Muscle Physiology, Motor Control, Cardiac Electrophysiology, and Hemodynamics: ANP Study Guide

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Muscle Physiology & Motor Control

Sarcomere Structure & Contractile Proteins

The sarcomere is the fundamental unit of skeletal muscle contraction, composed of organized thick and thin filaments. Its structure is essential for understanding muscle contraction mechanics.

Thick filament: Composed of myosin. Myosin heads contain actin-binding sites and ATP-binding sites.
Thin filament: Composed of actin, tropomyosin, and troponin (regulates contraction).
I band: Contains only thin filaments; shortens during contraction.
A band: Length of thick filament; remains constant during contraction.
H zone: Contains only thick filaments; shortens during contraction.

Example: During contraction, the I band and H zone decrease in width, while the A band remains unchanged.

Sliding Filament Theory

The sliding filament theory describes the process by which muscles contract, involving the interaction of actin and myosin filaments powered by ATP.

ATP binds to myosin, causing detachment from actin.
ATP is hydrolyzed to ADP + Pi, energizing the myosin head.
Myosin binds to actin, forming a cross-bridge.
Power stroke: Myosin pivots, pulling actin toward the M-line; ADP + Pi are released.
Filaments slide past each other, shortening the sarcomere.

Example: The power stroke is the pivotal movement that generates force for muscle contraction.

Excitation–Contraction Coupling (Skeletal Muscle)

Excitation–contraction coupling links the electrical signal from a motor neuron to muscle contraction, primarily through calcium release.

Acetylcholine (ACh) released from motor neuron.
ACh binds to nicotinic receptors, opening ligand-gated Na+ channels.
Na+ influx depolarizes the sarcolemma.
Action potential travels down T-tubules.
Voltage change alters dihydropyridine (DHP) receptors.
Ryanodine receptors open, releasing Ca2+ from the sarcoplasmic reticulum (SR).
Ca2+ binds troponin, shifting tropomyosin.
Cross-bridge cycling begins.

T-tubules: Deliver action potential deep into muscle fiber.

Example: The release of Ca2+ from the SR is the trigger for muscle contraction.

Role of Calcium & ATP

Calcium and ATP are essential for both muscle contraction and relaxation.

Ca2+: Binds to troponin, exposing myosin-binding sites on actin.
ATP: Required for detachment of myosin from actin and for pumping Ca2+ back into the SR via SERCA pumps.
Muscle relaxation: Occurs when Ca2+ is actively pumped back into the SR.

Example: Without ATP, muscles cannot relax, leading to rigor mortis.

Rigor Mortis

Rigor mortis is the postmortem stiffening of muscles due to ATP depletion.

Myosin heads cannot detach from actin without ATP.
Muscles remain contracted and stiff until proteins degrade.

Example: Rigor mortis is used in forensic science to estimate time of death.

Neuromuscular Disorders – Myasthenia Gravis (MG)

Myasthenia gravis is an autoimmune disorder affecting the neuromuscular junction.

Destruction of ACh receptors leads to weak muscle contractions.
Treatment: Acetylcholinesterase (AChE) inhibitors increase ACh levels.

Example: Patients with MG experience muscle fatigue that improves with rest.

Smooth Muscle Contraction

Smooth muscle contraction is regulated differently from skeletal muscle, primarily at the myosin level.

No troponin; Ca2+ binds to calmodulin.
Ca2+–calmodulin complex activates myosin light-chain kinase (MLCK).
MLCK phosphorylates myosin, enabling cross-bridge cycling.
Regulation occurs at myosin, not actin.

Example: Smooth muscle contraction is important in blood vessel regulation and gastrointestinal motility.

Reflexes & Motor Control

Types of Reflexes

Reflexes are rapid, automatic responses to stimuli, crucial for protection and homeostasis.

Stretch reflex: Prevents overstretching (e.g., knee-jerk reflex).
Golgi tendon reflex: Prevents excessive muscle tension.
Withdrawal (flexor) reflex: Removes limb from painful stimulus.
Crossed extensor reflex: Stabilizes opposite limb during withdrawal.

Example: The knee-jerk reflex is used clinically to assess spinal cord function.

Reflex Arc Components

The reflex arc is the neural pathway controlling reflex actions.

Receptor: Detects stimulus.
Sensory neuron: Transmits signal to spinal cord.
Integration center: Processes information (spinal cord).
Motor neuron: Sends command to effector.
Effector: Muscle or gland that responds.

Example: Touching a hot object triggers a withdrawal reflex via this arc.

Muscle Spindles & Golgi Tendon Organs

These sensory receptors regulate muscle activity and prevent injury.

Muscle spindles: Detect stretch, stimulate contraction via alpha motor neurons.
Golgi tendon organs: Detect tension, inhibit contraction to prevent damage.

Example: Muscle spindles are activated during sudden stretching, while Golgi tendon organs protect against overload.

Motor Control & CNS

Motor control is coordinated by various CNS structures.

Basal nuclei: Initiate and regulate movement; dysfunction leads to Parkinson’s disease.
Corticospinal tract: Carries voluntary motor commands from brain to spinal cord.
Interneurons: Prevent antagonistic muscle contraction, ensuring smooth movement.

