Smooth Muscle Contraction, Compared to Skeletal Muscle Contraction, Animation

Added: 2026-05-26

[00:00:02]Smooth muscle is found in the wall of hollow  internal organs, where it plays a vital role in their functions. Unlike skeletal muscle, which  is responsible for the body’s voluntary movements, smooth muscle contraction is involuntary.  Without a thought from the person, the autonomic nervous system and hormones  control smooth muscle contraction for automatic functions such as peristalsis during  digestion, air flow during respiration, and regulation of blood pressure, among others. Like skeletal muscle, smooth muscle contains thin (actin) and thick (myosin) filaments, but  these are not organized into parallel sarcomeres.

[00:00:46]This arrangement allows smooth muscle cells  to contract in not one, but many directions, reducing not just the length of the muscle,  but the volume of the organ it encloses.

[00:00:58]Smooth muscle also uses calcium for  contraction, but unlike skeletal muscle, which relies on intracellular calcium released  from the sarcoplasmic reticulum (the SR), smooth muscle contraction is initiated  by extracellular calcium influx.

[00:01:14]- Triggers (including membrane depolarization,  hormones and neurotransmitters), cause calcium to enter the cell via L-type voltage-gated calcium  channels. Intracellular calcium then stimulates the release of more calcium from the SR. - Instead of troponin as in skeletal muscle, calcium in smooth muscle binds  to and activates calmodulin.

[00:01:40]- Calmodulin then activates the enzyme  myosin light chain kinase (MLCK).

[00:01:47]- MLCK phosphorylates a regulatory light chain  on myosin. This phosphorylation produces a conformational change in the myosin  head, increasing its ATPase activity, and causing it to bind to actin. - Cross-bridge cycle then occurs, resulting in muscle contraction. Because myosin is phosphorylated during contraction, simply removing calcium  is not enough to produce relaxation.

[00:02:17]The activity of another enzyme - myosin light  chain phosphatase (MLCP) - is required to dephosphorylate myosin for relaxation to occur. This is also the mechanism by which nitric oxide, a vasodilator, acts to relax smooth  muscle in the wall of blood vessels.

[00:02:37]Because of the requirement for phosphorylation  and dephosphorylation, contraction in smooth muscle starts more slowly and lasts longer than in  skeletal muscle. This feature is essential for the functions of internal organs, which often require  smooth muscle to stay contracted for extended periods of time with minimum energy expenditure. Other factors that contribute to sustained contractions in smooth muscle include: - slower-acting ion channels in smooth muscle, which produce slower but  longer-lasting action potentials.

[00:03:09]- and lower ATPase activity, which translates to a  slower cycling speed and longer contraction time.

[00:03:16]There is also “latch state” mechanism  - when dephosphorylation occurs while myosin still attached to actin,  slowing their detachment rate.