Synaptic transmission Flashcards
What are the two types of transmission?
Electrical and chemical
What is electrical transmission?
Direct flow of ions from one neurone to another, hence direct influence of electric current from one to another
= Gap junction
How does electrical transmission work?
Ions pass through connexins
6 connexins = 1 connexon (hemichannel)
The connexon forms a pore to allow the ions through
How can we modulate gap junctions?
By pH, neurotransmitters and changes in intracellular calcium
Current can be passed down the neurone (using electrodes), in order to depolarise the first neurone, the second neurone will produce a very small amount of voltage as a small electrical transmission will take place
Where are gap junctions found?
Cell bodies, axons or dendrites
Found between axons of mossy fibres in hippocampus
Found between the dendrites in interneurones of the cerebellum
What are some examples where gap junctions in neurones are important?
Jabbing the tail of a crayfish leads to it tumbling forward as a survival mechanism, and a rapid escape response
Synchronisation of electrical activity in hippocampal interneurones
Important in causing secondary cell death after brain injury
What is chemical transmission?
Neurotransmitter substance released from presynaptic cell, diffuses across synaptic cleft, produces effect on postsynaptic neurone
= central synapse
One neurone can have up to 30,000 synapses connected to it
What are some factors of the chemical synapse?
Wide synaptic cleft
Neurotransmitter in the pre-synaptic membrane
Receptors in the post-synaptic membrane
Synaptic delay in the chemical synapse (larger gap)
What are the stages of neurotransmission?
Synthesis Storage Release Postsynaptic effects Inactivation
How is neurotransmitter synthesised?
Can be synthesised in the cell body using RER and Golgi (e.g. Peptides)
OR
Synthesised in the synaptic terminal using synthesising enzymes that were made in the cell body (e.g. Amines and amino acids)
How is neurotransmitter stored?
Stored in synaptic vesicles until needed, there are two types of vesicles:
Small/clear (40-60 nm) - contains small molecule neurotransmitter (around 100-250)
Large/dense core (90-250 nm) - neuropeptides (up to 1000)
A single neurone can synthesise/release more than one transmitter substance = co-existence or co-transmission
How is the neurotransmitter released into the synaptic cleft?
The action potential arrives at the pre-synaptic terminal (depolarisation)
Ca2+ voltage-gated channels open so Ca2+ flow into the terminal
Vesicles move to the presynaptic membrane (active zone) and dock there
The vesicles fuse with the membrane and releases the neurotransmitter into the synaptic cleft (exocytosis)
Uses SNARE proteins
What are SNARE proteins?
Synaptobrevin – vesicle bound protein which binds with SNAP-25 on plasma membrane (vesicle docking/priming)
Synaptotagmin – Ca2+ sensor (fusion)
What are the types of receptors on the postsynaptic membrane?
There are two types of receptors:
Metabotropic receptors - coupled to intracellular proteins that transduce the signal to the cell interior (slow responses)
Ionotropic receptors - form ion channels, that depolarise or hyperpolarise the postsynaptic cell (fast responses)
What are the different types of chemical neurotransmission?
ESPS - excitatory postsynaptic potential
When the neurotranmitter binds it changes the permeability of the postsynaptic membrane = Na+ ions flow inwards building up the ESPS
If ESPS exceeds threshold = action potential
IPSP - inhibitory postsynaptic potential (doesn’t reach threshold), as negative ions hyperpolarise the neurone
