Muscular and nervous tissues are able to transmit an electrochemical impulse down their cell membranes. These cells are excitable. This property sets them apart from other human tissues. In this chapter we will concentrate on muscular tissue and the junction between nervous and muscular tissue. Chapter twenty-one deals more fully with nervous tissue.
In addition to being excitable, muscle cells are contractile and elastic. That is they can shorten and then return to their original shape. The shortening of muscle cells is responsible for movement in and of the body.
Muscle cells are very long and are often referred to as muscle fibers. There are three basic types of muscle fibers (Figure 131): skeletal, cardiac, and smooth.
Skeletal muscle tissue is characterized by being multinucleate striated, and voluntarily controlled. Striated means that skeletal muscle tissue has regular transverse bands which are visible in the microscope. Skeletal muscles are usually attached to bones which the muscles move.
Cardiac muscle tissue is found only in the heart. These cells are not multinucleate, but they are striated. The striations are not as prominent as those of skeletal muscle. Individual fibers are bound to one another with specialized structures termed intercalated discs.
Cardiac fibers contract spontaneously, though the rate of contraction is under involuntary nervous control.
Figure 13.1 Muscle Tissues
Cardiac muscle Intercalated disc Nucleus Skeletal muscle Smooth muscle
Smooth muscle fibers are characterized by the lack of striations and by being involuntary. The fibers contain one nucleus and taper toward the ends from a thickened middle region. Smooth muscles are located in the walls of blood vessels, the stomach, intestines, and other hollow internal organs.
Figure 13.2 Skeletal Muscle
Deep fascia Endomysium Epimysium Fasciculus Mitochondrion Muscle fiber Myofibril
Opening of a transverse tubule Perimysium Sarcolemma Sarcomere Sarcoplasmic reticulum Transverse tubule
Details of skeletal muscles are shown in Figure 13 2. The muscle is covered with fibrous connective tissue called the deep fascia. The deep fascia has been reflected back in Figure 13 2. Underneath the deep fascia is another layer of fibrous connective tissue, the epimysium. The muscle can be seen as a bundle of structures termed fasciculi (sing. - fasciculus) (L. small bundle) held together by the epimysium. Each individual fasciculus is wrapped in yet another layer of fibrous connective tissue, the perimysium. A fasciculus is a bundle of muscle fibers or cells. Each fiber within the fasciculus is embedded in yet another layer of fibrous tissue called the endomysium.
Each skeletal muscle cell has a cell membrane called the sarcolemma. Extensions of the sarcolemma pass through the cell in the form of tubes called transverse tubules. Transverse tubules are also known as T tubules.
Inside the sarcolemma are nuclei, mitochondria, and other cell organelles. A special type of endoplasmic reticulum, the sarcoplasmic reticulum, surrounds long structures called myofibrils. Each myofibril is a stack of small contractile units, the sarcomeres, which are the functional unit of the muscle. Each sarcomere is a bundle of alternating thin and thick myofilaments (seen in cross section as small and large dots in the sarcomeres of Figure 13 2). Thin myofilaments are primarily composed of a protein called actin. The thick myofilaments are made of another protein, myosin.
The myofilaments overlap one another in distinctive and repetitive patterns. It is this overlapping that produces the striatums visible in the microscope. The various regions of the sarcomere have been given names. The ends of the sarcomeres where the thin myofilaments have their beginnings are the Z lines. They have a Z-like arrangement in electron micrographs. The clearer zone flanking the Z lines is an I band. The I bands are regions of thin myofilaments without overlapping thick myofilaments. The less dense zone in the center of the sarcomere contains thick myofilaments only. This region is the H zone. The band where the thin and thick filaments overlap plus the H zone is the A band.
We hav^ seen that the muscle is a bundle of fasciculi which are bundles of muscle fibers (cells) which are bundles of myofibrils which are stacks of sarcomeres which are bundles of myofilaments. Go over this material several times until you fully understand it.
Label Figures 13.1, 13-2, and 133- Pay special attention to the series of bundles and their "wrappers" which make up a muscle.
Examine microscopic slides of skeletal, smooth, and cardiac muscle tissues. Note the characteristic features of each.
Closely examine the model of the muscle cell. Locate: actin, mitochondrion, myofibril, myosin, opening of transverse tubule, sarcolemma, sarcomere, sarcoplasmic reticulum, and transverse tubule.
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