Types of muscle
Cardiac muscle and smooth muscle
Muscle contraction depends on the ATP-driven sliding of highly organized arrays of actin
filaments against myosin II filaments.
Skeletal muscle cells, also called muscle fibers
A muscle fiber is a huge single cell formed during development by the fusion of many
Typically 50 μm in diameter and up to several centimeters long
Contains the myofibrils
A myofibril consists of a long repeated chain of sarcomeres
The sarcomere is the contractile unit.
Partly overlapping array of parallel thin (actin) and thick (Myo II) filaments
Electron micrograph of an insect flight muscle viewed in cross section
Region of overlap between thin and thick filaments. Hexagonal lattice.
The ~300 myosin heads in a thick filament are not coordinated, therefore low
processivity is critical.
Organization of accessory proteins in a sarcomere
Actin filament plus end binds to the Z disc, which is built by CapZ and α-actinin
Nebulin is a large protein with an actin-binding repeat. Molecular ruler
Tropomodulin positions the thick filament midway between the Z discs. Molecular spring The force generated during contraction depends on the degree of overlap of the thick and thin
Muscle contraction is triggered by a nerve impulse that results in release of acetylcholine at the
neuromuscular junction (synapse).
A sudden rise in cytosolic [Ca ] initiates muscle contraction.
Ca enters the cell and triggers more Ca release from the sarcoplasmic reticulum that