Video Transcript
The diagram provided shows some of the structures that make up skeletal muscle
fibers. State the organelle described by the following statement: This is the structure
within skeletal muscles that stores and releases calcium ions needed to initiate
muscle contractions.
Skeletal muscle is a type of muscle tissue that is attached to the bones of the
skeleton involved in voluntary movements like consciously moving our legs to walk or
run. Skeletal muscle is composed of many bundles of muscle fibers. And depending on its size, a muscle may be made up of thousands of individual muscle
fibers. Each bundle of muscle fibers is surrounded by a protective layer of connective tissue
called the perimysium, which helps the muscle cells to withstand the pressure of
muscle contraction and provides a space for the blood and to nervous tissue to
connect to the individual muscle fibers.
Each skeletal muscle fiber is a long cylindrical muscle cell, surrounded by a plasma
membrane called the sarcolemma, or sometimes the myolemma. The prefix sarco- comes from the Greek word that means flesh, and myo- refers to
muscles themselves. Both of these prefixes are often used to describe different components of
muscles. The suffix -lemma derives from the Greek word for sheath as it forms a protective
layer around each muscle fiber.
As this question is asking us to identify a specific organelle found within muscle
fibers, let’s take a look at the structure of a section of muscle fiber on a
microscopic scale. Remember that the sarcolemma, a part of which we can see here, surrounds the whole
muscle fiber. The rest of the sarcolemma has been removed from the image so we can see some of the
other organelles more clearly. Muscle fibers are much longer than other cells as they formed by many individual
muscle cells fusing together when we were just embryos. As a result, one muscle fiber typically has many nuclei.
The cytoplasm within a muscle fiber is called the sarcoplasm, which contains the
organelles that are needed within the muscle fiber cell. Muscle fibers require a large amount of energy to contract, so they contain many
mitochondria. You may recall that the role of the mitochondria is to carry out cellular
respiration, so these organelles release the energy that is needed for muscular
contraction.
Each muscle fiber contains between 1000 and 2000 long, cylindrical organelles called
myofibrils arranged in parallel to each other. Myofibrils are made of protein fibers and are specialized for contraction. Muscle fibers contain a specialized endoplasmic reticulum called the sarcoplasmic
reticulum, which extends through the muscle fiber surrounding the myofibrils. The sarcoplasmic reticulum of a skeletal muscle fiber stores calcium ions, which are
released to initiate muscle contraction.
Now, we know the structure within skeletal muscles that stores and releases calcium
ions needed to initiate muscle contractions. This organelle is called the sarcoplasmic reticulum.
