Neuronal transmission - synapses and circuitsials

Question 1

action potential propagation

Answer 1

• Action potentials propagate (transmit) along axons
• They are the same size all along!
• As the action potential moves it depolarisesthe next bit of membrane and openssodium channels
• If enough sodium channels are opened, the membrane potential reaches the threshold potential and the action potential propagates along.
• The area that has just generated an action potential cannot fire another as the Na+channels are now inactivated.

Question 2

speed of action potential

Answer 2

0.1m/s to 100 m/s

Question 3

How fast does membrane potential change (and reach threshold)?

why is action potential propagation speed so variable ?

Answer 3

• Affected by leakiness/resistance of membrane – slower if more charge can leak out
• Affected by capacitance of cell –how easy it is to change the membrane voltage. Large membrane = large capacitance – takes a lot of charge to change membrane voltage

Question 4

How far can depolarisations spread along the axon?

why is action potential propagation speed so variable ?

Answer 4

• Affected by membrane resistance - more leaky membrane -> depolarisation spreadsless far
• Affected by diameter (internal resistance to flow down the axon) – big diametersconduct faster

Question 5

myelination

Answer 5

• Myelin insulates membrane – less charge loss.
• Action potentials travelling from one node of Ranvier to the next =saltatory conduction.
• It’s faster and moreefficient – fewer ionsflow, so less ATP neededto pump them back.

Question 6

action potential summary

Answer 6

• When the membrane reaches a threshold voltage, voltage-gatedsodium channels briefly open, depolarising the cell
• Voltage-gated potassium channels open and repolarise the cell.
• Depolarisation spreads along the membrane activating nearby sodiumchannels.
• Inactivation of sodium channels means action potential propagates inone direction and sets limit on firing frequency.
• Action potentials are All or Nothing and only occur once the thresholdis met!
• Myelin speeds action potentials and makes them more energy-efficient

Question 7

the synapse: presynaptic cell

Answer 7

1. AP potential arrives at axon terminal
2. membrane depolarization opens voltage-gated calcium channels
3. calcium inside cell causes vesicles of neurotransmitter to fuse with membrane
4. neurotransmitter diffuses through synaptic cleft
5. neurotransmitter binds to ligandgated ion channel
6. ions flow through the channel, depolarizing or hyperpolarizing the post-synaptic membrane
7. depolarisation of dendrites by io flow through glutamate receptors generates an excitatory post-synaptic potentional [EPSP] - drives membrane

Question 8

what is GABA

Answer 8

• GABA is the main inhibitory neurotransmitter in the brain
• It opens chloride channels (GABAA receptors), allowing negative charge into the), generating aninhibitory post-synaptic potential(IPSP), and/or making it harder to depolarize the membrane.
• It is harder for the threshold foraction potential firing to bereached so the cell is less likely tofire an action potential.

Question 9

what is neuronal computation

Answer 9

sum up inputs and produce an output

Question 10

strength of synaptic input relates to distance of synapses from axon hillock

Answer 10

dendrite synapses = proximal synapses have higher input than distal synapses
somatic synapses = somatic synapses have higher input than dendrite synapses
snapses along the axon are strongest

Question 11

what is the input weight affected by

synaptic integration

Answer 11

1. distance from axon hillock
2. shape of neuron
3. location relative to inhibitory inputs [gating]

Question 12

surface area of neuron affects effective distance from axon hollock

Answer 12

• small cell body size decreases distance to axon hillock
• high cell surfaces complexity increases distance to hillock

Question 13

synapses summary

Answer 13

• Information is passed between neurons at synapses.
• Depolarisation of the axon terminal leads to transmitter release
• Neurotransmitters activate receptors on the post-synaptic neuron’s dendrites.
• These receptors excite or inhibit the post-synaptic cell
• The soma integrates excitatory and inhibitory synapses to “decide” whether to fire an action potential

Question 14

what are neuronal networks

Answer 14

how neurons wore together affects computation they perform, and infomation they represent

Question 15

neuronal circuits and coding summary

Answer 15

• Information is coded by changing the frequency (and timing) of action potential firing
• Connecting excitatory and inhibitory neurons in different ways can produce different circuit effects
• Feedforward excitation and inhibition – knee extension reflex