THE SOURCE OF ENERGY FOR THE MUSCULOSKELETAL SYSTEM THE MUSCULAR SYSTEM THE SOURCE OF ENERGY FOR THE MUSCULOSKELETAL SYSTEM

TYPES OF MUSCLE 1. Skeletal: attached to bone 2. Cardiac: the heart 3. Smooth: found in organs

Basic Properties of all Muscle Cells 1. Elasticity – after being stretched, all muscle cells (fibers) are capable of resuming their original shape 2. Contractibility – upon being stimulated (an electrical event) muscle cells can shorten in length (contract) 3. Extensibility – after a contraction subsides, muscle cells can be stretched back to their natural length (relax) 4. Irritability/Sensitivity – muscle cells can respond to nerve stimulation

Muscle Functions 1. Produce movement (many kinds) a. gross (large) body movements across joints b. peristalsis (organ movement) c. shivering 2. Produce heat – negative feedback to regulate body temperature 3. Maintain body posture - fixators (muscle type) function to stabilize movements across joints to support the body and maintain posture

Skeletal Muscle Structure 1. Connective Tissues a. tendons – attach muscle to bone b. fascia – separates muscle c. aponeuroses – thin, sheet-like tendons d. epimysium – wraps entire muscle e. perimysium – wraps a bundle of muscle fibers (fascicle) f. endomysium – wraps a single muscle fiber or muscle cell

2. Skeletal Muscle Fibers (muscle  fascicles  muscle fibers  myofibrils  actin/myosin) a. fiber – a single cell that contracts in response to stimulation and then relaxes when stimulation ends - sarcolemma – cell membrane - sarcoplasm – cytoplasm b. myofibril – 4 to 20 protein filaments within a muscle fiber - actin – thin filament - myosin – thick filament repeating patterns provide skeletal muscle’s striated appearance

c. sarcomeres – functional units of skeletal muscle (contraction occurs here) I band (light band) – actin filaments connected to Z lines A band (dark band) – myosin band overlapping actin M line – central zone that anchors myosin Z line to Z line = one sarcomere

d. sarcoplasm - sarcoplasmic reticulum – like endoplasmic reticulum; membranous channels - transverse tubules (T tubules) – contain extracellular fluid - responsible for activation of muscle fiber contraction when stimulated

Sarcomere Relaxed

Sarcomere Contracted

Neuromuscular Junction 1. Motor neuron – axon of a nerve cell stimulates muscle fiber contraction - ends of neuron full of mitochondria and synaptic vesicles full of neurotransmitters 2. Neuromuscular junction – functional connection between the motor neuron and the muscle fiber - NO physical connection; space or gap called a synapse 3. When a nerve impulse reaches the end of the motor neuron axon, some synaptic vesicles release neurotransmitters into the synapse and stimulate muscle contraction

Contraction 1. Dependent on Ca2+ (calcium ion) concentration 2. Results when thin filaments are pulled toward one another 3. Myosin heads form crossbridges between actin and myosin molecules and then change shape (movement)

Sliding Filament Model 1. ATP bonds to the enzyme site on the myosin head 2. ATP  ADP + phosphate group; the energy released from this reaction activates the myosin head into a “cocked” position 3. Myosin heads and actin bind, releasing ADP and the phosphate group

4. The “power stroke” of the myosin head slides the actin filaments across the myosin filaments, shortening the sarcomere ( I band gets smaller, Z lines get closer together) 5. New ATP binds to myosin head and actin is released * If new ATP does not arrive, the muscle stays contracted; a condition known as rigor or rigor mortis after death

Factors Involved in Contraction 1. Tropomyosin – a strand-like protein that covers the active sites on actin while the muscle is at rest 2. Troponin – holds tropomyosin in position; binds with calcium ions 3. Calcium ions (Ca2+) - when attached to troponin, Ca2+ causes a conformational change (change in shape) that moves tropomyosin, exposing the active sites on actin, allowing myosin crossbridges to form

