BIO 252 Anatomy and Physiology Module 3
AP 3– Cardiovascular System
3.1 – Microscopic Anatomy
Cardiac Muscle Fibers
● Myocardium
○ Cardiac muscle tissue found only in the heart
○ Autorythmicity– ability to initiate its own electrical potential at a fixed rate that spreads
rapidly from cell to cell to trigger the contractile mechanism
■ Neither skeletal or smooth muscle can do this
○ Heart rate is modulated by input from the endocrine and nervous systems
Two Major Types of Cardiac Muscle Cells
● Myocardial contractile cells
○ 99% of cells
○ Responsible for contractions that pump blood flow through the body
○ Typically 1 nucleus
○ Internal membranes
■ Short T tubules
○ Active metabolism is aerobic
■ Primarily using lipids and carbohydrates
● Myocardial conducting cells
○ 1% of cells
○ Generally much smaller
■ Only have a few of of the myofibrils or filaments needed for contraction
○ Form the conduction system of the heart
○ Functions similar to neurons
■ Purkinje cells
● Specialized conducting cells that carry electrical impulse through to the ventricles
● Initiate and propagate the action potential (the electrical impulse) that travels throughout the
heart and triggers the contractions that propel the blood
Cardiomyocytes
● Cardiac muscle cells are called cardiomyocytes
● Branched structure
● Usually 1 central nucleus and many mitochondria
● Each cardiomyocyte thousands of myofibrils
,Myofibril
● Long chain of many links
● Long thin thread inside the muscle
● Made up of sarcomeres arranged end to end
● Myofibril contain sarcomeres
Sarcomere
● One link
● Smallest functional unit of a muscle contraction
● Runs from Z-line to Z-line
○ Z-line to Z-line is one contractile unit of a cardiomyocyte
○ As a muscle contraction is initiated, the Z-lines are pulled together and the muscle
cell shortens ● Contains actin (thin) and myosin (thick) filaments arranged in a repeating
pattern
● During contraction
○ Z-lines move together
○ All sarcomeres shorten → myofibril shortens → cell contracts
Structure of Cardiac Muscle vs Skeletal Muscle
● Similarities
○ Both contain sarcomeres that give them a striated appearance
○ Both use actin and myosin sliding filament mechanism
● Differences
○ Cardiomyocytes are shorter, branched, and connected by intercalated discs
○ Cardiomyocytes have…
■ Smaller T-tubules located at Z-lines
■ Fewer SR calcium stores
T-Tubules & Sarcoplastic Reticulum (SR)
● T Tubules
○ Larger in diameter but fewer in number
○ Located at Z-lines
○ Rapidly transmit action potentials deep into the cell
● SR
○ Stores less calcium than skeletal muscle
■ Thus, contraction relies on external Ca2+ from extracellular fluid
● This results in slower onset of contractions and longer contraction durations
Intercalated Discs
● Key feature of cardiac muscle
, ● They include…
○ Desmosomes
■ Anchor cells together
■ Prevent cells from pulling apart during forceful contractions
○ Gap Junctions
■ Allow ions to pass between cells
■ Enable cells to synchronize electrical impulses
■ Important for the heart functioning as a syncytium (unit)
Functional Features of Cardiac Muscle
● Contracts automatically (autorhythmicity)
● Cells electrically connected → heart contracts as a coordinated unit
● High mitochondria → fatigue-resistant
● Long refractory period → prevents tetany (sustained contraction)
Key Roles of Calcium in Cardiac Muscle
● Ca2+ ions have 2 essential roles in a heart
contraction ○ Initiate electrical activity /
conduction
■ Ca2+ entry during depolarization helps sustain the plateau phase of the cardiac
action potential
■ Slow Ca2+ entry → long refractory period → prevents tetany and allows the heart
to refill
○ Trigger contraction inside the cell
■ Ca2+ binds to troponin, shifting tropomyosin → exposes myosin-binding sites →
contraction begins
Steps of Excitation– Contraction Coupling
1. Action potential arrives
a. Depolarization spreads along sarcolemma and down T-tubules
2. Calcium enters the cell
a. Ca2+ moves through slow voltage-gated Ca2+ channels
b. Additional Ca2+ is released from the sarcoplasmic reticulum
i. Ca2+ induced Ca2+ release
3. Ca2+ binds to troponin
a. Causes conformational chane in the troponin-tropomyosin complex
b. Tropomyosin moves → myosin-binding sites on actin are exposed
4. Cross-bridge cycle begins
a. Myosin binds actin → power stroke → filaments slide → sarcomere shortens
b. Repeated cross-bridge formation/detachment continues for the contraction
