• Where two neurones meet, they do not actually come into?physical contact?with each other
• Instead, a very small gap, known as the?synaptic cleft, separates them
• The ends of the two neurones, along with the synaptic cleft, form a structure known as a?synapse
• Synapses act as the junctions?between any cells in the nervous system, e.g.
  • In the sense organs, there are synapses between?sensory receptor cells?and?sensory neurones
  • In muscles, there are synapses between?motor neurones?and?muscle fibres?

A synapse

Synaptic transmission

• Electrical impulses cannot ‘jump’ across the synaptic cleft
• When an electrical impulse arrives at the end of the axon on the?presynaptic neurone, the?membrane?of the presynaptic neurone becomes?depolarised, triggering an influx of?calcium ions?into the presynaptic cell via?calcium ion channels?in the membrane
• The calcium ions cause?vesicles?in the presynaptic neurone to move towards the presynaptic membrane where they fuse with it and?release chemical messengers?called?neurotransmitters?into the synaptic cleft
  • A common neurotransmitter is?acetylcholine, or?ACh
• The neurotransmitters?diffuse?across the?synaptic cleft?and?bind with receptor molecules?on the?postsynaptic membrane; this causes associated?sodium ion channels?on the postsynaptic membrane to open, allowing?sodium ions?to diffuse into the postsynaptic cell
• If enough neurotransmitter molecules bind with receptors on the postsynaptic membrane then an?action potential?is generated, which then travels down the?axon?of the?postsynaptic neurone
• The neurotransmitters are then?broken down?to prevent continued stimulation of the postsynaptic neurone
  • The enzyme that breaks down acetylcholine is?acetylcholinesterase

Synaptic transmission using the neurotransmitter acetylcholine

Unidirectionality

• Synapses ensure the?one-way transmission?of impulses
• Impulses can only pass in?one direction?at synapses because?neurotransmitter is released on one side?and its?receptors are on the other?– chemical transmission cannot occur in the opposite direction
• This prevents impulses from travelling the wrong way

• There are over 40 different known?neurotransmitters
  • Examples include dopamine and noradrenaline
• One of the key neurotransmitters used throughout the nervous system is?acetylcholine?(ACh)
  • ACh is produced in the?presynaptic neurone?by combing?choline?with an?acetyl group
  • Synapses that use the neurotransmitter ACh are known as?cholinergic synapses
• Acetylcholine is released into the?synaptic cleft?when?ACh-containing vesicles?fuse with the?presynaptic membrane, releasing ACh molecules into the?synaptic cleft
• ACh?binds to specific receptors?on the postsynaptic membrane, where it can?generate an action potential?in the postsynaptic cell by opening?associated sodium ion channels
• To prevent the sodium ion channels staying permanently open and to stop permanent depolarisation of the postsynaptic membrane, the?ACh molecules are broken down?and?recycled
  • The enzyme?acetylcholinesterase?catalyses the?hydrolysis?of ACh molecules into?acetate?and?choline
  • The products of hydrolysis are then?absorbed back into the presynaptic neurone, and the?active neurotransmitter?ACh is reformed

• Neonicotinoids?are synthetic compounds similar to nicotine that are commonly found in?pesticides
• Neonicotinoids can?block?synaptic transmission at?cholinergic synapses?in?insects?by binding to?acetylcholine receptors
  • This binding is?irreversible, as?acetylcholinesterase?cannot break down neonicotinoids
  • As the acetylcholine receptors are blocked,?acetylcholine is unable to bind, which?stops impulses?from being transmitted across synapses
  • This leads to?paralysis?and?death?in insects
• Neonicotinoids are considered to be especially suitable as pesticides because they're?not toxic to humans and other mammals
  • A much larger proportion of synapses in insects are cholinergic compared to mammals
  • Neonicotinoids bind much more strongly to acetylcholine receptors in insects
• There is a great deal of controversy over the use of neonicotinoid pesticides because of the impact that they are thought to have on essential pollinators such as bees