Comparison of Cardiac and Skeletal Muscle Physiology

Muscle Physiology: Cardiac vs. Skeletal Muscle

Overview of Muscle Types

Muscle tissue in the human body is classified into three main types: skeletal, cardiac, and smooth muscle. Cardiac muscle is found only in the heart, while skeletal muscle is attached to bones and is responsible for voluntary movements. Understanding the physiological differences between these two muscle types is essential for comprehending their unique roles in the body.

Structural Differences

• Cardiac Muscle: Composed of branched, striated fibers connected by intercalated discs. Each cell typically has one or two centrally located nuclei.

• Skeletal Muscle: Consists of long, cylindrical, multinucleated fibers. The nuclei are located at the periphery of the cell.

Physiological Differences

• Control:

  • Cardiac Muscle: Involuntary control, regulated by the autonomic nervous system and intrinsic pacemaker cells.

  • Skeletal Muscle: Voluntary control, regulated by the somatic nervous system.

• Contraction Initiation:

  • Cardiac Muscle: Action potentials originate from pacemaker cells (e.g., sinoatrial node) and spread via gap junctions.

  • Skeletal Muscle: Action potentials are initiated by motor neuron stimulation at the neuromuscular junction.

• Action Potential Duration:

  • Cardiac Muscle: Longer action potential duration (200–400 ms) due to a plateau phase caused by calcium influx.

  • Skeletal Muscle: Shorter action potential duration (1–5 ms), no plateau phase.

• Refractory Period:

  • Cardiac Muscle: Long refractory period prevents tetanus, ensuring rhythmic contractions.

  • Skeletal Muscle: Short refractory period allows for summation and tetanus.

• Calcium Source for Contraction:

  • Cardiac Muscle: Calcium enters from both the extracellular fluid and the sarcoplasmic reticulum.

  • Skeletal Muscle: Calcium is released almost exclusively from the sarcoplasmic reticulum.

• Energy Supply:

  • Cardiac Muscle: Relies primarily on aerobic metabolism; rich in mitochondria.

  • Skeletal Muscle: Can use both aerobic and anaerobic metabolism, depending on fiber type and activity.

Functional Implications

• Cardiac Muscle: Specialized for continuous, rhythmic contractions to pump blood throughout life.

• Skeletal Muscle: Specialized for rapid, forceful contractions to produce movement and maintain posture.

Comparison Table: Cardiac vs. Skeletal Muscle Physiology

Feature	Cardiac Muscle	Skeletal Muscle
Location	Heart	Attached to bones
Control	Involuntary	Voluntary
Cell Structure	Branched, single/dual nucleus, intercalated discs	Long, cylindrical, multinucleated
Action Potential Duration	Long (200–400 ms)	Short (1–5 ms)
Refractory Period	Long (prevents tetanus)	Short (allows tetanus)
Calcium Source	Extracellular fluid & sarcoplasmic reticulum	Sarcoplasmic reticulum only
Metabolism	Primarily aerobic	Aerobic & anaerobic

Key Terms

• Intercalated discs: Specialized connections between cardiac muscle cells that facilitate synchronized contraction.

• Pacemaker cells: Cells in the heart that generate spontaneous action potentials to regulate heartbeat.

• Refractory period: The time during which a muscle cell cannot be re-excited, important for preventing sustained contractions in the heart.

Example

During exercise, skeletal muscles contract rapidly and can experience fatigue due to lactic acid buildup from anaerobic metabolism. In contrast, cardiac muscle must contract continuously without fatigue, relying on a constant supply of oxygen and nutrients.