Muscle Tissue

Subtypes of Muscle Tissue

Muscle tissue is specialized for contraction and is classified into three main types, each with distinct structure and function:

• Skeletal Muscle: Voluntary, striated muscle attached to bones for movement.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs.

Microscopic Anatomy of Muscle Cells

• Sarcolemma: The plasma membrane of a muscle cell.

• Sarcoplasm: The cytoplasm of a muscle cell, containing organelles and myofibrils.

• Myofibrils: Cylindrical structures within muscle fibers, composed of repeating sarcomeres.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions (Ca2+).

• T-Tubules (Transverse Tubules): Invaginations of the sarcolemma that transmit action potentials into the muscle fiber.

• Triad: A structure formed by a T-tubule flanked by two terminal cisternae of the SR, crucial for excitation-contraction coupling.

Myofilaments

• Thick Filaments (Myosin): Composed of myosin molecules with heads that bind to actin during contraction.

• Thin Filaments (Actin): Composed of actin, tropomyosin, and troponin; actin provides binding sites for myosin.

• Associated Proteins:

  • Troponin: Binds Ca2+ and regulates tropomyosin position.

  • Tropomyosin: Blocks myosin-binding sites on actin in resting muscle.

  • Titin: Provides elasticity and stabilizes myosin.

Sarcoplasmic Reticulum and T-Tubules

• Triad: Consists of one T-tubule and two terminal cisternae of the SR; found at the A-I junction of sarcomeres. The triad ensures rapid transmission of action potentials and synchronized Ca2+ release.

• T-Tubules: Conduct action potentials from the sarcolemma into the muscle fiber's interior.

• Cisternae and SR: Store Ca2+; contain Ca2+ pumps (SERCA) and voltage-gated Ca2+ channels (ryanodine receptors).

• Triad Relationship: T-tubules are closely associated with SR cisternae; depolarization of T-tubules triggers Ca2+ release from SR via voltage sensors and ryanodine receptors.

Sliding Filament Theory of Contraction

• Cross-bridging: Myosin heads bind to actin, forming cross-bridges that pull thin filaments toward the center of the sarcomere.

• Essential Molecules: ATP and Ca2+ are required for contraction.

• Filament Length: Neither thick nor thin filaments shorten; instead, they slide past each other, shortening the sarcomere.

Physiology of Skeletal Muscle Fibers

• Excitation-Contraction Coupling:

  • Acetylcholine (ACh) released from motor neuron binds to nicotinic receptors on the motor end plate, causing Na+ influx and depolarization (end-plate potential).

  • Voltage-gated Na+ channels outside the NMJ propagate the action potential along the sarcolemma and into T-tubules.

  • Action potential triggers Ca2+ release from SR into sarcoplasm.

  • Increased cytosolic Ca2+ allows myosin-actin cross-bridge formation and contraction.

• Speed Comparison: Action potentials are much faster than muscle contraction.

Contraction of Skeletal Muscles

• Motor Unit: A motor neuron and all the muscle fibers it innervates; fibers are spread throughout the muscle.

Muscle Twitch

• Muscle Twitch: A single, brief contraction and relaxation cycle in a muscle fiber following a single action potential.

• Periods:

  • Latent Period: Time between stimulus and onset of contraction.

  • Contraction Period: Cross-bridging and sarcomere shortening occur.

  • Relaxation Period: Ca2+ returns to SR, muscle tension decreases.

• Summation: Increased frequency of stimulation leads to wave (temporal) summation, producing smoother, stronger contractions.

• Tetanus:

  • Unfused (Incomplete) Tetanus: Partial relaxation between stimuli.

  • Fused (Complete) Tetanus: No relaxation; maximal, sustained contraction.

• Recruitment: Increasing the number of active motor units increases force and speed of contraction.

• Size Principle: Smaller, more excitable motor units are recruited first; larger units are recruited as load increases.

Cardiovascular Physiology

Functional Anatomy of the Heart

• Pulmonary Circuit: Right atrium and ventricle pump deoxygenated blood to the lungs via pulmonary arteries; oxygenated blood returns via pulmonary veins.

• Systemic Circuit: Left atrium and ventricle pump oxygenated blood to the body via the aorta; deoxygenated blood returns via venae cavae.

• Coronary Circulation: Supplies heart muscle; arteries form anastomoses (networks) around capillary beds. During diastole, myocardium is perfused.

Heart Valves and Blood Flow

• Atrioventricular (AV) Valves: Prevent backflow from ventricles to atria.

• Semilunar Valves: Prevent backflow from arteries to ventricles.

• Chordae Tendineae: Prevent AV valve prolapse during ventricular contraction.

• Blood Flow Direction: Right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.

Cardiac Muscle Myocardium

• Cardiac muscle cells are arranged in a spiral pattern, enabling efficient chamber contraction.

• The left myocardium is thicker due to higher pressure needed to pump blood systemically.

