unit 13 review
The muscular system's cardiac and smooth muscles play vital roles in our body's function. Cardiac muscle forms the heart walls, pumping blood through the circulatory system. Smooth muscle lines hollow organs, facilitating substance movement through blood vessels, the digestive tract, and other organs.
Cardiac muscle cells are branched and interconnected, allowing coordinated contractions. Smooth muscle cells are spindle-shaped, arranged in parallel arrays. Both muscle types contract involuntarily, with cardiac muscle beating rhythmically and smooth muscle maintaining sustained contractions for organ regulation.
Key Concepts
- Cardiac muscle tissue forms the walls of the heart and generates the contractions that pump blood through the circulatory system
- Smooth muscle tissue lines the walls of hollow organs (blood vessels, digestive tract, uterus) and facilitates movement of substances through these organs
- Cardiac muscle cells are branched and interconnected by intercalated discs which allow for coordinated contraction and rapid transmission of electrical signals
- Smooth muscle cells are spindle-shaped and arranged in parallel arrays or sheets
- Contain a single nucleus and lack striations
- Contraction of cardiac muscle is involuntary, rhythmic, and coordinated by the heart's conduction system
- Smooth muscle contraction is involuntary, slow, and sustained, regulated by the autonomic nervous system, hormones, and local factors
- Calcium plays a crucial role in the contraction of both cardiac and smooth muscle through the sliding filament mechanism
- Disorders affecting cardiac and smooth muscle can lead to various cardiovascular and digestive pathologies (heart failure, hypertension, irritable bowel syndrome)
Muscle Types Overview
- Cardiac muscle is striated, involuntary, and found only in the heart
- Contracts rhythmically and continuously throughout life to pump blood
- Smooth muscle is non-striated, involuntary, and found in the walls of hollow organs
- Contracts slowly and maintains sustained contractions to regulate organ function
- Both cardiac and smooth muscle are under the control of the autonomic nervous system
- Cardiac muscle is autorhythmic, meaning it can generate its own electrical impulses for contraction
- Smooth muscle contraction is regulated by various stimuli (nervous, hormonal, mechanical)
- Cardiac muscle has a high resistance to fatigue due to its rich blood supply and mitochondrial content
- Smooth muscle can maintain prolonged contractions with low energy expenditure
Cardiac Muscle Structure
- Cardiac muscle cells (cardiomyocytes) are shorter and wider than skeletal muscle cells
- Cardiomyocytes are branched and interconnected by intercalated discs
- Intercalated discs contain gap junctions for electrical coupling and desmosomes for mechanical coupling
- Cardiac muscle fibers are arranged in a spiral pattern around the heart chambers
- Cardiomyocytes contain a single, centrally located nucleus
- Cardiac muscle has a striated appearance due to the organization of sarcomeres
- Sarcomeres are the basic contractile units containing thick (myosin) and thin (actin) filaments
- Cardiac muscle is richly supplied with blood vessels (coronary arteries) and mitochondria for energy production
- T-tubules are less developed in cardiac muscle compared to skeletal muscle
Smooth Muscle Structure
- Smooth muscle cells are spindle-shaped and elongated, with tapered ends
- Cells are arranged in parallel arrays or sheets, allowing for coordinated contraction
- Smooth muscle lacks the striated appearance of skeletal and cardiac muscle
- Myofilaments are not organized into sarcomeres
- Each smooth muscle cell contains a single, centrally located nucleus
- Smooth muscle cells are connected by gap junctions, allowing for the spread of electrical signals and coordinated contraction
- Smooth muscle contains actin and myosin filaments, but they are not arranged in a regular pattern
- Dense bodies and intermediate filaments help to anchor the myofilaments and transmit force during contraction
- Smooth muscle cells have a smaller diameter and are more variable in size compared to skeletal and cardiac muscle cells
Contraction Mechanisms
- Cardiac and smooth muscle contraction occurs through the sliding filament mechanism, similar to skeletal muscle
