Synaptic Transmission

• What is a synapse?

  Junction between 2 neurons or neurone and effector cell. Synaptic cleft= tiny gap between cells at synapse. Presynaptic neurone has swelling called synaptic knob, which contains synaptic vesicles filled with neurotransmitters.
• What might happen when neurotransmitters bind to receptors on postsynaptic membrane?

  Action potential triggered(neurone), hormone secreted(gland cell)or muscle contraction(muscle cell).
• What ensures that responses don't keep repeating?

  Neurotransmitters removed from synaptic cleft by taking them back into presynaptic neurone or broken down by enzymes and products taken into neurone.
• What is Acetylcholine (Ach)?

  Neurotransmitter that binds to cholinergic receptors.
• What happens when action potential arrives at synaptic knob of presynaptic neurone?

  Stimulates voltage-gated calcium ion channels in presynaptic neurone to open. Ca2+ diffuse into synaptic knob. (Pumped out afterwards via active transport). Influx of Ca2+ causes synaptic vesicles to fuse with presynaptic membrane. Vesicles release Ach into synaptic cleft by exocytosis.
• What is exocytosis?

  Vesicle inside cell moves to cell-surface membrane, fuses with membrane and releases its contents outside cell.
• What happens when Ach binds to specific cholinergic receptors?

  Sodium ion channels in postsynaptic neurone open. Influx of Na+ causes depolarisation. Action potential generated if threshold reached. Ach removed from cleft- broken down by acetylcholinesterase (AchE) and products re-absorbed by presynaptic neurone and used to make more Ach.
• What is an excitatory neurotransmitter?

  Depolarise postsynaptic membrane, making it fire an action potential if threshold reached. E.g, acetylcholine in cholinergic synapses in CNS and neuromuscular junctions.
• What are inhibitory neurotransmitters?

  Hyperpolarise postsynaptic membrane (make potential difference more negative), preventing it from firing an action potential. E.g, GaBa causes K+ channels to open on postsynaptic membrane when it binds to receptors, hyperpolarising neurone. E.g, Ach in heart.
• What is summation at a synapse?

  Where sum total of lots of smaller impulses triggers an action potential.
• What is spatial summation?

  2 or more presynaptic neurones release their neurotransmitters at same time onto same postsynaptic neurone. Small amount of neurotransmitter released from each of these neurones can be enough altogether to reach threshold and trigger action potential in postsynaptic neurone. If some neurones release inhibitory neurotransmitter then total effect might be no action potential.
• What is temporal summation?

  2 or more nerve impulses arrive in quick succession from same presynaptic neurone. Makes action potential more likely as more neurotransmitter is released into synaptic cleft. Impulses have to follow each other very quickly.
• What is a neuromuscular junction?

  Specialised cholinergic synapse between motor neurone and muscle cell. Use Ach neurotransmitter, binds to nicotinic cholinergic receptors. Postsynaptic membrane has many folds that form clefts to store AchE.
• What types of drugs can affect synaptic transmission?

  Agonist (nicotine), antagonist (curare), inhibit enzymes (nerve gases), stimulate release of neurotransmitters (amphetamines) and inhibit release of neurotransmitters (opioids).
• What 3 ways is a neuromuscular junction similar to a cholinergic synapse?

  1. Both involve the release of the neurotransmitter acetylcholine. 2. Both involve the binding of acetylcholine to receptors on the postsynaptic membrane. 3. Both result in the generation of an action potential in the postsynaptic cell. 4.Both use AchE
• Describe structure of A-band in myofibril. What does it look like under an electron microscope?

  Contains myosin filaments and some overlapping actin filaments. Dark band under microscope.