Cardiac Muscle Anatomy and Function

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Questions and Answers

What triggers the opening of ryanodine receptors (RyR) in cardiac muscle cells?

Stretching of muscle fibers

Influx of sodium ions

Influx of calcium ions (correct)

Release of acetylcholine

Which characteristic is NOT true of smooth muscle fibers?

They are uninucleate.

They contain sarcomeres. (correct)

They are under involuntary control.

They have a spindle shape.

How does calcium ion removal occur during relaxation in cardiac muscle?

Only through the Na+-Ca2+ exchanger (NCX)

By both Ca2+-ATPase pump and NCX (correct)

Exclusively via diffusion out of the cell

Only via Ca2+-ATPase pump into the extracellular fluid

What defines the absolute refractory period in cardiac muscle action potentials?

It prevents further stimulation to allow effective pumping. Signup and view all the answers

What primarily distinguishes cardiac muscle from skeletal muscle?

Cardiac muscle fibers are connected via gap junctions Signup and view all the answers

Which muscle type exhibits graded contraction force in response to stimuli?

Cardiac and smooth muscles Signup and view all the answers

Which of the following is true regarding the initiation of muscle contraction in skeletal muscle?

It requires acetylcholine from the motor neuron. Signup and view all the answers

Which statement about autorhythmic cells in cardiac muscle is true?

They initiate contraction signals automatically Signup and view all the answers

Which feature is characteristic of both skeletal and cardiac muscle but not smooth muscle?

Presence of troponin Signup and view all the answers

How do cardiac muscle cells behave during action potential generation?

All cells in the myocardium can be stimulated by a single action potential Signup and view all the answers

What is one major difference in the excitation-contraction coupling of cardiac muscle compared to skeletal muscle?

Calcium-induced calcium release is unique to cardiac muscle Signup and view all the answers

What role does calcium play in the contraction of smooth muscle?

It activates calmodulin to facilitate contraction. Signup and view all the answers

Which statement regarding the external structure of cardiac muscle is true?

Cardiac muscle fibers form branched structures. Signup and view all the answers

Which characteristic of cardiac muscle fibers allows them to link together electrically?

Presence of intercalated discs Signup and view all the answers

In terms of contraction speed, how does cardiac muscle compare to skeletal and smooth muscles?

Cardiac muscle contracts at an intermediate speed. Signup and view all the answers

What role does the autonomic nervous system play in cardiac muscle function?

It regulates heart rate and modifies pacemaker activity Signup and view all the answers

What is a primary function of the pacemaker cells in cardiac muscle?

To generate regular electrical signals for contraction Signup and view all the answers

Which of the following best describes the sarcoplasmic reticulum in cardiac muscle compared to skeletal muscle?

Smaller and less well-organized Signup and view all the answers

What is the primary reason for the high mitochondrial content in cardiac muscle cells?

To ensure adequate ATP production for continuous contraction Signup and view all the answers

What type of muscle is primarily responsible for involuntary reflexes in the body?

Smooth muscle Signup and view all the answers

Study Notes

Cardiac Muscle

Cardiac muscle shares characteristics of skeletal and smooth muscle.
It is striated, with myosin and actin filaments forming sarcomeres.
Contraction occurs via sliding thin filaments.
Unlike skeletal muscle fibers, cardiac muscle fibers are short, branched, and connected by intercalated discs (gap junctions).

Cardiac Muscle Cells Contract Without Innervation

Autorhythmic cells (pacemakers) generate signals for contraction, smaller than and with fewer contractile fibers compared to contractile cells.
They lack organized sarcomeres.
Contractile cells are striated and organized into sarcomeres.
Cardiac muscle cells are smaller than skeletal muscle cells, with a single nucleus per fiber.
They branch and connect to neighboring cells via intercalated discs.
Gap junctions allow electrical signal transmission between cells.
T-tubules are larger and branch more extensively than in skeletal muscle.
The sarcoplasmic reticulum is smaller in cardiac muscle than in skeletal muscle.
Mitochondria occupy one-third of the cell volume.

Cellular Structure of Cardiac Muscle

Cardiac muscle displays striations under a microscope.
Intercalated discs are visible between adjacent cardiac muscle cells.
Mitochondria are present, contributing significantly to the cell's volume.
Gap junctions and desmosomes are visible structural features of intercalated discs.

Myocardium

Myocardium is a mass of cardiac muscle cells interlinked by gap junctions.
Action potentials in one cardiac muscle cell stimulate all cells in the myocardium, creating a functional syncytium similar to single-unit smooth muscle.
The atria and ventricles of the heart have separate myocardia.

Pacemaker Potential

Cardiac muscle can generate action potentials automatically without nerve stimulation, originating in the pacemaker region.
The heart rate is influenced by autonomic nervous system input and hormones.

Excitation-Contraction Coupling in Cardiac Muscle

Action potentials from adjacent cells trigger voltage-gated Ca²⁺ channels (DHPR) opening.
This initiates calcium release from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyR).
Local Ca²⁺ release ("Ca²⁺ spark") leads to a larger rise in intracellular Ca²⁺.
Ca²⁺ binds to troponin, initiating contraction.
Relaxation occurs when Ca²⁺ is pumped back into the SR.
Calcium is exchanged with sodium using the Na⁺/Ca²⁺ exchanger (NCX).

Calcium Entry in Cardiac EC Coupling

Action potential begins in pacemaker cells.
Ca²⁺-induced Ca²⁺ release happens (different from skeletal muscle)
Voltage-gated L-type Ca²⁺ channels (DHPR) open in the cell membrane.
Ryandanine receptors open in the SR in response to Ca²⁺ inflow.
The release of Ca²⁺ from the SR occurs as "Ca²⁺ sparks."
Summed sparks form a Ca²⁺ signal leading to contraction via Ca²⁺ binding to troponin.
Relaxation removes Ca²⁺ from the cytoplasm.
Ca²⁺ is reabsorbed into the SR using Ca²⁺-ATPase pumps.
Excess Ca²⁺ is removed through the Na⁺-Ca²⁺ exchanger (NCX).

Action Potentials: Skeletal vs Cardiac Muscle

Cardiac muscle APs have a long absolute refractory period (~250ms) preventing tetanus, crucial for maintaining efficient heart pumping.
Skeletal muscle APs have a much shorter refractory period, allowing for tetanus.

Duration of Muscle Contraction

Contraction durations differ across muscle types (skeletal, cardiac, smooth). Cardiac muscle contraction duration is intermediate between skeletal and smooth.

Comparison of Muscle Types

Skeletal Muscle, Cardiac Muscle, and Smooth Muscle differ greatly in structure, function, control and initiation.
Cardiac Muscle is Striated, has intercalated discs that interconnect cells, uses Ca²⁺ and calmodulin, has intrinsic pacemaker activity, and is influenced by hormones.

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Description

Explore the unique features of cardiac muscle, which shares characteristics with both skeletal and smooth muscle. This quiz covers aspects such as the structure of cardiac muscle cells, contraction mechanisms, and the role of autorhythmic cells in heart function.