MUSCLE TISSUE

Muscle tissue is responsible for movement of the body and its parts and for changes in the size and shape of internal organsCharacterized by aggregates of specialized, elongated cells arranged in parallel array, whose primary role is contraction

Myofilament interaction is responsible for muscle contraction

Thin filaments is composed primarily of the protein actinThick filaments is composed of the protein myosin II

Muscle is classified on the basis of the appearance of the contractile cells

2 principal types of muscle are recognized:Striated muscle: in w/c the cells exhibit cross-striations at the light microscope levelSmooth muscle: in w/c cells do not exhibit cross striations

Striated muscle tissue is further subclassified on the basis of its location:

Skeletal muscle is attached to bone and is responsible for movement of the axial and appendicular skeleton and for maintenance of body position and postureVisceral striated muscle is morphologically identical w/ skeletal muscle but is restricted to the soft tissues (tongue, pharynx, lumbar part of the diaphragm, & upper part of esophagus)Cardiac muscle is found in wall of the heart and in the base of the large veins that empty into the heart

Skeletal Muscle

Is a multinucleated syncytiumConsists of striated muscle fibers held together by a connective tissueEndomysium is the delicate layer of reticular fibers that immediately surrounds individual muscle fibersPerimysium is a thicker connective tissue layer that surrounds a group of fibers to form a bundle or fascicleEpimysium is the sheath of dense connective tissue that surrounds a collection of fascicles that constitutes the muscle

3 types of skeletal muscle fibers are now described as:

Type I ( slow oxidative fibers). Small fibers, appear red in fresh specimens, contain many mitochondria and large amounts of myoglobin and cytochrome complexesType I fibers are slow-twitch fatigue-resistant motor units; (a twitch is a single brief contraction of the muscle)

3 types of skeletal muscle fibers are now described as:

Type IIa (fast oxidative glycolytic fibers).These are intermediate fibers seen in fresh tissueThey are medium size w/ many mitochondria and a high myoglobin contentThey make up fast-twitch fatigue-resistant motor units that generate high peak muscle tension

3 types of skeletal muscle fibers are now described as:

Type IIb (fast glycolytic fibers).Are large fibers, w/c appear light pink in fresh specimens, contain less myoglobin & fewer mitochondria than type I & IIa fibers.Have low levels of oxidative enzymes but exhibit high anaerobic enzyme activity and store a considerable amount of glycogenThese fibers are fast-twitch fatigue-prone motor units & generate high peak muscle tension

Cardiac Muscle

Has same types and arrangements of contractile filaments as skeletal muscleCardiac muscle nucleus lies in the center of the cellNumerous large mitochondria and glycogen stores are adjacent to each myofibrilThe intercalated disks represent junctions between cardiac muscles

Components of intercalated disk contain specialized cell to cell junction between adjoining cardiac muscle cells:

Fascia adherens (adhering junction) is the major constituent of the transverse component of the intercalated disk and is responsible for its staining in routine H&E preparations Maculae adherentes (desmosomes) bind the individual muscle cells to one another. They help prevent the cells from pulling apart under the strain of regular repetitive contractions

Components of intercalated disk contain specialized cell to cell junction between adjoining cardiac muscle cells:

Gap junctions (communicating junctions) constitute the major structural element of the lateral component of the intercalated disk. They provide ionic continuity between adjacent cardiac muscle cells allowing informational macromolecules to pass from cell to cell

Smooth Muscles

Generally occurs as bundles or sheets of elongated fusiform cells w/ finely tapered ends.Its cells possess a contractile apparatus of thin and thick filaments and a cytoskeleton of desmin and vimentin intermediate filaments

Thin filaments containing actin, the smooth muscle isoform of tropomyosin and 2 smooth muscle specific proteins, caldesmon and calponinThick filaments containing myosin II molecules are oriented in one direction on one side of the filament

Tropomyosin is present in smooth muscle, spanning seven actin monomers and is laid out end to end over the entire length of the thin filaments. In striated muscle, tropomyosin serves to enhance actinmyosin interactions, but in smooth muscle, its function is unknown.Calponin molecules may exist in equal number as actin, and has been proposed to be a load-bearing protein.Caldesmon has been suggested to be involved in tethering actin, myosin and tropomyosin, and thereby enhance the ability of smooth muscle to maintain tension.

Functional aspects of smooth muscle

Specialized for slow, prolonged contractionNerve terminals in smooth muscles are observed only in the connective tissue adjacent to muscle cellsSmooth muscles also secrete connective tissue matrix