2 types of synapses
1) electrical
2) chemical
How to electrical synapses conduct?
ions move through membrane spanning gap junctions to the next cell
Describe structure of gap junction channel
- Formed by two hemichannels called Connexons
- Each connexon has 6 subunits called Connexins
- Each connexin has 4 membrane spanning regions
Cells of CNS where electrical synapses occur
1) Hormone-secreting cells of the hypothalamo-pituitary axis
2) Some spinal motor neurons
3) Hippocampal pyramidal cells
4) Mesencephalic nuclear cells
5) Retinal cells
6) connect Astrocytes (K+ buffering and Ca2+ signalling)
Cells of PNS where electrical synapses occur
Schwann cells
Speed and direction of electrical synapses
Fast (0.1 ms) and can be uni or bidirectional
Where does transmitter vesicle release occur in chemical synapses
from presynaptic cells at the ACTIVE ZONE of the synaptic Bouton
3 locations on the neuron where neuronal chemical synapses occur
1) Axodendritic (on dendrites)
2) Axosomatic (on cell bodies)
3) Axoaxonic ( on axon hillock and near synaptic terminal)
________ and __________ synapses are mainly inhibitory
Axoaxonic and Axosomatic
__________ synapses are mainly excitatory
Axodendritic
How do Axosomatic and Axoaxonic inhibition differ?
Axosomatic inhibit Action potential
Axoaxonic inhibit transmitter release (AP do occur)
What are Tripartite synapses
occur btw astrocyte , postsynaptic and presynaptic neuron
What is role of Astrocyte in Tripartite synapses?
uptake of transmitter released and release others fr uotake by neurons (eg glutamate to glutamine)
5 traditional criteria for substance to be classified as neurotransmitter:
- synthesis in presynaptic neurons
- storage in preparation for release
- release by presynaptic neurons, typically in a calcium-dependent fashion
- binding to specific postsynaptic receptors
- removal from synapses by specific mechanisms
5 groups of LOW molecular weight neurotransmitters:
1) Acetylcholine
2) Monoamines
3) Amino Acids
4) Purines
5) Gases
3 Groups of Monoamines and examples
1) Catecholamines- Dopamine, Norepi and Epi
2) Indoleamines - Serotonin and Tryptamine
3) Others - Histamine and taurine
5 amino acid neurotransmitter
1) Glycine
2) Glutamate
3) Aspartate
4) Homocysteine
5) GABA
4 Purine neurotransmitters
1) Adenosine
2) ATP
3) ADP
4) AMP
2 Gases neurotransmitters
1) NO
2) CO
Describe synthesis of Low Molecular weight Neurotransmitters
- synthesized in cytosol
- loaded into clear vesicles (some in dense core vesicles)
- tethered to cytoskeleton near active zone
Describe synthesis of High Molecular weight Neurotransmitters
- synthesized in soma as propeptides
- transported by anterograde axonal transport in Large dense core vesicles
- converted along the way into multiple transmitter molecules
- the vesicles are stored further away from active zones
- eg : opioid peptides of pain regualtion
Describe synthesis of Nitric Oxide (NO) Neurotransmitter
- Ca2+ enters cell through NDMA receptor
- Ca2+-Calmodulin complex forms => activates NO synthase => converts L-arginine to L-citrulline and NO
Isoform of NO synthase in neurons
nNOS
Is NO stored?
no , it is synthesized on demand by Ca release which can come from presynaptic neuron
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