9.9 Smooth muscle is nonstriated involuntary muscle Anat 1571

During excitation-contraction coupling,

calcium ions enter the cell from the extracellular space, bind to calmodulin, and activate an enzyme, myosin light chain kinase, powering the cross-bridging cycle.

Unitary smooth muscle,

called visceral muscle, is the most common type of smooth muscle.

Multi-unit smooth muscle

consists of cells that are structurally independent of each other, has motor units, and is capable of graded contractions.

Unitary smooth muscle

contracts rhythmically as a unit, is electrically coupled by gap junctions, and exhibits spontaneous action potentials.

Smooth muscle lacks

neuromuscular junctions, but has varicosities: numerous bulbous swellings that release neurotransmitters to a wide synaptic cleft.

Smooth muscle has

no striations, no sarcomeres, a lower ratio of thick to thin filaments compared with skeletal muscle, and has tropomyosin but no troponin.

Smooth muscle fibers exhibit

slow, synchronized contractions due to electrical coupling by gap junctions.

Smooth muscle contracts more

slowly and consumes less ATP than skeletal muscle.

Smooth muscle cells are

small, spindle-shaped cells with one central nucleus, and lack the coarse connective tissue coverings of skeletal muscle.

Because the muscle filaments have an irregular overlapping pattern,

smooth muscle stretches more and generates more tension when stretched than skeletal muscle.

Smooth muscle initially contracts when

stretched, but contraction is brief, and then the cells relax to accommodate the stretch.

actin and myosin interact

by the sliding filament mechanism; contraction is triggered by a rise in intracellular calcium level, and the process is energized by ATP.

peristalsis

Contraction of the opposing layers of muscle leads to a rhythmic form of contraction, called which propels substances through the organs.

signals for contraction

Hormones and local factors, such as lack of oxygen, histamine, excess carbon

Autonomic nerve endings release either

acetylcholine or norepinephrine, which may result in excitation of certain groups of smooth muscle cells, and inhibition of others.

Hyperplasia,

an increase in cell number through division

hypertrophy

an increase in individual cell size.

Smooth muscle cells are usually

arranged into sheets of opposing fibers, forming a longitudinal layer and a circular layer.

Smooth muscle cells have a less

developed sarcoplasmic reticulum, sequestering large amounts of calcium in extracellular fluid within caveolae in the cell membrane.

Multi-unit smooth muscle

is located in large airways to the lungs, large arteries, arrector pili muscles in hair follicles, and the iris of the eye.

Smooth muscle fibers contain

longitudinal bundles of noncontractile intermediate filaments anchored to the sarcolemma and surrounding tissues via dense bodies.