Muscle Anatomy and Functions

Muscle Function

• Muscle is an organ composed of muscle tissue, blood vessels, nerves, fat, and connective tissue
• Muscle makes up nearly 50% of the body's mass
• Muscle tissue is responsible for movement, maintaining posture, stabilizing joints, and generating heat

Muscle Types

• There are three types of muscle tissue: skeletal, cardiac, and smooth
• Skeletal muscles are the longest type
• Skeletal muscles are voluntary and striated, while cardiac muscle is involuntary and striated
• Smooth muscle is found in the walls of hollow organs and other areas, it is involuntary and lacks striations

Muscle Characteristics

• All muscle tissues share four characteristics: excitability, contractility, extensibility, and elasticity

Skeletal Muscle Structure

• Skeletal muscle is organized in a hierarchy: muscle, fascicle, muscle fiber
• Muscle fibers are multinucleated, cylindrical cells that are the functional units of skeletal muscle
• Muscle fibers contain numerous mitochondria and are surrounded by the sarcolemma and sarcoplasm

Muscle Attachments

• Muscles span joints and attach to bones
• The origin is the attachment to the stationary bone, while the insertion attaches to the movable bone

Muscle Actions

• Muscles work in groups to produce movement
• Prime movers (agonists) produce a specific movement, antagonists oppose or reverse that movement, and synergists assist prime movers

Microscopic Anatomy

• Myofibrils are the contractile units within muscle fibers
• Sarcomeres are the repeating units within myofibrils, composed of actin and myosin filaments
• The A band contains both actin and myosin filaments, while the I band only contains actin
• Titin and nebulin are proteins that stabilize and regulate the structure of the sarcomere

Actin and Myosin

• Myosin filaments are thick and composed of myosin protein dimers, with heads that bind to ATP
• Actin filaments are thin and composed of two actin chains twisted into a helix
• Tropomyosin blocks the binding sites on actin, while troponin is a calcium-binding protein that regulates tropomyosin's position

Muscle Contraction

• Calcium binds to troponin, causing a conformational change that shifts tropomyosin and exposes myosin binding sites on actin
• The cross-bridge cycle is initiated when ADP-bound myosin heads bind to actin
• The "power stroke" of the cross-bridge cycle is powered by the release of ADP and the stored energy from ATP hydrolysis
• ATP binding to myosin heads causes their detachment from actin, and ATP hydrolysis resets the myosin head for another power stroke.

Sources of ATP

• Muscle fibers store oxygen (myoglobin) and glucose (glycogen)
• Glycolysis is a rapid, anaerobic process that produces a small amount of ATP
• Aerobic respiration, which requires oxygen, produces a larger amount of ATP

Cardiac Muscle

• Cardiac muscle cells are shorter than skeletal muscle fibers and possess a less pronounced striation pattern
• Cardiac muscle is regulated by pacemaker cells, the autonomic nervous system, and epinephrine

Pacemaker Cells

• Pacemaker cells spontaneously depolarize and initiate the heart's rhythmic contractions
• The sinoatrial (SA) node depolarizes the atria, causing atrial contraction
• The impulse travels to the ventricles through the atrioventricular (AV) node and AV bundle, leading to ventricular contraction

Electrocardiography

• An electrocardiogram (ECG) records the electrical signals in the heart
• The P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization

Cardiac Cycle

• The cardiac cycle describes the blood flow through the heart during one heartbeat
• Systole is the period of ventricular contraction, during which blood is ejected to the pulmonary trunk and aorta
• Diastole is the period of ventricular relaxation, during which the ventricles fill with blood

Blood Vessels

• Arteries carry oxygenated blood away from the heart, except in the pulmonary circulation and umbilical vessels
• Veins carry deoxygenated blood toward the heart, except in the pulmonary circulation and umbilical vessels
• Capillaries are tiny vessels that connect arteries to veins and facilitate the exchange of nutrients, gases, and waste products with tissue cells

Blood Pressure

• Blood pressure is the force of blood exerted on the walls of blood vessels
• It is measured in millimeters of mercury (mmHg) and is typically measured in arteries near the heart
• Systolic pressure is the peak pressure during ventricular contraction, while diastolic pressure is the pressure during ventricular relaxation
• Healthy blood pressure is typically below 120/80 mmHg, readings above this can indicate problems with the heart or blood vessel elasticity

Blood Pressure Distribution

• Arteries have the highest blood pressure, followed by arterioles, capillaries, venules, and veins
• The low blood pressure in veins is assisted by the muscular pump and venous valves to ensure blood return to the heart

The Baroreceptor Reflex

• Baroreceptors are specialized sensory receptors located in the walls of blood vessels that detect changes in blood pressure
• When blood pressure decreases, baroreceptors send signals to the brain, which triggers an increase in heart rate and vasoconstriction to increase blood pressure
• When blood pressure increases, baroreceptors send signals to the brain, which triggers a decrease in heart rate and vasodilation to decrease blood pressure