basic functions of synapses,

8/12/2019 Basic Functions of Synapses,

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Basic Functions of Synapses,Basic Functions of Synapses,

“Transmitter Substances”“Transmitter Substances”



Impulses are transmitted from oneImpulses are transmitted from onenerve cell to another cell atnerve cell to another cell at synapses.synapses. These are the junctions where theThese are the junctions where theaxon or some other portion of one cellaxon or some other portion of one cell(( presynaptic cell) presynaptic cell) terminates on theterminates on thedendrites, some or axon of anotherdendrites, some or axon of another

neuron (neuron ( postsynatic cell postsynatic cell ))



Types of synapses:Types of synapses: A.A. AnatomicAnatomic

• axo-somatic – 5 to ! per centaxo-somatic – 5 to ! per cent• axo-axonalaxo-axonal• axo-dendritic – "! to #" per centaxo-dendritic – "! to #" per cent



$.$. %h&siolo'ic%h&siolo'ic

(functional)(functional)• hemicalhemical

• lectricallectrical

Chemical synapseChemical synapse – the first neuron– the first neuronsecretes at its nerve endin' s&napse asecretes at its nerve endin' s&napse achemical su*stance called achemical su*stance called aneurotransmitter (transmitterneurotransmitter (transmitter

su*stance) and this transmitter in turnsu*stance) and this transmitter in turn



the& alwa&s transmit si'nals in onethe& alwa&s transmit si'nals in onedirection , form the neuron thatdirection , form the neuron that

secretes the transmitter su*stancesecretes the transmitter su*stance(pres&naptic neuron) to the neuron(pres&naptic neuron) to the neuronon which the transmitter actson which the transmitter acts(posts&naptic neuron) –(posts&naptic neuron) – Principle ofPrinciple of

one-way conductionone-way conduction

Electrical synapseElectrical synapse ––characteri+ed *& direct opencharacteri+ed *& direct open

fluid channels that conductfluid channels that conductelectricit& from one cell to theelectricit& from one cell to thenextnext

often transmits si'nals in eitheroften transmits si'nals in eitherdirectiondirection



echanism *& hich an Actionechanism *& hich an Action

%otential auses Transmitter%otential auses Transmitterelease from the %res&napticelease from the %res&napticTerminals –Terminals – Role of Calcium IonsRole of Calcium Ions

Action of the TransmitterAction of the Transmitter/u*stance on the %osts&naptic/u*stance on the %osts&naptic

0euron – 1unction of 2eceptor0euron – 1unction of 2eceptor%roteins)%roteins)



Components Of TheComponents Of The

Postsynaptic Receptor ProteinsPostsynaptic Receptor Proteins

3.3. $indin' omponent$indin' omponent – protrudes– protrudesoutward from the mem*rane into theoutward from the mem*rane into thes&npatif clefts&npatif cleft

.. Ionophore omponentsIonophore omponents T&pes4T&pes4

Ion hannelIon hannel

-- ation channelsation channels – most often– most oftenallowsallows sodium ion to pass sodium ion to pass

-- Anion channelsAnion channels – allows mainl&– allows mainl&chloridechloride to pass to pass



/econd essen'er Activator/econd essen'er Activator

1our han'es that can occur41our han'es that can occur4

a.a. penin' specific ion channelspenin' specific ion channelsthrou'hthrou'h the posts&naptic cellthe posts&naptic cell

mem*ranemem*rane*.*. Activation of c&clic adenosineActivation of c&clic adenosinemonophosphate (cA%) pr c&clicmonophosphate (cA%) pr c&clic'uanosine monophsphate (c6%) om'uanosine monophsphate (c6%) om

the neruonal cellsthe neruonal cellsc. Activation of one or morec. Activation of one or moreintracellularintracellular en+&mesen+&mesd. Activation of 'ene transcriptiond. Activation of 'ene transcription



Excitatory or Inhibitory Receptors inExcitatory or Inhibitory Receptors in

the Postsynaptic Membranethe Postsynaptic Membrane

3.3. xcitationxcitation penin' of sodium channels to allow lar'epenin' of sodium channels to allow lar'e

num*er of positive electrical char'es to flow tonum*er of positive electrical char'es to flow tothe interior of the posts&naptic cellsthe interior of the posts&naptic cells

7epressed conduction throu'h chloride of7epressed conduction throu'h chloride ofpotassium channels or *othpotassium channels or *oth

8arious chan'es in the internal meta*olism of8arious chan'es in the internal meta*olism ofthe psots&naptic neuronthe psots&naptic neuron

.. Inhi*itionInhi*ition

penin' of chloride ion channels throu'h thepenin' of chloride ion channels throu'h theposts&naptic neuronal mem*raneposts&naptic neuronal mem*rane Increase in conductance of potassium ions outIncrease in conductance of potassium ions out

of the neuronof the neuron Activation of receptor en+&mesActivation of receptor en+&mes



Chemical Substances ThatChemical Substances That

Function as Synaptic Transmitter Function as Synaptic Transmitter



/mall-olecule apidl& Actin'/mall-olecule apidl& Actin'

