2. Objectives
1.Define synapse
2. Discuss the classification of synapses
3.Describe the functional anatomy of synapse
4.Describe the electrical events occurring at the synapse
5.Discuss the properties of synapse
3. 1.Define synapse
A junction point where axon or other part of
nerve cell (presynaptic cell) terminates on
the dendrites, soma, or axon of another
neuron( postsynaptic cell).
7. IV :Size and shape
of vesicle Size & shape of vesicles
1.TYPE---1 Excitatory
2. TYPE---2 Inhibitory
III:Anatomical ( Responsive action)
8. 1. One to One
1. Multiple to One
1. 3: One to Multiple
V:Depending on number of neurons involved
9. Clear core
Dense core
Cholinergic synapses
Adrenergic synapses
VI: Depending upon contents in synaptic
vesicles
VII: Depending on type of neuro-transmitter
10. VIII .Direction of release of NT in Synapses
DIRECTED-
SYNAPSES
NON-DIRECTED SYNAPSES
Gray’s I synapses
Usually Excitatory
IX: CNS membrane differentiations
Gray’s II synapses
Usually inhibitory
11. Types of chemical synapses
Type I
• Asymmetric structure
• synaptic wide
• Post synaptic
membrane thickened
• ECF in cleft present
• Vesicle spherical
• NT:5HT,Glutamate,NE,E
,DOPA
• Excitatory
• Axo-dendritic
Type II
• Symmetric structure
• Narrow
• Thin
• Absent
• Flat
• NT GABA,Glycine
• Inhibitory
• Axo-somatic
13. Structure of chemical synapse
1. Presynaptic
a flat, 20-μm2 membrane area,
Synaptic knob , tuft, bulb of axon
2. Synaptic cleft (20-30 nm)
2. Postsynaptic neuron
(grid,subsynaptic memebrane,
subsynaptic web).
14.
15. Post synaptic receptors - 2 components
1. Binding site
that face the
cleft to bind the
neurotransmitter
2. Ionophore: It passes through
the membrane to the interior. 2
types
Ion channels
Cation channels
Na+ (most common)
K+,Ca++
Opening of Na+ channels
🡪 ↑ membrane potential in
positive direction toward
threshold level of excitation
🡪 (+) neuron
Anion channels
Cl¯ (mainly)
Opening of Cl¯
channels 🡪 diffusion
of negative charges
into the membrane 🡪
↓ membrane
potential making it
more negative 🡪
away from threshold
level 🡪 (-) neuron
16. 2nd messenger system in the post-synaptic
membrane.
Important mechanism where prolonged post-synaptic
changes are needed to stay for days, months . . Years
(memory).
As channels close in milliseconds.
17.
18.
19. Differences : chemical & electrical synapses
CHEMICAL
1.Liberation of NT.
2.Most common.
non-economical
3.Synatic cleft 20-30nm
4.Delay-1-3msec.
2msec-000,ms
5.Hypoxia-sensitive
6.Active process
ELECTRICAL
1. NO.
2. Economical
i.e. hippo,C.C, glial
3. 2nm.
4.Absent
5.Insensitive
6. passive
20. Chemical
7.Slower transmission.
8.Unidirectional-pre to post.
9.Membrane Polarity changes
with the graded potential.
10. Can have inhibitory effect.
11. Can have longer-term
actions mediated by second
messengers.
12. Transmission of Ion
current from outside to inside
Electrical
7. Extremely fast
8. Bi / Unidirectional (rare )
9. No.
10. cannot have inhibitory
effect.
11. Cannot have longer-term
actions mediated by second
messengers
12 From cell inside to inside
21.
22. Types of neural circuit
1:Convering
2:Diverging
3:Reverberating
Passage of impulse from presynaptic neuron and again
back to presynaptic neuron to cause continuous stimulation
of presynaptic neuron.
Used in:breathing, coordinated muscular activities, waking
up, & short term memory.
