Cardiac Muscle: Structure, Function, and Comparison to Skeletal Muscle

Cardiac Muscle

Overview of Muscle Types

Muscle tissue in the human body is classified into three main types: skeletal, cardiac, and smooth muscle. Each type has unique structural and functional characteristics that enable specialized roles in the body.

• Skeletal Muscle: Voluntary, striated muscle attached to bones for movement.

• Cardiac Muscle: Involuntary, striated muscle found only in the heart.

• Smooth Muscle: Involuntary, non-striated muscle found in walls of hollow organs.

Cardiac Muscle: Cellular Organization

Cardiac muscle cells, or cardiomyocytes, are specialized for continuous, rhythmic contraction to pump blood throughout the body. Their unique structure supports this function.

• Location: Found exclusively in the heart wall.

• Cell Shape: Shorter fibers than skeletal muscle; often branched.

• Nuclei: Uni- or bi-nucleate (1 or 2 nuclei per cell).

• Intercalated Discs: Specialized junctions connecting adjacent cells, containing desmosomes (for mechanical strength) and gap junctions (for electrical connectivity).

• Mitochondria: Numerous, supporting high energy demand.

Desmosomes are crucial for resisting mechanical stress during contraction, while gap junctions allow ions to flow between cells, enabling synchronized contraction.

Comparison: Cardiac vs. Skeletal Muscle

While both cardiac and skeletal muscle are striated and rely on similar contractile proteins, they differ in several key aspects:

• Fiber Structure: Cardiac fibers are shorter, branched, and interconnected; skeletal fibers are long and cylindrical.

• Cellular Junctions: Cardiac muscle has intercalated discs; skeletal muscle does not.

• Nuclei: Cardiac cells are uni- or bi-nucleate; skeletal muscle cells are multinucleate.

• Control: Cardiac muscle is involuntary and modulated by the autonomic nervous system; skeletal muscle is voluntary and controlled by the somatic nervous system.

• Calcium Source: Cardiac muscle uses both extracellular and sarcoplasmic reticulum (SR) calcium; skeletal muscle relies mainly on SR calcium.

Cardiac Muscle: Action Potentials

Cardiac muscle cells generate action potentials (APs) that are essential for coordinated contraction. These APs have unique features compared to those in skeletal muscle and neurons.

• Resting Membrane Potential: Approximately -90 mV in cardiac muscle cells.

• Action Potential Duration: Cardiac APs are much longer (~300 ms) than skeletal muscle APs, due to a prolonged "calcium plateau" phase.

• Phases of Cardiac AP:

  1. Rapid depolarization (Na+ influx)

  2. Plateau phase (Ca2+ influx balances K+ efflux)

  3. Repolarization (K+ efflux)

• Refractory Period: Long absolute refractory period prevents tetanus, ensuring rhythmic contractions.

Equation: The Nernst equation can be used to calculate equilibrium potentials for ions:

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

Excitation-Contraction Coupling in Cardiac Muscle

Excitation-contraction coupling describes the process by which an action potential leads to muscle contraction. In cardiac muscle, this process involves both the sarcoplasmic reticulum and extracellular calcium.

• Trigger: Action potential opens voltage-gated L-type Ca2+ channels (DHPRs) in the cell membrane.

• Calcium-Induced Calcium Release (CICR): Influx of Ca2+ triggers further release from the SR via ryanodine receptors (RyRs).

• Contraction: Increased cytosolic Ca2+ binds to troponin, initiating contraction.

• Relaxation: Ca2+ is pumped back into the SR and out of the cell, allowing relaxation.

Comparison Table: Excitation-Contraction Coupling

Feature	Skeletal Muscle	Cardiac Muscle
AP Source	Somatic motor neuron	Pacemaker cells & autonomic input
Calcium Source	Sarcoplasmic reticulum (SR)	SR & extracellular
Junctions	No intercalated discs	Intercalated discs (desmosomes & gap junctions)
Refractory Period	Short	Long (prevents tetanus)

Regulation of Cardiac Muscle Contraction

Cardiac muscle contraction is modulated by the autonomic nervous system:

• Sympathetic Stimulation: Norepinephrine (NE) and epinephrine increase heart rate and contractility.

• Parasympathetic Stimulation: Acetylcholine (ACh) decreases heart rate.

Pacemaker cells in the heart generate spontaneous action potentials, setting the rhythm of contraction. The autonomic nervous system can upregulate or downregulate this intrinsic rate.

Key Concepts Summary

• Cardiac muscle cells are structurally similar to skeletal muscle but possess unique features (intercalated discs, branching, uni-/bi-nucleate).

• Action potentials in cardiac muscle have a prolonged plateau phase due to Ca2+ influx, resulting in a long refractory period.

• Excitation-contraction coupling in cardiac muscle relies on both extracellular and SR calcium.

• Cardiac muscle contraction is regulated by both intrinsic pacemaker activity and autonomic nervous system input.