Physiology properties of synapses For mbbs, MSc physiology and MD physiology

1. Properties of Synapses

2. SCHEME OF PRESENTATION
• Introduction
• Properties of synapses-
1. Law of forward conduction
2. Synaptic Fatigue
3. Synaptic delay
4. Law of Divergence
5. Law of convergence

3. 6. Synaptic Inhibitions
7. Summation
8. Occlusion
9. Subliminal Fringe
10. Synaptic Plasticity

4. Introduction
Synapses are neuro-neuronal junctions
through which information from one neuron
passes to the other.

5. PROPERTIES OF SYNAPSES
Law of Forward Conduction
- Impulse travels only in one direction
- Presynaptic terminal to post synaptic terminal.

6. Synaptic Fatigue
Progressive decline in rate of discharge of the pre
synaptic neuron, following intense prolonged
stimulation of the pre synaptic neuron.

7. Mechanism
- Depletion of the stores of the neurotransmitter
- No enough time re-synthesis and reuptake
Significance
- Protects CNS from over excitability

8. Synaptic Delay
• The time required by the impulse to be transmitted
through synapse.
• It is about 0.5 ms.
• Helps to find out about the number of synapses
present in a neural pathway.

9. Reason for synaptic delay

10. Law of Divergence & Convergence
• Divergence- One to many projections
• Convergence- Many to one projection

11. Importance
 Convergence- Allows information from many sources
to influence a cells activity
 Divergence- Allows one information source to affect
multiple neurons thus help in physiological
facilitation of impulses.

12. Synaptic Inhibitions
-- Synaptic inhibition stabilizes the neurons and prevents
unnecessary spread of impulses.
-- Types-
o Direct v/s Indirect Inhibition
o Feedback v/s Feedforward Inhibition

13. Direct Inhibition
 Is also called post synaptic inhibition
 Occurs by the formation of IPSP on the post synaptic
membrane
 Example-
1. Anterior horn cells by descending inhibitory fibers
2. Afferent inputs from golgi tendons that inhibit
Anterior horn cell.

14. Indirect Inhibition
o Passage of impulse through synapse is inhibited by a
a separate neuron terminating on the pre synaptic
ending.
o Inhibition occurs due to decreased release of
neurotransmitters from pre synaptic neuron.

15. • Mechanism-
Release of neurotransmitter from inhibitory
interneuron
Activation of pre synaptic
receptors
Facilitation of chloride conductance
Decrease in AP, Decreased entry of calcium
Decrease in neuro transmitter release

16. Feedback Inhibition
• Neurons inhibit themselves in a
feedback fashion
• Spinal motor neuron gives a
collateral that synapses
Renshaw cell which is inhibitory
interneuron .
• Renshaw cell, sends back
axons that inhibit the spinal
motor neuron.

17. Feedforward Inhibition
• Pre synaptic neuron directly inhibits the post
synaptic neuron by means of inhibitory
neurotransmitter.
• EXAMPLE – Inhibition of Purkinje cell output by
parallel fibers originating from granule cells in
cerebellum.

18. Feed forward inhibition in cerebellum

19. Synaptic facilitation
This is the process by which the transmission through a
synapse is increased.
It is usually caused by increased transmitter release
from the pre synaptic terminal.
Is of the following types-
1. Summation
2. Occlusion
3. Habituation
4. Sensitization
5. Potentiation

20. Summation
• Summation refers to adding up.
• As, synaptic potentials are graded individual
potential changes can summate to result in a larger
post synaptic potential.
• Two types of summation exist- Temporal summation
and spatial summation.

21. Temporal Summation
• The same input stimulates
post synaptic membrane
repetitively.
• The second potential
arrives before the
membrane could recover
from the first synaptic
potential.
• Thus post synaptic
potential overlap in time,
elevating the firing level

22. Spatial Summation
• In this two or more separate inputs arrive
simultaneously at the post synaptic membrane.
• If the inputs are in same direction, the post synaptic
response becomes larger than individual responses.
• Thus occur when many synaptic knobs converge on
one post synaptic membrane are stimulated
simultaneously

23. Spatial Summation & Trigger zone

24. Temporal & Spatial summation
•

25. Occlusion
• If two pre synaptic neurons share their discharge
zone, with each neuron activating another 5
neurons, and both of them share two neurons.
• Then the firing of these two synaptic neuron
simultaneously will result in activation of 8 neurons
instead of 10. This is called Occlusion.

26. Schematic representation of occlusion

27. Synaptic Plasticity
• Activity-dependent changes in the effectiveness of
synapses, and some of their mechanisms.
• This helps in learning & memory.

28. • Mechanisms involving synaptic plasticity can impact
for short and longer duration
1. Short duration mechanisms
a. Habituation
b. Tetanic Potentiation
c. Sensitization
d. Depression
2. Long duration mechanisms
a. Long term Potentiation
b. Long term depression

29. Habituation
• Gradual decrease of transmitter release in response
to repeated transmission of impulses.
• Due to
- Prolonged inactivation of the calcium channels
- Decreased number of vesicles

30. • Experiment on Aplysia california

31. Sensitization
• Occurs when a stimulus is accompanied by an
unpleasant or painful sensation.
• Increases by the means of pre synaptic facilitation
• Sensitization is short lived and example of short
term synaptic plasticity.

32. Experimental demonstration of sensitization

33. Potentiation
• It is the enhancement of synaptic transmission for a
prolonged duration.
• In Post tetanic Potentiation the duration is shorter as
compared to long term Potentiation.

34. Post tetanic Potentiation
• Rapid repeated stimulation of the pre-synaptic
terminal, for a period of time ----- shows ↑
presynaptic discharge above normal for a period of
seconds or even minutes.

35. • Mechanism
It is caused mainly by excess rise of the concentration
of Ca2+ ions in the synaptic knob.
• Significance
Role in the short-term memory processes

36. Long term Potentiation
• Repeated stimulation of synapses in brain can
produce more persistent changes in effectiveness.
• It resembles post-tetanic Potentiation, but is much
more prolonged and can last for several days & has a
different mechanism of action
• Can persist for days to weeks

37. • Mechanism
It occurs due to increased calcium in post synaptic
neuron due to opening of NMDA channels along with
increase in intracellular protein.
• Significance
It has a special significance in the hippocampus; which
plays an important role in learning and memory.

39. References
• Guyton & hall, Textbook of Physiology, 13th
Edition
• Prof. G.K. Pal, Textbook of Physiology, 2nd Ed
• Berne & Levi, Physiology, 2nd Ed
• Boron & Boulpeap, Textbook of Medical
Physiology, 6th Ed
• Ganong, Review of Physiology, 23rd Ed