Chapter 9 - Skeletal Muscle Tissue


     Muscle tissue is one of the four types of tissue. The other three tissues are epithelial, connective and nervous tissue. Muscle tissue consists of muscle fibers that are capable of contraction along their longitudinal axis. In this unit we will concentrate only on skeletal muscle.
Functions of Skeletal Muscle
  1. Produce skeletal movement
     Muscles move the bones of the skeleton by their connective tissue attachments. This enables all kinds of voluntary movement.
  2. Maintains posture and body position
     When not moving the various parts of the supportive skeleton are held in position by isometric muscle contraction.
  3. Supports soft tissue
     The weight of the abdominopelvic viscera is supported by the muscles of the abdominal wall and the pelvic floor.
  4. Guards entrances and exits
     Skeletal muscle forms sphincters around orifices of the digestive and urinary tracts.
  5. Maintains body temperature
     Muscle contraction generates heat that is involved in maintaining normal body temperature.

Anatomy of Skeletal Muscles
  Gross Anatomy
  Connective Tissue
     Skeletal muscle can only exert force on a structure if it is harnessed in some way to that structure. Connective tissue performs this role. The organization of fibers within the muscle gives rise to three concentric wrappings of connective tissue.
  1. Endomysium
     Each individual muscle fiber is surrounded by a delicate network of reticular fibers called endomysium. Endomysium binds fibers to their neighbors and supports the capillaries that supply the fibers. 
  2. Perimysium
     The muscle fibers bound together by the endomysium form a bundle called a fascicle. The fascicles are surrounded by connective tissue containing collagen and elastic fibers that supports the blood vessels and nerves that serve the fibers of the fascicles. This connective tissue is called the perimysium.
  3. Epimysium
     The entire muscle is surrounded by dense irregular connective tissue that separates the muscle from the surrounding tissue and organs. This is the epimysium.
  Tendons and Aponeuroses
     The fibers of the endomysium, perimysium and epimysium converge on the points of attachment of the muscle to bone, skin or muscle. This fibrous connection transfers the force of contraction of the muscle to the attachment point. When the attachment is round and resembles a thick cord or cable it is called a tendon. When the attachment is broad and forms a sheet it is called an aponeurosis (sing.).
  Nerves and Blood Vessels
     Skeletal muscle uses and demands nutrients and oxygen that are supplied by blood vessels that enter the muscle along the connective tissue partitions and eventual form capillary networks that surround each muscle fiber. Each muscle fiber is also controlled by a nerve that forms a single attachment point with each fiber at the neuromuscular junction, a.k.a. myoneural junction.
Microanatomy of Skeletal Muscle
     Skeletal muscle cells develop when hundreds of individual embryonic cells called myoblasts fuse. The resulting cells' cell membrane is called the sarcolemma and the cytoplasm is called sarcoplasm. Some of the unique characteristics of skeletal muscle cells (fibers) are:
       Skeletal muscles are very large. Their diameter is about 100 microns (about 13 times the diameter of a red blood cell) and they can be as long as 30 to 40 cm (10 to 16 in.).
       Skeletal muscles are multinucleate. The nuclei of the myoblasts that fuse to form the cell remain within the cell. Some myoblasts do not fuse but remain associated with the fiber as satellite cells that can divide and differentiate to repair damaged cells.
       There are internal tubular extensions of the sarcolemma into the sarcoplasm called transverse tubules or T tubules.
  Myofibrils and Myofilaments
     Within each skeletal muscle cell or fiber there are cylindrical structures called myofibrils. The myofibrils are about 1/100 the diameter of the cell but extend the full length of the cell and attach to the cell membrane at either end. Contraction of the myofibrils result in contraction of the entire cell. Transverse tubules form loops around myofibrils along their entire length. In between the loops of transverse tubule, there is a network of tubules of smooth endoplasmic reticulum called the sarcoplasmic reticulum. The sarcoplasmic reticulum surrounds the myofibril like a sleeve. The sarcoplasmic reticulum on either side of the loops of transverse tubules fuse to form expanded chambers called terminal cisternae. The combination of a transverse tubule flanked by two terminal cisternae is called a triad.
      The energy for muscle contraction is supplied by the numerous mitochondria that surround the myofibrils. Glycogen granules that also surround the myofibrils as a ready source of fuel.       
     The myofibrils are made-up of myofilaments. The myofilaments are organized in repeating units along the length of the myofibril called sarcomeres.
  Sarcomere Organization
     The primary microfilaments in myofibrils are actin (thin) and myosin (thick) filaments. Within each sarcomere there is a precise arrangement of these filaments. The sarcomeres of neighboring myofibrils are in alignment and they create the banding pattern visible with the light microscope. 
     The difference in the size and density of the thick and thin filaments is responsible for the banding pattern. The center of the sarcomere has a narrow band called the H band where there are only thick filaments. The H band is in the center of the A band that includes a zone of overlap between the thick and thin filaments. In the zone of overlap, each thick filament is surrounded by six thin filaments and each thin filament is surrounded by three thick filaments. The I band is where the thin filaments do not overlap with the thick filaments at either end of the sarcomere, and straddling the Z line.    
     The thick filaments are linked in the center of the sarcomere by proteins that form the M line. At either end of the sarcomere, the actin filaments are attached to one another by proteins that form the Z line. The Z lines mark the boundaries of each sarcomere.
  Thin Filaments
     The thin filament is composed of 300-400 G-actin protein molecules that form a linear molecule called F-actin. The F-actin molecules are held together by another protein called nebulin. Each G-actin molecule has an active site that binds to a site on the myosin molecule. This binding is prevented by another protein called tropomyosin that covers the active site on the actin. Tropomyosin is held in position by a protein called troponin. When calcium ions bind to troponin, the position of tropomyosin changes to uncover the binding site and permits the binding of actin to myosin.
     At either end of the sarcomere the actin filaments are attached to the Z line or disc by a protein called actinin.
  Thick Filaments
     Thick filaments are composed of a bundle of about 500 myosin molecules. Each myosin molecule consists of two strands, of which each has a tail that twists around the other, and a head. The head projects outward toward the thin filaments and are called cross-bridges because they attach to the thin filaments during contraction.
     The thick filaments are associated with a protein called titin. Titin contributes to the normal alignment of the thick and thin filaments and has elastic properties that restores the sarcomere to its original resting position after being stretched.