Cardiac Muscle Cell Types and Properties

• Autorhythmic (Pacemaker) Cells: Generate spontaneous action potentials; set heart rhythm.

• Contractile Cells: Perform mechanical work of contraction.

• Comparison with Skeletal Muscle:

  • Cardiac muscle has longer refractory periods, preventing tetanus.

  • Action potentials in cardiac contractile cells have a plateau phase due to Ca2+ influx.

  • Autorhythmic cells have unstable resting potentials (pacemaker potentials).

Electrical Conduction Through the Heart

• SA Node: Primary pacemaker; initiates heartbeat.

• Conduction Pathway: SA node → AV node → Bundle of His → bundle branches → Purkinje fibers.

• If the SA node fails, the AV node can take over at a slower rate.

• ECG/EKG: Records electrical activity; P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).

• Ventricular contraction follows the QRS complex.

Control of Cardiac Function

• Autonomic Regulation:

  • Sympathetic stimulation increases heart rate and contractility (norepinephrine on β1-adrenergic receptors).

  • Parasympathetic stimulation decreases heart rate (acetylcholine on muscarinic receptors).

• Chronotropic Substances: Affect heart rate.

• Inotropic Substances: Affect contractility.

Heart Measurements and Formulas

• Cardiac Output (CO): Volume of blood pumped per minute.

  • Formula:

• Stroke Volume (SV): Volume of blood pumped per beat.

  • Formula:

• Frank-Starling Law: Increased EDV stretches cardiac muscle, increasing force of contraction and SV.

• Increasing sympathetic activity reduces ESV, increasing SV and CO.

Blood Pressure and Flow

Structure of Blood Vessel Walls

• Tunica Intima: Inner layer; endothelium provides smooth surface for blood flow.

• Tunica Media: Middle layer; smooth muscle controls vessel diameter.

• Tunica Externa (Adventitia): Outer layer; connective tissue provides support.

Vasoconstriction and Vasodilation

• Vasoconstriction: Narrowing of blood vessels, increasing resistance and blood pressure.

• Vasodilation: Widening of blood vessels, decreasing resistance and blood pressure.

Arterial System

• Elastic Arteries: Large, stretch to accommodate blood; contain more elastin.

• Muscular Arteries: Distribute blood to organs; more smooth muscle.

• Arterioles: Regulate blood flow into capillary beds; major site of resistance.

Capillaries

• Composed of a single layer of endothelial cells.

• Types:

  • Continuous: Most common; found in muscle, skin, brain.

  • Fenestrated: More porous; found in kidneys, small intestine.

  • Sinusoidal: Most permeable; found in liver, bone marrow.

• Precapillary sphincters regulate blood flow through capillary beds.

Venous System

• Veins have thinner walls, larger lumens, and valves to prevent backflow.

• Function as blood reservoirs (contain ~60% of blood volume).

• Blood flow aided by skeletal muscle pump and thoracic pump.

Systemic Blood Pressure

• Highest in arteries, drops in arterioles and capillaries, lowest in veins.

• Blood Flow (F): Volume of blood flowing through a vessel per unit time.

• Blood Pressure (P): Force exerted by blood on vessel walls.

• Resistance (R): Opposition to flow; mainly determined by vessel radius.

• Relationship:

• Mean Arterial Pressure (MAP): Average pressure in arteries.

  • Formula:

• Pulse Pressure: Difference between systolic and diastolic pressure.

Factors Influencing MAP

1. Blood Volume: Increased by fluid intake, decreased by fluid loss; kidneys regulate long-term blood volume.

2. Cardiac Output (CO): Increased by higher HR or SV.

3. Peripheral Resistance: Controlled by arteriole diameter (vasoconstriction increases, vasodilation decreases resistance).

4. Blood Viscosity: Higher viscosity increases resistance.

5. Vessel Length: Longer vessels increase resistance.

6. Elasticity of Arteries: Less elasticity increases systolic pressure.

Regulation of Blood Pressure

• Short-term Regulation: Baroreceptor reflex detects changes in pressure; increased pressure increases baroreceptor firing, decreasing sympathetic and increasing parasympathetic activity, lowering HR and vasodilation.

• Low pressure decreases baroreceptor firing, increasing sympathetic activity, raising HR, contractility, and vasoconstriction.

• Adrenergic Receptors: α1 causes vasoconstriction; β2 causes vasodilation.

• Intrinsic vs Extrinsic Factors: Local metabolites (intrinsic) and nervous/hormonal control (extrinsic) regulate tissue perfusion.

Table: Comparison of Arteries and Veins

Key Equations

• Cardiac Output:

• Stroke Volume:

• Flow:

• Mean Arterial Pressure:

Example Applications

• If heart rate increases, cardiac output increases (assuming SV is constant).

• Vasoconstriction increases resistance, raising blood pressure.

• Baroreceptor reflex quickly adjusts blood pressure during posture changes.

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.