This document summarizes key concepts about synapses and synaptic transmission. It defines a synapse as the junction point between neurons. It classifies synapses based on their structure and function. It describes the anatomical components of a synapse including the presynaptic terminal, synaptic cleft, and postsynaptic membrane. It explains the events that occur during synaptic transmission including the release of neurotransmitters from the presynaptic terminal and their effects on the postsynaptic membrane. It also discusses properties of synapses such as synaptic delay, summation, inhibition, and plasticity which are important for neural communication and functions like learning and memory.

1. Lecture13
Synapses & their transmission
Dr Shamshad
Majamaah University

2. Objectives
1.Define synapse
2. Discuss the classification of synapses
3.Describe the functional anatomy of synapse
4.Describe the events occurring at the synapse
5.Discuss the properties of synapse
Reference:
Guyton and Hall-TB of Medical Physiology 13th edition: page 543-55

3. Defn:
A junction point where axon or other part of nerve
cell (presynaptic cell) terminates on the dendrites or
other part of another neuron (postsynaptic cell).

4. I:Functional classification

6. II:Anatomical types

7. III:Anatomical ( Responsive action)

8. Type I (Gray’s I
synapses)
Type II (Gray’s II
synapses)
Structure Asymmetric symmetric
Synaptic space wide Narrow
Post synaptic
membrane
thickened thin
ECF in cleft present absent
NTs. NT:5HT,Glutamate,
NE,E,DOPA
NT GABA,Glycine
Action Excitatory Inhibitory
Type Axo-dendritic Axo-somatic

9. Structure of chemical synapse
1. Presynaptic
• A flat,
• 20-μm2 membrane area,
• Synaptic knob ,
• tuft, bulb of axon
2. Synaptic cleft
• (20-30 nm)
3. Postsynaptic neuron
• Grid
• Sub synaptic membrane,
• Sub synaptic web.

11. Post synaptic receptors - 2 components
1. Binding site that face
the cleft to bind the NT
2. Ionophore: It passes through
the membrane to the interior.

12. Ion channels
Channels Cation channels Anion channels
Ions Na+ (most common) Cl¯ (mainly)
Other ions K+,Ca2
+
Action Opening of Na+ channels
increase membrane
potential in positive
direction toward
threshold level of
excitation  (+) neuron.
Opening of Cl¯ channels 
diffusion of negative charges
into the membrane decrease
membrane potential making
it more negative away
from threshold level  (-)
neuron.

13. Second messenger system in the post-synaptic membrane.
Significance: when prolonged post-synaptic changes are
needed to stay for days,months,Years
Ex: memory storage.

14. Chemical (common) Electrical
NT NT liberated No NT
Synaptic Cleft 20-30 nm 2 nm
Delay 1-3 msec Absent
Sensitive hypoxia Sensitive Insensitive
Process Active Passive
Transmission Slower Fast
Direction of flow Unidirectional pre-post. Bi/Unidirectional
Inhibitory Effect Inhibitory +/- No
Post synaptic
duration
Longer-mediated by
second messengers.
No
Direction of Ions
current transmission
From outside to inside From cell inside to
inside.

15. https://media.sciencephoto.com/image/k0039491/p
review/K0039491-
Synaptic_transmission,_animation.mp4
https://media.sciencephoto.com/image/k004
4312/preview/K0044312-
Electrical_synapse,_animation.mp4

17. Types of neural circuit
1:Convering 2:Diverging

18. 3:Reverberating
Defn: Impulse pass from presynaptic neuron & again back to
presynaptic neuron to cause continuous stimulation of
presynaptic neuron.
Significance:
Breathing, coordinated muscular activities,
waking up, & short term memory.

19. 4:Parallel after discharge circuits:
Defn: Involve a single presynaptic cell that stimulates a group of
neurons ,which then synapse with a common postsynaptic cell.
Significance :
Precise activities like calculations

20. Graded Potentials
• Amplitude varies with conditions of
initiating
• Can be summed up.
• No threshold, refractory period
• Duration varies with initiating conditions
• Can be depolarization/ hyperpolarization
• Mechanism depends on ligand-sensitive
channels or other chemical or physical
changes
• Initiated by environmental stimulus
(receptor)
• Affects only limited portion of cell
membrane.

