Human Physiology: Skeletal Muscle & Cardiovascular System Study Guide

A. Skeletal Muscle Physiology

Muscle Structure and Function

Skeletal muscle is composed of organized bundles of fibers that contract to produce movement. Understanding the microanatomy and molecular mechanisms is essential for grasping muscle physiology.

• Muscle structure: Muscle → fascicles → muscle fibers → myofibrils → filament proteins (actin and myosin).

• Sarcomere: The functional unit of muscle contraction, defined by Z line, A band, I band, H zone, and M line.

• Proteins: Actin (thin filaments), myosin (thick filaments), troponin, tropomyosin.

• Excitation-contraction coupling: Action potential (AP) travels via T-tubules, causing Ca2+ release from the sarcoplasmic reticulum (SR). Ca2+ binds troponin, moving tropomyosin and exposing actin binding sites.

• Role of ATP: ATP binds to myosin, allowing detachment from actin; hydrolysis of ATP energizes the myosin head for the next power stroke.

• Crossbridge cycle: ATP binding (detaches), ATP hydrolysis (energizes), Pi release (power stroke), ADP release (rigor until ATP binds again).

• Neuromuscular junction: Motor neuron releases acetylcholine (ACh), which binds to receptors on the muscle membrane, initiating contraction.

• Relaxation: Ca2+ is actively pumped back into the SR by Ca2+-ATPases.

Example: During a muscle contraction, the myosin head binds to actin, performs a power stroke, and then detaches when ATP binds, repeating the cycle as long as Ca2+ and ATP are present.

B. Cardiovascular System – Structure & Function

Heart Anatomy and Circulation

The cardiovascular system consists of the heart and blood vessels, responsible for transporting blood throughout the body.

• Heart chambers: Right side → pulmonary circulation; left side → systemic circulation.

• Valves: AV (atrioventricular) valves (tricuspid, mitral); semilunar valves (pulmonary, aortic).

• Cardiac muscle: Specialized for electrical conduction and contraction; contains intercalated discs for rapid signal transmission.

• Conduction system: SA node (pacemaker), AV node (delay), Bundle of His, left/right bundle branches, Purkinje fibers.

Example: The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery, while the left ventricle pumps oxygenated blood to the body via the aorta.

C. Cardiovascular Physiology – Electrical Activity & ECG

Pacemaker Cells and Action Potentials

Electrical activity in the heart is initiated by pacemaker cells, which generate spontaneous action potentials and coordinate contraction.

• Pacemaker cells: Found in the SA node; slow depolarization to threshold via Na+ and Ca2+ channels.

• Action potential phases: Non-pacemaker cells (ventricular muscle) have rapid depolarization, plateau, and repolarization phases.

• ECG (Electrocardiogram): P wave = atrial depolarization; QRS complex = ventricular depolarization; T wave = ventricular repolarization.

Example: The QRS complex on an ECG represents the rapid depolarization of the ventricles, which precedes ventricular contraction.

D. Cardiovascular Physiology – Cardiac Cycle, Hemodynamics, and Control

Phases of the Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat, including contraction (systole) and relaxation (diastole).

• Phases: Ventricular filling (AV valves open), isovolumetric contraction (all valves closed), ventricular ejection (semilunar valves open), isovolumetric relaxation (all valves closed).

• Systole: Contraction; Diastole: Relaxation.

• Heart sounds: S1 (AV valves close), S2 (semilunar valves close).

• Stroke volume (SV): SV = EDV (end diastolic volume) − ESV (end systolic volume).

• Cardiac output (CO): CO = HR × SV.

• Blood flow: Depends on pressure gradient (ΔP) and vessel resistance (R). Poiseuille’s law: $Q = \frac{\Delta P}{R}$

• ANS regulation: Sympathetic (NE, β1 receptors) increases HR and contractility; parasympathetic (ACh, muscarinic receptors) decreases HR.

Example: During exercise, sympathetic stimulation increases heart rate and contractility, raising cardiac output to meet metabolic demands.

Summary Table

Topic	Key Points
Skeletal Muscle	Sarcomere = functional unit; Actin, Myosin, Troponin, Tropomyosin; Crossbridge cycle (ATP hydrolysis → power stroke); Ca2+ release; ACh (motor neuron); Muscle relaxation = Ca2+ reuptake
Excitation-Contraction Coupling	AP → T-tubules → DHPR/ryanodine receptors → SR Ca2+ release → binds troponin → tropomyosin shifts → crossbridge formation
Heart Structure	Right heart = pulmonary; Left heart = systemic; Valves: AV, semilunar; Myocardium; Coronary arteries supply heart muscle
Conduction System	SA node (pacemaker); AV node (delay); Bundle of His; Purkinje fibers; Gap junctions allow rapid conduction
ECG	P wave = atrial depolarization; QRS = ventricular depolarization; atrial repolarization hidden in QRS; T wave = ventricular repolarization
Cardiac Output	CO = HR × SV; Stroke Volume = EDV − ESV; Flow depends on ΔP and vessel radius (radius4); Sympathetic (NE, β1 receptor) ↑HR, Parasympathetic (ACh, mAChRs) ↓HR

Additional info:

• Poiseuille’s Law for blood flow: $Q = \frac{\Delta P \cdot \pi r^4}{8 \eta l}$, where Q = flow, ΔP = pressure difference, r = vessel radius, η = viscosity, l = length.

• ATP hydrolysis is essential for muscle contraction and relaxation.

• Cardiac output increases with exercise due to increased heart rate and stroke volume.