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In the name of God

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Session 5

1- skeletal muscle fiber 2- skeletal muscle contraction

M.Bayat PhD

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http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__function_of_the_neuromuscular_junction__quiz_1_.html

http://www.dnatube.com/video/5034/Contraction-of-muscle-function-of-neuromuscular-junction

How can motor neuron stimulatesmuscle fiber?

Generation of AP in muscle

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Motor unit

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Motor end plate

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End plate potential

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Motor end plate

EPSP

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EPSP

AP

AP

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• How can AP produce contraction in muscle fiber ?

• Generation of muscle contraction (excitation & contraction coupling)

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• Ca release from sarcoplasmic reticulum?

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Ryanodine receptor

RYR1 functions as a calcium release channel in the sarcoplasmic reticulum, as well as a connection between the sarcoplasmic reticulum and the transverse tubule

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__action_potentials_and_muscle_contraction.html

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• http://www.dnatube.com/video/4812/Control-of-voluntary-muscle-contraction

• Please read the following link before the next class

• MUSCLE TISSUE PHYSIOLOGY

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http://www.getbodysmart.com

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Actin

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Myosin

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Power stroke

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• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__breakdown_of_atp_and_cross-bridge_movement_during_muscle_contraction.html

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http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__myofilament_contraction.html

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__sarcomere_contraction.html

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Types of Muscle Contractions• Isometric: No change in length but tension

increases• Isotonic: Change in length but tension

constant

Isometric ContractionIsometric muscle contractions occur when the muscle doesn't shorten. For example when pushing against an immovable object like a wall or holding a dumbbell directly in front of you at a constant level.

Isotonic Contraction Isotonic contractions are those when the tension on a muscle remains constant but the muscle shortens as in lifting a static amount of weight.

The basic difference between the two forms of contraction is that isometric contraction does not require much sliding of the filaments among each other, but force is developed. No external work is done. In isotonic contraction, sliding of the filaments occurs, and a load is moved: external work is performed.

Force Summation.

• Summation occurs in two ways: • (1) by increasing the number of motor units contracting

simultaneously, which is called multiple fiber summationMotor unit Recruitment

• (2) by increasing the frequency of contraction, which is called frequency summation and can lead to tetanization.

• (3) by hypertrophy and hyperplasia

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Motor units

Motor unit Recruitmenthow many motor neurons are activated in a particular muscle?how many muscle fibers of that muscle are activated?

Motor units are generally recruited in order of smallest to largest (smallest motoneurons to largest motoneurons (and thus slow to fast twitch)) as contraction increases. This is known as Henneman's Size Principle

Motor unit rotation in prolong low level contraction

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Frequency Summation and Tetanization

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Multiple fiber summation – When a weak signal is sent by the CNS to contract a muscle, the smaller motor units, being more excitable than the larger ones, are stimulated first. As the strength of the signal increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger. A concept known as the size principle, allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required.

Frequency summation – For skeletal muscles, the force exerted by the muscle is controlled by varying the frequency at which action potentials are sent to muscle fibers. Action potentials do not arrive at muscles synchronously, and, during a contraction, some fraction of the fibers in the muscle will be firing at any given time. In a typical circumstance, when a human is exerting a muscle as hard as he/she is consciously able, roughly one-third of the fibers in that muscle will be firing at once, though this ratio can be affected by various physiological and psychological factors (including Golgi tendon organs and Renshaw cells). This 'low' level of contraction is a protective mechanism to prevent avulsion of the tendon—the force generated by a 95% contraction of all fibers is sufficient to damage the body.

Slow Fibers• (1) Smaller fibers. • (2) Also innervated by smaller nerve fibers. • (3) More extensive blood vessel system and

capillaries to supply extra amounts of oxygen. • (4) Greatly increased numbers of mitochondria,

also to support high levels of oxidative metabolism. • (5) Fibers contain large amounts of myoglobin, an

iron containing protein similar to hemoglobin in red blood cells.

• The myoglobin gives the slow muscle a reddish appearance and the name red muscle, whereas a deficit of red myoglobin in fast muscle gives it the name white muscle.

Fast Fibers

• (1) Large fibers for great strength of contraction.• (2) Extensive sarcoplasmic reticulum for rapid

release of calcium ions to initiate contraction. • (3) Large amounts of glycolytic enzymes for rapid

release of energy by the glycolytic process. • (4) Less extensive blood supply because oxidative

metabolism is of secondary importance.• (5) Fewer mitochondria, also because oxidative

metabolism is secondary.

Sources of Energy for MuscleContraction

• The concentration of ATP in the muscle fiber, about 4 millimolar, is sufficient to maintain full contraction for only 1 to 2 seconds at most. 1-2 sec

1. The first source of energy that is used to reconstitute the ATP is the substance phosphocreatine. 5-8sec

2. The second important source of energy, which is used to reconstitute both ATP and phosphocreatine, is “glycolysis” of glycogen previously stored in the muscle cells. 1min

3. The third and final source of energy is oxidative metabolism. More than 95 per cent of all energy used by the muscles for sustained, long-term contraction is derived from this source.