Muscle Tissue

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is classified into three main subtypes, 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; located at the A-I junction of sarcomeres. This arrangement ensures rapid transmission of action potentials and synchronized Ca2+ release.

• T-Tubules: Conduct action potentials deep into the muscle fiber, triggering Ca2+ release from the SR.

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

• Triad Relationship: T-tubule depolarization activates voltage sensors (dihydropyridine receptors), which mechanically interact with ryanodine receptors on the SR, causing Ca2+ release.

Sliding Filament Theory of Contraction

The sliding filament theory explains how muscles contract by the sliding of thin (actin) filaments past thick (myosin) filaments, shortening the sarcomere without changing the length of the filaments themselves.

• Cross-bridging: The process where myosin heads bind to actin, forming cross-bridges and pulling actin filaments inward.

• Essential Molecules: ATP (energy source) and Ca2+ (regulatory ion).

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

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, leading to contraction.

  • Contraction is faster than the action potential; relaxation occurs as Ca2+ is pumped back into the SR.

Contraction of Skeletal Muscles

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

Muscle Twitch and Summation

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

• Periods of Twitch:

  • Latent Period: Time between stimulus and contraction onset.

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

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

• Summation: Increased frequency of stimulation leads to greater tension (temporal/wave summation).

• Tetanus:

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

  • Fused (Complete) Tetanus: No relaxation; 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 blood to heart muscle; arteries form anastomoses (networks) for redundancy.

• Perfusion: Myocardium is perfused mainly during diastole.

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 Structure and Function

• Arrangement: Cardiac muscle cells are branched and interconnected, forming a functional syncytium for coordinated contraction.

• Left Myocardium: Thicker due to higher pressure needed to pump blood systemically.

• Cell Types: Autorhythmic (pacemaker) cells and contractile cells.

• Comparison to Skeletal Muscle: Cardiac muscle has longer refractory periods, preventing tetanus; action potentials differ in duration and ion channel involvement.

Cardiac Action Potentials

• Autorhythmic Cells: Generate spontaneous action potentials (pacemaker potential) due to "funny" Na+ channels and Ca2+ influx.

• Contractile Cells: Action potential includes rapid depolarization (Na+ influx), plateau phase (Ca2+ influx), and repolarization (K+ efflux).

• Refractory Period: Longer in cardiac muscle, preventing summation and tetanus.

Electrical Conduction Through the Heart

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

• Pacemaker Cells: SA node sets the rhythm; if SA node fails, AV node or Purkinje fibers can take over at slower rates.

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

• Cell Connections: Gap junctions in intercalated discs allow action potential spread between cells.

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: The heart pumps more forcefully when filled with more blood (increased EDV increases SV).

• Increasing CO: Increase HR or SV; decrease ESV via sympathetic stimulation.

Blood Pressure and Flow

Structure of Blood Vessel Walls

• Tunica Intima: Inner layer; smooth endothelium reduces friction.

• Tunica Media: Middle layer; smooth muscle and elastic fibers for vasoconstriction/dilation.

• Tunica Externa (Adventitia): Outer layer; connective tissue for support and protection.

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

• Types of Arteries:

  • Elastic Arteries: Large, with many elastic fibers; dampen pressure fluctuations.

  • Muscular Arteries: More smooth muscle; distribute blood to organs.

  • Arterioles: Smallest arteries; regulate blood flow into capillary beds.

Capillaries

• Structure: Single layer of endothelial cells; thin walls for exchange.

• Types:

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

  • Fenestrated: Pores for filtration; found in kidneys, small intestine.

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

• Precapillary Sphincters: Regulate blood flow into capillary beds.

Venous System

• Veins: Thinner walls, larger lumens, valves to prevent backflow; act as blood reservoirs (contain ~60% of blood volume).

• Blood Flow in Veins: Assisted by skeletal muscle pump, respiratory pump, and venous valves.

Systemic Blood Pressure

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

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

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

• Mean Arterial Pressure (MAP):

  • Formula:

  • Or: (DP = diastolic pressure, SP = systolic pressure)

• 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).

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 Vessels: Less elasticity (as in aging) increases MAP.

Regulation of Blood Pressure

• Short-term Regulation: Baroreceptor reflex detects changes in pressure and adjusts sympathetic/parasympathetic output.

• Sympathetic Stimulation: Increases HR, contractility, and vasoconstriction (via α1-adrenergic receptors).

• Parasympathetic Stimulation: Decreases HR.

• Baroreceptor Reflex:

  • High BP: Baroreceptors fire more, decrease sympathetic and increase parasympathetic activity, lowering BP.

  • Low BP: Baroreceptors fire less, increase sympathetic and decrease parasympathetic activity, raising BP.

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

Comparison Table: Arteries vs Veins

Key Equations

• Cardiac Output:

• Stroke Volume:

• Mean Arterial Pressure:

• Pulse Pressure:

Examples and Applications

• Increasing HR increases CO and thus MAP, assuming SV and resistance remain constant.

• Vasoconstriction increases resistance, raising MAP.

• Frank-Starling Law: If venous return increases (e.g., via muscular pump), EDV increases, leading to greater SV and CO.

Additional info: Where content was brief or implied, standard academic explanations were added for completeness and clarity.