The smallest functional unit of skeletal muscle is the sacromere
. A sacromere is composed of many strands of two protein filaments, the thick and the thin filament
, laid side by side to form a cylindrical segment.
Skeletal muscle is multinucleate
- The thick filament of a sacromere is made of the protein myosin. The thin filament is co mposed mainly of a polymer of the globular protein actin. Myosin and actin work together sliding alongside each other to create the contrctile force of skeletal muscle. Each myosin head crawls along the actin in a 5 stage cycle.
- -First, tropomyosin covers an active site on the actin preventing the myosin head from binding. The myosin head remains cocked in a high energy position with a phosphate and ADP group attached. Second, in the presence of Ca2+ ions, troponin pulls the tropomyosin back, exposing the active site, allowing the myosin head to bind to the actin. Third the myosin head expels a phosphate and ADP and bends into a lower energy position. This is called the power stoke because it causes the shortening of the sacromere and the muscle contraction. Fourth, ATP attaches to the myosin head, releasing the myosin head from the active site. Fifth, ATP splits to inorganic phosphate and ADP causing the myosin head to cock into the high energy position.
A muscle contraction begins with an action potential. A neuron attaches to a muscle cell forming a neuromuscular synapse
. The action potential of the neuron releases acetylcholine
into the synaptic cleft, creating an action potential. The action potential moves deep into the muscle cell via small tunnels in the membrane called T-tubules
. T-tubules allow for a uniform contraction of the muscle by allowing the action potential to spread through the muscle cell more rapidly. The Ca2+ ions begin the 5 stage cycle above. At the end of each cycle, Ca2+ is actively pumped back into the sacromere.