Lecture 10: Information Processing in the Nervous System

Question 1

How does a post-synaptic cell decide to fire an action potential?

Answer 1

• Motor neuron cell body
• Many 1000s of inputs
• Some excitatory
• Some inhibitory
• Must combine information before firing – lots of layers and converge or diverge information

Question 2

SYNAPTIC INTEGRATION

Answer 2

change strength

Question 3

The events of a chemical synapse:

Answer 3

1. At fast synapses NTs directly act upon ligand-gated ion channels – inotropic
   - Nicotinic AChR
2. At slow synapses NTs act through second messengers (G-proteins)
   - Muscarinic AChR

Question 4

Mechanism 1:

Answer 4

on the post-synaptic membrane, there is a receptor which has a G-protein or direct message allowing the NT in and causing depolarisation

1. a volt meter shown in the bottom right corner
2. it would be zero to begin with but as NAChR is released, there is slight depolarisation of the post-synaptic membrane (+10 – (-50))

Question 5

Mechanism 2:

Answer 5

GABA (a) channel opens allowing Cl- in

2. GABA (b) receptor opening to allow K+ out
3. Causes a net reponse causing a negative response in the cell membrane

Question 6

Analysis:

Answer 6

Overall answer is +10

• going in is +10mv
• going out is -20mv
• this is because due to EPSPs, the sodium gated channels open and cause depolarisation of the membrane
• then due to IPSPs, the sodium gated channels close and K+ begins to come in
• this leads to the action potential being generated

Question 7

AP Threshold:

Answer 7

1. AP is triggered by a depolarisation of the plasma membrane
2. When a threshold is achieved the AP will follow

Question 8

Decision to fire an action potential depends on:

Answer 8

1. Net sign of combined input (neurons integrate both EPSP’s and IPSP’s to determine response – can hyperpolarize)
2. Strength of synaptic input (this can differ for different input to same neuron)
3. However, decision is also influenced by location of synapse and firing frequency of the presynaptic neuron

Question 9

1. Spatial

Answer 9

• How close to the axon hillock (end of the axon that connects it to the cell body)
• Dendrites Surface Area (site of synaptic input) > Cell Body and axon
• Neither have many Voltage gated Na+ channels – signals in dendrites fade

EPSP unlikely to reach AP threshold
- The further EPSPs are from Hillock less influence they have - PSP will dissipate (fade)
- Graded PSPs travel to the axon hillock in a passive, decremental fashion
¬ Inputs further from axon hillock have less influence on the final output
¬ Location of synapse is key
¬ Spread of electrical charge is measured and the further away you go from the cell body, the less spread of electrical charge there is

Question 10

Dendritic Cable Properties:

Answer 10

Spread of a PSP along the membrane depends on the time constant () and the length constant () of the membrane and on the diameter, membrane resistance (Rm) and internal resistance (Ri) of the dendrite – dendritic cable properties
Cannot be myelinated axon

Question 11

Enormous implications:

Answer 11

Solutions:

• Myelinate axons
• Cannot make it fatter but myelination helps
• Too much heat would denature enzymes

Question 12

2. Temporal

Answer 12

• How often are the input firing
• To be effective PSPs must be additive
• If new APs arrive before previous PSP decayed then will be compounded
• Diagram shows post-synaptic terminals
• An action potential usually lasts around 1ms, so 800 per second is very high (most neurons would never get above 500)
• First EPSP is shown to be around 200mv in the first diagram
• Same amount of vesicles are released, causing same amount of sodium to come into the post-synaptic membrane
• Slightly bigger elevation leads to slightly bigger depolarisation
• Bigger residual amount results in bigger PSPs as you fire in succession – higher mv
• IPSPs would show the diagram but flipped around and in negative values

Question 13

Spatial and Temporal Summation of PSPs:

Answer 13

• Spatial is where they are occurring on the dendritic field – on the axon
• Temporal is the frequency of firing
• Diagram shows that the EPSPs increase if more action potentials are stimulated from various number of neurons
• third part shows that the frequency increases of action potentials in the same neuron but the threshold value is still achieved

Question 14

Location of synapse is key for ‘shunting inhibition’:

Answer 14

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

• If the inhibitory synapse is timed correctly, it will prevent the Cl- ions from entering and thus will prevent an action potential being stimulated (NO EPSPs)
• Can cause the values to be negative

Question 15

AM to FM Conversion:

Answer 15

Reasons to encode are:

1. Prevent firing at low stimulation levels and is specific to an effector
2. To fire at a frequency proportional to the stimulus
3. FM Improved signal quality
4. Coding in a fixed amplitude is a better way to encode
5. Only intervals between potentials is measured which does not rely on the number of potentials or the size of them – will remain fixed

• In a cell, it will set up a competition within itself to decide which electrical impulse is best to fire by an amplitude signal (either small or large EPSP)
• It will then turn that into different action potentials at different frequencies
• AP DOES NOT VARY BETWEEN INDIVIDUALS IT IS ALL OR NOTHING

Question 16

Neurons use an FM code:

Answer 16

• Both the frequency and pattern of action potentials convey information
• Diagram shows the change to the same number of action potentials keeping flow constant but a different effect on the postsynaptic cell
• It shows the neuron firing 12 action potentials
• What you can see is that the frequency is different on both diagrams
• The post-synaptic cell would be different even though the number of action potentials is the same
• Going to be peaks and troughs
• This depends on amount of neurotransmitter it is exposed to and the pattern of firing
• More reliable method – codes more information