Lecture Synapses, Properties & Transmission Dr. Roomi
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Transcript of Lecture Synapses, Properties & Transmission Dr. Roomi
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SYNAPSES
Definition: it is the site of functional contact
b/w two neurons at which an electric impulse
is transmitted from one neuron to another.
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Types of synapses- on the basis of site of contact
1. Axodendritic synapses(most common type)
2. Axosomatic synapses
3. Dendrodenritic
synapses
4. axosaxonic synapses
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Types of synapses- on the basis of method
of signal transmission
Chemical synapses:
Most common type
Signal transmission isdelayed for about 0.5 m
sec in these synapses.Electrical synapses (nexus):
Less common
Flow of ions from one
neuron to another via gapjunctions.
Signal transmission isnearly instantaneous.
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Events occurring at a chemical synapse during
signal transmission (Synaptic Transmission Mechanism)
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EPSP and IPSP
Depending on type of neurotransmitter& type ofchange in permeability of post-synapticmembrane, post-synaptic neuron is either excitedor inhibited.
Neuro-transmitter binds with receptor on post-synaptic membraneopening of ion channelslocalized change in membrane potentialpost-synaptic membrane potential (PSP)
2 types Excitatory (EPSP),
Inhibitory (IPSP).
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EPSP
Resembles EPP (end platepotential). There is localizedhypo-polarization due toNa+influx.
Resting potential of cellbody of neuron is-65mV.
When EPSP is producedhypo-polarizationpotential becomes lessnegativereach thresholdof excitation (-45mV)ACTION POTENTIAL in cellbody.
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IPSP:
Produced when post-synaptic neuron is inhibited.
Neuro-transmitter is ofinhibitory type (GABA,Glycine)
It binds with receptors onpost-synaptic membranechange in permeability ofmembrane for K+or Cl-
(there is opening ofK+or Cl-
channelsefflux of K+
cell becomes more negativehyper-polarization / IPSP.
Opening of Cl- channelsextra-cellular Cl- moves intothe cellmore negative
hyper-polarization / IPSP.
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Effect of IPSP:
Because of IPSP, resting potential which is
-65mV, becomes -70 to -75mVPost-synapticneuron is inhibitedPOST-SYNAPTIC INHIBITION.
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Properties of Synaptic Transmission
1. DALES LAW:
2. LAW OF FORWARD CONDUCTION:
3. SYNAPTIC DELAY
4. FATIGUE OF SYNAPTIC TRANSMISSION
5. SUMMATION:
Temporal summation
Spatial summation
6. POST-TETANIC FACILITATION OR POTENTIATION
7. EXCITATION AND INHIBITION OF SYNAPTIC TRANSMISSION
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Properties of Synaptic Transmission
DALES LAW:
At a given synapse, only 1 type of neurotransmitter is
released, it may be excitatory or inhibitory.
Later on it was found that in certain cases releaseof additional substances at a given synapse
e.g.,in noradrenergic synapses: along with nor-
epinephrine, some dopamine, neuropeptide Y &prostaglandins are also released.
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SYNAPTIC DELAY
At a synapse, there is
delay due to time taken
in events duringsynaptic transmission.
Through each synapse,
there is delay of 0.5
milli seconds.
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FATIGUE OF SYNAPTIC
TRANSMISSION
If impulses are conducted
through a synapse
repeatedlyfatigue due to
exhaustion of storesor
progressive inactivation of
receptorson post-synaptic
membrane. Significance of fatigue??
Fatigue of synaptic
transmission is protectivein
naturetermination of
epileptic fit.
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TEMPORAL
Impulses transmitthrough 1 or fewsynaptic knobs
repeatedlyeffects onpost-synaptic neuronsare addedstimulation.
Second stimulus mustfall when effect of 1stone is still there.
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SPATIAL
Impulses are conductedalong a number ofsynapsessimultaneouslyeffects on postsynapticneuron are addedexcitation.
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POST-TETANIC FACILITATION OR POTENTIATION
If impulses are conductedthrough a synapse rapidlythen rest is given tosynapsethen againimpulses are conductedresponse of post-synaptic neuron isincreased.
Mechanism: Calcium ionsenter in synaptic knob in
each transmission, beforefatigue occursincreaseno. of calcium accumulatein knobmoreneurotransmitterreleasedmore EPSP.
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ALKALOSIS INCREASE EXCITABILITY OF SYNAPSES,
ACIDOSIS DEPRESSES SYNAPTIC TRANSMISSION
Increase excitability
Caffeine(cerebral
stimulant)
Theophylline
Strychnine/ Kuchla
Decreased calcium
(tetany)
Decrease excitability
Anesthetics
Hypoxia
Increased calcium
(stabilize the
membrane)