Synapses and Networks

Question 1

What is the neuron doctrine?

Answer 1

• brains are composed of separate neurons and other cells
• cells are independent
• neurons are polarised cells
• information is transmitted from cell to cell across tiny gaps

Question 2

What are the different types of neurones?

Answer 2

• multipolar neuron: receives input from many different neurones and has multiple polarities around the soma
• bipolar neuron: only has 1 dendrite and 1 axon
• unipolar neuron: found in invertebrates mainly, cell body lies in the middle of the conduction zone

Question 3

What are the typical locations of ion channels?

Answer 3

• voltage-gated channels (Na+, K+): axonal hillock (integration zone of the axon), axon (conduction zone)
• leak channels and ion pumps: entire neural membrane
• voltage-gated Ca2+ channels: axon terminals (ouput zone)
• ligand-gated channels: dendrites and soma (input zone)

Question 4

What are the typical locations of synaptic processes?

Answer 4

• axo-dendritic
• axo-somatic
• axo-axomic
• dendro-dendritic

Question 5

What occurs in a signal transmission within a spiking neuron?

Answer 5

• depolarisation: (graded potential before reaching threshold) when the neuron is excited
• action potentials are generated in the integration zone if depolarisation reaches threshold (signal transmission inside neuron)
• signal transmission to next neuron starts when action potentials reach the output zone and neurotransmitter is released
• (signal transmission is accomplished if graded potential in input zone of postsynaptic neuron is elicited)

Question 6

How is a signal transformed during synaptic transmission?

Answer 6

• at the presynaptic neuron: depolarisation of axon terminal membrane opens the Ca2+ channels and allows those ions to enter the terminal
• increase in Ca2+ concentration stimulates the release of neurotransmitter
• postsynaptic neuron: neurotransmitter crosses cleft and interacts with iontropic receptors (embedded in membrane of dendrite/soma) of the neuron
• iontropic receptors are ligand-gated ion channels (open when bound by neurotransmitter molecules), different types vary in affinity to certain neurotransmitter/drug

Question 7

What are metabotropic receptors?

Answer 7

• are G protein-coupled receptors (GPCRs) are slow but influence adjacent ion channels
• receptors coupled to G protein consist of 3 subunits
• when activated by conformational shape change, the protein can interact directly with ion channels/ control release of another second messenger molecule inside the postsynaptic cell

Question 8

What is an electric synapse?

Answer 8

• gap junctions connect the cytoplasm of 2 neurons
• instantaneous current flow (fast transmission of electrical signal across connexions producing coupling effect)
• gap between membrane as little as 20-40mm
• fast action: commanding escape responses
• synchronised activity: inhibitory neurons in mammalian brain, eye-moving muscles

Question 9

How does the transmission of neural signals differ over short and long distances?

Answer 9

• neurons with long axons transmit the signal with action potentials along the axon, signal is sustained effectively by the population of voltage-gated ion channels in the axonal membrane
• neurons with short axons/no axon don’t generate action potentials, signal spreads passively to the output zone
• signals weaken the longer it travels, longer the distance the stronger the attenuation
• long neurons have thick axons to reduce costs, myelinisation of axons in spiking neurons is another solution
• neuroglia cells assist the signal propagation, provide nutrients to neurons and clear debris and mediate immune response

Question 10

What are neurons and how do they pick up a signal?

Answer 10

• each neuron forms many synapses
• collect information from few to hundreds of other neurons
• when/which signal is picked up depends on the type of and duration of the neurotransmitter release
• type of neurotransmitter and receptor defines whether a postsynaptic potential is excitatory/inhibitory

Question 11

What are the typical transmitters at excitatory and inhibitory synapses?

Answer 11

• excitatory: glutamate, aspartate, nicotinic acetylcholine, muscarinic ach
• inhibitory: GABA, glycine, muscarinic, acetylcholine
• the more excitatory input the stronger the output signal, the probability an action potential is generated increases
• the more inhibitory input arrives the weaker the output signal, may not even transmit any output signal
• voltage threshold has to be crossed for action potential to be occurred of if total sum of the PSPs is more positive than negative, it still has to be enough to cross threshold

Question 12

What is temporal summation and spatial summation?

Answer 12

• temporal summation: if the neurotransmitter is released for longer time into synaptic cleft then the postsynaptic potential is stronger
• spatial summation: when postsynaptic potentials arrive together in the integration they’re summed up, the closer they are the larger the effect of summation

Question 13

What is the vocabulary of the neural language?

Answer 13

• neuron responds in binary code (yes/no)
• yes: signal gets transmitted inside neuron to output zone and sufficient neurotransmitter is released
• no: signal doesn’t reach output zone or is too weak to release sufficient neurotransmitter so signal not transmitted
• if both EPSPs are present at same time and sum to depolarisation that passes threshold then action potential is generated
• if IPSPs are present and stronger than the EPSPs then action potential isn’t generated

Question 14

How is information coded in neural networks?

Answer 14

• spatial and temporal summation at synapses determine strength of signal when it’s passed from 1 neuron to the next
• in feedforward circuits the signal is distributed to many neurons through divergent connections or is determined by collecting signals from many converging neurons
• feedback loops provide direct/indirect input influencing signals and thus information
• simple neural language rules and binary code, complexity of neural language comes with number of connections, layers and types of connectivity between neurons in networks