[Music] [Laughter] skeletal muscle also called striad muscle because of its striped appearance under the microscope is used to move the skeleton skeletal muscles are under the Direct Control control of the nervous system and can produce contractions ranging from Quick Twitches the powerful sustained tension individual muscle cells called muscle fibers are among the largest cells in the human body ranging from 10 to 100 micrometers in diameter each muscle fiber runs the entire length of the muscle which may be as much as 35 CM or about 14 in in a human thigh each muscle fiber in turn contains many individual contractile subunits myofibrils extending from one end of the fiber to the other each cylindrical myofibril is surrounded by sarcoplasmic reticulum a complex of membranes forming a network of interconnected holot tubes the pyop plasmic reticulum contains a fluid rich in calcium ions deep indentations of the muscle cell membrane called transverse or t tubules extend down into the muscle fiber passing very close to portions of the sarcoplasmic reticulum this arrangement of T tubules and sarcoplasmic reticulum is crucial as we will see to controlling muscle contraction biof fibrils contained subunits called sarir made up of precise Arrangements of actin and meios filaments sarir are attached end to end throughout the length of the myofibril and their Junction points are called zines attached to the Z lines are strands of actin and two accessory proteins that form the thin filaments suspended between the thin filaments are thick filaments composed of the protein myosin the alternating thick and thin filaments give myofibrils their striped appearance Small Arms called cross Bridges extend from the strands of bosin and contact the thin filaments the actin protein of the thin filament is formed from a double chain of subunits resembling a twisted double strand of pearls each subunit has a binding sight for a myosin crossbridge in a relaxed muscle however these sights are covered by two accessory proteins which prevent the mein cross Bridges from attaching to the thin filaments but when a muscle contracts the accessory proteins on the thin filaments are moved aside so meas and cross Bridges can attach to the binding sites on the actin subunits of the thin filaments [Music] [Music]