Synapses and networks

Question 1

Neuron doctrine

Answer 1

1. Brains are composed of separate neurons and other cells
2. cells are independent
3. neurons are polarised cells
4. information is transmitted from cell to cell across tiny gaps

Question 2

typical locations of synaptic processes

Answer 2

1. axo-dendritic
2. axo-somatic (axon terminal connects directly to cell body)
3. axo-axonic (between axons)
4. dendro-dendritic (very rare, function is unclear)

Question 3

chemical and electrical synapses

Answer 3

1. chemical connection - most common, presynaptic neuron meets post synaptic neuron
2. electrical connection - rare

Question 4

interaction of neurons

Answer 4

single neurons function is to transmit or not transmit a neuronal signal

Question 5

signal transformation during synaptic transmission

Answer 5

1. presynaptic neuron - depolarisation of the axonal terminal membrane opens calcium channel and calcium ions enter the terminal, the increase in calcium concentration stimulates the release of the neurotransmitter that is stored in the vesicle, when these vesicles fuse with the presynaptic membrane the neurotransmitter diffuses into the synaptic cleft
2. post synaptic neuron - ionotropic receptors that are embedded in the membrane of the dendrite or soma of the post synaptic neuron, neurotransmitter binds to ionotropic receptors, opening their ion channels

Question 6

ionotropic receptors

Answer 6

1. ligand gated ion channels
2. neurotransmitter binds directly to the channel protein
3. channel opens immediately
4. ions flow across membrane for a brief time

Question 7

metabotropic receptors

Answer 7

1. coupled to a G protein consisting of 3 subunits (GPCRs, G protein-coupled receptors)
2. slower
3. control ion channels indirectly from the inside
4. control different enzymes within the cell (second messenger molecules)
5. neurotransmitter binds to GPCR, G protein is activated, activated G protein subunit moves to adjacent ion channel which causes a brief delay, channel opens, ions flow across membrane for a longer period of time

Question 8

in the synapse

Answer 8

1. postsynaptic - ionotropic receptor, metabotropic receptor
2. presynaptic - reuptake transported, autoreceptor, heteroreceptor
3. enzymes - for degradation

Question 9

synapse overview

Answer 9

1. action potential is propagated over the presynaptic membrane
2. depolarisation of the presynaptic terminal leads to influx of calcium ions
3. calcium ions promote exocytosis, fusion of vesicles with the presynaptic membrane, releases transmitter into cleft
4. binding of transmitter to receptor molecules in the postsynaptic membrane opens channels, permitting ion flow and initiating an excitatory or inhibitory post synaptic potential
5. excitatory or inhibitory postsynaptic potentials spread passively over dendrites and the cell body to the axon hillock
6. enzyme present in the extracellular space breaks down excess transmitter
7. reuptake of transmitter slows synaptic action and recycles transmitter for subsequent transmission
8. transmitter binds to autoreceptors in the presynaptic membrane

Question 10

each neuron forms many synapses

Answer 10

1. a neuron can collect information from few to hundred of other neurons
2. when and which signal is picked up by a neuron depends on the type of synapse and associated neurotransmitter (inhibitory or excitatory), number synapses and spatial position on the dendrites of the input zone, duration and synchrony of neurotransmitter release from different synapses

Question 11

type of neurotransmitter and receptor define whether a postsynaptic potential is excitatory

Answer 11

1. typical transmitters at excitatory synapses - glutamate, acetylcholine
2. inhibitory - GABA
3. when an excitatory presynaptic neuron fires it shows a normal action potential and causes depolarisation in postsynaptic neuron
4. when an inhibitory presynaptic neuron fires it also shows a normal action potential but it causes hyperpolarisation in postsynaptic neuron

Question 12

temporal summation

Answer 12

if the neurotransmitter is released for a long time into the synaptic cleft, then the postsynaptic potential is stronger

Question 13

spatial summation

Answer 13

if postsynaptic potentials arrive together in the integration, they are summed up

Question 14

spiking neurons

Answer 14

1. graded potentials set up trains of action potentials
2. if the membrane at the integration zone is polarised above the threshold an action potential will be generated
3. the more excitatory input arrives, the stronger the output signal
4. the more inhibitory input arrives, the weaker the ouput signal and the neuron may even not transmit any output signal

Question 15

how is information coded in neural networks

Answer 15

1. spatial and temporal summation at the synapses determine how the signal travels through a network
2. the connectivity of the network determines when or where a signal travels faster or slower, is amplified or reduced, or muted
3. divergence - neuron broadcasts to many others
4. convergence - neuron listens to many others, has high sensitiver and/or can be a gatekeeper
5. feedback loops provide direct or indirect input influence signals and thus information

Question 16

deep neural networks

Answer 16

1. artificial networks for solving AI problems may not require reference to cognitive mechanisms or neurobiological circuits, do not aim to explain the details of how the brain works
2. DNNs have been shown to predict experimental outcomes

Question 17

myelinated neurons

Answer 17

1. conduction velocity increases with increase of diameter and with myelinisation of axons
2. myelinated neurons with thin axons can reach similar conduction velocities as those with unmyelinated thick axons