Cardiac and Skeletal Muscle Physiology: Structure, Function, and Control

Muscle Structure and Contraction

Anatomy of the Sarcomere

The sarcomere is the fundamental contractile unit of striated muscle, including both skeletal and cardiac muscle. It is defined by the region between two Z discs and contains several key components:

• I band: Region containing only thin filaments (actin).

• A band: Region containing the entire length of thick filaments (myosin), including overlapping thin filaments.

• H zone: Central region of the A band with only thick filaments.

• M line: Center of the sarcomere, where thick filaments are anchored.

• Z disc: Defines the boundaries of each sarcomere; anchors thin filaments.

Example: During muscle contraction, the I band and H zone shorten, while the A band remains the same length.

Contraction in Skeletal vs. Cardiac Muscle

• Chronological sequence of contraction: Action potential → Calcium release → Cross-bridge cycling → Muscle contraction.

• Key ions: Calcium (Ca2+) is essential for contraction in both muscle types, but the source and regulation differ.

• Heart muscle: Calcium-induced calcium release from the sarcoplasmic reticulum is triggered by extracellular Ca2+ influx.

• Skeletal muscle: Calcium is released from the sarcoplasmic reticulum via direct coupling to the voltage sensor.

Additional info: Cardiac muscle cells are connected by intercalated discs, allowing coordinated contraction.

Cardiac Physiology and Control

Heart Rate Regulation

Heart rate is controlled by intrinsic and extrinsic factors:

• Intrinsic: Pacemaker cells in the SA node generate spontaneous action potentials.

• Extrinsic: Autonomic nervous system (sympathetic increases, parasympathetic decreases heart rate), hormones (e.g., epinephrine).

Example: During exercise, sympathetic stimulation increases heart rate and contractility.

Blood Pressure and Cardiac Output

• Cardiac output (CO): The volume of blood pumped by the heart per minute.

• Equation: $CO = HR \times SV$ (Heart Rate × Stroke Volume)

• Total peripheral resistance (TPR): Resistance to blood flow in systemic circulation; affects blood pressure.

• Mean arterial pressure (MAP): Average pressure in arteries during one cardiac cycle.

• Equation: $MAP = CO \times TPR$

Action Potentials in Cardiac Cells

• SA node: Initiates the heartbeat; action potentials here are responsible for pacemaking.

• Cardiac muscle cells: Action potentials lead to contraction via calcium influx and release.

• Phases: Rapid depolarization, plateau (unique to cardiac muscle), repolarization.

Channels and Excitation-Contraction Coupling

• Ion channels: Sodium (Na+), calcium (Ca2+), and potassium (K+) channels regulate action potentials and contraction.

• Excitation-contraction coupling: The process by which an action potential leads to muscle contraction.

• Calcium's role: In cardiac muscle, extracellular Ca2+ triggers further Ca2+ release from the sarcoplasmic reticulum.

Example: Drugs that block calcium channels can reduce heart contractility.

Muscle Fiber Types and Control

Motor Units and Recruitment

• Motor unit: A single motor neuron and all the muscle fibers it innervates.

• Recruitment: Increasing the number of active motor units increases muscle force.

• Neuronal control: Autonomic (involuntary) vs. somatic (voluntary) control.

Force Regulation in Muscle

• Length-tension relationship: Optimal sarcomere length allows maximal force generation.

• Frequency of stimulation: Higher frequency leads to summation and increased force.

Calcium Regulation in Muscle Contraction

Role of Calcium in Cardiac vs. Skeletal Muscle

Feature	Cardiac Muscle	Skeletal Muscle
Source of Ca2+	Extracellular influx & SR release	Sarcoplasmic reticulum (SR) only
Excitation-contraction coupling	Ca2+-induced Ca2+ release	Direct coupling via voltage sensor
Duration of contraction	Longer (due to plateau phase)	Shorter

Additional info: Cardiac muscle contraction is more sensitive to changes in extracellular Ca2+ concentration.

Summary Table: Key Differences Between Cardiac and Skeletal Muscle

Characteristic	Cardiac Muscle	Skeletal Muscle
Control	Involuntary (autonomic)	Voluntary (somatic)
Cell connections	Intercalated discs	No intercalated discs
Action potential	Plateau phase present	No plateau phase
Pacemaker cells	Present (SA node)	Absent