6.5 Neurons and Synapses

• Neuron


  Specialised cells that transmit electrical impulses
• Central nervous system

  Brain and spinal cord
• Structure of a neuron

  • Dendrites: short-branched fibres that convert chemical info received from other neurons → electrical signals
  • Soma: contains nucleus and organelles where metabolic processes occur to maintain cell survival
  • Axon: elongated fibres that transmit electrical signals to terminal regions for communication with other neurons or effectors. May be myelinated
  • Axon terminal: joins to other neurons (neurons) or muscle cells (effectors)
• Membrane potential


  Measure of the unequal distribution of ions on either side of the membrane (basically the charge difference)
• Resting potential


  Membrane potential when nerve is NOT firing, generated/maintained by active transport of Na and K across neuron membrane via sodium-potassium pumps
• Action potential


  Rapid change in charge across the membrane when neuron IS firing
• Action potential generation


  1. Depolarisation: stimulus at dendrite triggers Na channels to open, Na+ rushes into neuron, membrane potential becomes more positive

  2. Repolarisation: K channels open and Na channels close, K+ rushes out of neuron, membrane potential becomes more negative

  3. Refractory Period: repolarisation causes membrane potential to be more negative than resting potential, ensuring neuron can't propagate another action potential straight away
• Nerve impulse


  Action potentials propagated along the axon of a neuron
• Na and K channels are voltage-gated - membrane potential must reach the threshold potential (~-55mV) for channels to open
• If depolarisation in one part of the axon reaches the threshold potential, it will cause adjacent Na channels to open and a rapid increase in membrane potential to propagate down the axon (wave of depolarisation), and depolarisation/repolarisation process to be repeated along length of axon
• Oscilloscope trace


  Measures membrane potential, time is milliseconds, membrane potential is millivolts
• Myelination


  • Some neurons' axons are insulated by myelin produced by Schwann cells, increases speed of electrical transmission via saltatory conduction
• Synapse


  Junctions between neurons and between neurons and receptor or effector cells
• Synaptic transfer process


  Action potential reaches axon terminal, triggers voltage-gated Ca channels to open, Ca2+ diffuse into neuron and promotes synaptic vesicles (containing neurotransmitters) to fuse with the cell membrane, releasing NTs via exocytosis into the synaptic cleft, NTs bind to specific receptors in post-synaptic neuron, causing ion channels to open in post-synaptic neuron - this causes depolarisation in post-synaptic neuron, which may lead to action potential, NTs recycled by reuptake or degraded by enzymes
• Acetylcholine (ACh)


  Neurotransmitter used in synapse between neuron and muscle (effector cell), when bind to muscle fibre, stimulate muscle contraction, created in axon terminal by combine choline + acetyl CoA and then released by synaptic transfer process, broken down into component parts by acetylcholinesterase (AChE), choline transported back into presynaptic axon terminal to make more ACh
• Neonicotinoid pesticides


  Irreversibly bind to acetylcholine receptors in cholinergic synapses (because can't be broken down by AChE) and trigger sustained response, causing overstimulation - cause paralysis and death, highly toxic and effective on insects because they have different composition of ACh receptors which bind to neonicotinoids more strongly, may have negative impact on beneficial insects (e.g. honeybees), and hence bird populations