23. 4:Parallel after –discharge circuits: Involve a single presynaptic
cell that stimulates a group o f neurons ,which then synapse with
a common postsynaptic cell.
Used in :Precise activities like calculations
24. Graded Potentials
🡪 Amplitude varies with conditions of initiating
🡪 Can be summed
🡪 Has no threshold
🡪 Has no refractory period
🡪 Is conducted decrementally (amplitude ↓ with distance)
🡪 Duration varies with initiating conditions
🡪 Can be a depolarization or a 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.
26. 🡪 One way conduction
🡪 Synaptic delay
🡪 Post Synaptic Potential:
🡪 EPSP
🡪 Summation
:Spatial and
:Temporal summation
🡪 IPSP:
:Presynaptic inhibition
:Postsynaptic inhibition
:Reciprocal inhibition
27. 🡪 Conversions and divergence
🡪 Occlusion and subliminal fringe phenomenon
🡪 Facilitation
🡪 Synaptic fatigue
🡪 Recruitment
🡪 Synaptic plasticity ,learning and habituation
🡪 Reverberation
🡪 Reciprocal inhibition
🡪 After discharge
🡪 Effect of
Acidosis and alkalosis
Hypoxia
Drugs
28. One way conduction
Law of dynamic polarity /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 is. If
produce no effect.
Importance: for orderly conduction of impulses in
one direction only.
29. Synaptic delay
Time passed between arrival of an action potential to the
synaptic knob & the occurrence of response in the
postsynaptic neuron.
Mechanism: Release and diffusion of the NT
Binding of the NT to the post SM.
Generation of PSP and its summation to generate AP.
Minimum delay :0.5 msec.
Importance: can be used to determine the number of
synapses present in a polysynaptic reflex.
30. Graded potentials EPSP &IPSP
EPSP:
An electrical charge on
the post synaptic
membrane(post SM)
🡪 Caused by the
binding of the
excitatory NT &
makes post SM
generate an AP.
IPSP:
An electrical charge on
the post SM
🡪 Caused by the
inhibitory NT &
makes Post SM
membrane less likely
to generate an AP.
31. 🡪 Caused by flow of
+vly charged ions.
🡪 A depolarization.
🡪 Brings the Post SM
towards threshold.
🡪 Makes the Post SM
excited.
🡪 Facilitated the firing of
an AP on Post SM.
🡪 Generated by flow of
glutamate or Na+
aspartate ions.
🡪 Caused by flow of –
vly charged ions.
🡪 A hyperpolarization.
🡪 Takes the post SM
away from threshold.
🡪 Makes the post SM
less excited.
🡪 Lowers the firing of an
AP on the post SM.
🡪 Generated by flow of
glycine and GABA Cl-
.
35. 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.
36. Feedforward inhibition. Neuron connected through 2
pathways Excitatory and inhibitory.
It allows brief and precisely timed excitation.
EX : Deep neural circuit of cerebellum
Importance:For restriction over neurons and muscles
to react properly and appropriately
Feedback (Renshaw cell inhibition)
In spinal alpha motor neuron
Neuron inhibit those neuron which excite it
Importance:
It serves to limit excitability of motor
neurons.
37. 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.
Importance: protects CNS from over excitability.
38. Dale law
Only one type of NT is released at one synapse.
Either excitatory or Inhibitory.
39. Occlusion
Response to stimulation of 2 presynaptic neurons is
less than sum total of the response obtained when
stimulated separately.
40. Synaptic plasticity , and learning
Synaptic transmission can be increased or
decreased on the basis of past experience
Post tetanic potentiation: When presynaptic axon is
stimulated with several consecutive individual stimuli
each evokes larger post synaptic potential than
previous stimuli
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.
Importance: Not known. May be short term memory
41. 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
Importance: seen in many parts of CNS
Mainly in hippocampus
Play role in long term memory and learning.
