Stimulation of Muscular Action Potentials
nerve impulse axon terminal synaptic end bulb neurotransmitters released synapse motor end plate i) Release of neurotransmitter (Acetylcholine = Ach): Arrival of a nerve impulse at the SEB causes synaptic vesicles to undergo exocytosis where the vesicles fuse with the motor neuron’s plasma membrane emptying ACh into the synaptic cleft. ACh then diffuses across to the motor end plate. ii) Activation of ACh receptors: Binding of 2 ACh molecules to the receptor on the motor end plate opens an ion channel. Once the channel is open, small ions (mostly Na+) can flow across the membrane. iii) Action Potential: As Na+ flows into the muscle fibre (down its electrochemical gradient) the inside becomes more positively charged. This change in membrane potential triggers an action potential. The AP then propagates along the sarcolemma into the T-tubules system. This AP causes a release of Ca++ from the sarcoplasmic reticulum into the sarcoplasm and the muscle fibre contracts. iv) Termination of neurotransmitter activity : The effects of ACh is temporary because it is rapidly broken down by the enzyme Acetycholinesterase (AChE). **2 nd nerve impulse allows repeat of steps 2 & 3 for continued contraction.
Excitation-Contraction Coupling
Increased Ca++ in the cytosol starts muscle contraction and a decrease stops it – when a muscle fibre is relaxed, the concentration of Ca++ in the cytosol is low. However, there is a huge amount of Ca++ stored in the Sarcoplasmic Reticulum (SR). The process of muscle action potentials facilitating a contraction cycle are called the Excitation-Contraction Coupling Mechanism and are described below:
i) An action potential (AP) is generated in muscle fibre
ii) AP is propagated along the sarcolemma and through T-tubules
iii) Ca ++ release channels in sarcoplasmic reticulum (SR) open iv) Ca ++ is released into the cytosol and flows around the thick and thin filaments. v) The released Ca ++ binds to the regulatory protein Troponin causing it to change shape.
vi) This change in shape moves tropomyosin away from the myosin-binding sites on actin.
vii) Once the myosin-binding sites are free, myosin heads are free to bind to myosin-binding sites
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