAPTIC CLEFT) which prevents action
potential from moving directly to receiving neuron
– ACTION POTENTIAL (electrical) converted to CHEMICAL
SIGNAL at synapse (molecules of neurotransmitter) then
generate ACTION POTENTIAL (electrical) in receiving neuron

18. Overview of Transmission of Nerve Impulse
• Action potential
synaptic knob
opening of Ca+channels
neurotransmitter vesicles fuse with
membrane
release of neurotransmitter into synaptic cleft
binding of neurotransmitter to protein receptor
molecules on receiving neuron membrane
opening of ion channels
triggering of new action potential.

19. NEUROTRANSMISSION
•
Presynaptic neuron
• Vesicles
• [Calcium channels]
• Synaptic cleft
• Postsynaptic neuron
• Neurotransmitter receptor

20. NEUROTRANSMISSION
• Action potential
synaptic knob
opening of Ca+
channels
neurotransmitter
vesicles fuse with
membrane
release of
neurotransmitter into
synaptic cleft
Ca2+

21. NEUROTRANSMISSION
• Actionpotential
neurotransmitter
vesicles fuse with
membrane
release of
neurotransmitter into
synaptic cleft

22. NEUROTRANSMISSION
• Action potential
binding of
neurotransmitter to
protein receptor
molecules on receiving
neuron membrane
opening of sodium
channels
triggering of new
action potential

23. THANKS