Synapses

What are Cholinergic synapses?

Synapses that use the neurotransmitter acetylcholine (ACh)

Describe transmission across a cholinergic synapse

• Pre-synaptic neurone: depolarisation of pre-synaptic membrane causes opening of voltage-gated Ca²⁺ channels - Ca²⁺ diffuse into pre-synaptic neurone

• Causing vesicles containing ACh to move and fuse with pre-synaptic membrane - releasing ACh into the synaptic cleft (by exocytosis)

• At post synaptic membrane neurone: ACh diffuses across synaptic cleft to bind to specific receptors on post-synaptic membrane

• Causing ligand-gated Na⁺ channels to open - Na⁺ diffuse into post-synaptic knob causing depolarisation. If threshold is met, an AP is initiated

Explain what happens to ACh after synaptic transmission

• It’s hydrolysed by acetylcholinesterase

• Products are reabsorbed by the presynaptic neurone

• To stop overstimulation - if not removed it would keep binding to receptors, causing depolarisation

Remaking ACh

• ATP released by mitochondria is used to recombine acetyl (ethanoic acid) and choline - recycling ACh

• More ACh can be made at the SER

• Na⁺ ion channels close in the absence of ACh at their receptor sites

Explain how synapses result in unidirectional nerve impulses

• Neurotransmitter only released from pre-synaptic neurone

• Receptors only on post-synaptic membrane

Explain summation by synapses

• Addition of a no. of impulses converging on a single post-synaptic neurone

• Causing rapid buildup of neurotransmitter (NT)

• So threshold more likely to be reached to generate an AP

Describe Spatial summation

• Many pre-synaptic neurones share one post-synaptic neurone

• Collectively release sufficient NT to reach threshold to trigger an AP

Describe Temporal summation

• One pre-synaptic neurone releases NT many times over a short time

• Sufficient NT to reach threshold to trigger an AP

Inhibitory synapses

• NT can prevent the generation of an AP in a postsynaptic neurone - this is inhibition - the impulses stops at the synapse

• Inhibitory NT hyperpolarise postsynaptic membrane as: Cl^- channels open → Cl^- diffuse in, K⁺ channels open → K⁺ diffuse out

• More Na⁺ required for depolarisation

• Reduces likelihood of threshold being met at post-synaptic membranes

Explain the effect of drugs on synapses - more APs created

Drugs can stimulate the nervous system by creating more APs in postsynaptic neurones:

1. Stimulate the release of more NT

2. Attach to receptors if they’re complementary on shape (mimic the neurotransmitter)

3. Inhibiting the enzyme that hydrolyses the NT

Explain the effect of drugs on synapses - Fewer APs created

Drugs can stimulate the nervous system by creating fewer APs in postsynaptic neurones:

1. Inhibit the release of the NT

2. Block receptors on Na⁺ channels on the postsynaptic neurone