The response of muscle to injury depends on the muscle type. The wound closure mechanism always involves the proliferation of fibroblasts
A. Skeletal Muscle: Small, mononucleatcd satellite cells are scattered in adult skeletal muscles within the basal lamina of the mature fibers. While mature skeletal muscle fibers are incapable of mitosis, the normally quiescent satellite cells can divide following muscle injury, differentiate into myoblasts, and fuse to form new skeletal muscle fibers.
B. Cardiac Muscle: Cardiac muscle has little regenerative ability beyond early childhood. Lesions of the adult heart are repaired by replacement with connective tissue scars.
C. Smooth Muscle: Smooth muscle contains a population of relatively undifferentiated mono nucleated smooth muscle precursors that proliferate and differentiate into new smooth muscle fibers in response to injury. The same mechanism appears to be involved in adding new muscle to the myometrium as the uterus enlarges during pregnancy to accommodate the growing fetus.
RESPONSE OF MUSCLE TO INJURY
Smooth Muscle Biology
C. Organization of Smooth Muscle: Unlike striated-muscle fibers, which abut end-to-end, smooth muscle fibers overlap to various degrees and attach to one another by fusing their endomysial sheaths. The sheaths are interrupted by many gap junctions, which transmit the ionic currents that initiate contraction. Smooth muscle fibers form fascicles that vary in size but are usually smaller than those in striated muscle. The fascicles, each surrounded by a meager perimysium, are often organized in layers separated by the thicker epimysial connective tissue. Fibers in adjacent layers often lie perpendicular to one another.
D. Mechanism of Contraction: The mechanism of smooth muscle contraction is a modification of the sliding-filament mechanism. At the beginning of the contraction, the myosin filaments appear and the actin filaments are pulled toward and between them. Continued contraction involves forming more myosin filaments and further sliding of the actin filaments. The sliding actin filaments pull the attached dense bodies closer together, shortening the cell. Unlike striated muscle fibers, individual smooth muscle fibers may undergo partial peristaltic, or wavelike, contractions. During relaxation, the myosin filaments decrease in number, disintegrating into soluble cytoplasmic components.
E. Initiation of Smooth Muscle Contraction: Like cardiac muscle fibers, smooth muscle fibers are capable of spontaneous contraction that may be modified by autonomic innervation. Motor end-plates are not present. Neurotransmitters diffuse from terminal expansions of the nerve endings between smooth muscle cells to the sarcolemma. Both sympathetic (adrenergic) and parasympathetic (cholinergic) endings are present and exert antagonistic (reciprocal) effects. In some organs, contractile activity is enhanced by cholinergic nerves and decreased by adrenergic nerves, whereas in others the opposite occurs
SMOOTH MUSCLE
A. Histogenesis: Most smooth muscle cells differentiate from mesenchymal cells of mesodermal origin in the walls of developing hollow organs of cardiovascular, digestive, urinary, and reproductive systems. During differentiation, the cells elongate and accumulate myofilaments. Smooth muscles of the iris arise from ectoderm.
B. Smooth Muscle Cells: Mature smooth muscle fibers are spindle-shaped cells with a single central ovoid nucleus. The sarcoplasm at the nuclear poles contain abundant mitochondria, some RER, and a large Golgi complex. Each fiber produces its own basal lamina, consisting of proteoglycan-rich material and type III collagen fibers.
1. Myofilaments a. Thin filaments. The actin filaments of smooth muscle are like those of skeletal and cardiac muscle. They are always present in the cytoplasm and are anchored by alpha-actinin dense bodies associated with the plasma membrane. b. Thick filaments. The myosin filaments of smooth muscle are less stable than those in striated muscle cells; they are not always present in the cytoplasm but seem to form in response to a contractile stimulus. Unlike the thick filaments in striated muscle cells those in smooth muscle have heads along most of their length and bare areas at the ends of the filaments. c. Organization of the myofilaments. The filaments run mostly parallel to the long axis of smooth muscle fibers, but they overlap much more than those of striated muscle, accounting for the absence of cross striations. The greater overlap of thick and thin filaments results from the unique organization of the thick filaments and permits greater contraction. The ratio of thin to thick filaments in smooth muscle is about 12:1, and the arrangement of the filaments is less regular and crystalline than in striated muscle.
