Muscle Physiology Overview

Muscle Physiology

• Types of Muscles: Skeletal, Cardiac, Smooth
• Striated Muscle: Contains numerous cross-striations (transverse lines). Skeletal and Cardiac muscle are striated.
• Non-striated Muscle: Lacks cross-striations. Also called smooth muscle. Found in visceral organs.
• Voluntary Muscle: Controlled by conscious will. Skeletal muscle is voluntary. Innervated by somatic nerves.
• Involuntary Muscle: Not consciously controlled. Cardiac and smooth muscles are involuntary. Innervated by autonomic nerves.
• Skeletal Muscle Location: Associated with bones, forming 40-50% of body mass. Skeletal muscle fibers are parallel, anchored to bones by tendons.
• Cardiac Muscle Location: Forms the heart's musculature. These muscles are striated and involuntary. Supplied by autonomic nerve fibers.
• Smooth Muscle Location: Associated with viscera. Also called visceral muscle, it differs from skeletal/cardiac muscles due to lack of cross-striations. Smooth muscles line various visceral organs and are supplied by autonomic nerve fibers.
• Skeletal Muscle Cell Structure: Multinucleated, cylindrical, long cells surrounded by sarcolemma. Skeletal muscle fibers are composed of myofibrils. Myofibrils are subdivided into individual filaments. Filaments are composed of contractile proteins.
• Cardiac Muscle Cell Structure: Cells are short, branched, and interconnected. Contains 1-2 nuclei near center.
• Smooth Muscle Cell Structure: Cells are spindle-shaped. Contains 1 nucleus near cell's center. Sarcomeres are absent.

Skeletal Muscle Composition

• Skeletal muscle composition (Solids): 75% water, 25% solids.
• Proteins: Myosin, Actin, Tropomyosin, Troponin, Actinin, Desmin, Nebulin, Titin, Myoglobin.
• Other organic substances: Carbohydrates (glycogen, hexophosphate), Lipids (neutral fat, cholesterol, lecithin, steroids), Nitrogenous substances (ATP, adenylic acid, carnosine, carnitine, urea, uric acid, xanthine, hypoxanthine).
• Inorganic substances: Potassium, Sodium, Calcium, Magnesium, Chloride, Phosphate, Sulfate. Potassium is the primary mineral.

Skeletal Muscle Fiber Structure

• Skeletal muscle fiber structure: Each muscle fiber is a single cell, multinucleated, cylindrical and long. Surrounded by a cell membrane called sarcolemma. Muscle fibers are made up of myofibrils which are subdivided into fine filaments. Filaments are made of contractile proteins.
• Myofibril: Actin, Myosin, Sarcoplasmic Reticulum are components. Fibers are subdivided into sarcomeres.
• Sarcomere: The repeating structural unit of myofibrils.

Skeletal Muscle Sarcomere

• Sarcomere Structure: Contains actin (thin filaments), myosin (thick filaments), Z-discs, I bands, A bands, H zones.
• Actin (thin filaments): Part of the sarcomere.
• Myosin (thick filaments): Part of the sarcomere.

Sarcomere Contractile Proteins

• Actin: Thin filaments.
• Myosin: Thick filaments.
• Troponin: Attached to actin.
• Tropomyosin: Covers active sites on actin when muscle is relaxed.
• Titin: Stabilizes myosin filaments, and elasticity.

Propagation of Action Potential in Skeletal Muscle

• Transverse Tubules (T-tubules): Narrow membranous tunnels continuous with the sarcolemma. Conduct action potential allowing stimulation of voltage-gated Calcium Channels.
• Sarcoplasmic Reticulum: Modified endoplasmic reticulum, surrounding myofibrils. Stores Ca ions.

Skeletal Muscle Innervation

• Innervation: The axon of a motor neuron terminates at a specialized junction known as the neuromuscular junction.

Skeletal Muscle Properties

• Excitability: Tissue's response or reaction to irritation or stimulation. Physicochemical change.
• Contractility: Muscle's response to stimulation. Contraction is change in length or tension of fibers.
• Muscle Tone: Continuous and partial contraction with certain degree of vigor and tension.
• Action potential: Initiated in the transverse tubules, Ca+2 moves from the SR into the cytoplasm, which then causes muscle contraction by stimulating the movement of actin and myosin.

Excitation-Contraction Coupling

• Excitation-Contraction Coupling Steps:(1-3): Acetylcholine (ACh) release, ACh receptor binding, depolarization, voltage-gated channels activating, Ca²⁺ diffuse into cytoplasm stimulating muscle contraction.

Role of Calcium in Muscle Contraction

• Calcium Role: Calcium causes a shift in the troponin complex position on actin filaments. Exposes myosin binding sites, allowing Myosin to bind, cross-bridges form, ADP and Pi release produces power stroke driving muscle contraction.

Cardiac Muscle (Myocardium)

• Cardiac Muscle (Myocardium) Structure: Striated muscle, involuntary, interconnected cells called myocardial cells with intercalated disks. Intercalated disks are gap junctions, allowing electrical impulses to be conducted from cell to cell.
• Pacemaker: Specialized group of cardiac cells that spontaneously depolarize, setting the heart's rate and rhythm.

Cardiac Muscle Properties (Excitable)

• Excitability: The ability of a tissue to respond to a stimulus. Initial response is an electrical event- action potential followed by mechanical event- contraction.
• Rhythmicity: The capacity of cardiac tissues to generate their own impulses and beat regularly.

Cardiac Muscle Properties (Conductive)

• Conductivity: The special quality to transmit electrical impulses throughout the heart from the SA node to the other parts, via the AV node, and bundle of His. and Purkinje fibers.

Cardiac Muscle Properties (Contractility)

• Contractility: The ability of cardiac tissues to shorten in length (contraction). After receiving stimulus.

Smooth Muscle

• Smooth Muscle Presence: Sheets of smooth muscle surround hollow organs and tubes such as the stomach, intestine, urinary bladder, uterus, blood vessels, and airways in lungs.
• Smooth Muscle Function: Contraction of muscles that surround hollow organs may propel contents through the organ, regulating blood flow by changing tubing/tube diameters.
• Smooth Muscle Structure: Lacks sarcomeres, thus lacks striations. Cells are spindle shaped containing one single nucleus. Dense bodies and gap junctions are present.
• Smooth Muscle Control: Controlled by autonomic nervous system, hormones, and other chemical signals. Contraction and relaxation are slow.

This quiz covers the fundamental concepts of muscle physiology, including the types of muscles: skeletal, cardiac, and smooth. It explains the differences between striated and non-striated muscles, as well as voluntary and involuntary control. Understand the locations and functions of each muscle type in the body.