Muscle Fiber
A muscle fiber (also technically known as a myocyte) is a single cell of a muscle. Muscle
fibers contain many myofibrils. Muscle fibers are very long; a single fiber can reach a length of
30 cm. Muscle fibers can be grouped according to what kind of tissue they are found in skeletal
muscle, cardiac muscle and smooth muscle. Skeletal muscle fibers can be further divided into
two basic types: type I (slow twitch fibers) and type II (fast twitch fibers). The muscle cells of
heart muscle tissue are called cardiomyocytes.
Component Variation among Fiber Types
There are differences in the myofibrils and mitochondria of all three muscle tissue fibers.
With regards to the myofibrils; there are a greater number and size of the these components
in the white, fast twitch muscle fibers than in the red, fast twitch muscle fibers, and a greater
number and size in the red, fast twitch muscle fibers, than in the red, slow twitch muscle fibers.
Muscle Composition
At the highest level, the (whole) muscle is composed of many strands of tissue called
fascicles. These are the strands of muscle that we see when we cut red meat or poultry. Each
fascicle is composed of fasciculi, which are bundles of muscle fibers. These muscle fibers are,
in turn composed of tens of thousands of thread-like myofybrils, which can contract, relax, and
elongate (lengthen). The myofybrils are (in turn) composed of up to millions of bands laid end-to-
end called sarcomeres. Each sarcomere is made of overlapping thick and thin filaments called
myofilaments. The thick and thin myofilaments are made up of contractile proteins, primarily actin
and myosin.
Skeletal Muscle (figure 3)
There are seven types of skeletal muscle. The structure of a muscle is specific to its purpose and
function. The types of skeletal muscle are:
- Fusiform - a muscle that has the shape of a spindle, which is wider in the middle and
narrows at both ends. This allows for greater range of motion but limited strength. - Quadrate - a muscle that is square shaped, with parallel fibers that run directly from
origin to insertion - Triangular - a wide origin that converges to a narrow insertion resembling a
triangular shape. - Pennate muscles - fibers run obliquely with respect to the tendon.
- Unipennate - fibers are on the same side of the tendon
- Bipennate - fibers on both sides of the central tendon
- Multipennate - central tendon branches within a pennate muscle
- Longitudinal - parallel fibers consisting of tendinous intersections that run perpendicular
to the direction of the fibers.
Most skeletal muscles are either fusiform or pennate fibers.