lecture synapses, properties & transmission dr. roomi

8/13/2019 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)

lecture synapses, properties & transmission dr. roomi

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