Synapse
Functional junction between 2 neurons
Impulses are transmitted from one neuron to another at a
synapse
Sherrington described as site of contiguity
Contiguity means contact without continuation
There are minimum 2 neurons in any synapses
Presynaptic neuron : carries action potential towards synapse
Post synaptic neuron :that carries action potential away from
synapse
TYPES OF SYNAPSES
Anatomical types
Depending upon the manner an axon terminates on other
neurons
Type of Part of Part of location
synapses presynaptic postsynaptic
cell cell
Axodendritic axon dendrite Motor
(common) neurons of
spinal cord
Climbing
fibers of
cerebellum
axosomatic axon soma Motor
neurones in
the spinal
cord
Basket cells
of cerebellum
Autonomic
ganglia
axoaxonal axon axon Spinal cord
dendrodendritic dendrite dendrite Very less in
CNS
Synapse
between
mitral and
granule cells
in olfactory
bulb
Physiological types
Chemical synapses Electrical synapses Conjoint synapses
Transmission of Pre and post Synapse where
impulse from pre to synaptic membrane both chemical and
post synaptic occurs comes very close to electrical
due to liberation of form gap transmission
chemical mediator junctions .These coexists
junctions act as low
resistance bridges
through which ins
can pass with
relative ease
One way Bidirectional
conduction from transmission
pre synaptic to post Can conduct in both
synaptic neuron directions
Synaptic cleft is Synaptic cleft is
present replaced by low
resistance bridges
Synaptic delay Synaptic delay
present(0.5ms) absentt
They are sensitive Insensitive to
to hypoxia,fatigue hypoxia
and pH
Helps in processing There are not too
of many synapses on
information .helps the same
in the process of neuron,transmisssion
learning and across is rapid and
memory efficient
Most synapses in Retina
human nervous Olfactory bulb in
system nasal epithelium
Structure ofa a synapse
1. Synaptic knob or bouton
The axon of presynaptic neuron loses its myelin sheath and
divides in to numerous filaments which terminates in button
like expansion called synaptic knob or bouton
Mitochondria provides ATP for the synthesis of
neurotransmitter
Microtubules helps to transport vesicles to reach the
presynaptic membrane (axoplasmic flow)
Synaptic vesicles
Small clear synaptic vesicles Acetyl choline ,GABA,glycine
Small dense core synaptic catecholamines
vesicles
Large dense core synaptic Neuropeptides
vesicles
Presynaptic membrane : axonal membrane lining the
synaptic knob.contains large no of voltage gated calcium
channels.
Synaptic cleft :gap between pre and post synaptic membrane
20-40 nm wide
Contains ECF and glycoproteins
Neurotransmitter is released in to this cleft
from presynaptic membrane
Enzymes present in cleft destroy the
neurotransmitter
Post synaptic membrane :
contains large number of receptor proteins
Receptor proteins are of 2 types
Ion channel receptor proteins and enzymatic type of receptor
protein
Ion channel receptor proteins /ionotropic receptors :
responsible for closing or opening of ion channels
Enzymatic type of receptor protein /metabotropic receptors :
act through second messenger system
Process of chemical synaptic transmission
When an action potential arrives at the presynaptic terminal
Depolarization of presynaptic membrane
Voltage gated calcium channels open and enters the presynaptic
terminal
Increase intracellular calcium triggers the fusion of synaptic
vesicles with the presynaptic membrane (DOCKING)
Packets (quanta) of neurotransmitter molecule release in to
synaptic cleft ( KISS AND RUN DISCHARGE)
Usually one type of neurotransmitter is released from all
terminals of a single neuron.First proposed by Dale,so called
Dales phenomenon
Neurotransmitter binds with specific receptor on post synaptic
membrane
Opening of ligand gated sodium channels
Sodium ion moves in to the post synaptic terminal
Depolarization of post synaptic membrane by excitatory
neurotransmitter (GLUTAMATE)
INFLUX OF SODIUM IONS CAUSES A LOCALIZED NON
PROPAGATED DEPOLARIZATION OF MEMBRANE
CALLED AS EPSP (EXCITATORY POST SYNAPTIC
POTENTIAL )
EPSP
Local potential(EXCITATORY) develops on the post
synaptic membrane
When an excitatory neurotransmitter acts on the post
synaptic membrane receptors ,a local depolarizing current
develops due to sodium influx in to the post synaptic nerve
terminal .This is called EPSP
Depolarization start with a latency of 0.5 ms rises to a peak
in 1-1.5 ms and then declines slowly over the next 4 ms
During this potential excitability of neuron to other stimuli is
increased ,therefore potential is called EPSP
LOCAL RESPONSE
NON PROPAGATED
DOESNOT FOLLOW ALL OR NONE LAW
MONOPHASIC
GRADED RESPONSE
When EPSP reaches a threshold value ,triggers an action
potential in initial segment of post synaptic neuron and is
conducted in both directions ,down the axon and back in to
the soma
EPSP can be studied by inserting a microelectrode in to
ventral horn cells of spinal cord and stimulating the sensory
fibers in dorsal root
IPSP
Local potential (inhibitory ) that develops on the post
synaptic membrane
When an inhibitory neurotransmitter(GLYCINE,GABA) acts
on the post synaptic membrane receptors ,a local
HYPERPOLARIZING current develops due to chloride
influx or potassium efflux
Impulse conduction is blocked
Magnitude of IPSP is - 2 mV.It begins at a latency of 2 ms
attaining its maximum at 4 ms and declines exponentially
with a time constant of about 3 ms
Recording of IPSP is similar to EPSP
During this potential excitability of neuron to other stimuli is
decreased ,therefore potential is called IPSP
Development of action potential
Summated potential whether it is EPSP or IPSP produced by
excitatory or inhibitory neurotransmitter will determine
synaptic transmission
Summated potential pass to initial segment ,axon hillock
If ppotential is large enough to depolarize initial segment (6-
10mv) ,initial spike is generated
Magnitude :30-40 mV
INITIAL SPIKE ONCE INITIATED CAUSES FURTHER
DEPOLARIZATIONS BY OPENING VOLTAGE GATED
SODIUM CHANNELS ON AXON HILLOCK
Generate action potential ,travels in both
directions ,orthograde and retrograde propagation
Fate of released neurotransmitter
Diffusion of transmitter out off cleft
Enzymatic degradation of transmitter>..>
Acetylcholinesterase
Active neurotransmitter re-uptake : active transport back in
to presynaptic terminal )
EXTRA NOTES
1. Portion of cell with the lowest threshold for the production of
a full fledged action potential is the initial segment
2. Type of synapse which most commonly exists in the nervous
system ….. axodendritic
3. A neuron postsynaptic to one group of cells can be
presynaptic to another : true
4. NEUREXINS are protein that hold the presynaptic and post
synaptic membranes together at the synapse ,providing
structural stability to the synapse
5. Neurexins are present in the presynaptic membrane
6. The binding of synaptic vesicle to presynaptic membrane
requires integral membrane protein called SNARE
PROTEINS
7. SNARE PROTEIN present in the vesicular membrane is
synaptobrevin
8. SNARE PROTEIN present in the presynaptic membrane is
syntaxin
9. Fusion of synaptic vesicle with cell membrane is facilitated
by synaptobrevin
10.The number of synapses in the nervous system can increase
or decrease with use and experience ….. true
11.Term synapse was coined by sherrington meaning to clasp
12.
