KIN 425,  Concepts of Strength & Conditioning            

Muscle Physiology

 

Any contraction of a muscle pulls on a tendon and, in turn, on bone.  How?

-        Fibrous connective tissue (epimysium) covers the body’s more than 430 skeletal muscles

    -        The epimysium is continuous with the tendons at the ends of the muscle

        -        The tendons are attached to bone periosteum (a specialized connective tissue covering all bones)

 

What are the two attachments for limb muscles called?

-        Proximal / Origin (closer to the trunk or center of the body)

-        Distal / Insertion (away from the trunk or center of the body)

 

Muscle cells are often called what?

-        Muscle fibers

 

What is a fasciculi?

-        Under the epimysium the muscle fibers are grouped in bundles (fasciculi) that may consist of up to 150 fibers

 

What is a neuromuscular junction? (also called the motor end plate)

-        The junction between a motor neuron (nerve cell) and the muscle fibers it innervates

 

How many neuromuscular junctions does each muscle fiber have?

-        Each fiber has only one neuromuscular junction

 

How many muscle fibers does a single motor neuron innervate?

-        A single motor neuron innervates many muscle fibers, sometimes as many as several hundred

 

What is a motor unit?

-        A motor neuron and all the muscle fibers it innervates are called a motor unit

 

What is the all-or-none principle of muscle stimulation?

-        All the muscle fibers of a motor unit are stimulated to contract simultaneously together or none at all

 

What is the interior structure of a muscle fiber (sarcoplasm) composed of?

-        The sarcoplasm contains:

-        Hundreds to thousands of myofibrils (each about 1/100 the diameter of a human hair)

-        Contractile components (protein filaments-myofilaments)

-        Stored glycogen

-        Fat particles

-        Enzymes

-        Mitochondria

-        Sarcoplasmic reticulum (calcium storage and movement channels) and T-tubules

 

What do myofibrils contain?

-        Two types of myofilaments responsible for muscle fiber contraction and two other proteins:

-        Myosin – thick filaments (1/10,000 the dia. of a hair), contain protruding globular heads called cross-bridges

-        Actin – thin filaments, consist of two strands arranged in a double helix

-        Troponin – a protein situated at regular intervals along the actin filament that has a high affinity for calcium ions

-        Tropomyosin – a protein strand that runs along the length of the actin filament in the groove of the double helix

 

What is the smallest contractile unit of a muscle fiber?

-        A sarcomere is the smallest contraction unit of a muscle fiber

 

The regulation of what mineral controls muscle fiber contraction?

-        The discharge of an action potential (an electric nerve impulse) arrives nearly simultaneously from the surface to all depths of the muscle fiber and signals the release of Calcium from the sarcoplasmic reticulum into the myofibril producing a coordinated contraction and tension development.

 

What theory of muscular contraction involves the actin and myosin filaments and results in shortening of muscle fibers?

-        In simple terms, the sliding filament theory states that actin filaments at each end of the sarcomere slide inward on myosin filaments, pulling the z-lines toward the center of the sarcomere and thus shortening the muscle fiber.

 

What are the five phases of the sliding filament theory of muscular contraction?

-        Resting phase

-        Very few of the myosin cross-bridges are bound to actin.  No tension is developed in the muscle, so the muscle is said to be at rest.  During sleep.

-        Excitation-Contraction coupling phase

-        Before myosin cross-bridges can flex, they must first attach to the actin filament (coupling).  The sarcoplasmic reticulum is stimulated to release calcium ions; calcium binds with troponin along the actin filament.  This causes the tropomyosin to attract the myosin cross-bridge heads to move onto the actin filament. 

-        Contraction phase

-        Once the myosin cross-bridge head has attached to the actin one ATP is hydrolyzed for energy and the head swivels causing tension/movement to occur (power stroke).

-        Another molecule of ATP must replace the ADP on the myosin cross-bridge head to detach from the active actin site and recock.  This allows the contraction process to be continued or relaxation to occur.

-        Recharge phase

-        Measurable muscle shortening occurs only when the coupling and contraction phases are repeated over and over again throughout the muscle fiber.  Recharge is the return to ready.

-        Relaxation phase

-        Relaxation occurs when the stimulation of the motor nerve stops.

-        Calcium is pumped back into the SR, which prevents the link between actin and myosin.  

 

 

What unit of energy is responsible for a single contraction phase (cross-bridge flexion) of the sliding filament theory?

-        The energy for cross-bridge flexion comes from the hydrolysis (breakdown) of ATP to ADP and i-phosphate, a reaction catalyzed by the enzyme myosin ATPase.

 

What are the three primary muscle fiber types?

-        Type I – slow twitch:

-        Resist fatigue by having a large number of mitochondria (aerobic powerhouse), high aerobic enzyme activity, and dense capillary concentration.

-        Type IIb – fast glycolytic:

-        True fast twitch fiber.  Fatigues rapidly, have few mitochondria, low aerobic enzyme activity and few capillaries.

-        Type IIa – fast twitch oxidative glycolytic:

-        Have some of the fatigue-resistant qualities of Type I fibers with more mitochondria, greater aerobic enzyme activity, and greater capillary density than Type IIb fibers.

-        Referred to as intermediate fiber – adapts according to training stimulus.

 

Which muscle fiber type is capable of developing the highest forces?

-        Type II, or fast twitch, muscle fibers are capable of developing higher forces, especially at higher velocities of muscle action, than type I, or slow twitch, muscle fibers.

 

What are the three types of muscle actions?

-        Concentric muscle actions – occurs when the total tension developed in all the cross-bridges of a muscle is sufficient to overcome any resistance to shortening.  Muscle force produced through cross-bridge flexion exceeds any resistance to shortening, and joint angle decreases through positive movement.

-        Isometric muscle actions – occurs when the tension in the cross-bridges equals the resistance to shortening, and the muscle length remains relatively constant.  Tension balances or equals resistance. 

-        Eccentric muscle actions – occurs when tension developed in the cross-bridges is less than the external resistance, and the muscle lengthens despite contact between the myosin cross-bridge heads and the actin filaments.

 

What dictates how much force is being produced in a muscle?

-        The number of myosin cross-bridges that are attached to actin filaments at any instant in time dictates how much force is being produced in that muscle.

 

What are the two main ways that force production is controlled through motor unit recruitment?

-        Frequency of stimulation of motor units

-        Number of motor units activated

 

Prepared by Greg Werner for KIN 425