Neurones, synapses and neurotransmitters

Question 1

The human brain

Answer 1

• contains ~100 billion neurones
• 1.3-1.4kg in weight
• 150,000-180,000km of nerve fibres; huge nerve packing and complexity
• 150,000 billion synapses in cortex
• 1 neurone lost every second

Question 2

Neurone specialisation

Answer 2

Features that are specialised for communication:

• dendrites; pick up input from other neurones
• axon hillock; action potential generation
• axon; action potential propagation

Question 3

Synapses

Answer 3

Points of communication between cells of the nervous system (neurones and effectors).

Question 4

Electrical synapse/gap junction

Answer 4

Physical connection between cell membranes via connexons (6 connexins), allowing transfer of ions and small molecules.

• fastest and most primitive
• bi-directional conduction
• allows synchronous activity between cells
• rarely found between neurones; common in glia-glia/neurone-glia connection and cardiomyocytes

Question 5

Chemical synapse

Answer 5

Synaptic cleft between the axon terminal of a presynaptic neuron and a postsynaptic dendrite/membrane.

• release of neurotransmitter from vesicles to a receptor
• uni-directional transfer of information

Question 6

Neurotransmission process

Answer 6

1) action potential invades nerve terminal (Na+ influx)
2) depolarisation triggers Ca2+ channel opening (Ca2+ influx)
3) neurotransmitter is released from vesicles into the intracellular space via exocytosis
4) neurotransmitter diffuses across the synapse and binds to a receptor
5) post-synaptic effect may be activation or inhibition
6) signal is terminated by re-uptake or enzymatic breakdown of neurotransmitter

Question 7

The major neurotransmitters

Answer 7

• amino acids eg. glutamate, GABA, glycine
• monoamines eg. noradrenaline, dopamine, 5-HT (serotonin)
• acetylcholine (ACh)
• neuroactive peptides: >100 different kinds
  eg. opioid peptides (endorphin, enkephalin), tachykinins (substance P, neurokinin A)

Question 8

Receptors

Answer 8

Transmembrane proteins serving as recognition sites for neurotransmitters to bind to, causing a structural change. Each receptor is specific to one NT, but each NT may bind to several receptor subtypes.

Question 9

Ionotropic signalling mechanism

Answer 9

Neurotransmitter binds to a ligand-gated ion channel, causing a conformational change which opens the channel, allowing the movement of ions. Signal transmission is fast.

Question 10

Excitatory ionotropic receptor

Answer 10

Na+ influx leads to membrane depolarisation and propagation of the action potential; eg. glutamate AMPA/NMDA/kainate receptor subtypes, nicotinic ACh receptor.

Question 11

Inhibitory ionotropic receptor

Answer 11

Cl- influx leads to membrane hyperpolarisation, counteracting any depolarisation; eg. GABAa receptor subtype, glycine receptor.

Question 12

Metabotropic signalling mechanism

Answer 12

Neurotransmitter binds to a G-protein coupled receptor (GPCR), causing a conformational change which activates the G-protein, subsequently activating effector systems eg. ion channels, enzymes, secondary messengers; eg. muscarinic ACh receptor, GABAb/monoamine receptors.

Question 13

Spatial summation

Answer 13

Summing of post synaptic potentials generated at separate synapses.

Question 14

Temporal summation

Answer 14

Summing of post synaptic potentials generated at the same synapse, if they occur in rapid succession.