BIO 201 Anatomy & Physiology I

• Course at Arizona State University, Downtown Phoenix Campus

• Fun fact: "Why was myosin cast in the + play? It had a good grip on actin."

Universal Characteristics of Muscle

• Excitability (Responsiveness): Ability to respond to stimuli like chemical signals, stretch, and electrical changes.

• Conductivity: Local electrical excitation leads to a wave of excitation that travels along the fiber.

• Contractility: Ability to shorten when stimulated.

• Extensibility: Capability to be stretched between contractions.

• Elasticity: Returns to original length after being stretched.

Skeletal Muscle

• Properties: Voluntary, striated muscle mostly attached to bones.

• Striations: Alternating light and dark transverse bands due to arrangement of contractile proteins.

• Muscle Fiber: A muscle cell, also known as a myofiber, can be up to 30 cm long.

Connective Tissue Wrappings in Skeletal Muscle

• Endomysium: Connective tissue surrounding individual muscle fibers.

• Perimysium: Connective tissue wrapping around muscle fascicles (bundles of fibers).

• Epimysium: Connective tissue surrounding the entire muscle.

• Tendons: Attach muscles to bones; composed of collagen that is somewhat extensible and elastic.

Muscle Fiber Structure

• Sarcoplasmic Reticulum (SR): Smooth ER that forms a network around myofibrils, acting as a calcium reservoir.

• Sarcoplasm: Cytoplasm of muscle fiber containing myofibrils, glycogen, and myoglobin (oxygen-binding pigment).

• Multiple Nuclei: Flattened nuclei pressed against the sarcolemma, resulting from myoblast fusion.

• Satellite Cells: Unspecialized myoblasts that aid in muscle tissue regeneration.

Myofilaments

• Thick Filaments: Composed of myosin; each molecule has a head and a shaft-like tail, organized in a helical array.

• Thin Filaments: Composed of actin, tropomyosin, and troponin - necessary for contraction regulation.

• Elastic Filaments: Titin protein stabilizes and positions thick filaments, preventing overstretching.

Muscle Contraction Mechanism

• Contractile Proteins: Myosin (thick) and actin (thin) work together for contraction.

• Regulatory Proteins: Tropomyosin and troponin control binding sites and contractile activity, sensitive to calcium levels.

• Striations in Muscle: Result from the arrangement of thick and thin filaments, forming A-bands (dark) and I-bands (light).

Sarcomere

• Definition: Functional unit of a muscle fiber, involved in muscle contraction by sliding filament action between the thick and thin filaments.

• Sliding Filament Theory: Thick and thin filaments slide past each other during contraction without changing their lengths.

Nervous System Interaction with Muscle

• Skeletal muscle contraction requires nerve stimulation; nerve severance leads to paralysis.

• Motor Unit: A motor neuron and all muscle fibers it innervates work together to sustain contraction over time.

Muscle Fiber Types

• Fast-Twitch Fibers: Provide quick bursts of speed, higher fatigue rate; ideal for sprinting.

• Slow-Twitch Fibers: Provide endurance; better for long-duration activities like marathon running.

Muscle Fatigue

• Factors: Potassium accumulation or depletion of glycogen can lead to fatigue.

• EPOC (Excess Postexercise Oxygen Consumption): Oxygen debt post-exercise necessary for ATP replenishment, repairing tissues, and clearing lactate.

Cardiac vs. Smooth Muscle

• Cardiac Muscle: Striated, involuntary, has intercalated discs, rhythmically contracts without external stimulation.

• Smooth Muscle: Non-striated, slower contraction, can be stimulated by various factors including hormones and stretch.

Muscular Disorders

• Muscular Dystrophy: Genetic disorders resulting in muscle degeneration due to dystrophin abnormalities.

• Myasthenia Gravis: Autoimmune condition that affects communication between nerves and muscles, reducing muscle strength.