What happens to a muscle when it relaxes?
When a muscle relaxes, several physiological changes occur:
1. Reduction in Action Potential: The relaxation process begins when the action potential propagated along the motor neuron during muscle contraction reaches the neuromuscular junction. The motor neuron stops releasing acetylcholine (ACh), the neurotransmitter responsible for muscle contraction.
2. Cessation of Calcium Release: The absence of ACh binding to receptors on the motor end plate prevents the opening of voltage-gated calcium channels on the muscle cell membrane. As a result, calcium ions can no longer enter the muscle cell from the extracellular fluid.
3. Reuptake of Calcium Ions: Calcium pumps located on the sarcoplasmic reticulum, the muscle cell's internal calcium storage, actively transport calcium ions back into the sarcoplasmic reticulum. This process lowers the calcium concentration in the muscle cell's cytoplasm, reducing its availability for muscle contraction.
4. Detachment of Myosin Heads from Actin: With decreased calcium levels, the calcium-binding protein troponin on the actin filament undergoes conformational changes. This change alters the shape of the troponin-tropomyosin complex, allowing the myosin head (crossbridge) to detach from the actin filament's myosin-binding site.
5. Disengagement of Crossbridges: The release of calcium from troponin causes a loss of the high-energy state that powered the crossbridge formation during contraction. As a result, the crossbridges between actin and myosin filaments break, and the muscle fibers can no longer slide past each other.
6. Muscle Fiber Lengthening: The elastic recoil of the muscle tissue and the antagonistic muscles' action, which oppose the contracted muscle's movement, cause the relaxed muscle fibers to lengthen and return to their resting length.
7. Metabolic Recovery: During muscle relaxation, the muscle also undergoes metabolic recovery, replenishing the adenosine triphosphate (ATP) stores depleted during contraction. ATP is essential for the reuptake of calcium ions into the sarcoplasmic reticulum and for preparing the muscle for subsequent contractions.
In summary, muscle relaxation involves the cessation of action potential and calcium release, the reuptake of calcium into the sarcoplasmic reticulum, the detachment of myosin heads from actin, and the recovery of metabolic resources, leading to a return of the muscle to its resting state and allowing it to extend or be elongated.