2. Sarcoplasmic reticulum.Smooth muscle cells contain a poorly organized sarcoplasmic reticulum that participates in the sequestration and release of Ca but does not divide the myofilaments into myofibrillar bundles. Abundant surface-associated membrane-limited ves icles termed caveolae appear to aid in Ca2+ uptake and release. The small size and slow contraction of these fibers make an elaborate stimulus-conducting system unnecessary; these fibers have no T tubules, dyads, or triads.
3. Types of smooth muscle fibers. Although smooth muscle cells exhibit similar morphology in histologic section, they can be classified according to developmental, biochemical, and functional differences.
a. Visceral smooth muscle derives from splanchnopleural mesenchyme and is found in the walls of the hollow thoracic, abdominal, and pelvic organs of the respiratory, digestive, urinary, and reprc;ductive systems. In addition to thick myosin and thin actin filaments, its sarcolemma-associated dense bodies are linked by desmin-containing intermediate filaments. Because of their poor nerve supply, the cells transmit contractile stimuli to one another through their abundant gap junctions, acting as a functional syncytium. Contraction is slow and in waves. Visceral smooth muscle is classed as unitary smooth muscle.
b. Vascular smooth muscle differentiates in situ from mesenchyme around developing blood vessels. Its cells have intermediate filaments containing vimentin as well as desmin. It has the same functional features as visceral smooth muscle and is also classed as unitary smooth muscle, although its waves of contraction are not sustained and are localized.
CARDIAC MUSCLE
A. Histogenesis: Cardiac muscle arises as parallel chains of elongated splanchnic mesenchymal cells in the walls of the embryonic heart tube. Cells in each chain develop specialized junctions between them and often branch and bind to cells in nearby chains. As development continues, the cells accumulate myofilaments in their sarcoplasm. The branched network of myobiasts forms interwoven bundles of muscle fibers, but cardiac myoblasts do not fuse.
B. Cardiac Muscle Cells: Cardiac muscle fibers are long, branched cells with one or 2 ovoid central nuclei. The sarcoplasm near the nuclear poles contains many mitochondria and glycogen granules and some lipofuscin pigment. Mitochondria lie in chains between the myofilaments. The arrangement of myofilaments yields striations like those of skeletal muscle.
1. Sarcoplasmic reticulum and T tubule system. The sarcoplasmic reticulum in cardiac muscle fibers is less organized than that of skeletal muscle and does not subdivide myofilaments into discrete myofibrillar bundles. Cardiac T tubules occur at the Z line instead of the A-I junction. In most cells, cardiac T tubules associate with a single expanded cisterna of the sarcoplasmic reticulum; thus, cardiac muscle contains dyads instead of triads.
2. Intercalated disks. These unique histologic features of cardiac muscle appear as dark transverse lines between the muscle fibers and represent specialized junctional complexes. With the EM, intercalated disks exhibit 3 major components arranged in a stepwise fashion. a. The fascia adherens, similar to a zonula adherens, is a half Z line found in the vertical (transverse) portion of the step. Its cr-actinin anchors the thin filaments of the terminal sarcomeres. b. The macula adherens (desmosome) is the second component of transverse portion of the junction. It prevents detachment of the cardiac muscle fibers from one another during contraction. c. The gap junctions of intercalated disks form the horizontal (lateral) portion of the step. They provide electrotonic coupling between adjacent cardiac muscle fibers and pass the stimulus for contraction from cell to cell.
