CAM102 Lecture Notes - Lecture 26: Nicotinic Acetylcholine Receptor, Axon Terminal, Myocyte
Muscle Contraction
Sequence:
1. Activation (excitation)
2. Excitation-contraction coupling
3. Molecular interaction at filaments
4. Sarcomere shortening
5. Tension development
Plugging In
• Muscle fibres are activated by synapses, where nerve endings contact receptors on the
sarcolemma
• Motor axon will terminate in a bouton, which is the activating part of a neuromuscular
junction or synapse
Neuromuscular Junction (NMJ)
Three parts:
• Presynaptic Terminal: axon terminal with transmitter vesicles
• Synaptic Cleft: fluid space though which transmitter moves
• Postsynaptic Membrane: with neurotransmitter receptors
How to Excite a Muscle
1. Action potential: release of acetylcholine into synaptic space
2. Diffusion of Ach onto nicotinic receptors
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3. Depolarisation (end-plate potential)
4. Action potential in the muscle fibre
5. Breakdown of Ach by acetylcholinesterase; choline is recycled
Turning Excitation into Contraction
• AP propagates along sarcolemma, and into the interior of fibre via transverse (T) tubules which
are parts of the surface membrane reaching inside, thus brining the AP into the fibre
• These are close to the sarcoplasmic reticulum which stores CA2+ ions
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• As an AP propagates into the T-Tubule, it activates voltage gated calcium channels, by yanking
those plugs, releasing calcium stores in Sarcoplasmic reticulum lumen.
• This quickly releases Calcium ions into the inferior of the fibre
What Calcium Does
• Calcium ions bind to troponin on the thin filaments, lifting tropomyosin away from actin
binding sites
Cross-Bridge Cycling
1. Actin sites exposed
2. Cross-bridges form
3. Binding of myosin heads flexes them, pulling the thin filament along the thick
4. Fresh ATP binds to myosin heads
5. Cross-bridges release
6. Myosin heads return to resting position
7. Steps 2-6 repeat while calcium ions and ATP are present
Synapse
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Document Summary
Sequence: activation (excitation, excitation-contraction coupling, molecular interaction at filaments, sarcomere shortening, tension development. Plugging in: muscle fibres are activated by synapses, where nerve endings contact receptors on the sarcolemma, motor axon will terminate in a bouton, which is the activating part of a neuromuscular junction or synapse. Three parts: presynaptic terminal: axon terminal with transmitter vesicles. Synaptic cleft: fluid space though which transmitter moves: postsynaptic membrane: with neurotransmitter receptors. How to excite a muscle: action potential: release of acetylcholine into synaptic space, diffusion of ach onto nicotinic receptors, depolarisation (end-plate potential, action potential in the muscle fibre, breakdown of ach by acetylcholinesterase; choline is recycled. What calcium does: calcium ions bind to troponin on the thin filaments, lifting tropomyosin away from actin binding sites. Sarcomeres pull on series elastic components: muscle shortening, relaxation time: during which tension returns to baseline levels, re-uptake of calcium ions into sarcoplasmic reticulum, cessation of cross-bridge cycling.
