Synapses Flashcards
Is it possible to get different size spikes?
No, there are just spikes.
Name two types of synapse?
Chemical, gap junction
Describe an axon to dendrite gap junction
Axon connected to a dendrite by a gap junction where there is a small hole directly connecting the inside of one neuron through to the inside of the other.
What can happen to the ions at a gap junction?
Some of the charged ions may diffuse through the gap changing the charge in the dendrite.
What are gap junctions thought to be responsible for in the hippocampus?
The dynamics which supports very rapid oscillations.
What type of synapse is most common in a mammalian brain?
Chemical. This allows for a more variable effect of a pre-synaptic spike on the voltage of the post-synaptic dendrite.
In a chemical synapse, the pre-synaptic spike does not affect the post synaptic voltage directly it instead…
cause a cascade of bio-electrodynamics which ultimately causes a transient change in conductance of the post synaptic membrane.
What is the terminal bouton held by ?
astrocytes
What is the terminal bouton filled with?
What do these contain?
Tiny bubbles called vesicles which contain neurotransmitters.
What happens when a spike reaches the terminal bouton?
It causes calcium gates to open to the cellular membrane , the resulting influx of calcium ions cause some of the vesicles to migrate to the membrane separating the bouton with the synaptic cleft. They burst releasing neurotransmitters in to the cleft
The membrane of the dendritic is pieced by what time of gated ion channels?
Ligand gated channels. They contain a receptor site which binds with a particular type of molecule, like a key designed for the receptor site’s lock.
When the receptor has a molecule bound to it, the gate is open so ions can pass through the channel. The channel is ion specific.
What happens once neurotransmitters are released in to the synaptic cleft?
Hence, after a spike arrives at the synapse the cleft is filled with neurotransmitter and some of that neurotransmitter binds to the gated channels, causing them to open. This in turn allows a flow of ions in or out of the dendrite, changing the voltage there.
What happens to the neurotransmitters once in the cleft?
They get quickly reabsorbed through reuptake pumps. Some of the neurotransmitter is absorbed into the bouton, some in to the spine and some is absorbed by the astrocyte.
What happens to the concentration of neurotransmitters?
It falls rapidly
Why do the neurotransmitters unbind from receptors on the ligand gated channels?
They unbind as a result of random collisions and thermal variations. as they do the channels close again and the conductivity of the dendritic spine’s membrane falls back towards zero.
How do excitatory synapses affect the post synaptic neuron?
They make the post synaptic neuron more likely to spike by increasing the voltage. Opening the ligand gated channels causes a positive current to enter into the cell.
Typically what channels are associated with excitatory synapses?
Calcium or Sodium channels. When they open sodium or calcium ions to move in to the dendrite.
Typically what channels are associated with inhibitory synapses?
Potassium gates allowing potassium ions to leave the dendrite.
Also, chlorine gates allowing NEGATIVELY charged chlorine to flow IN.
The post synaptic change in potential due to the pre synaptic spike is called…
post synaptic potential.
What does EPSP stand for?
Excitatory post synaptic potential
What does IPSP stand for?
Inhibitory post synaptic potential
How does the profile of PSPs reflect the neurotransmitter dynamics?
It rises fast as the neurotransmitter floods the cleft and the ion channels open, it then decays back to zero following an exponential decay, reflecting the constant rate unbinding process.
The post-synaptic conductivity is often taken to be an alpha function.
Is(t)=g_s *s(t)(E_s-V)
What are the variables?
Is(t) is the synaptic current
Es is the reversal potential
g_s*s(t) is the conductance, g_s is a constant describing the strength of the synapse
s(t)=texp(-t/taus)
Describe the alpha function
The rising part of the alpha function models the period when there is neurotransmitter in the cleft. This is binding to the channels increasing the conductance; the falling part represents the period where the unbound neurotransmitter has been cleared from the cleft and the bound neurotransmitter is unbinding randomly due to the thermal motion of molecules.
What are some details of the synapse that can effect the dynamics?
the strength of the synapse, that is, the amplitude of the change in conductance that follows a pre-synaptic spike, depends on the number of vesicles released, on the size of the cleft, the speed of reuptake and the number of ligand-gated channels; the shape and duration of the PSP depends on the detailed dynamics of binding, reuptake and unbinding and the effect of the PSP on the post-synaptic soma depends on the shape of the dendritic tree and the details of the diffusive process which conducts the PSP to the soma, a distal synapse will have a different effect to a proximal one.
How does the length of the timescale differ between excitatory and inhibitory synapses?
excitatory synapses tend to have shorter time constants than inhibitory ones
Give an example of short term plasticity
if two spikes arrive one soon after the other, the first might have already opened a substantial quantity of the gates, reducing the effect of the second.
What is short term depression?
If there is a high rate of pre-synaptic spike arrivals the vesicles may be depleted reducing the size of PSPs.
What is short term facilitation?
build up of calcium ions in response to a high rate of pre-synaptic spiking can increase the size of PSPs
What is synaptic plasticity?
Synaptic plasticity usually refers to the long-term changes in synapse strength, a long term increase in synaptic strength is called long term potentiation of LTP, a decrease is called long term depression or LTD
Plasticity rule
If one neuron tends to cause another to fire, the synapse from the first to the second gets stronger
Neurons that fire together wire together
Plasticity rule formula
∂wij = µxixj
where wij is the stength of the synapse for neuron i to j, xi and xj are the states of the two neurons and µ is the learning rate