TransmitterTransmitter

s&nthesi+ed in the c&tosol of thes&nthesi+ed in the c&tosol of thepres&naptic terminalpres&naptic terminal

continuall& rec&cled and used over andcontinuall& rec&cled and used over andoverover

a'aina'ain

0europeptides0europeptides not s&nthesi+ed in the c&tosol of thenot s&nthesi+ed in the c&tosol of the

pres&naptic terminals, instead the& arepres&naptic terminals, instead the& are

s&nthesi+ed as inte'ral parts of lar'e-s&nthesi+ed as inte'ral parts of lar'e-proteinprotein

molecules *& ri*osomes in the neuronalmolecules *& ri*osomes in the neuronalcellcell

*od&*od&



Electrical Events urin! "euronalElectrical Events urin! "euronal

ExcitationExcitation

9estin' em*rane %otential of9estin' em*rane %otential ofthethe

0euronal /oma (-:5mv0euronal /oma (-:5mv



ffect of /&naptic xcitation onffect of /&naptic xcitation onthe %osts&naptic em*rane –the %osts&naptic em*rane –

xcitator& %osts&naptic %otentialxcitator& %osts&naptic %otential



Transmitter acts on the mem*rane toTransmitter acts on the mem*rane toincrease the membrane permeabilityincrease the membrane permeabilitytoto sodiumsodium thus the mem*ranethus the mem*ranepotential increases from –:5 to –;5potential increases from –:5 to –;5mvmv

The positive increase in volta'e toThe positive increase in volta'e toless ne'ative value isless ne'ative value is calledcalledexcitatory excitatory postsynaptic potential postsynaptic potential (or(orEPSP) or +! m" EPSP) or +! m"

#hreshold for excitation#hreshold for excitation of neuron isof neuron isa*outa*out $%& m" $%& m"



Electrical Events urin! "euronalElectrical Events urin! "euronal

InhibitionInhibition ffect of Inhi*itor& /&napses on theffect of Inhi*itor& /&napses on the

%osts&naptic em*rane-%osts&naptic em*rane-Inhi*itor& %osts&naptic %otentialInhi*itor& %osts&naptic %otential-- The inhi*itor& s&npases open mainl&The inhi*itor& s&npases open mainl&

chloride channels, allowin'chloride channels, allowin'

passa'e ofpassa'e of chloride ionschloride ions-- penin' of potassium channels willpenin' of potassium channels willallowallow

positivel& char'ed potassium ionspositivel& char'ed potassium ions

toto move to themove to the exteriorexterior-- 'oth chloride influx and potassium'oth chloride influx and potassiumefflux efflux increase the de'ree ofincrease the de'ree ofintracellularintracellular ne'ativit& which calledne'ativit& which calledhyperpolari(ationhyperpolari(ation

- em*rane potential is –<! mv, 5mv- em*rane potential is –<! mv, 5mv



Types of Synaptic InhibitionTypes of Synaptic Inhibition

3.3. %osts&naptic%osts&naptic

.. %res&naptic%res&naptic

caused *& release of an inhi*itor&caused *& release of an inhi*itor&su*stance onto the outsides of thesu*stance onto the outsides of thepres&naptic nerve fi*rilspres&naptic nerve fi*rils

inhi*itor& transmitter su*stance isinhi*itor& transmitter su*stance is

6A$A6A$A=.=. enshaw cell inhi*itionenshaw cell inhi*ition



22/patial /ummation> in 0eurons/patial /ummation> in 0eurons



Temporal /ummationTemporal /ummation

/uccessive dischar'es from a sin'le/uccessive dischar'es from a sin'lepres&naptic terminal, can add topres&naptic terminal, can add toone another@ that is, the& summateone another@ that is, the& summate

221acilitation> of 0eurons1acilitation> of 0eurons The summated posts&napticThe summated posts&naptic

potential is excitator& *ut has notpotential is excitator& *ut has notrisen enou'h to reach the thresholdrisen enou'h to reach the thresholdfor firin' *& the posts&napticfor firin' *& the posts&naptic

neuron. The neuron is said to *eneuron. The neuron is said to *efacilitated facilitated



=.=. ffect of 7ru's on /&naptic Transmissionffect of 7ru's on /&naptic Transmission Increase neuronal excita*ilit& *& reducin' theIncrease neuronal excita*ilit& *& reducin' the

threshold for excitation of neuronsthreshold for excitation of neurons

BB caffeinecaffeine

BB theoph&llinetheoph&lline

BB theo*rominetheo*romine Increase neuronal excita*ilit& *& inhi*itin' theIncrease neuronal excita*ilit& *& inhi*itin' the

action of some normall& inhi*itor& transmitteraction of some normall& inhi*itor& transmittersu*stances especiall& inhi*itor& effect ofsu*stances especiall& inhi*itor& effect of *lycine*lycine

in the spinal cordin the spinal cordBB /tr&chnine/tr&chnine

;.;. /&naptic 7ela& – a*out !.5 millisecond/&naptic 7ela& – a*out !.5 millisecond dischar'e of the transmitter su*stance *& thedischar'e of the transmitter su*stance *& the

pres&naptic terminalpres&naptic terminal

diffusion of the transmitter to the posts&napticdiffusion of the transmitter to the posts&napticneuronal mem*raneneuronal mem*rane action of the receptor to increase the mem*raneaction of the receptor to increase the mem*rane

permea*ilit&permea*ilit& inward diffusion of sodium to raise theinward diffusion of sodium to raise the

excitator& posts&naptic potential to a hi'hexcitator& posts&naptic potential to a hi'h

enou'h level to elicit an action potentialenou'h level to elicit an action potential

basic functions of synapses,

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