Smooth muscle comes in various types including: multi-unit smooth muscle, unitary (single unit) smooth muscle, and vascular smooth muscle. Unitary smooth muscle is the most common type of smooth muscle and is present in the uterus, gastrointestinal tract, ureter, and bladder. It is spontaneously active (has slow waves) and has “pacemaker” activity that is modulated by hormones and neurotransmitters. Thanks to electric coupling this muscle subtype is able to have coordinated contraction.

Multi unit smooth muscle is present in the iris, ciliary muscle of the lens, and vas deferens. It has little to no electric coupling between cells thus acts as separate motor units. Because of this it is controlled by neural innervation, and is thus densely innervated. Vascular smooth muscle shares properties of both multi-unit and single unit smooth muscle.

The sympathetic nervous system signals the activation of the fight-or-flight response in the body (including increased heart, blood vessel and eye pupil dilation, and increased stress hormone release). The sympathetic nervous system signal is transmitted via two chains of neurons — the preganglionic neuron and the postganglionic neuron. The preganglionic neuron uses the neurotransmitter acetylcholine. The postganglionic neuron uses the neurotransmitter norepinephrine.

ATP binds to myosin head regions and is hydrolyzed to ADP and inorganic phosphate when muscle relaxes. The release of these products allows the contraction to occur as the myosin head changes position. The binding of new ATP releases the myosin head from actin, and allows the muscle to relax prior to another round of hydrolysis. This explains why, in the absence of adequate ATP, muscle can remain in a contracted state. This phenomenon, when seen in the deceased, is called rigor mortis.

Troponin (also called troponin C) is the most direct structure that interacts with ions involved in initiating muscle contractions. Once bound to calcium ions, troponin facilitates the movement of tropomyosin away from actin binding sites, allowing myosin to bind and, ultimately, contract. Without the binding of calcium ions to troponin, the myosin-binding site on actin remains obscured by tropomyosin and contraction cannot occur.

During muscular contraction, the myosin heads pull the actin filaments toward one another resulting in a shortened sarcomere. While the I band and H zone will disappear or shorten, the A band length will remain unchanged. This is because the A band corresponds to the full length of the myosin filament, or thick filament. Since the myosin filament does not actually change length, the A band remains constant.

The I band corresponds to the region of action that does not overlap with myosin. The length of the actin filament does not change during contraction, but the region of overlap increases. This results in a decrease of the non-overlapped I band.

The H zone refers to the region of myosin that is not overlapped by action. As the region of overlap grows, the H zone shrinks.

When calcium is released in muscle cells, it binds to troponin. This binding allows tropomyosin to change its orientation, exposing the myosin-binding sites on actin. Myosin heads can then bind to actin, and contraction can occur.

Calmodulin is a molecule that can bind calcium; however, it plays important roles in cell signal cascades and is not involved in skeletal muscle contraction.

The H-zone is an area made up of only thick filaments (myosin). The I-band is thin filaments (actin) only, and the A-band is where there are thick and thin filaments. The Z-disc is dividing feature between sarcomeres and appears as dark lines in electron micrographs. 

Muscle contraction results in both the H-band and I-bands shortening, but the A-band remains the same length (A band is Always the same). The Z-band is a static structure and doesn't change size.  

Thick filaments contain myosin. Thick filaments are present in the A band in the center of the sarcomere. Myosin has six polypeptide chains, including one pair of heavy chains and two pair if light chains. Each myosin molecule has two “heads” attached to a single “tail.” The myosin heads are capable of ATP hydrolysis and bind ATP and actin, and are involved in cross bridge formation.

Troponin is the regulatory protein that permits cross bridge formation when it binds . Troponin is a complex of three globular proteins. Troponin C () is the  binding protein, that when bound to , permits the interaction of actin and myosin via conformational change that reveals the myosin binding site on actin, and thus muscular contraction.