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Cardiac Muscle, Smooth Muscle and Neuromuscular Junction


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Cardiac Muscle
-striated, single nucleus, eccentric
-branch and anastomose
-linked electrically and mechanically by intercalated discs
-endo and peri
-highly developped capillary network
fine structure
-t-tuble located at zline, larger than skeletal
-diad only one contact btw tubule and sarcoplasmic ret.
-larger more numerous mitochondria

intercalated discs
desmosomes and fasciae adherentes on transvers region of disc

cardiac muscle receives blood from only one main vessel, no colateral blood supply
Atrial vs. Ventricular
atrial myocytes smaller
atrial myocytes fewer t-tubules
atrial myocytes contain dense granules w/ ANP
ANP causes medullary ducts of kidney to excrete Na and H2O and smooth muscles of blood vessels relaz
Cardiac Impulse Conducting Fibers
Purkinje fibers are larger and paler staining than myocytes
contain more cytoplasm and fewer myofibrils
larger are near nucleus contain glycogen
lack t-tubules contain mito and sarcoplasmic ret.
contain acetylcholinesterase
Muscle contraction
-Contraction mechanism within myocytes similar to skeletal muscle
-Action potential spreads through gap junctions
-orderly sequential contraction
-conduction velocity determined by number of gap junctions and type of connexion
-cardiac muscles formed from splanchnic mesoderm surrounding endocardial tubes
-division lost after birth
-enlargement during disease due to hypertrophy not hyperplasia
-damage results in death of myocytes and repair by CT proliferation
Smooth Muscle
-spindle shaped w/ central cigar nucleus
-cell surrounded by basal lamina and reticular fibers
-no t-tubules
-take up Ca by endocytosis
-attache to neighbors via gap junctions
Smooth muscle contraction
-impulse transmission by gap junctions
-greater more sustained but slower
-contraction by phosphorylation of myosin2
Innervation and Impulse Conduction
-sympathetic or parasympathetic
1.unitary, visceral organs. all muscle cells are not innervated, have extensive gap juctions and act syncytium, peristalsis type
2.Multiunit, iris. Lack gap junctions individual innervation for precise contraction
Hypertrophy and Hyperplasia
-by own limited ability of mitosis
-pericapillary mesenchymal cell derivation
-mostly heal via scar tissue
Neuromuscular Junction
-somatic efferent nerve fibers innervate skeletal muscles
-motor unit
-motor end plate
-large motor units are readily fatigued
-small ones are first to fire last to cease
-Neuromuscular Junction comprised of:
Presynaptic neuronal end plate containing mitochondria and acetylcholine vesicles

postsynaptic muscle sole plate trough shaped

Neurplemmocytes covering motor end plate

Acetylcholinesterase enzyme to destroy acetylcholine

Acetylcholine binds to receptors on muscle sole plate to open cation-channels leading to depolarization
Muscle spindle
-very elaborate arrangement for stretch detection
-elongated capsule derived from perineural epitheloid cells
-have afferent and efferent innervation
-2 types of muscle fibers
--nuclear bag intrafusal: one or two, middle dilated zone filled w/nuclei, lack myofibrils in nuclear region, straited region innervated by fusimotor motor neurons
--nuclear chain intrafusal: chains of nuclei in the middle area
2 types of receptors:
--primary ending, single large myelinated axon spirals around nuclear bag region
--secondary ending: myelinated axon forms flower-spray configuration of terminal branches

Receptors are stimulated by rate and degree of stretch
Information is subconscious i.e. adjusting posture, regulating muscle tone, coordinating movements

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