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:zap:Synapses (Structural features (Chemical synapse (Presynaptic neuron…
:zap:Synapses
Structural features
Anatomically specialised junctions
Between two neurons
Electrical activity of presynaptic neurone
Affects electrical/metabolic activity in postsynaptic
Types of synapse
Electrical synapse
Neurons connected by GAP JUNCTION
Two way communication
Local potential passes rapidly between neurons
Mainly cardiac and smooth muscle tissue
Chemical synapse
Neurons connected by synaptic cleft
One way communication
Mainly in neurons
Longer time taken to pass local potential
Chemical synapse
Presynaptic neuron
Axonic terminals are swollen
vesicles store neurotransmitters
Also store co-transmitter released along with the neurotransmitter
Contain V-gated Ca channel
also Na and K channels
Postsynaptic neuron
Contains high density of membrane proteins
Each membrane protein has active sites
Each active site specific to a receptor
Synaptic cleft
10-20 micron space
separate pre and post synaptic neurons
Direct propagation of AP is prevented from pre to post
Synaptic junctions
Convergence
Many presynaptic neurons CONVERGE on a single postsynaptic neuron
Integration and Processing of Information from multiple sources
Divergence
A single presynaptic neurons synapses with many postsynaptic neurons
Allows multiple effects in different pathways
Excitatory and inhibitory chemical synapses
Graded potential
Produced by ligand/mechanically gated ion channels at dendrite or soma
Magnitude is proportional to the amount of graded potential (stimulus strength)
The more frequent the stimulus, the greater the strength
Spatial summation
Temporal summation
Decremental - inversely proportional to distance of stimulation
Counteracted by in/efflux of opposing ions to balance the ionic equilibrium
Local and transient - does not transfer far
Reversible
If subthreshold, resting membrane potential will be swiftly restored
Excitatory postsynaptic membrane potential
Depolarisation
Opening of Na/Closing of K channels
Increases membrane potential closer to threshold
Inhibitory postsynaptic membrane potential
Decreases positive membrane potential, bringing it further from threshold
Opening of K and Cl channels
Hyperpolarisation
Factors affecting synapses
Presynaptic
NT availability
Precursor molecules
Rate-limiting enzyme activity
Axonic terminal potential
Axonic terminal [Ca]
Activation of membrane receptors at axonic terminals
Axo-axonic synapses
Autoreceptors
Other receptors
Pathophysiology of the other pathways
Postsynaptic
Immediate past history of postS membrane potential
Other EPSP/IPSF
Temporal summation
Spatial summation
Other NT or neuromodulators acting on PM membrane
Diseases related to the below
Others
Surface contact area
Geometric diffusion path
Enzymatic destruction/reuptake
NT and NM
NT
Substance that directly produce EPSP/IPSP on post-synaptic membrane potential
Acts rapidly
Swiftly degraded into inactive metabolites
ACh, NA
Acts by ligand-gated ion channels
1-1-1 neuron-NT-neuron
NM
Are released as cotransmitters
May include paracrine, autocrine or hormone substance
Modify postsynaptic membrane response to the neurotransmitter
Increase sensitivity or duration of effect
Affect synthesis/release/reuptake/metabolism of neurotransmitter
Act by GPCR and Vgated Ca channels
Affect the pre/postsynaptic neurons metabolic machinery
Do not degrade or active after degradation
Diffuse to large areas and affect them
Long-term effects
Serotonin, dopamine
Events of synapse
AP arrives and depolarises axon terminal
Voltage gated Ca channels open
CA influx triggers fusion of vesicles with presynaptic membrane via snare proteins
Exocytosis of NT from fusion - diffuse across syn cleft
Binds to receptors on postsynaptic membrane
Affect PostS membrane potential, permeability towards certain ions
Transient effect
Active reuptake
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Inactivation and reuptake
Diffusion away from active site
Also triggers production of more NT vesicles