Lecture 9: Anatomy of Muscle Tissue
I. Anatomy of Skeletal Muscle
A. General Features of Cells
1. multinucleate
2. striations caused by intracellular structure
B. Fascia - fibrous connective tissue surrounding muscle
1. superficial - just below the skin
a. fat storage area
b. insulation and protection
c. nerve and vessel passage
2. deep - lines body wall, binds muscles together
3. epimysium - encapsulates a single muscle
4. perimysium - around muscle fiber bundles (fasciculi)
5. endomysium - around each individual muscle fiber
6. tendon - continuation of connective tissue to bone
7. tendon sheath - synovium of a tendon (lubricated)
8. aponeurosis - broad, sheet-like connective tissue
C. Nerve and Blood Supply
1. artery & veins enter alongside the nerve
2. capillaries are within each endomysium
D. Anatomy of a Muscle Fiber (Myofiber)
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10-100 um diameter; up to 30 cm long
sarcolemma - plasma membrane of muscle cell (fiber)
sarcoplasm - cytoplasm of muscle cell
sarcoplasmic reticulum - SER with Calcium ions
a. terminal cisterns - sacs around myofibrils
b. T (transverse) tubules - perpendicular
5. myofibrils - girder-like structures in cell
a. thin myofilaments - actin
b. thick myofilaments - myosin
6. sarcomere - a compartment housing myofibrils
a. Z line - termination of a sarcomere
b. A-band - length of thick/thin filament overlap
c. I-band - length of thin filaments ONLY
d. H-zone - length of thick filaments ONLY
e. M-zone - union of the thick filaments
E. Sliding Filament Theory
1. Components of Thin (Actin) Filaments
a. myosin binding site
b. tropomyosin
c. troponin
2. Components of Thick (Myosin) Filaments
a. golf-club like appendages
b. cross-bridges - the head that attaches to actin
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i. actin binding site
ii. ATP binding site
3. Contraction through Actin-Myosin "Pulling Motion"
a. cross-bridges pull myosin along actin filaments
b. action is like "oars on a boat" pulling forward
c. ATP responsible for "ratchet" motion
F. Neuromuscular Junction
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motor neuron - nerve cell that innervates muscle
motor end plate - where axon meets muscle cell
neuromuscular junction - entire muscle/nerve site
synapse - general name axon terminal junction
synaptic vesicles - inclusions in the axon terminal
a. neurotransmitter - chemical messenger
i. acetylcholine ACh (for muscular synapse)
6. synaptic cleft - space between axon and cell
7. motor unit - a neuron and all myofibers acted upon
G. Sequence of Events at the Neuromuscular Junction
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action potential moves down the axon of the nerve
electricity causes synaptic vesicles to release ACh
ACh crosses synaptic cleft
ACh receptors on sarcolemma transfer message inside
Calcium is released from sarcoplasmic reticulum
Calcium activates movement of myosin along actin
acetylcholine esterase (enzyme) breaks down ACh
H. Hypertrophy and Atrophy
1. muscular hypertrophy
a. increase in size of myofibers (muscle cells)
b. allows for increased strength
2. muscular atrophy
a. progressive loss of myofibrils
i. disuse atrophy
ii. denervation atrophy (neuron lost)
II. Cardiac Muscle
A. General Features
1. involuntary muscle
2. one, centrally located nucleus
3. mitochondria larger and more numerous
B. Structure of Tissue
1. muscle fibers branch and interconnect
2. intercalated disc - thickening of sarcolemma
3. cells connected by gap junctions
a. allow passage of ions like Calcium
b. makes adjacent cells electrically linked
c. allows for rhythmic, domino-like contraction
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III. Smooth Muscle
A. General Features
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non-striated
5-10 um in diameter; 30-200 um long
thick in middle; thinner, tapering off to the end
single oval centrally located nucleus
actin and myosin fibers not arranged as sarcomere
lack of organization (no bands) --> smooth muscle
also contain intermediate filaments
B. Difference in Contraction
1. intermediate filaments attach to dense bodies
2. as muscle cell contracts, twists like corkscrew
3. caveolae - like T tubules of skeletal muscle
C. Two Kinds of Smooth Muscle
1. visceral (single unit) muscle
a. small arteries and veins
b. viscera - stomach, intestines, uterus, bladder
c. continuous network with gap junctions
d. action spreads from one cell to another
2. multiunit muscle
a. each fiber (cell) has it own nerve ending
b. no gap junctions
c. large arteries, large airways, arrector pili
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