Example: Damage to the corticospinal tract results in loss of voluntary movement.

Cardiac Electrophysiology & ECG

SA Node Action Potential Phases

The sinoatrial (SA) node is the heart’s natural pacemaker, generating rhythmic action potentials.

Pacemaker potential: Slow Na+ influx via If channels.
Depolarization: Ca2+ influx through L-type channels.
Repolarization: K+ efflux.

SA node sets heart rate; lacks stable resting membrane potential.

Example: The SA node’s automaticity is essential for maintaining a regular heartbeat.

Myocardial Contractile Cell Action Potential Phases

Myocardial contractile cells generate action potentials with a unique plateau phase, ensuring coordinated contraction.

Rapid depolarization (Na+ influx).
Brief repolarization (K+ out).
Plateau phase (Ca2+ in, K+ out).
Final repolarization (K+ out).

Plateau prevents tetany and allows effective pumping.

Example: The plateau phase distinguishes cardiac muscle from skeletal muscle.

Electrocardiogram (ECG)

An ECG records the heart’s electrical activity, providing diagnostic information.

P wave: Atrial depolarization.
QRS complex: Ventricular depolarization.
T wave: Ventricular repolarization.
AV node delay: Allows ventricular filling before contraction.

Example: ECGs are used to diagnose arrhythmias and conduction disorders.

Heart Sounds

Heart sounds are produced by valve closures during the cardiac cycle.

LUB (S1): AV valves close at start of ventricular systole.
DUB (S2): Semilunar valves close at end of systole.

Example: Abnormal heart sounds (murmurs) may indicate valve disease.

Conduction System Disorders

Disorders of the cardiac conduction system affect heart rhythm and rate.

SA node failure: AV node takes over, resulting in slower heart rate.
AV block: Impaired conduction between atria and ventricles; abnormal P-QRS relationship on ECG.

Example: AV block can cause bradycardia and require pacemaker implantation.

Hemodynamics, Blood Pressure & Cardiac Cycle

Blood Pressure Basics

Blood pressure (BP) is the force exerted by blood against vessel walls, determined by cardiac output and peripheral resistance.

BP = CO × Peripheral Resistance
Systolic BP: Pressure during ventricular contraction.
Diastolic BP: Pressure during ventricular relaxation.
Pulse pressure: Difference between systolic and diastolic BP.

Example: Normal BP is typically around 120/80 mmHg.

Cardiac Output (CO)

Cardiac output is the volume of blood pumped by the heart per minute.

CO = HR × SV
SV = EDV − ESV
Stroke volume depends on end-diastolic and end-systolic volumes.

Example: Increased heart rate or stroke volume raises cardiac output.

Formulas:

Preload, Afterload, Contractility

These factors influence cardiac performance and output.

Preload: Degree of ventricular stretch (EDV).
Afterload: Resistance the ventricle must overcome to eject blood.
Contractility: Strength of myocardial contraction, independent of preload and afterload.

Example: Increased preload enhances stroke volume via the Frank–Starling mechanism.

Frank–Starling Law

The Frank–Starling law states that increased ventricular filling (preload) leads to increased stroke volume.

Ensures both ventricles pump equal volumes.

Example: During exercise, increased venous return boosts stroke volume.

Peripheral Resistance

Peripheral resistance is the opposition to blood flow, mainly determined by vessel radius.

Vasodilation decreases resistance and BP.
Vasoconstriction increases resistance and BP.
Small changes in radius cause significant BP changes.

Example: Arterioles regulate blood flow and pressure via changes in diameter.

Formula:

Baroreceptor Reflex

The baroreceptor reflex maintains stable blood pressure by detecting arterial stretch.

Located in carotid sinus and aortic arch.
Low BP triggers increased heart rate and vasoconstriction.

Example: Standing up quickly activates the baroreceptor reflex to prevent fainting.

Cardiac Cycle Events

The cardiac cycle consists of coordinated events ensuring efficient blood flow.

Isovolumetric contraction: AV valves closed; ventricles contract with all valves closed.
Ventricular ejection: Semilunar valves open; blood is pumped out.
Isovolumetric relaxation: All valves closed; ventricles relax.

Example: The cardiac cycle is visualized in pressure-volume loops.

Clinical Correlations

Various conditions and medications affect cardiac function.

Sick sinus syndrome: SA node dysfunction causes abnormal rhythms.
Calcium channel blockers: Decrease contractility and heart rate.
Ivabradine: Blocks If channels, lowering heart rate without affecting contractility.

Example: Ivabradine is used in heart failure management to reduce heart rate.

Summary Table: Muscle Types and Contraction Mechanisms

Muscle Type	Regulatory Protein	Calcium Target	Contraction Regulation
Skeletal	Troponin	Troponin	Actin level
Smooth	Calmodulin	Calmodulin	Myosin level
Cardiac	Troponin	Troponin	Actin level

Additional info: Table summarizes muscle types and their contraction regulation mechanisms for comparison.