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functions of skeletal muscle

force production for locomotion and breathing
fore production for postural support
heat production during cold stress

human body contains over... skeletal muscles which accounts for...% body weight

400, 40-50% total body weight

connective tissue covering skeletal muscle

epimysium, perimysium, endomysium

epimysium

surrounds entire muscle

perimysium

surrounds bundles of muscle fibers (fasicles)

endomysium

surrounds individual muscle fibers

sarcolemma

muscle cell membrane

myofibrils:

threadlike strands within muscle fibers (cells). actin (thin filament), myosin (thick filament)

sarcomere

contractile unit. Z-line, M-line, A-ban and I-band

Titan

connects Z band to M line
spring-like characteristics
passive component force
assists in maintaing shape of cell

Within the sarcoplasm

Sarcoplasmic reticulum: Storage site for calcium
Transverse tubules
Terminal cisternae
Mitochondria

The Neuromuscular Junction

Where motor neuron meets the muscle fiber
Motor end plate
Neuromuscular cleft
Motor unit

motor end plate

Pocket formed around motor neuron by sarcolemma

Neuromuscular cleft

Gap between neuron and sarcolemma

Motor unit:

Motor neuron and all of the fibers it innervates

The sliding filament model

Muscle shortening occurs due to movement of the actin filament over the myosin filament
Formation of cross-bridges between actin and myosin filaments--> "power stroke"
Sarcomere shortens

Actin

Actin (globules), troponin, tropomyosin
Regulatory binding site for Ca++ (troponin)

myosin

Myosin head
Myosin tail

contraction cycle

1. Rest; cross-bridges weakly bound (no Ca++)
2. Ca++ binds; cross-bridges strongly bound (to troponin)
3. Pi released; movement starts
4. ADP released; movement finishes
5. ATP attaches; weak binding state. ATP breakdown-->cross-bridge "energized"
Ca++ removal-->weak binding state (rest)

excitation-contraction coupling

Depolarization of motor end plate (excitation) is linked to contraction: Nerve impulse travels down T-tubles and causes release of Ca** from SR
Ca** binds to troponin and causes position change in tropomyosin, exposing active sites on actin
Permits strong binding state between actin and myosin, and contraction occurs
Continues until Ca** is removed

ATP is required for muscle contraction

1. Myosin ATPase breaks down ATP as fiber contracts
2. Calcium ATPase breaks down ATP for calcium pump in SR
3. Sodium-potassium pump on sarcholemma re-establishes membrane potential

sources of ATP

1. Phosphocratine (PCr)
2. Glycolysis
3. Oxidative phosphorylation

biochemical properties of muscle fibers

oxidative, glycolytic function, type of ATPase

contractile properties of muscle fibers

maximal force production, speed of contraction, muscle fiber efficiency (economy)

isometric muscle contraction

force but no external change in length. used by postural muscles

isotonic (dynamic)

concentric, eccentric

concentric muscle contraction

muscle shortens during force production

eccentric muscle contraction

muscle produces force but length increases

regulation of muscle force is dependent on

1. Size of the muscle
2. Number of motor units recruited (recruitment)
3. Rate at which they are recruited ("rate coding")

muscle weakness

relative lack of strength in an unfatigued muscle. strength is measured as the force of power produced during a maximum voluntary contraction (MVC)

muscle fatigue

acute fall of maximum force generating capacity of the muscle in response to muscular activity. measured as decrease in mvc as a result of contraction(s)

muscular dystrophy

lack of dystrophin (structural protein that helps transmit force developed by actin and myosin to connective tissue to move limb)

amyotrophic lateral sclerosis

loss of motor neurons

myasthenia gravis

Ach Receptors on sarcolemma blocked or destroyed.

McArdle's disease

no glycogen phosphorylase cannot break down glycogen

mitochondrial myopathy

various mutations interferes with ability to make energy from oxidative phosphorylation

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