`. The microstructure of muscle. myofiber (muscle fiber)- a single muscle cell sarcolemma- muscle cell membrane sarcoplasm- muscle cell cytoplasm myofibril- long contractile protein structure actin and myosin sarcomere- the contractile unit between two z-lines. Muscle Terminology.
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myofiber (muscle fiber)- a single muscle cell sarcolemma- muscle cell membrane sarcoplasm- muscle cell cytoplasm myofibril- long contractile protein structure actin and myosin sarcomere- the contractile unit between two z-lines Muscle Terminology
sarcoplasmic reticulum- storage and release site of calcium transverse tubule- also involved in calcium flux More Terms
1) Impulse travels down motor neuron 2) at end of neuron, acetylcholine released 3) Acetylcholine diffuses across synaptic cleft 4) acetylcholine binds to receptors on sarcolemma causing permeabilty 5) sodium enters cell causing depolarization and muscle contraction Neuromuscular Junction
functions to produce force for locomotion force for breathing force for postural support heat production in cold (no force) Muscular Contraction
Sliding filament model of contraction the interaction of actin and myosin How do skeletal muscles contract?
Sliding Filament Animation • -- sliding filament animation.htm • http://intro.bio.umb.edu/111-112/112s99Lect/muscle/contract.html
The myosin head binds to the actin filament in a weak state initially (or unbound) the signal to contract initiates a strong binding state Binding of calcium to troponin regulates this strong-weak state How do the Actin and Myosin Interact?
The Contraction Itself • during the strong binding the myosin pulls the actin past • this effectively shortens or contracts the muscle
ATP is necessary for each contraction cycle to occur each contraction cycle results in a shortening of the muscle by 1% some muscles can shorten by up to 60 % of their resting length therefore many shortening cycles must occur for a single contraction Where does the energy for contraction come from?
Excitation-Contraction Coupling Fig 8.9
Crossbridge Animation • Quicktime - Actin Myosin Crossbridge 3D Animation.htm • http://www.sci.sdsu.edu/movies/actin_myosin.html
at rest actin and myosin are weakly bound (or unbound) an excitation impulse from the a motor nerve causes an end-plate potential the potential depolarizes the muscle cell beginning at the sarcolemma Steps in Excitation - Contraction coupling
depolarization travels down the T-tubules to the sarcoplasmic reticulum the impulse reaches the SR and calcium is released calcium binds to troponin and causes the strong binding state Excitation- Contraction cont’d
during strong binding, myosin head cocks this action moves actin filament along myosin Binding of ATP causes the weak binding (or release) again enabling another contraction Excitation- Contraction (one more)
depolarization causes release of calcium by SR calcium enables the strong binding state ATP provides energy for cocking of myosin head, BUT binding of ATP causes the weak binding state (or release) of actin and myosin Important Points
contraction can continue as long as calcium is available to enable strong binding AND ATP is available for energy of cocking and release of strong binding the signal to stop contraction is the loss of an impulse and uptake of calcium A couple more important points
Muscle fatigue is characterized by a reduced ability to generate force
Properties of Muscle Fiber Types • Biochemical properties • Oxidative capacity • Type of ATPase • Contractile properties • Maximal force production • Speed of contraction • Muscle fiber efficiency
Fast fibers Type IIx fibers Fast-twitch fibers Fast-glycolytic fibers Type IIa fibers Intermediate fibers Fast-oxidative glycolytic fibers Slow fibers Type I fibers Slow-twitch fibers Slow-oxidative fibers Individual Fiber Types
Muscle Fiber Types Fast FibersSlow fibers Characteristic Type IIx Type IIa Type I Number of mitochondria Low High/mod High Resistance to fatigue Low High/mod High Predominant energy system Anaerobic Combination Aerobic ATPase Highest High Low Vmax (speed of shortening) Highest Intermediate Low Efficiency Low Moderate High Specific tension High High Moderate
type II are fast twitch muscles type IIa are sort of like slow twitch but faster type I are slow twitch muscles therefore IIb will have the fastest shortening velocity and type I will have the slowest Type I vs Type II (velocity)
Endurance exercise training induced changes in fiber type in skeletal muscle
an isometric contraction is occurs when there is no change in muscle length when force is being produced trying to push a car out of the snow holding up a table so it can be leveled Isometric Muscle Action
an isotonic contraction occurs when there is a change in muscle length concentric when muscle shortens bicep curl, lifting eccentric when muscle lengthens tug o war, negatives in weights, putting down a beer Isotonic Muscle Action
Stimulus Strength vs Force of Contraction • Weak stimulus does not recruit many motor units • Stronger stimulus recruits more motor units • When all motor units are recruited, no more force can be applied regardless of stimulus strength
Length Tension Relationship • There exists an optimal length of muscle at which it produces the greatest force • Typically between 100-120 % resting length • Maximal tensions at lengths longer or shorter than the optimal length will be less
Tetanus • If twitches become more frequent, greater force can be developed during summation than for a single twitch • If twitches become to frequent, tetanus will develop and the muscle will not relax • Typically results only from electrical stimulation
GTO • Provides info to the CNS about tension development in the muscle • Acts like a governor to prevent damaging tension from being generated • Can be overridden to a certain extent by training • Supraphysiological strength in crisis