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SYNAPSES AND DRUGS

SYNAPSES AND DRUGS. What is a synapse?. A synapse is the junction between 2 neurones. There is a very narrow gap of about 20nm between neurones called the synaptic cleft .

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SYNAPSES AND DRUGS

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  1. SYNAPSES AND DRUGS

  2. What is a synapse? • A synapse is the junction between 2 neurones. • There is a very narrow gap of about 20nm between neurones called the synaptic cleft. • An action potential cannot cross the synaptic cleft, so nerve impulses are carried by chemicals called neurotransmitters.

  3. A Synapse • Pre-synaptic neurone = neurone sending impulse • Post-synaptic neurone = neurone receiving impulse

  4. Neurotransmitter • Neurotransmitter is made by the pre-synaptic neurone and is stored in synaptic vessels at the end of the axon. • The membrane of the post-synaptic neurone has chemical-gated ion channels called neuroreceptors. These have specific binding sites for neurotransmitters.

  5. Acetylcholine is a common transmitter. Synapses that have acetylcholine transmitter are called cholinergic synapses. Some neurones form more than 1 synapse. This is an electron micrograph of synapses between nerve fibres and a neurone cell body. Cholinergic Synapses

  6. What happens at a cholinergic synapse? Stage 1 • An action potential arrives at presynaptic membrane. Voltage gated calcium channels in the presynaptic membrane open, calcium ions enter the presynaptic neurone.

  7. What happens at a cholinergic synapse? Stage 2 • Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft.

  8. What happens at a cholinergic synapse? Stage 3 • Acetylcholine diffuses cross the synaptic cleft and binds to specific neuroreceptor sites in the post synaptic membrane.

  9. What happens at a cholinergic synapse? Stage 4 • Sodium channels open. Sodium ions diffuse into the postsynaptic membrane causing depolarisation, which may initiate an action potential.

  10. What happens at a cholinergic synapse? Stage 5 • Acetylcholinesterase breaks down acetylcholine. The products diffuse back into the presynaptic neurone where acetycholine is resynthesised using ATP from the mitochondria.

  11. Neuromuscular Junctions • Same stages as cholinergic synapses, but in this case the postsynaptic membrane is the muscle fibre membrane, (Sarcolemma). Depolarisation of the sarcolemma leads to contraction of muscle fibre.

  12. Drugs • Drugs which have molecules of similar shape to transmitter substances can affect protein receptors in postsynaptic membranes. • Drugs that stimulate a nervous system are called AGONISTS • Drugs that inhibit a nervous system are called ANTAGONISTS.

  13. DRUG ACTION EFFECT Mimic a neurotransmitter Switch on a synapse Stimulate the release of a neurotransmitter Switch on a synapse Open a neuroreceptor channel Switch on a synapse Block a neuroreceptor channel Switch off a synapse Inhibit the breakdown enzyme Switch on a synapse Inhibit the Na+K+ATPase pump Stop action potentials Block the Na+ or K+ channels Stop action potentials Various effects of drugs on synapses:

  14. Effect of nicotine and atropine

  15. Summary • A synapse is the point where 2 nerve cells meet. Tiny gap = synaptic cleft. • Chemical transmitter released from presynaptic neurone diffuses across synaptic cleft & fits into receptors on postsynaptic membrane. May cause postsynaptic neurone to depolarise & set up action potential. • Neuromuscular junction = motor neurone connects with muscle fibre – similar to a synapse. • Many drugs affect synapses.

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