AP 3– Cardiovascular System
3.1 – Microscopic Anatomy
Cardiac Muscle Fibers
● Myocardium
○ Cardiac muscle tissue found only in the heart
○ Autorythmicity– ability to initiate its own electrical potential at a fixed rate that spreads
rapidly from cell to cell to trigger the contractile mechanism
■ Neither skeletal or smooth muscle can do this
○ Heart rate is modulated by input from the endocrine and nervous systems
Two Major Types of Cardiac Muscle Cells
● Myocardial contractile cells
○ 99% of cells
○ Responsible for contractions that pump blood flow through the body
○ Typically 1 nucleus
○ Internal membranes
■ Short T tubules
○ Active metabolism is aerobic
■ Primarily using lipids and carbohydrates
● Myocardial conducting cells
○ 1% of cells
○ Generally much smaller
■ Only have a few of of the myofibrils or filaments needed for contraction
○ Form the conduction system of the heart
○ Functions similar to neurons
■ Purkinje cells
● Specialized conducting cells that carry electrical impulse through to the ventricles
● Initiate and propagate the action potential (the electrical impulse) that travels throughout the
heart and triggers the contractions that propel the blood
Cardiomyocytes
● Cardiac muscle cells are called cardiomyocytes
● Branched structure
● Usually 1 central nucleus and many mitochondria
● Each cardiomyocyte thousands of myofibrils
,Myofibril
● Long chain of many links
● Long thin thread inside the muscle
● Made up of sarcomeres arranged end to end
● Myofibril contain sarcomeres
Sarcomere
● One link
● Smallest functional unit of a muscle contraction
● Runs from Z-line to Z-line
○ Z-line to Z-line is one contractile unit of a cardiomyocyte
○ As a muscle contraction is initiated, the Z-lines are pulled together and the muscle
cell shortens ● Contains actin (thin) and myosin (thick) filaments arranged in a repeating
pattern
● During contraction
○ Z-lines move together
○ All sarcomeres shorten → myofibril shortens → cell contracts
Structure of Cardiac Muscle vs Skeletal Muscle
● Similarities
○ Both contain sarcomeres that give them a striated appearance
○ Both use actin and myosin sliding filament mechanism
● Differences
○ Cardiomyocytes are shorter, branched, and connected by intercalated discs
○ Cardiomyocytes have…
■ Smaller T-tubules located at Z-lines
■ Fewer SR calcium stores
T-Tubules & Sarcoplastic Reticulum (SR)
● T Tubules
○ Larger in diameter but fewer in number
○ Located at Z-lines
○ Rapidly transmit action potentials deep into the cell
● SR
○ Stores less calcium than skeletal muscle
■ Thus, contraction relies on external Ca2+ from extracellular fluid
● This results in slower onset of contractions and longer contraction durations
Intercalated Discs
● Key feature of cardiac muscle
, ● They include…
○ Desmosomes
■ Anchor cells together
■ Prevent cells from pulling apart during forceful contractions
○ Gap Junctions
■ Allow ions to pass between cells
■ Enable cells to synchronize electrical impulses
■ Important for the heart functioning as a syncytium (unit)
Functional Features of Cardiac Muscle
● Contracts automatically (autorhythmicity)
● Cells electrically connected → heart contracts as a coordinated unit
● High mitochondria → fatigue-resistant
● Long refractory period → prevents tetany (sustained contraction)
Key Roles of Calcium in Cardiac Muscle
● Ca2+ ions have 2 essential roles in a heart
contraction ○ Initiate electrical activity /
conduction
■ Ca2+ entry during depolarization helps sustain the plateau phase of the cardiac
action potential
■ Slow Ca2+ entry → long refractory period → prevents tetany and allows the heart
to refill
○ Trigger contraction inside the cell
■ Ca2+ binds to troponin, shifting tropomyosin → exposes myosin-binding sites →
contraction begins
Steps of Excitation– Contraction Coupling
1. Action potential arrives
a. Depolarization spreads along sarcolemma and down T-tubules
2. Calcium enters the cell
a. Ca2+ moves through slow voltage-gated Ca2+ channels
b. Additional Ca2+ is released from the sarcoplasmic reticulum
i. Ca2+ induced Ca2+ release
3. Ca2+ binds to troponin
a. Causes conformational chane in the troponin-tropomyosin complex
b. Tropomyosin moves → myosin-binding sites on actin are exposed
4. Cross-bridge cycle begins
a. Myosin binds actin → power stroke → filaments slide → sarcomere shortens
b. Repeated cross-bridge formation/detachment continues for the contraction
