The Muscular System

Muscle Tissues Cardiac Smooth Skeletal Involuntary striated muscle Found only in heart Natural contraction cycle determined by pacemaker cells Smooth Lines blood vessels, digestive organs, urinary system, and parts of respiratory system, pupils of eyes Involuntary non-striated muscle Skeletal Voluntary striated muscle Multinucleated cells called muscle fibers Controlled by motor nerve cells

Functions of Skeletal Muscle Produce Movement Contractions pull on tendons and move bones Maintain posture and body position Continuous contractions maintain posture Support/protect soft tissues Abdominal wall Floor of pelvic cavity Guard entrances and exits Voluntary control of swallowing, defecation, and urination Maintain body temp Some energy from contractions lost as heat

Gross Anatomy of Skeletal Muscle Each cell is called a “muscle fiber” Contains several tissues Connective Epimysium – collagen fibers surrounding entire muscle Perimysium – divide skeletal muscles into bundles of fibers (fascicles) Endomysium – surrounds ea/fiber Tendons – connect skeletal muscle to periosteum of bones Blood vessels Nerves – control contractions Skeletal muscle

Microanatomy of Skeletal Muscle Sarcolemma – cell membrane Sarcoplasm – cytoplasm Myofibrils – bundles of myofilaments Thin filaments – actin proteins Thick filaments – myosin proteins Sarcoplasmic reticulum – smooth ER (stores Ca+) Sarcomeres – repeating units of myofilaments

Muscle Fiber Contraction

Muscle Tone Tone – resting tension Stabilizes the position of your joints Any skeletal muscle not stimulated on a regular basis will atrophy – fibers become smaller and weaker Initially atrophy is reversible Extreme atrophy is permanent

Energetics of Muscle Activity Active Skeletal Muscle fibril requires 600 trillion ATP/sec Sources of ATP Stored (ADP + Creatine-P  ATP) Lasts 15 sec Aerobic Metabolism (Krebs  Oxidative Phosphorylation) Provides 30% of ATP needed during peak exertion Anaerobic Metabolism (glycolysis) Main E source Lactic acid builds up Ineffective Muscle Fatigue – no contraction despite stimulation Lack of ATP or lactic acid build up Recovery Period – returns to pre-exertion levels

Muscle Performance Force and endurance depends on: Types of muscle fibers Fast Twitch (white) Powerful contractions Fatigue rapidly (few mitochondria) Slow Twitch (red) Extended contraction (many mitochondria) Extensive capillary network Myoglobin binds O2 Physical conditioning ( increase power and endurance) Anaerobic Frequent, brief intense workouts (hypertrophy of muscle fibrils) Aerobic Sustained low levels of activity Carb-load the day before; drink glucose rich sports drinks