4.0 MUSCLE a. Define and use properly the following words: Muscle fiber/myofiber, Myofibrils, b. c.
Myofilaments, Sarcolemma, Sarcoplasm, Sarcoplasmic reticulum, Skeletal muscle,
Striated, Multinucleated cell, Syncytium, Myoblasts, Mesoderm, Connective Tissue sheaths, Endomysium, Perimysium, Fascicle, Epimysium, Fascia, Motor end-plate,
Sarcomere, Cytoplasmic Filaments, Actin, Myosin, T tubules, Sarcoplasmic reticulum,
Cardiac muscle, Striated fibers, Intercalated disks, Purkinje fibers, Glycogen, PAS stain,
Smooth muscle, Spindle-shaped, Pinocytotic vesicles, Gap junctions, Myoepithelial cells,
Muscle regeneration.
Describe and associate basic structure/function for: 1) all structures listed above; 2) regeneration of muscle tissue.
Identify by microscopy the cells, organelles, tissues, and organs listed above.
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I. INTRODUCTION
A. Muscle is an organ as well as a tissue. We are quite familiar with the bulk muscle that moves bones and joints (Muscle as an organ). Muscle as a tissue is found in every organ system.
B. Types of muscle
1. Smooth 2. Cardiac 3. Skeletal
Cell Biology Terminology
Cell
Fibrils
Filaments
Plasmalemma
Cytoplasm
Endoplasmic reticulum
Muscle synonyms
Muscle fiber/ myofiber myofibrils myofilaments sarcolemma (sarc-flesh) sarcoplasm sarcoplasmic reticulum
II. SMOOTH MUSCLE (Visceral Muscle)
A. Light microscopy
1. Elongated, tapered, overlapping spindle-shaped cells
2. nuclei located in center (uninucleate)
3. cork screw cigar shaped nucleus (when contracted)
4. synthesizes a basal lamina
5. no perimysium or epimysium (connective tissue sheaths)
6. location: surrounds tubular organs (intestine), blood vessels, uterus, epididymis, respiratory system
B. TEM
1. Cytoplasmic organelles include filaments, mitochondria and scattered smooth endoplasmic reticulum
2. numerous pinocytotic vesicles
3. Filaments -- mostly thin actin microfilaments; thick filaments can be seen with special EM techniques, but no crystalline array; attached to dense bodies.
C. Contractions
1. Not well understood, but impulses passed through gap junctions between cells; thus each cell may not be directly innervated.
D. Myoepithelial cells
1. smooth muscle-like cells lining glandular epithelium
2. always basal and surrounded by basal lamina
3. actin filaments, dense bodies, hemidesmosomes
4. contractile
III. SKELETAL MUSCLE
A. Light microscopy
1. striated (light & dark bands)
2. Multinucleated cell--SYNCYTIUM (fusion of embryonic myoblasts--Mesoderm)
3. Nuclei located at periphery of cell
4. function as an organ at times (moves bone; tongue; pharynx; upper esophagus)
5. make up 1/3 to 1/2 of body weight
B. Organization of Skeletal Muscle as an Organ
1. organized in bundles
2. very long cells (30 cm)
3. up to 100 µm diameter
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4. Connective Tissue sheaths
C. Components of Skeletal muscle:
1. Endomysium - CT of each fiber (cell) is a basal lamina & reticular fibers
2. Perimysium - CT surrounding muscle bundles
3. Fascicle - a bundle of perimysium-bound fibers
4. Epimysium - (on top) external dense CT sheath called FASCIA a. cannot suture muscle (does not hold) but can suture this surrounding CT b. connects muscle to tendon as dense regular CT or elastic CT
5. Motor end-plate = the nerve-muscle junction (covered under nervous tissue)
D. TEM: Organization of the Muscle Cell (myofiber)
1. Nucleus is located next to the cell membrane
2. Cytoplasm contains mostly microfilaments, actin and myosin, that are highly organized into contractile units called sarcomeres. Mitochondria and endoplasmic reticulum are arranged between the sarcomeres.
3. Sarcomere a. contractile unit (23 µm in length) in the cytoplasm b. repetitive, in parallel c. some parts recognized at light microscopic level (band A, I, H, Z)
4. Cytoplasmic Filaments a. actin filaments (thin; 5 nm)- light areas b. both actin and myosin- dark areas c. only myosin filaments (thick; 15 nm)- only small area
5. Other cytoplasmic organelles of skeletal muscle a. T tubules (transverse tubules) b. finger-like invaginations of the plasmalemma that are embedded in the SER at the I - A band junction of every fibril. c. Provides for rapid transmission of the depolarization signal for contraction. d. Sarcoplasmic reticulum i. series of enmeshed smooth endoplasmic reticulum tubules surrounding the I & A bands of a group of sarcomeres. ii. at the edge of the A band the tubules fuse to form an enlarged channel that encircles the sarcomere e. Mitochondria -- between sarcomeres & along the periphery (2% vol) f. glycogen is stored throughout the cytoplasm
E. CONTRACTION OF SKELETAL MUSCLE
Cross-Bridge, ratchet mechanism between myosin and actin (see Banks 13.16). Thin filaments move past the thick filament toward the center of sarcomere, which shortens. The myosin head projects outward toward the actin filament and when Ca ++ is present there is a conformational change so that myosin head can bind actin forming a cross-bridge while
ATPase releases the energy of ATP. The binding of the head moves the thin filament about 10 nm, so this process is repeated numerous times during one contraction. As the head bends it increases the space between the thick & thin filaments -- bulging of muscle.
Rigor Mortis occurs when the actin-myosin bridges become fixed because no new ATP is available as energy to permit their detachment.
Stretched muscle = I and H bands are wider
Contracted muscle = I and H bands are more narrow
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IV. CARDIAC MUSCLE
A. Light microscopy
1. striated fibers
2. branched cells
3. single nuclei located in the center (halo artifact)
4. INTERCALATED DISKS are the junctions between branching cells
5. Purkinje fibers are larger striated cells (appear swollen) found primarily in the septum.
These cells are pale-staining but contain glycogen (PAS+). They conduct impulses to the ventricles
B. TEM
1. Sarcoplasmic reticulum wanders irregularly
2. Intercalated disks seen in LM as a junctional complex without a tight junction
3. An adhering junction, desmosomes & Gap junctions between sarcomere make up the intercalated disk
4. Numerous large mitochondria -- parallel to & between sarcomeres (40% of cell volume)
C. Contraction of Cardiac Muscle
1. branching of the cells allows one cell to form junctions between many cardiac muscle cells; this allows the contractions to spread widely across the tissue
2. contraction depends upon specialized cardiac muscle cells (Purkinje cells) for passing the impulses.
V. REGENERATION OF MUSCLE TISSUE
A. Cardiac muscle has no regenerative capacity (damaged areas filled in by connective tissue)
1. The only exception is the use of transplanted stem cell technology.
B. Skeletal muscle regeneration
1. Basically no division...
2. Expands by adding new protein in sarcomeres.
3. Some division takes place in Satellite cells that are undifferentiated
4. thought to be activated by injury or extensive exercise. This process is dependent upon an intact basement membrane.
C. Smooth muscle regeneration
1. modest degree of mitosis ...
2. replaced when injured and expands with new blood vessels into exercised tissues.
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