General Characteristics and functions: Made up of elongated cells containing the contractile proteins myosin and actin. These proteins allow muscle to contract - i.e., change their length and exert tension - alowing them to generate mechanical force for locomotion, the pumping action of the heart, vasoconstriction, intestinal peristalsis, etc.

Germ Cell Origin: Mesoderm.

Types of Muscle:

• Skeletal muscle - Organized into long, cylindric bundles of muscle fibers called fascicles bound by dense connective tissue. Individual muscle fibers within a fascicle are formed from the fusion of hundreds of precursor cells called myoblasts creating a multinucleated syncytium. Nuclei are located at the periphery of fibers - just beneath the plasma membrane.

  Microscopically, skeletal muscle fibers have characteristic cross-striations. This appearance is the result of actin and myosin molecules being arranged in tight overlapping parallel bundles. Each group of thicker myosin filaments (dark bands) and their associated flanking actin filaments (light bands) are referred to as a sarcomere. This is the functional contracting unit of skeletal muscle. Sarcomeres are associated with a complex tubular network within the muscle fiber called the sarcoplasmic reticulum which plays an important role in controlling the contraction-relaxation state of muscle.

  Skeletal muscle is responsible for voluntary movements of the limbs, digits, and other structures. Skeletal muscle is also found in the diaphragm (breathing), upper esophagus (swallowing), tongue and pharynx (speech). Skeletal muscle associated with the axial skeleton plays a role in maintaining posture. A small population of satellite cells in adult skeletal muscle can divide and form new muscle after injury.

  
  

  

  
  

  

  
  Skeletal muscle fibers vary in diameter and in their concentration of the oxygen-binding pigment myoglobin :

  1. Red fibers (slow-twitch fibers) - Small fibers that contain large amounts of myoglobin and numerous mitochondria. They exhibit a great deal of resistance to fatigue, and are suitable for maintaining sustained contraction. Red fibers predominate in the muscles of the limbs. They are also found in the intrinsic muscles of the back responsible for maintaining erect posture.

  2. White fibers (fast-twitch fibers) - Large fibers with less myoglobin and fewer mitochondria. While white fibers fatigue faster than red fibers, they are capable of generating large amounts of tension over a short period of time. This makes them ideally suited for rapid, precise, fine movements. White fibers predominate in the digits and extraocular muscles of the eyes. White fibers are more generously innervated than red fibers.

  Many muscles of the body have fibers whose size, and myoglobin and mitochondrial content lie somewhere between the extremes of red and white fibers. They are appropriately called intermediate fibers .

• Cardiac muscle - Found in the heart and the bases of the large veins that supply the heart. Unlike the syncytial organization of skeletal muscle, cardiac muscle is composed of individual, branched cells lying parallel to each other with central nuclei. Like skeletal muscle, cardiac muscle is also striated.

  An important distinguishing characteristic of cardiac muscle is the presence junctional complexes called intercalated disks . These structures join individual cardiac muscle cells in an end-to-end fashion. More importantly, intercalated disks contain gap junctions that facilitate transmission of the contractile stimulus between muscle cells. This insures that the pumping action of the heart is coordinated. Cardiac muscle has a greater concentration of mitochondria, and a greater blood supply, than skeletal muscle. Cardiac muscle cells are innervated by the autonomic nervous system and, unlike skeletal muscle, their contraction is not under voluntary control.

  Cardiac muscle also has an intrinsic ability to contract spontaneously and rhythmically (automaticity). This activity is generated from a specialized subset of cardiac muscle cells called cardiac conduction cells (also called Purkinje cells). They are located in the sino-atrial and atrio-ventricular nodes, as well as the bundle of His, bundle branches, and Purkinje fibers of the ventricles. Unlike skeletal and smooth muscle which retain some regenerative ability, injured cardiac muscle cells cannot renew themselves. When cardiac muscle cells die, they are lost forever. Injured cardiac muscle is replaced by scar tissue.

• Smooth muscle - Bundles and sheets of individual fusiform cells (thicker in the middle than at the ends) with centrally located nuclei. Not as well organized as skeletal or cardiac muscle. Nevertheless, groups of smooth muscle cells are often bound together by connective tissue so that they can function as a unit. Additionally, gap junctions between smooth muscle cells help coordinate their contraction. Smooth muscle is not striated because actin and myosin filaments are not arranged in the tight, parallel bundles seen in skeletal and cardiac muscle. Instead, these contractile proteins are anchored to the plasma membrane and arranged in a crossing pattern within individual cells.

  Smooth muscle is found in the walls of blood vessels, and hollow viscera such as the gut (and other gastrointestinal organs), bladder, ureter, uterus, and bronchi. The iris and ciliary body of the eye also contain smooth muscle. Contraction of smooth muscle is slower and more prolonged than skeletal muscle. Smooth muscle cells are innervated by autonomic nerves and their contraction is not under voluntary control. Smooth muscle contraction can also be stimulated by two hormones secreted by the posterior pituitary - oxytocin and vasopressin. Smooth muscle cells are capable of dividing in response to tissue injury. Like fibroblasts, they can also secrete extracelluar matrix such as collagen, elastin fibers, and ground substance.