Synapses and drugs


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Synapses and drugs

  2. 2. What is a synapse? <ul><li>A synapse is the junction between 2 neurones. </li></ul><ul><li>There is a very narrow gap of about 20nm between neurones called the synaptic cleft . </li></ul><ul><li>An action potential cannot cross the synaptic cleft, so nerve impulses are carried by chemicals called neurotransmitters . </li></ul>
  3. 3. A Synapse <ul><li>Pre-synaptic neurone = neurone sending impulse </li></ul><ul><li>Post-synaptic neurone = neurone receiving impulse </li></ul>
  4. 4. Neurotransmitter <ul><li>Neurotransmitter is made by the pre-synaptic neurone and is stored in synaptic vessels at the end of the axon. </li></ul><ul><li>The membrane of the post-synaptic neurone has chemical-gated ion channels called neuroreceptors . These have specific binding sites for neurotransmitters. </li></ul>
  5. 5. Cholinergic Synapses <ul><li>Acetylcholine is a common transmitter. </li></ul><ul><li>Synapses that have acetylcholine transmitter are called cholinergic synapses. </li></ul><ul><li>Some neurones form more than 1 synapse. </li></ul><ul><li>This is an electron micrograph of synapses between nerve fibres and a neurone cell body. </li></ul>
  6. 6. What happens at a cholinergic synapse? Stage 1 <ul><li>An action potential arrives at presynaptic membrane. Voltage gated calcium channels in the presynaptic membrane open, calcium ions enter the presynaptic neurone. </li></ul>
  7. 7. What happens at a cholinergic synapse? Stage 2 <ul><li>Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft. </li></ul>
  8. 8. What happens at a cholinergic synapse? Stage 3 <ul><li>Acetylcholine diffuses cross the synaptic cleft and binds to specific neuroreceptor sites in the post synaptic membrane. </li></ul>
  9. 9. What happens at a cholinergic synapse? Stage 4 <ul><li>Sodium channels open. Sodium ions diffuse into the postsynaptic membrane causing depolarisation, which may initiate an action potential. </li></ul>
  10. 10. What happens at a cholinergic synapse? Stage 5 <ul><li>Acetylcholinesterase breaks down acetylcholine. The products diffuse back into the presynaptic neurone where acetycholine is resynthesised using ATP from the mitochondria. </li></ul>
  11. 11. Neuromuscular Junctions <ul><li>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. </li></ul>
  12. 12. Drugs <ul><li>Drugs which have molecules of similar shape to transmitter substances can affect protein receptors in postsynaptic membranes. </li></ul><ul><li>Drugs that stimulate a nervous system are called AGONISTS </li></ul><ul><li>Drugs that inhibit a nervous system are called ANTAGONISTS. </li></ul>
  13. 13. Various effects of drugs on synapses:   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
  14. 14. Effect of nicotine and atropine
  15. 15. Summary <ul><li>A synapse is the point where 2 nerve cells meet. Tiny gap = synaptic cleft. </li></ul><ul><li>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. </li></ul><ul><li>Neuromuscular junction = motor neurone connects with muscle fibre – similar to a synapse. </li></ul><ul><li>Many drugs affect synapses. </li></ul>