Skeletal Muscles Anatomy of Skeletal Muscle Skeletal muscle

Skeletal Muscles

Anatomy of Skeletal Muscle Skeletal muscle also known as striated muscle -bundled muscle fibers (cells) Endomysium-delicate tissue sheath wrapped around each muscle fiber Fascicle-bundle of muscle fibers wrapped with perimysium Epimysium-tough tissue layer that wraps several Fascicles (musle fiber bundles) Tendons-tough connective tissue that connect muscle to bone

Anatomy of a muscle fiber Myofibrils-long ribbon-like organelles that fill the cell -site of contraction Sarcomeres-segmented sections of the myofibrils which are chains of contracting units Myofilaments-threadlike protein inside the sarcomeres which give the stripe appearance of muscle Myosin-thick filaments of protein Actin-thin filaments of protein

Muscle contraction: Sliding Filament Theory When stimulated by the nerves the Myosin heads begin to contract (walking motion). The contracting of the myosin cause the Actin to pull towards each other. As the Actin pull towards each other they overlap which shortens the overall length of the muscle causing muscle contraction. The filaments never shorten just pull together and overlap. The myosin filaments are like a centipede with hundreds of legs and as it walks it pulls the actin filaments together.

Neuromuscular junction -area where nerve connects to the muscle fiber Axon -nerve fiber that reaches the muscle fibers Axon terminals -ends of axons that connect with the sarcolema (covering of muscle fiber) Synaptic cleft -gap between the axon terminals and the muscle fiber When a muscle contracts the nerve releases a chemical (neurotransmitter) at the junction site. The neurotransmitter that stimulates skeletal muscle is acetylcholine (ACh). The chemical Ach allows the muscle to temporarily absorb sodium ions (NA+) and release potassium ions (K+). This gives the muscle fiber an excess of positive ions which causes and electrical current called an action potential. This electrical current causes the contraction of the muscle cell.

Types of Muscle Contractions Isotonic -muscle contracts and shortens causing movement Ex: contracting bicep and moving ulna and radius Isometric -muscle contracts but does not shorten because the object is too big for the muscle to move it. Ex: trying to bench press 400 pounds (not going to happen) unless you’re Mr. Schwarz

Use it or Lose it Muscles must be used or they will waste away, “atrophy” Atrophy is common when after an injury and the muscle has been mobilized. Ex: You break your arm and have to wear a cast for 9 weeks. When the cast comes off your arm will appear that it has shrunk because your muscle has lost size.

So how does exercise effect muscles? Aerobic exercise (swimming, jogging, walking, biking, etc) -more flexible muscles -greater resistance to fatigue -blood supply to muscles increase -able to store more oxygen in muscle fibers -overall body metabolism improves -enlarges heart which allows to pump more blood Resistance training (weightlifting) -key is forcing muscles to contract with as much force as possible -requires little time -increase in muscle size and strength -enlargement of individual muscle cells (make more filaments, actin and myosin)

Muscle movements Origin area where muscle is attached to the immovable or less movable bone Insertion area where muscle is attached to the movable bone -the insertion moves towards the origin when the muscle contracts

5 Golden Rules of Skeletal Muscles 1. Most skeletal muscles cross at least one joint 2. The majority of the muscle lies proximal to the joint 3. All skeletal muscles have at least two attachments: origin and insertion 4. Skeletal muscles only pull; never push 5. During contraction, the insertion moves towards the origin