- Myosin heads bind to actin filaments and pull them towards the center of the sarcomere, shortening the muscle fiber
- Calcium plays a crucial role in initiating and regulating muscle contraction
- In cardiac muscle, calcium enters the cell through L-type calcium channels and triggers calcium release from the sarcoplasmic reticulum
- In smooth muscle, calcium enters through voltage-gated calcium channels and is released from the sarcoplasmic reticulum
- Calcium binds to troponin in cardiac muscle, causing a conformational change that allows myosin to bind to actin
- In smooth muscle, calcium binds to calmodulin, activating myosin light chain kinase (MLCK) which phosphorylates myosin and enables cross-bridge cycling
- Removal of calcium from the cytoplasm by active transport mechanisms leads to muscle relaxation
- Cardiac muscle contraction is an all-or-none response, while smooth muscle can generate graded contractions
Regulation and Control
- Cardiac muscle contraction is primarily regulated by the heart's conduction system
- Sinoatrial (SA) node generates the initial electrical impulse, setting the heart rate
- Atrioventricular (AV) node delays the impulse, allowing for sequential contraction of atria and ventricles
- Autonomic nervous system modulates heart rate and contractility
- Sympathetic stimulation increases heart rate and contractility
- Parasympathetic stimulation decreases heart rate
- Smooth muscle contraction is regulated by the autonomic nervous system, hormones, and local factors
- Sympathetic stimulation generally causes smooth muscle contraction (except in blood vessels, where it causes relaxation)
- Parasympathetic stimulation typically causes smooth muscle relaxation
- Hormones (e.g., epinephrine, angiotensin II) can modulate smooth muscle tone
- Local factors (e.g., nitric oxide, endothelin) released by endothelial cells can cause smooth muscle relaxation or contraction
- Stretching of smooth muscle can trigger contraction (myogenic response)
Physiological Functions
- Cardiac muscle contraction is responsible for pumping blood through the circulatory system
- Atrial contraction fills the ventricles with blood
- Ventricular contraction ejects blood into the pulmonary and systemic circulations
- Smooth muscle in blood vessels regulates blood pressure and blood flow distribution
- Vasoconstriction narrows blood vessels, increasing resistance and decreasing blood flow
- Vasodilation widens blood vessels, decreasing resistance and increasing blood flow
- Smooth muscle in the digestive tract (e.g., esophagus, stomach, intestines) facilitates peristalsis and mixing of digestive contents
- Smooth muscle in the respiratory tract (e.g., bronchi, bronchioles) regulates airway diameter and resistance to airflow
- Smooth muscle in the urinary tract (e.g., ureters, bladder) enables urine transport and storage
- Smooth muscle in the reproductive tract (e.g., uterus, vas deferens) is involved in reproductive functions (e.g., labor, ejaculation)
Clinical Significance
- Disorders affecting cardiac muscle can lead to heart failure, arrhythmias, and cardiomyopathies
- Myocardial infarction (heart attack) occurs when coronary blood flow is blocked, leading to cardiac muscle damage
- Atrial fibrillation is a common arrhythmia characterized by rapid, irregular atrial contractions
- Hypertension (high blood pressure) can result from increased smooth muscle tone in blood vessels
- Asthma involves excessive smooth muscle contraction in the airways, leading to bronchoconstriction and difficulty breathing
- Irritable bowel syndrome (IBS) is associated with abnormal smooth muscle function in the digestive tract, causing abdominal pain and altered bowel habits
- Premature labor can occur due to excessive smooth muscle contraction in the uterus
- Erectile dysfunction can result from impaired smooth muscle relaxation in the blood vessels of the penis
- Medications targeting cardiac and smooth muscle function are used to treat various conditions
- Beta-blockers reduce heart rate and contractility in hypertension and heart failure
- Calcium channel blockers relax smooth muscle in blood vessels to treat hypertension and angina
- Bronchodilators relax airway smooth muscle in asthma and chronic obstructive pulmonary disease (COPD)