MUSCLE STRUCTURE

The organization of muscle structures makes
them strong. Imagine layers of stretchy cylinders
9
Muscles are made up of smaller fibers, which give them great
strength and flexibility.
inside other stretchy
cylinders then stacked into
bundles. Such bundles would
resemble muscles. They are
packed, multi-layered, and
hard, yet flexible.
Even the largest,
firmest muscle on a body
builder is made up of delicate
threadlike structures
called muscle fibers. These
fibers are actually cells,
the smallest basic unit
in an organism. Under a
microscope, a single muscle
cell, or fiber, looks thin and
fragile. Yet each fiber is
packed with many filaments.
Myofibril filaments are
coated cylinders. Within them are thick and thin myofilaments. The thicker ones are made of a chemical protein substance called actin. The thin myofilaments contain myosin proteins. Groups of myofilament cylinders are bundled into units called sarcomeres. Inside sarcomeres, actin and myosin proteins, slide past each other. This sliding makes muscles move. It is hard to imagine that tiny muscle fibers could possibly contain even more structures, but they do. Muscle spindles inside muscle fibers react to muscle stretching. They send messages to the brain that one muscle is stretched out. The brain then causes electrical and chemical changes to relax the paired muscle. Muscle spindles also communicate with the brain about where
muscles are located. They tell the brain things like how an arm is bent or if a leg is up or down. The brain then adjusts the movements of other body parts for balance. Even if you close your eyes, muscle spindles and the brain’s messengers (called neurons) work together to tell you your position. All muscle fiber structures are individually coated with connective tissue, mainly made up of collagen. This natural protein substance
strengthens everything it encloses. Bundles of filaments, which are called the fascicles, are also covered with connective tissue. These wrapped, coated bundles form the muscle itself and make each one incredibly strong. Muscles do not just float around loosely inside the body. Cord-like connective tissues called tendons attach skeletal muscles to bones, skin, or to other muscles. Muscles are also threaded with networks of tiny blood vessels called capillaries and tubules. These carry nutrients and oxygen in blood into the muscles. When muscles are being used, they release chemicals that cause the heart to direct more blood to the muscles from

Collagen can be found in different parts of the body. This connective tissue strengthens muscles.

The pink and red tubes shown here are capillaries found in muscles. The blood vessels’ many loops and

bends allow them to adjust in size and length as the muscles contract and expand.

the other parts of the body. This extra blood gives the muscles an extra

boost of oxygen and nutrients so they can move faster and longer.