synapses and neurotransmitters Flashcards
whats a synapse
-A synapse is a junction between two neurons allowing signals to pass from one to the other
-The process of signaling via synapses is synaptic transmission
-Synapse comes from Greek, ‘syn’ = together, ‘haptein’ = to fasten, join (coined by Sherrington in 1897)
-Your brain has ~100 trillion (1014) synapses, compared to ~100 billion (1011) neurons
whats the evidence for neurones
-Golgi stain (Ramón y Cajal, late 1800s)- labelling single neurones sparsely
-Physiological evidence from study of reflexes (Sherrington, around 1900)- depending on type of reflex, there is a delay to the reflex, this is due to gaps between neurones
-Final evidence from electron microscopy (1950s)
how do synapses enable flexible processing
-with a very simple animal: sensory input -> behaviour
-with more specialisation, flexibility: sensory input -> muscle-> behaviour
-even more flexibility integration: sensory input , sensory neuron and internueron -> motor neurone -> muscle -> muscle -> behaviour
wha† are electrical synapses formed of
-gap junctions that allow current to pass directly between neurons
-gap junctions are pores that ions can travel through
what happens When two neurons are connected by gap junctions
-electrical changes in one neuron pass directly to the other
-depolarisation and hyper polarisation can pass through gap junctions
How to tell if neurons are connected by gap junctions
-Small molecules like dyes diffuse from one neuron to the other.
Here, researchers filled the green neuron with red dye (so it’s yellow), and saw another neuron got filled with red dye
-Both hyperpolarising and depolarising stimuli are passed from one neuron to the other. This is blocked by deleting a connexin gene
what are electrical synapses good for
-Fast communication
-Synchronising neurons
whats The first evidence of chemical synapses 100 years ago
-Demonstrated using two isolated frog hearts that nerves release a chemical which slows the heartbeat
whats A prototypical chemical synapse
-The post-synaptic cell could be another neuron, or a non-neuronal cell:
-motor neuron-> skeletal muscle
-autonomic neuron -> hormonal gland, smooth muscle, or heart
-presynpatic element: lots of mitochondria present, synaptic vesicles (contain chemical messengers), secreting granules
what are the Steps in chemical synaptic transmission
1)Package neurotransmitters in vesicles, put them at the pre-synaptic terminal
2)Action potential arrives-> voltage-gated Ca2+ channels open
3)Ca2+ influx-> vesicles fuse to membrane, neurotransmitters released- After fusing, the vesicle is taken up again by endocytosis (not for peptides)
4)Neurotransmitters diffuse across the synaptic cleft, activate receptors on the postsynaptic cell -> further signaling
5)Neurotransmitters are removed from the cleft
difference between synaptic vesicles and dense-core secretory granules in stage 1 of chemical synaptic transmission
-synaptic vesicles:
=clear , small (40-50nm)
=small molecules NT
=filled by transporter proteins at the presynaptic terminal
=recycled by endocytosis
-dense-cire secretory granules
=dense, large (100nm)
=peptide NT
=created and filled by ER/ Golgi secretory apparatus
=one and done
what happens when Vesicles fuse via SNAREs
-When Ca2+ binds to synaptotagmin, a conformational change makes the SNAREs ‘zipper’ together, forcing the vesicle to fuse to the plasma membrane
-SNAREs are targets for toxins (botulinum toxin, tetanus toxin)
what happens when Neurotransmitters affect the postsynaptic neuron by binding to receptors
-Ligand-gated ion channels (ionotropic receptors) ->directly depolarise or hyperpolarise the postsynaptic cell
-G-protein-coupled receptors (metabotropic receptors) -> more complex effects
-Note: the neurotransmitter itself DOES NOT enter the postsynaptic cell
what happens when NT are removed
1)They diffuse away
2)They are actively taken up by transporters for recycling (into the presynaptic neuron or glia)
3)They are destroyed in the synaptic cleft by enzymes
Electrical vs. chemical synapses
-electrical:
=sig all pass in both directions
=signals are passed directly, can only be attenuated
=fast (<0.3ms)
-chemical:
=signals pass in one direction
=signals can be radically transformed (inverted, amplified and modulated )
=slower (0.3-0.5 ms)
-Both:
=are ‘plastic’ (i.e., can be modified), but chemical synapses probably more so
=allow summing up inputs by the post-synaptic neuron
-Most synapses are chemical synapses
what are neuromuscular junctions
-Fast and reliable neurotransmission
-Motor neuron action potentials always cause muscle cell action potentials
-Uses the neurotransmitter acetylcholine
How does the NMJ achieve such efficient transmission?
-One of the largest synapses in the body- large number of active zones (where NT are released) on the pre-synaptic terminal
-Presynaptic:
=Large number of active zones
-Postsynaptic (motor end-plate):
=Contains junctional folds, densely filled with neurotransmitter receptors
-Active zones and junctional folds are precisely aligned
How did we figure out that neurotransmitters are released from vesicles
-stimulate motor nerve, record from muscle
-never changed much in amplitude
-Notice how the evoked responses are all integer multiples of a spontaneous potential
