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Muscle Excitation and Contraction

Calcium Ions and Muscle Excitation

• Calcium Ion Diffusion: Calcium ions diffuse following the opening of voltage-gated calcium channels located on the membrane of the axon terminal. This opening is triggered in response to an action potential.

• Release of Acetylcholine:
    - In the axon terminal, there are membrane-bound synaptic vesicles that contain the neurotransmitter acetylcholine (ACh).
    - The influx of calcium ions stimulates the vesicles to release acetylcholine into the synaptic cleft.

• Receptor Binding:
    - Acetylcholine binds to receptors located on the postsynaptic membrane, specifically in the junctional folds of the synapse.
    - These receptors are chemically gated ion channels that open when acetylcholine attaches to them, facilitating the influx of ions which contributes to muscle excitation.

Excitation-Contraction Coupling

• Action Potential Propagation in Muscle Fiber:
    - An action potential is generated and propagated along the sarcolemma (muscle cell membrane) due to the opening of voltage-gated sodium channels.
    - The action potential travels through the entire muscle fiber and down into the T-tubules, which also contain voltage-gated channels.

• Calcium Release:
    - Voltage-gated proteins in the T-tubules trigger calcium release channels in the sarcoplasmic reticulum (SR) to release calcium ions into the sarcoplasm.

• Calcium Activation of Sarcomeres:
    - Calcium ions from the SR activate the sarcomeres of each myofibril, thereby initiating muscle contraction.
    - Each actin filament has binding sites for myosin heads, but these sites are covered by tropomyosin in the absence of calcium.
    - When calcium ions are released from the terminal cisterns, they bind to troponin, which causes tropomyosin to shift and expose the binding sites on the actin.

Contraction Mechanics

• Cross-Bridge Formation:
    - Myosin heads can now bind to the exposed active binding sites on actin, forming cross bridges, which are essential for contraction.

• Initiation of Power Stroke:
    - The power stroke begins when myosin heads pivot in a high-energy state, fueled by the hydrolysis of ATP, releasing ADP and Pi (inorganic phosphate).
    - The pivoting action pulls the thin filaments along the thick filaments towards the center of the sarcomere.

• Detachment of Myosin Heads:
    - Myosin heads remain bound to actin until ATP binds to them, which weakens the attachment between actin and myosin, facilitating detachment.
    - The myosin head remains in a low-energy state post-detachment.

Muscle Relaxation

• Cessation of Calcium Release:
    - Motor neuron signals cease, and therefore, action potentials no longer conduct along the sarcolemma.
    - The terminal cisterns stop releasing calcium ions.

• Calcium Ion Transport:
    - Calcium ions are actively transported out of the sarcoplasm back into the sarcoplasmic reticulum through calcium ion pumps.
    - This reduction in calcium concentration below the threshold for excitation-contraction coupling causes troponin to return to its original state.

• Return of Tropomyosin:
    - Tropomyosin glides back over the binding sites on actin, which inhibits myosin heads from binding even if they are still in the high-energy state.
    - The sarcomeres can return to their original length as a result of this process.