Synaptic Integration

Question 1

How many synapses does each neuron in the substantia nigra make?

Answer 1

around 500000

Question 2

What is convergence and divergence?

Answer 2

• Convergence – multiple inputs into a single cell

* Divergence – single input e.g. sensory receptor makes contact with multiple neurons

Question 3

How does a postsynaptic cell decide to fire an action potential?

Answer 3

• 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

Question 4

How do you work out the input?

Answer 4

EPSP - IPSP

Question 5

What is the AP threshold?

Answer 5

• 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

Question 6

What does the decision to fire an action potential depend on?

Answer 6

• 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

Question 7

What is Spatial and Temporal summation?

Answer 7

• Spatial – how close to the axon hillock

* Temporal – how often are the inputs firing

Question 8

Why is location of the synapse key?

Answer 8

• 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

Question 9

What are Dendritic cable properties?

Answer 9

• 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

Question 10

Give features of spatial summation

Answer 10

• 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

Question 11

What is temporal summation?

Answer 11

• 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

Question 12

What is ‘Shunting Inhibition’?

Answer 12

A single inhibitory synapse, located close to the soma, can switch off all other inputs to that dendrite

Question 13

What is encoding?

Answer 13

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

Question 14

What sort of codes do neurons use?

Answer 14

An FM code

Question 15

What of action potentials conveys information?

Answer 15

Both the frequency and pattern of action potentials

Question 16

Talk about the reward pathway

Answer 16

• 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

Question 17

What are reasons for ‘encoding’?

Answer 17

• To prevent firing at low stimulation levels

- To fire at a frequency proportional to stimulus