Motor Unit A motor neuron and all the muscle fibers it stimulates to contract

- at rest, Ca2+ is stored in transverse tubules (T tubules) and sarcoplasmic reticulum (SR) - low levels of Ca2+ in sarcoplasm equals relaxation - high levels of Ca2+ in sarcoplasm equals contraction

Stimulus for Contraction 1. Acetylcholine (ACh) – neurotransmitter released to trigger muscle contraction - when ACh binds to receptors on the sarcolemma (cell membrane), Na+ rushes into the cell creating electrical activity - SR and T tubules immediately release Ca2+ into sarcoplasm - Ca2+ binds to troponin, which shifts the position of tropomyosin and exposes the active sites on actin

- myosin head crossbridges form, initiating the “power stroke” movement - the more Ca2+ is released, the more troponin bound, the greater the contraction 2. Relaxation – no nerve impulses (no ACh) - release of Ca2+ ends and ATP is used to pump Ca2+ back into T tubules and SR - without Ca2+ , troponin returns to its resting state and tropomyosin again covers the active sites on actin

Energy Sources for Contraction 1. ATP – adenosine triphosphate; energy for muscle contraction 2. Creatine phosphate (CP) – high-energy compound/phosphate storage molecule - transfers a phosphate group to ADP to create ATP * When the supply of ATP and CP are depleted, muscles rely on the metabolism of glucose for energy, known as cellular respiration. The type of respiration that occurs is dependent upon the availability of oxygen.

3. Aerobic Respiration – requires oxygen to make ATP and occurs in the mitochondria in the sarcoplasm 4. Anaerobic Respiration – occurs when oxygen is not available (oxygen debt) - results in fewer ATP and build-up of pyruvic acid (precursor to lactic acid) - leads to muscle fatigue and pain from lactic acid build-up 5. Less than half of the energy produced can be used for contraction, the rest is released as heat

Muscular Responses 1. Threshold Stimulus – minimal strength required to cause a contraction 2. All-or-None Response – muscle fibers contract completely or not at all - increases in intensity, rate and duration of impulses do NOT strengthen the contraction 3. Summation – contractions add onto each other & results in greater contraction - tetanus – sustained maximum contraction

4. Myogram – graph of muscle contraction - twitch – contraction of a muscle fiber - latent period – delay between the time the stimulus was applied and the time the muscle fiber responded 5. Recruitment – increase in number of motor units activated as stimulation intensity increases 6. Tonus – muscle tone during normal posture 7. Graded response – a variable response depending upon the number of contracting muscle fibers in a muscle

Myogram example

Label: contraction stimulus latent period relaxation resting phase maximum tension level

Skeletal Muscle Actions 1. Origin – muscle end attached to a bone by a tendon; immovable end 2. Insertion – movable end 3. During contraction the insertion is pulled toward the origin (Example: gastrocnemius origin is by the distal part of the femur and its insertion is on the calcaneus).

Interactions of Skeletal Muscle 1. Prime mover (agonist) – muscle responsible for a particular movement (biceps brachii move elbow) 2. Synergists – muscles that contract and assist the prime mover. Fixators are synergists that stabilize the origin of a prime mover by preventing movement at another joint. 3. Antagonists – resist a prime mover’s action and cause movement in opposite direction (triceps brachii extends the elbow)

Smooth and Cardiac Muscle 1. Smooth Muscle - spindle shaped cells, single nucleus, no striations, involuntary control, no sarcomeres - actin and myosin present in cytoplasm contract more slowly than skeletal muscle, but sustain longer contraction 2. Cardiac Muscle - heart only - single nucleus, striated, involuntary control, lots of mitochondria, branched, intercalated discs

Smooth or Cardiac?

Smooth or Cardiac?

Add these to your notes.. Isotonic contraction- tension rises and muscle length changes. (example: lifting a book) Isometric contraction- muscle do not change, and the tension produced never exceeds the resistance. (example: pushed on a closed door)