Synaptic Integration Flashcards
How many synapses does each neuron in the substantia nigra make?
around 500000
What is convergence and divergence?
- Convergence – multiple inputs into a single cell
* Divergence – single input e.g. sensory receptor makes contact with multiple neurons
How does a postsynaptic cell decide to fire an action potential?
- Excitatory inputs:
Stimulate presynaptic terminal – releases glutamate
Allows sodium to go into cell
Slight positive change on voltmeter – EPSP
Bringing the cell towards threshold - Inhibitory inputs
GABBA cases K+ to leave cell or Cl- to enter cell
Causes hyperpolarisation: IPSP
How do you work out the input?
EPSP - IPSP
What is the AP threshold?
- AP is triggered by depolarisation of the plasma membrane
- When a threshold is achieved the AP will follow
- If the input is above the AP threshold the APs will fire
What does the decision to fire an action potential depend on?
- Net sign of combined input
- Strength of synaptic input (this can differ for different inputs to the same neuron)
- However, decision is also influence by location of synapse and firing frequency of the postsynaptic neuron
What is Spatial and Temporal summation?
- Spatial – how close to the axon hillock
* Temporal – how often are the inputs firing
Why is location of the synapse key?
- Dendrites Surface Area > Cell body
- Neither have many voltage gated Na+ channels
- EPSP unlikely to reach AP threshold
- The further EPSPs are from Hillock the less influence they have – PSP will dissipate
- Graded PSPs travel to the axon hillock in a passive, decremental fashion
- Input further from axon hillock have less influence on the final output
What are Dendritic cable properties?
- Spread of a PSP along the membrane depends on the time constant and the length constant oft the membrane and on the diameter, membrane resistance and internal resistance of the dendrite – dendritic cable properties
- To get efficient transmission we want cables with big length constants
- Know length constant depends of Rm (leakiness), Ri (conductivity) and diameter
Give features of spatial summation
- placement of synapse on dendritic field determines how the axon hillock will respond (whether enough signal will cause firing or not)
- Synapses move closer to or further out from dendritic fields
What is temporal summation?
- To be effective PSPs must be additive
- If new APs arrive before previous PSP decays, then will become compounded (PSPs add to each-other)
- As more and more action potentials occur per second more calcium builds up, more vesicles released, more action potential
What is ‘Shunting Inhibition’?
A single inhibitory synapse, located close to the soma, can switch off all other inputs to that dendrite
What is encoding?
A small change in amplitude at axon hillock is converted to a low frequency action potential signal and a large change in amplitude at hillock is converted to a high frequency action potential
What sort of codes do neurons use?
An FM code
What of action potentials conveys information?
Both the frequency and pattern of action potentials
Talk about the reward pathway
- Ventral tegmental area releases dopamine on nucleus accumbens
- Expected stimulus – tonic (1-8Hz)
- Better than expected – phasic (15-20 Hz)
- Worse than expected – pause in activity
- High DA release = reward. May explain why routes of drug administration that achieve fast/high concentrations are more rewarding
What are reasons for ‘encoding’?
- To prevent firing at low stimulation levels
- To fire at a frequency proportional to stimulus
