Acetylcholine will stimulate sodium channels on the sarcolemma, which will consequently trigger the release of calcium ions from the sarcoplasmic reticulum. The calcium will then attach to troponin, which pulls tropomyosin away from the active site on actin. With the active site available, myosin heads are able to attach to the actin filament.

Troponin binds free calcium once it is released from the sarcoplasmic reticulum, causing a conformational change in tropomyosin. This change exposes the myosin binding site on actin, allowing for cross-bridge formation and contraction.

Small motor units, typically consisting of one nerve and a few muscle cells, are recruited first. As the muscle contracts for a longer period of time or is required to lift a heavier load, intermediate and large muscle motor units are recruited. As intermediate and large muscle motor units are recruited, more action potentials begin to fire, releasing more calcium from the sarcoplasmic reticulum and increasing the overall strength of the muscle contraction.

Before a contraction, calcium is released from the sarcoplasmic reticulum and attaches to troponin. Troponin will then remove tropomyosin from the active site on actin where myosin is able to attach.

If calcium is never pumped back into the sarcoplasmic reticulum, the active site on actin will stay exposed, which allows myosin to attach at all times.

Note that calcium is also responsible for initiating acetylcholine release from the neuron at the neuromuscular junction; however, this process involves extracellular calcium ions and is not linked to the sarcoplasmic reticulum.

During muscle contraction, the I-band, the H-zone, and the area between the Z-lines get decreased in length. The A-band remains in constant length. Overall, the sarcomere constricts in size, allowing the muscle to contract.

The sliding filament model of muscle contraction states that as the cross-bridge forms between actin and myosin, the myosin head bends (the power stroke), causing actin to move (slide) in the direction of the M-line. When all the actin filaments slide toward the M line like this, the muscle fiber contracts. Calcium is needed for the binding of myosin head to actin. ATP binding leads to the detachment of myosin head from actin. ATP hydrolysis is needed for the unbending of myosin head.

The mechanism for troponin and tropomyosin’s interaction with calcium is comparable to a bike chain with a lock and key. Tropomyosin is the “chain” that blocks the binding sites on actin from the myosin heads. The calcium ion acts like a key to “unlock” troponin and move tropomyosin off of actin’s binding sites. This allows myosin heads to bind to actin and complete their power stroke.

Smooth muscle is found is the bladder, intestines, blood vessels, and a lot of other places that have involuntary motion. Skeletal muscle connects bones and muscles and allows us to move things voluntarily. Cardiac muscle is found in one place—the heart; therefore, the only correct match is the bladder to smooth muscle.

There are three classifications of muscle tissue: skeletal, cardiac, and smooth. A major distinction between these groups is that only skeletal muscle can be voluntarily controlled. Smooth muscle (such as that around blood vessels) and cardiac muscle (in the heart) are not consciously controlled. Each muscle type is unique to its specific function.

Skeletal muscle is the only tissue type that allows for voluntary control; cardiac and smooth do not. Unlike the muscles in your arm, you cannot simply will the other muscle types to work. Think about it: you cannot stop your heart from pumping simply by trying to.