Each muscle cell is made up of around 1000 “strings” (cylinders) shown
at "5". What are these called?
Myofibrils Myo=muscle Fibril=thread-like
What is the repeating pattern of lines, shown at "6," called?
The sarcomere is the repeating pattern of lines (made of
proteins) found inside the myofibrils of the muscle cell.
What is the name of a muscle fiber's cell membrane, shown at "7"?
What is the name of the tubes shown at "8"?
Transverse Tubules or T-tubules
What is the purpose of these tubes (8) in helping the muscle to
T-tubules carry the electrical current (called an action
potential) produced by nerves from the surface of the muscle cell
into its interior. This makes sure that all of the myofibrils
inside the cell (all 1000 of them) take part in the contraction.
The myofibrils are “strings” filled with sarcomeres, which are
where the proteins responsible for contraction of the cell are
What structure is shown at “10” in the diagram?
Sarcoplasmic reticulum (SR)
Structure “10” stores a substance needed to start the muscle
contraction. What is this substance and what causes its release?
The SR stores calcium. The release of the SR’s calcium inside
skeletal muscle cells is caused by the electrical currents that
nerves create. These electrical currents enter the muscle cell
(by flowing down T-tubules) and cause the sarcoplasmic reticulum
to release its calcium.
How is structure “10” also important in ending/stopping a muscle
To end a contraction, the calcium in the muscle must be
returned/pumped back into the sarcoplasmic reticulum. If calcium
remains in the muscle, it cannot relax (stays contracted). (This
is because the myosin heads stay attached to actin and cannot
separate). See questions #33 and #34 for more information about
the SR and calcium.