21. 1:One way conduction
2:Synaptic delay
3:Post Synaptic Potential:
A: EPSP
I: Spatial and
II: Temporal summation
B: IPSP:
I :Presynaptic inhibition
II :Postsynaptic inhibition
III :Reciprocal inhibition
Properties of synapses

22. 6: Conversions and divergence
7: Occlusion and subliminal fringe phenomenon
8: Facilitation
9: Synaptic fatigue
10: Recruitment
11: Synaptic plasticity ,learning and habituation
12: Reverberation
13: Reciprocal inhibition
14: After discharge
15: Effect of Acidosis & alkalosis,Hypoxia,Drugs..

23. One way conduction
Bell Magendie law:
Forward /Orthodromic conduction.
Synapses allow only one way conduction from pre to post
synaptic neuron.
Antidromic /backward conduction
IF spread back over soma or dendrites.
It produce no effect.
Significance: for orderly conduction of impulses in one
direction only.

24. Synaptic delay
Defn: Time between arrival of an AP to the synaptic knob & the
occurrence of response in the postsynaptic neuron.
Generation of Post Synaptic Potential & its summation to generate
AP.
Mechanism: Release and diffusion of the NTs.
Binding of the NTs. to the post Synaptic membrane.
Minimum delay : 0.5 msec.
Significance: can be used to determine the number of synapses
present in a polysynaptic reflex.

25. EPSP IPSP
Cause by binding of the
excitatory NT & makes
post SM generate an AP.
by the inhibitory NT &
makes Post SM membrane
less likely to generate an AP
Ions
Flow
+vly ions. –vly ions.
Leads to Depolarization.
Brings the Post SM
towards threshold.
Hyperpolarization.
Takes the post SM away
from threshold.
Effect
on Post
SM
Excited.
Facilitated the firing of
an AP on Post SM.
Less excited.
Lowers the firing of an AP
on the post SM.
NTs &
ions
involved
Glutamate or Na+
aspartate ions
Glycine and GABA Cl-.

26. Temporal (Time):
When repeated stimuli applied in short duration.
Spatial (Space):
When post synaptic membrane receives impulses
simultaneously from large number of presynaptic terminals.

27. Inhibition
Postsynaptic : Direct by releasing inhibitory NT due to refractory
period. By development of IPSP .
Golgi-tendon inhibition
Presynaptic: Release of GABA opens K+ or Cl channels—leads to
diffusion of K ions or Cl ions .
Hyperpolarization of post synaptic membrane by inhibitory.

28. Location: In spinal alpha motor neuron
Neuron inhibit those neuron which excite it
Significance:
It serves to limit excitability of motor neurons.
Feedback (Renshaw cell inhibition)

29. Neuron connected through 2 pathways Excitatory and inhibitory.
It allows brief and precisely timed excitation.
EX : Deep neural circuit of cerebellum
Significance :
For restriction over neurons and muscles to react properly
and appropriately
Feed forward inhibition.

30. Fatigue
Defn:
Progressive decline in rate of discharge of the Post SN,
Following intense prolonged stimulation of the PreSN .
If sever can lead to compete stop called synaptic block
Temporary in nature.
Cause:
Fatigue is mainly due to exhaustion of Nm substances .
Due to lack of time to resynthesize and reuptake of NT.
Significance:
Protects CNS from over excitability.

31. Dale law
Defn:
Only one type of NT is released at one synapse.
Either excitatory or Inhibitory.
Occlusion
Defn:
Response to stimulation of 2 presynaptic neurons is
less than sum total of the response obtained when
stimulated separately.

32. Synaptic plasticity , and learning
Defn:
Synaptic transmission can be increased or decreased
on the basis of past experience
Defn:
When presynaptic axon is stimulated with several
consecutive individual stimuli each evokes larger post synaptic
potential than previous stimuli
Post tetanic potentiation:

33. Synaptic plasticity , and learning
Also called post tetanic facilitation or potentiation.
Mechanism:
By excess rise of Ca ions in the synaptic knob which
causes more vesicle to release NT ,producing a greater
response of the Post SN.
Significance: Not known. May be short term memory

34. Long term potentiation
Similar to post tetanic potentiation.
Last for several days.
Mechanism:
It is initiated by an increase of intracellular calcium
ion in post SN through opening of Ca 2+ channels in post
SM after binding of glutamate to its specific NMDA
receptors

35. Significance:
Seen in many parts of CNS
Mainly in hippocampus
Play role in long term memory and learning.