Muscle Physiology

Phases of Muscle Contraction

Muscle contraction occurs in four distinct phases, each involving specific cellular and molecular events that enable skeletal muscle fibers to generate force and movement.

• Excitation: The action potential from a motor neuron arrives at the axon terminal, causing the release of acetylcholine (ACh) into the synaptic cleft. ACh binds to receptors on the muscle fiber, opening Na+ channels and depolarizing the motor end plate. This generates an action potential in the sarcolemma.

• Excitation-Contraction Coupling: The action potential travels down T-tubules, triggering the release of Ca2+ ions from the sarcoplasmic reticulum. Ca2+ binds to troponin, causing a conformational change in the troponin-tropomyosin complex, which exposes active sites on actin filaments.

• Contraction Cycle: Myosin heads bind to exposed actin sites, forming cross-bridges. The myosin head pivots, using energy from ATP hydrolysis, resulting in the power stroke that moves actin filaments. After the power stroke, ADP and phosphate are released, and a new ATP binds to myosin, causing detachment from actin. The cycle repeats as long as Ca2+ and ATP are present.

• Relaxation: Ca2+ is actively transported back into the sarcoplasmic reticulum, and troponin-tropomyosin returns to its resting position, covering actin's active sites. The muscle fiber returns passively to its resting length.

Example: Skeletal muscle contraction during voluntary movement, such as lifting an object.

Cardiovascular System

Conducting System of the Heart

The heart's conducting system coordinates the rhythmic contraction of cardiac muscle, ensuring efficient blood flow.

• Sinoatrial (SA) node: The primary pacemaker, located in the right atrium, initiates electrical impulses.

• Atrioventricular (AV) node: Receives impulses from the SA node and delays transmission to allow atrial contraction.

• AV bundle (Bundle of His): Conducts impulses from the AV node to the ventricles.

• Bundle branches and Purkinje fibers: Distribute impulses throughout the ventricles, triggering coordinated contraction.

Example: The normal heartbeat relies on the sequential activation of these components.

Heart Valves: Structure and Function

Heart valves ensure unidirectional blood flow and prevent backflow during the cardiac cycle.

• Atrioventricular (AV) valves: Located between atria and ventricles (tricuspid on the right, mitral on the left). They have cusps and are attached to chordae tendineae and papillary muscles, which prevent prolapse during ventricular contraction.

• Semilunar valves: Located between ventricles and major arteries (pulmonary and aortic valves). They prevent backflow from arteries into ventricles and do not have muscular braces.

Function Table:

Respiratory System

Trachea and Alveolar Structure

The trachea and alveoli are specialized for efficient air conduction and gas exchange.

• Trachea: Lined with ciliated epithelium and supported by cartilaginous rings to maintain airway patency.

• Alveoli: Consist of type I pneumocytes (thin, flat cells for gas exchange), type II pneumocytes (secrete surfactant to reduce surface tension), and alveolar macrophages (remove debris and pathogens).

Efficient Gas Exchange Features:

• Thin barrier (type I cells + capillary endothelium)

• Large surface area

• Moist surface with surfactant

• Short diffusion distance for O2 and CO2

Example: Rapid oxygen uptake in the lungs during exercise.

Digestive System

Gastric Glands and Cell Types

The stomach contains specialized cells in its gastric glands, each with distinct functions in digestion.

• Stem cells: Regenerate epithelial lining.

• Parietal cells: Secrete hydrochloric acid and intrinsic factor (for vitamin B12 absorption).

• Chief cells: Secrete pepsinogen, an inactive enzyme converted to pepsin by HCl.

• Mucous neck cells: Secrete mucus to protect the stomach lining.

• Enteroendocrine cells: Secrete gastrin, stimulating HCl secretion and gastric motility.

Example: Pepsinogen activation to pepsin for protein digestion.

Small and Large Intestine Specialization

The intestines are specialized for absorption and processing of nutrients and waste.

• Small intestine: Contains permanent circular folds, villi, and microvilli to increase surface area for absorption. Rich blood and lymph supply for efficient transport.

• Large intestine: Has three longitudinal muscle bands (teniae coli) forming haustra. Specialized for water and electrolyte absorption, contains bacteria for fermentation and vitamin production.

Example: Absorption of glucose and amino acids in the small intestine.

Renal System

Nephron Structure and Function

The nephron is the functional unit of the kidney, responsible for filtration and regulation of fluid and electrolyte balance.

• Parts of the nephron: Includes Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.

• Cells targeted by aldosterone: Principal cells in the collecting duct and DCT.

Nephron Table:

Aldosterone Effects

Aldosterone, produced by the adrenal cortex, regulates sodium and potassium balance.

• Increases sodium reabsorption and potassium secretion in principal cells.

• Promotes water retention, increasing blood volume and pressure.

• Stimulates synthesis of Na+ pumps and channels.

Equation:

Reproductive System

Male and Female Reproductive Tract Structures

The reproductive system includes specialized organs for gamete production and transport.

• Male: Testes, epididymis, vas deferens, seminal vesicles, prostate gland, urethra, penis.

• Female: Ovaries, fallopian tubes, uterus, cervix, vagina.

Example: Sperm maturation in the epididymis; ovum transport in the fallopian tube.

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for exam preparation.