Term
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Definition
-Innervated by large, myelinated nerves
-Nerve contacts muscle fiber in the middle
-Usually only one junction per muscle fiber
-Nerve terminus branches extensively at junction (called motor end plate)
-Nerve does not penetrate muscle fiber plasma membrane
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Term
Neuromuscular Junction (cont.) |
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Definition
-Muscle fiber has invaginations at motor end plate (synaptic trough)
-Smaller folds in synaptic trough (subneural clefts)
-Small space between nerve terminal and muscle fiber (synaptic cleft)
-Junction covered by Schwann cell (insulation)
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Term
Excitation-Contraction Coupling |
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Definition
-Coupling the reception of a nerve action potential with the contraction of myofilaments
-Involves influx of Ca2+ upon stimulation by action potential
-98% of muscle fibers are innervated by one nerve interacting at a neuromuscular junction near its center
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Term
Molecular Characteristics of Myosin Filaments |
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Definition
-Myosin molecule is composed of 2 heavy chains and four light chains
-Heavy chains are globular at one end
-Light chains associated with globular portion
-Filament is composed of 200 myosin molecules
-Myosin molecules arranged with heads extended outward and tails forming the thick portion of the filament
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Term
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Definition
-Composed of three different proteins: actin, troponin, and tropomyosin
-Backbone is double-stranded F-actin molecule wound in a helix
-F-actin polymer is composed of individual G actin molecules
-One end of the actin filament is attached to the Z-disk
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Term
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Definition
-Active protein sitting on the F-actin strands
-Composed of three loosely bound protein subunits: Troponin I , Troponin C, and Troponin T
-Involved in binding of Ca2+ ions during muscle contraction
-Allows for interaction of actin and myosin during muscle contraction
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Term
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Definition
-Another protein closely associated with actin
-Large protein molecule lining the actin molecule, mainly at seam between the F-actin strands
-Thought to block “active” sites on actin prohibiting actin/myosin interactions
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Term
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Definition
-Ca2+ ion movement into the cell and binding to Troponin C
-Movement of Tropomyosin complex deeper into the groove exposing active sites
-Myosin heads interact with actin active sites
-“Walk along” theory of contraction
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Term
Mechanism for Myosin Head Movement |
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Definition
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uHeads of myosin bind ATP
uSet in perpendicular position
-Binding to active site
-Conformational change once bound to active site
-ADP + Pi released
-ATP binds → Myosin head releases
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Term
Muscle Cell Action Potentials |
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Definition
-Resting potential = -90 mV
-Activation at the neuromuscular junction by ACh-gated Na+ /K+ channels
-AP travels down sarcolemma (Na+ channels)
-AP moves into T tubules (Na+ channels)
-T-tubules are in close proximity to sarcoplasmic reticulum (SR)
-SR has Ca2+ channels (voltage gated)
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Term
Cessation of Muscle Contraction |
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Definition
-Muscle contraction continues as long as Ca2+ is in the sarcoplasm
-Ca2+ pump on SR moves calcium inside (concentrates 10,000-fold)
-Activated milliseconds after Ca2+ release
-Aided by calsequestrin
-Brings cytosolic concentration back to resting levels
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Term
Termination of Muscle Stimulation at the Neuromuscular Junction |
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Definition
Mediated by two methods:
1. Acetylcholinesterase
2. Diffusion of acetylcholine from neuromuscular junction
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Term
Drugs Affecting
Neuromuscular Transmission |
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Definition
Acetylcholine antagonists (blockers):
1. Atropine
2. Other curariform drugs
Acetylcholine agonists (copy-cat compound):
1. Methacholine
2. Carbachol
3. Nicotine
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Term
Acetylcholinesterase inhibitors
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Definition
has same effect as acetylcholine agonists
–increases the length of time that the acetylcholine molecule is present in the neuromuscular junction
–found in some pesticides and poisons
–Ex. Diisopropyl fluorophosphate, neostigmine, physostigmine
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Term
ENERGY FOR MUSCLE CONTRACTION |
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Definition
- cellular ATP (1-2 sec.)
- phosphocreatine (5-8 sec.)
- intramuscular glycogen (1 min.)
- oxidative metabolism (hours)
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Term
Treatment of Myasthenia Gravis |
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Definition
-Give patient acetylcholinesterase inhibitor such as neostigmine or physostigmine
-Do not give acetylcholine analogs
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