Smooth muscle is found throughout the body around various organs and tracts. Smooth muscle cells have a single nucleus, and are spindle-shaped. Smooth muscle cells can undergo hyperplasia, mitotically dividing to produce new cells. The smooth cells are nonstriated, but their sarcoplasm is filled with actin and myosin, along with dense bodies in the sarcolemma to anchor the thin filaments and a network of intermediate filaments involved in pulling the sarcolemma toward the fiber’s middle, shortening it in the process. Ca++ ions trigger contraction when they are released from SR and enter through opened voltage-gated calcium channels. Smooth muscle contraction is initiated when the Ca++ binds to intracellular calmodulin, which then activates an enzyme called myosin kinase that phosphorylates myosin heads so they can form the cross-bridges with actin and then pull on the thin filaments. Smooth muscle can be stimulated by pacesetter cells, by the autonomic nervous system, by hormones, spontaneously, or by stretching. The fibers in some smooth muscle have latch-bridges, cross-bridges that cycle slowly without the need for ATP; these muscles can maintain low-level contractions for long periods. Single-unit smooth muscle tissue contains gap junctions to synchronize membrane depolarization and contractions so that the muscle contracts as a single unit. Single-unit smooth muscle in the walls of the viscera, called visceral muscle, has a stress-relaxation response that permits muscle to stretch, contract, and relax as the organ expands. Multiunit smooth muscle cells do not possess gap junctions, and contraction does not spread from one cell to the next.
calmodulin
regulatory protein that facilitates contraction in smooth muscles
dense body
sarcoplasmic structure that attaches to the sarcolemma and shortens the muscle as thin filaments slide past thick filaments
hyperplasia
process in which one cell splits to produce new cells
latch-bridges
subset of a cross-bridge in which actin and myosin remain locked together
pacesetter cell
cell that triggers action potentials in smooth muscle
stress-relaxation response
relaxation of smooth muscle tissue after being stretched
varicosity
enlargement of neurons that release neurotransmitters into synaptic clefts
visceral muscle
smooth muscle found in the walls of visceral organs
1. Smooth muscles differ from skeletal and cardiac muscles in that they ________.
A) lack myofibrils
B) are under voluntary control
C) lack myosin
D) lack actin
A
2. Which of the following statements describes smooth muscle cells?
A) They are resistant to fatigue.
B) They have a rapid onset of contractions.
C) They cannot exhibit tetanus.
D) They primarily use anaerobic metabolism.
A
1. Why can smooth muscles contract over a wider range of resting lengths than skeletal and cardiac muscle?
Smooth muscles can contract over a wider range of resting lengths because the actin and myosin filaments in smooth muscle are not as rigidly organized as those in skeletal and cardiac muscle.
2. Describe the differences between single-unit smooth muscle and multiunit smooth muscle.
Single-unit smooth muscle is found in the walls of hollow organs; multiunit smooth muscle is found in airways to the lungs and large arteries. Single-unit smooth muscle cells contract synchronously, they are coupled by gap junctions, and they exhibit spontaneous action potential. Multiunit smooth cells lack gap junctions, and their contractions are not synchronous.