The Muscular System

Muscle Tissues

• Cardiac – 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