Lecture 13: Synaptic Strength and Short Term Plasticity Flashcards
What factors determine the magnitude of a PSP generated by a single presynaptic AP?
- availability of Ca2+
- presence of a toxin
How does the availability of Ca2+ affect the magnitude of a PSP generated by a single presynaptic AP?
less available Ca2+ (less enters presynaptic terminal) = less vesicles fusing (due to less synaptotagmin activated) = less neurotransmitter release = less response to receptors
How does the presence of toxins, particularly receptor antagonists, affect the magnitude of a PSP generated by a single presynaptic AP?
antagonists of receptors in the synapse affect the number of receptors available
less receptors = smaller response (even if the same amount of neurotransmitter is released)
Are postsynaptic voltage responses from a single synapse (ie. EPSPs) always the same size?
NO – they are variable in size
they are not always (or usually) sufficiently large to depolarize a postsynaptic neuron to threshold
What is synaptic strength?
relative size of the response in the postsynaptic neuron when the presynaptic neuron is stimulated with a stimulus of equivalent magnitude
What factors govern the magnitude of a PSP for a given stimulus?
- presynaptic factors
- cleft factors
- postsynaptic factors
- (other postsynaptic factors)
What are presynaptic factors?
how many quanta of neurotransmitter (ie. vesicles) are released for a given stimulation (number of APs)
What are cleft factors?
how rapidly neurotransmitters are broken down/removed from synaptic cleft
What are postsynaptic factors?
how many receptors are present at the PSD (and capable of binding neurotransmitter and generating currents)
What are other postsynaptic factors?
what subtype(s) of receptors are present – this affects the response due to:
- different transmitter binding affinities
- different response kinetics
- different ion selectivity
- different voltage sensitivity
- different conductance
What is synaptic plasticity?
ability of synapses to change their synaptic strength over time
synaptic plasticity = synaptic strength + plasticity
What does the quality of synaptic plasticity allow?
allows nervous system to adapt the ways in which it integrates and transfers information to better coordinate behaviour to the environment
What happens if synapses are used repeatedly?
changes their strength over short time scales
How is synaptic strength measured?
by peak amplitude, or (more often) rate of rise of the PSP (ie. change in amplitude for a given amount of time)
What happens to synaptic strength when stimulated by rapid repeated stimuli?
initially increases (facilitation)
but if the stimuli continue to occur, the strength decreases (depression)
How can you determine that synaptic plasticity (facilitation and depression) is occurring, rather than temporal summation occurring by itself?
temporal summation does not change the amplitude/rate of rise of the PSP (ie. synaptic strength)
True or False: synaptic plasticity only occurs when more than one synapse is activated
false
True or False: synaptic plasticity only occurs within the brain
false
True or False: temporal summation is the only thing that can occur if the PSPs overlap in time, so if you see overlap, there is no synaptic plasticity
false
Sometimes in synaptic plasticity, we use different terms to indicate separable processes (meaning processes that look superficially similar use different underlying biochemical mechanisms).
ie. terms that describe different biological processes that all create increases in synaptic strength, but persist for different amounts of time
- facilitation
- augmentation
- potentiation
Sometimes in synaptic plasticity, we use the same term to describe changes in synaptic strength that look similar, even though we know they also have different underlying mechanisms.
ie. the same word is used to describe decreases in synapse strength over different time-scales, even though these have different underlying mechanisms
- rapid depression
- short-term depression
- long-term depression
What is facilitation?
synaptic strength increases that last for ~10s of ms
What is augmentation?
synaptic strength increases that last for ~ 0.5 - 10s
What is potentiation?
describes synaptic strength increases that lasts ~ >30s
- short-term potentiation: up to ~30 min
- long-term potentiation: > ~60 min
What is rapid depression?
decreases in synapse strength that occur within ~10s of ms and last for up to ~1-10s
What is short-term depression?
decreases in synaptic strength that take longer to begin (1-10s) and last for 1-30 minutes
What is long-term depression?
decreases in synaptic strength that will last for > 60min once induced
What is short-term plasticity?
any use-dependent changes (changes in strength that occur by manipulating the history of activity of the synapse) in synaptic strength which occur over time scales up to ~10 min
When does short-term plasticity occur?
occurs when exposing a synapse to one or more stimuli transiently affects the strength of its response to stimuli that occur at later time points
What do characteristics of short-term plasticity (STP) depend on?
- type of stimuli
- type of synapse stimulated
How do different types of synapses (belonging to different neurons, or in different locations on the same neuron) differ?
show distinct patterns of short-term plasticity responses to a particular pattern of stimuli
What are 2 extra features used to categorize all forms of STP?
- frequency dependence
- temporal dependence
What is frequency dependence?
amount of change in synaptic strength varies based on the length of the interval between stimuli
(how much the interval between stimuli affects level of potentiation)
ie. in PPF example, varying the delay between the 2 stimuli leads to different amounts of increase in synaptic strength
What is temporal dependence?
amount (and direction) of change in synaptic strength varies based on how many stimuli have happened
(pattern of getting larger or smaller)
ie. in Katz’s example, the first 4-5 stimuli in the train produce augmentation, but as more stimuli occur, synaptic strength is then depressed below the original level
Do different synapses in CNS show characteristically different STP profiles to the same stimuli?
yes