Mainly Exocytosis

1. PROSUN DE
M.Sc In HUMAN PHYSIOLOGY
SERAMPORE COLLEGE
1

2. WHAT IS SYNAPSE?
 Communication between neurons and
communication between neurons &
muscle occurs at specialized junctions
called SYNAPSE.
 Synapse are of 2 types-
▪ Electrical synapse
▪ Chemical synapse
2

3. Typical Chemical Synapse
3

4. VESICULAR RELEASE
 Two processes- 1.EXOCYTOSIS,
2.ENDOCYTOSIS
 Most Neurotransmitter(NT) release occurs
by transmitter loaded synaptic vesicles
fusing with Pre-synaptic membrane and
discharging their contents into synaptic
cleft; EXOCYTOSIS.
 After release the vesicle membrane is
recycled from Pre Synaptic membrane to
form new vesicle by ENDOCYTOSIS.
4

5. MECHANISM OF
EXOCYTOSIS
POOLS
DOCKING
PRIMING
FUSION
5

6. BIOCHEMISTRY OF
EXOXYTOSIS
6

7. 7

8. RELEASE IS QUANTAL!
 Neurotransmitter is secreted in discrete
packets or QUANTA.
 Each quantum represents the release of
the contents of a single vesicle,about 4000
molecules of NT.
 At CNS synapses this causes a miniature
postsynaptic potential(mpsp), either
excitatory or inhibitory, due to release of
transmitter from the vesicle acting on 30-
100 postsynaptic receptors beneath the
active zone.
8

9. Synaptic Vesicle inside a neuron
9

10. 2 pools-
Exocytosis from Small Synaptic
Vesicle (SSV) involves several
linked steps,most of which need
calcium.
• RELEASABLE POOL is
located at the active zone and
take part in repeated cycles of
exocytosis and endocytosis at
low firing frequencies.
• The RESERVE POOL consists
of vesicles tetherd to
cytoskelatal proteins, and can
be recruited by repetitive
stimulation to join the
RELEASABLE POOL.
10

11. Presynaptic proteins and
their roles in synaptic vesicle cycling
Model of the molecular organization of
a synaptic vesicle
Role of Proteins in Synaptic vesicle
cycling
11

12. DOCKING
12

13. • Vesicles are aligned at
specific sites in the
active zone by
DOCKING.
• A vesicle-associated
protein, Synaptobrevin(v-
SNARE) binds with high
affinity to a presynaptic
membrane protein,
Syntaxin(t-SNARE).
• Syntaxin is closely
associated with voltage
dependent calcium
channels, ensuring that
release machinery is
optimally placed to
receive calcium signal.
• Sub & Stx (following fig),
together with a third
protein crucial for
docking, SNAP-25.
13

14. PRIMING
14

15. 15

16. After DOCKING comes
another calcium
dependent step,
PRIMING.
 In this process a
number of soluble
cytoplasmic proteins
form a transient
complex with
SNAREs.
 Resulting in partial
fusion of vesicle and
presynaptic
membranes.
 This step involves the
hydrolysis of ATP.
16

17. FUSION
17

18. 1
• Primed vesicles are poised for exocytosis
2
• Requires only a large pulse of calcium to permit complete fusion of the vesicle
and presynaptic membranes
3
• Opening of Fusion Pore through which Exocytosis occurs.
18

19. 19
1.When the axon
terminal is
depolarized,
voltage gated
calcium channels
open
2.Calcium ions
rush into the
axon terminal
3.Some of the
calcium ion bind to
a protein on
synaptic vessicle
membrane,
SYNAPTOTAGMI
N
4.After
calcium
binding with
synaptotagmi
n , vesicle
fuse with the
axon terminal
membrane
5.Release the
transmitters into the
synaptic cleft.

20. THANK YOU
20