Examination of Muscle Tissue

Below is a description of skeletal muscle tissue. Click on the terms for information on smooth muscle or cardiac muscle tissue.


Skeletal Muscle: Macroscopic

Skeletal muscle attaches to bone for the movement of the skeleton, to other muscles and to the base membrane of the skin. The approximatley 700 skeletal muscles in the body vary greatly in size and shape.

Each muscle is composed of parallel bundles of individual muscle cells. Each cell is surrounded by a delicate connective tissue called the endomysium, which connects the individual muscle cells together. Bundles of these cells, called fascicles are wrapped in another layer of connective tissue, the perimysium, which holds the bundle together, defining the fascicle. Many fascicles are wrapped in yet a third tissue, the epimysium making a single muscle. Each muscle is wrapped by a another sheath of connective tissue called a fascia, which supports and holds the entire muscle together , seperating it from other muscles. These connective tissue wrappings are actually continuous with each other, meeting at the end of the muscle where they unite to form an extremely tough, fibrous connective tissue material called a tendon. Tendons attach muscle to bone by intertwining with the fibers of the periosteum of the bone.

Muscles are usually attached to the skeleton at two locations. One end is attached to a 'stationary' location, called the origin, and the other end is attached to the bone that will move called the insertion. For instance, the bicep originates on the humerus and inserts on the ulna/radius. Muscles usually exist in antagonistic pairs which allow for movement in two dimensions. The tricep moves the forearm in the opposite direction of the bicep. If both contracted at the same time, no movement would occur, so coordination of the two muscles (all muscles!) is imperitve.


Skeletal Muscle: Microscopic

Skeletal muscle cells are tubular in shape, contain many nuclei per cell, and appear striated or striped. These muscle cells can be very long, ranging from 1mm to 30cm in length with a diameter of 1-10µm. The longer the muscle, the longer the cell!

Muscle cells contain all the same cellular parts other cells have, but some have specialized functions. The smooth endoplasmic reticulum forms into parallel sac-like compartments called the sarcoplasmic reticulum, or SR. On the outer surface of the cells membrane are the openings of tiny tubes called transverse tubules, or T tubules. These tubules are extensions of the cell membrane that run deep into the cell and lie close to the SR. They relay the electrical signals that trigger contraction.

 

Within each muscle and running it's entire length are numerous protein fibers called myofibrils. Myofibrils are the structures responsible for contration. Each myofibril consists of a series of units called sacromeres. The orderly arrangement of the sacromeres give a striated appearence to skeletal muscle cells.
Each sacromere is made up of a complex arragement of proteins. Two of these proteins are actin and myosin. Myosin filaments lie primarily within dark bands seen within muscle cells, a region called A bands, while actin filaments are primarily within the lighter bands of the sacromere. These lighter bands are called I bands. Where the individual actin filaments attach to one another is a dark line, the Z line, which appears within the I band. It is the distance between Z lines that defines any one sacromere. Within the A band is a lighter region, the H band, which is an area of only myosin. A thin line within this region, formed by connections between myosin filaments if the M line. As we'll see, the molecular interaction between the two directly lead to muscle contraction.