3. Types of cardiac muscle fibers a. Atrial cardiac muscle fibers are small and have fewer T tubules than ventricular cells. They contain many small membrane-limited granules that contain a precursor of atrial natriuretic factor, a hormone secreted in response to increased blood volume that opposes the action of aldosterone and acts on the kidneys to cause sodium and water loss. b. Ventricular cardiac muscle fibers are larger cells with more T tubules and no granules.
C. Organization of Cardiac Muscle: Because of the abundant capillaries in the endomysium, cardiac muscle fibers appear more loosely arranged in histologic section than those of skeletal muscle. The whorled arrangement of cardiac muscle fibers in the wall of the heart accounts for the ability of the myocardium to "wring out" blood in the heart chambers.
D. Mechanism of Contraction: Although the arrangement of the sarcoplasmic reticulum and T tubule complex of cardiac muscle fibers differs from that of skeletal muscle, the composition and arrangement of myofilaments are almost identical. Thus, at the cellular level, skeletal and cardiac muscle contractions are essentially the same.
E. Initiation of Cardiac Muscle Contraction: Unlike skeletal muscle fibers, which rarely contract without direct motor innervation, cardiac muscle fibers contract spontaneously with an intrinsic rhythm. The heart receives autonomic innervation through axons that terminate near, but never form synapses with, cardiac muscle cells. The autonomic stimulus cannot initiate contraction but can speed up or slow down the intrinsic beat. The initiating stimulus for contraction is normally provided by a collection of specialized cardiac muscle cells called the sinoatrial node; it is delivered by other specialized cells, called Purkinje fibers, to the other cardiac muscle cells. The stimulus is passed between adjacent cells through the gap junctions of the intercalated disks. The gap junctions establish an ionic continuity among cardiac muscle fibers that allows them to work together as a functional syncytium.
E. Relaxation
E. Relaxation: When neural stimulation ends, all the membranes repolarize, allowing the sar coplasmic reticulum to sequester Ca from the sarcoplasm by active transport. This removes Ca" from the TnC and returns the TnI to a position in which it inhibits binding of the myosin head to the actin filament. Muscle phsiology
F. Energy Production: Muscles use glucose (from stored glycogen and from the blood) and fatty acids (from the blood) to form the ATP and phosphocreatine that provide chemical energy for contraction. When ATP is not available, actin-myosin binding become stabilized, accounting for rigor mortis, the muscular rigidity that occurs shortly after death.
G. Organization of the Skeletal Muscles: Named muscles leg, biceps brachii) are bundles of muscle fascicles surrounded by a sheath of dense connective tissue termed the epimysium. Each fascicle is a bundle of muscle fibers surrounded by a dense connective tissue sheath called the perimysium, which consists of septumlike inward extensions of epimysium. Each muscle fiber is a bundle of myofibrils surrounded by a delicate connective tissue sheath termed the endo mysium, which consists of a basal lamina and a loose mesh of reticular fibers. Each myofibril is a bundle of myofilaments surrounded by an investment of sarcoplasmic reticulum, with a triad at both A-I junctions of each sarcomere. The connective tissue investments are continuous with one another.
H. Muscle-Tendon Junctions: The attachment of muscle to tendon must be secure to prevent the muscle from tearing away during contraction. The tendon's collagen fibers blend with the epi mysium and penetrate the muscle along with the perimysium. Near the junction with the tendon, the ends of the muscle cells taper and exhibit many infoldings of their sarcolemmas. Collagen and reticular fibers enter the infoldings, penetrate the basal lamina, and attach directly to the outer surface of the sarcolemma. The attachment of actin filaments to the inner surface of the sar colemma helps stabilize the association between the collagen fibers and the muscle cell.
I. Pattern of Innervation: Each motor neuron has a single axon that may terminate on a single muscle fiber or undergo terminal branching (arborization) and terminate on multiple muscle fibers. A motor neuron and all the muscle fibers it innervates (one to > 100) is termed a motor unit. Muscles responsible for delicate movements leg, extraocular muscles) are composed of many small motor units; those responsible for coarser movements leg, gluteus maximus) are composed of a few large motor units.