MICROSCOPIC ANATOMY OF THE STRIATED MUSCLE TISSUE
The striated muscle fiber is a syncytium (para 5-3b).
a. The fiber, as a whole, is surrounded by a membrane known as the
sarcolemma. The sarcolemma has specialized invaginations that enter the interior of
the fiber at right angles to the sarcolemma. These are called transverse tubules
(T-tubules). The T-tubules connect with the extracellular space and allow interstitial
fluid to flow in and through the striated muscle fiber.
b. The fiber is filled with a type of intracellular fluid called sarcoplasm.
SARCO = flesh
c. Within the sarcoplasm is a tubular system called the sarcoplasmic reticulum
that stores calcium, which is necessary for the muscle activation and contraction.
d. Myofilaments are found in the sarcoplasm.
MYO = muscle
FIL = thread
Myofilaments are long complexes of protein molecules, either actin or myosin. Thus,
there are two main types of myofilaments: actin and myosin. The myosin filaments are
thicker and have appendages known as myosin "bridges." The myosin filaments are
surrounded by the thinner actin filaments.
e. Great numbers of well-developed mitochondria (the "powerhouse" elements
of cells) are found in striated muscle fibers.
5-5. CONTRACTION OF A STRIATED MUSCLE FIBER
a. "Sliding Filament" Theory. The current consensus of opinion of how a
striated muscle fiber contracts is known as the "sliding filament" theory (Figure 5-1).
This theory emphasizes the role of the myosin bridges. Energy is provided by the
mitochondria in the form of ATP. With this energy, the myosin bridges swing and draw
the actin filaments over the myosin filaments. The length of the striated muscle fiber is
b. "All-or-None" Phenomenon. When stimulated to contract by a nervous
impulse, a striated muscle fiber contracts totally or not at all. This is the "all-or-none"
phenomenon. The striated muscle fiber has a threshold of stimulation. Below this
threshold, the fiber will not act. When stimulated at or above this threshold, the fiber will
contract totally every time.