The human endoskeleton provides a framework for body tissues and organs. Bones attach to one another via tough connective tissue called ligaments and by cartilaginous, fluid-filled joints. Muscles attach to bones via tendons so that you can leverage your bones to move body parts. Skeletal muscles are striated muscles, as opposed to heart muscle or the smooth muscle of organs like the intestines. There are more than 600 skeletal muscles, which constitute about 40 percent of body weight.
Arrangement
Muscles attach to bones so that they can act as levers working around a fulcrum on the bone. Muscles are usually arranged by pairs in an antagonistic fashion --- one muscle contracts while the other relaxes. Muscles that move structures away from the body midline are called abductors; those that move body parts toward the body midline are called adductors. Some other terms that describe muscle movement relative to the bones to which they attach are flexion/extension, elevation/depression and rotation.
Origins and Insertions
Muscles are said to originate and insert with respect to the bones they attach to. The origin end of a muscle is the end that attaches to the bone that has the least movement. The insertion end attaches to the bone that moves the most. Some muscles have a synergistic arrangement, where they aid each other in a movement.
Tendons
There are typically tendons at each end of a muscle that attach the muscle to the points of origination and insertion. Tendons consist of a connective tissue called collagen, which has elasticity due to the crimp contained in collagen's molecular structure. Tendons are divided into subunits called fascicles, which contain the crimped fibril units and cells called fibroblasts, which create the tendon materials. Tendons carry tensile forces from muscle to bone and act like pulleys on the bone.
Muscle Cells
Striated muscle consists of long, multinucleate cells formed by precursor myoblast cells that fuse together. Each cell is filled with long thread-like myofibrils so that muscle cells look banded or striated when viewed under a microscope. The striations are due to subcellular arrangements of two proteins: thick filaments of myosin and thin filaments of actin. Thin dense Z-lines mark the end and beginning of each functional muscle unit, called a sarcomere.
Muscle Structure
Each body muscle consists of subunits. First individual muscle cells form small units called primary bundles, which have a connective tissue covering called the perimysium. Primary bundles are further assembled together to form muscles. Entire muscles are covered with a tough connective tissue sheath called the epimysium.
Contraction
For muscles to leverage bones, they have to shorten and lengthen. This is accomplished through the contractility of the myosin and actin filaments sliding against each other. Contractions occur when calcium is released into the sarcomere. Under usual conditions, the energy for contraction comes from molecules of ATP within the muscle fibers. Calcium activates binding sites in the actin, which surrounds the myosin. The myosin fibers have double heads that attach to actin binding sites. The process of binding, unbinding and rebinding occurs many times over a short period of time, sliding myofibrils past each other in a telescoping action and shortening the muscle fiber. To lengthen, the calcium is pumped out, the actin binding sites are deactivated and the actin and myosin relax.
