lecture synapses, properties & transmission dr. roomi.ppt

7/29/2019 Lecture Synapses, properties & Transmission Dr. Roomi.ppt

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Synapses, properties & Transmission

BY

DR. MUDASSAR ALI ROOMI (MBBS, M. PHIL)


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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 synapses3. 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

msec 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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Anatomy of a typical synapse (synaptic morphology)

Axon terminals

Pre-synaptic membrane

Post-synapticmembrane

Synaptic cleft (20-30nm

wide)

Synaptic vesicles.


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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 of change in permeability of post-synaptic membrane, post-synaptic neuron iseither excited or inhibited.

Neuro-transmitter binds with receptor onpost-synaptic membrane opening of ionchannels localized change in membrane

potential

post-synaptic membranepotential (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 lessnegative reach thresholdof excitation (-45mV)ACTION POTENTIAL in cellbody.


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Purpose of EPSP:

To bring potential of

membrane to threshold

(-45mV)

It is graded like EPP(directly proportional to

amount of neuro-

transmitter released).


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IPSP:

Produced when post-synapticneuron is inhibited.

Neuro-transmitter is of inhibitorytype (GABA. Glycine)

It binds with receptors on post-synaptic membrane change inpermeability of membrane for K+or Cl- (there is opening ofK+ or Cl-

channels efflux of K+ cellbecomes more negativehyper-polarization / IPSP.

Opening of Cl- channels extra-cellular Cl- moves into the cellmore negative hyper-polarization / IPSP.


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Effect of IPSP:

Because of IPSP, resting potential which is

-65mV, becomes -70 to -75mV Post-synapticneuron is inhibitedPOST-SYNAPTIC INHIBITION.

PRE-STNAPTIC INHIBITION:Synaptic knob has additional synapse with other

nerve terminals release of inhibitory neuro-transmitter from additional synapse synaptic knob

is inhibited no further transmission from synapsenow to post-synaptic neuron.


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EPSP Vs ACTION POTENTIAL:

Property EPSP or IPSP or

Graded potential

Action Potential

Magnitude Low High

Propagation &

Duration

Nil; it remains localized

( up to 20 msec)

Self propagating ( up

to 2 msec)

Refractory period absent present

All or none law Not obeyed. It is

graded.

obeyed

Summation Present absent

Decrement (decline of

size with distance)

present Absent. Size is

constant

Increased permeability

to ions

To Na+ & K+ at one

time but Na+ influx >

Na+ Influx , then K+

efflux


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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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LAW OF FORWARD CONDUCTION:

Through synapses, impulses are conducted

always from pre-synaptic to post synaptic

neuron, never in backward direction.

(NO REVERSE GEAR!!)


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SYNAPTIC DELAY

At a synapse, there is delay due to time taken

in events during synaptic transmission.

Through each synapse, there is delay of0.5

milli seconds.


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FATIGUE OF SYNAPTIC TRANSMISSION

If impulses are conducted through a synapse

repeatedly fatigue due to exhaustion of

stores or progressive inactivation of receptors

on post-synaptic membrane.

Significance of fatigue??

Fatigue of synaptic transmission is protective in

nature termination of epileptic fit.


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IN UNITY RESTS STRENGTH!

SUMMATION:

Adding up of effects of stimuli particularly if stimuliare subthreshold.

On a single motor neuron, thousands of synapticknobs terminate to form synapses.

About 80% of these synapses are on dendrites,remaining on cell body & few on axons.

So, single impulse coming to motor neuron through asynapse, cant excite a motor neuron &

there must be summation of effects of stimuli.


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TEMPORAL

Impulses transmitthrough 1 or fewsynaptic knobs

repeatedly effects 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

(Rest is best for test!) If impulses are conducted

through a synapse rapidly then rest is given tosynapse then again

impulses are conductedresponse of post-synapticneuron is increased.

Mechanism: Calcium ionsenter in synaptic knob ineach transmission, beforefatigue occursincreaseno. of calcium accumulatein knobmoreneurotransmitter releasedmore 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.ppt

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