Synapses Flashcards
Anatomy of a neuron
Contains cell body/soma (which contains a nucleus)
Contains dendrites (branching out from cell body)
Contains axon
What is a synapse?
Neuron to neuron connection or between neuron and target cell connection
What’s an electrical transmisson and what’s a chemical transmission and how does the speed between these vary?
Electrical transmission - flow of ions directly from one neuron to another
Chemical transmission - release of neurotransmitters
Electrical synapses transmit signals faster than chemical synapses
(Some synapses can be both chemical and electrical)
What are the sequence of events in synaptic transmission?
Action potential arrives at axon terminal
This activates voltage gated Ca2+ channels and these channels open
Ca2+ ions enter pre-synaptic terminal
Ca2+ entry causes synaptic vesicles to fuse with axon terminal membrane, and release NT into synaptic cleft
The NT diffuse across synaptic cleft and bind to receptor proteins on post synaptic terminal cell
Activation of postsynaptic receptors leads to opening or closing of ion channels in cell membrane
The opening or closing of ion channels can be depolarising (making inside of cell more positive) or it can be hyperpolarising (making inside of cell more negative)
At the post synaptic membrane, following events are based on the kind of receptors present.
What happens if the neurotransmitter activates a ligand gated ion channel and what happens if it activates a G protein linked receptor?
Activates ligand gated ion channel- then synaptic transmission is short lived and rapid (fast synaptic transmission)
Activates G protein linked receptor- slower synaptic transmission and lasts longer
How can signals be terminated?
Enzymatic degradation of NT
Reuptake of NT by pre synaptic neuron
NT diffuse away
NT is ‘mopped up’ by nearby glial cells
Why would signals need to terminated?
To stop the message coming, e.g person has already taken their hand off the hot plate
Briefly describe an electrical synapse
A direct physical connection
Takes the form of a channel (gap junction), which allows current (ions) to flow directly from one cell to another
The type or intensity of an action potential reaching the synaptic terminal will determine which vesicle is released at the pre synaptic membrane
What are the two types of presynaptic inhibition?
Homotropic modulation
Heterotropic modulation
What is homotropic modulation?
Neurotransmitter (NT) has an inhibitory effect on itself (negative autocrine signal) by binding to presynaptic autoreceptors
Presynaptic autoreceptors are different from the post synaptic receptors for the same NT. (These are useful in drug development- can target one or other receptors without affecting the other receptor)
What is heterotropic modulation?
Either there are 2 NT’s or 2 neurones involved. E.g acetylcholine is excitatory to exocrine gland but is inhibitory to the same parasympathetic fibre
What is co-transmission?
The release of multiple neurotransmitters
Co-transmission requires NT’s to be separated into distinct populations of synaptic vesicles
The release of which and how many vesicles are based on terminal depolarisation, that is determined by differential Ca2+ sensitivities or the spatial segregation of vesicles
Describe two examples of co-transmission
Example 1: Vesicles contain different NT’s
Which vesicle is released depends on Ca2+ intensity or frequency of Ca2+ in the terminal
e.g. a single action potential may release one set of NT but multiple action potentials may release all/both sets of NT
Example 2: Different boutons/terminals contain different NT’s
Unique info is transmitted to different synaptic targets - neuron gets signal and can release both/either
What is co-release?
CO-RELEASE IS NOT CO-TRANSMISSION
Both NT’s are packaged into same vesicle
Both NT’s are released by the same action potential
