Study Notes on Cardiac Anatomy and Electrical Activity

Overview of Cardiac Anatomy and Function

The human heart consists of four chambers: left atrium, left ventricle, right atrium, right ventricle.
Interventricular spaces separate the ventricles.
The anatomical orientation is critical for understanding blood flow.

Common slang name for the left anterior descending artery (LAD) is the "Widow Maker".
- Reason for designation: Blockage in this artery often leads to heart attacks and is a common death cause post-heart attack.
- The left ventricle is crucial because it pumps blood to the entire body, unlike other chambers.

The atria have thin walls as they only move blood between chambers using gravitational assistance under normal circumstances.
The right ventricle has thicker walls compared to the atria, and the left ventricle is the thickest due to its extensive workload.
- Left ventricle primarily pumps blood from the head to toe.
Damage to the left ventricle results in significant detrimental effects on overall body blood supply.

The Q wave and S wave appear as downward deflections due to the direction of current flow in the heart, unrelated to intensity.
Understanding the electrical activity in cells requires knowledge of action potentials.

Action Potential: A rapid rise and fall in voltage across a cell membrane.
- Threshold potential: Around -70 mV, where an influx of sodium ($Na^+$) occurs through voltage-gated sodium channels.
- Sodium's role: Sodium is a cation that enters the cell and contributes to the membrane potential change.

Ion movement across membranes: Requires channels due to lipid nature of cell membranes; ions cannot diffuse freely.
- Gated channels (ligand, voltage, mechanically gated) determine ion flow.
- Sodium channels, when opened (usually by ligand binding), allow sodium ions to enter.

Resting potential and depolarization process depend on the influx of Na+.
Ligand-gated channels allow a small amount of sodium to enter first, triggering further depolarization and opening of voltage-gated channels.
Voltage Gated Channels: Characterized by their activation and inactivation states, crucial for action potential flow.

Rapid Depolarization: Sodium flows into the cell, causing a steep increase in membrane potential.
- Peaks at approximately +35 mV.
Plateau Phase: Characteristic of cardiac myocytes, balances calcium inflow and potassium outflow, maintaining contraction without a quick decline.
Repolarization: Initiated by the closure of calcium channels and the opening of potassium channels, restoring the negative membrane potential.

Cardiac muscle cells utilize calcium-induced calcium release for contraction.
Contraction allows for efficient blood pumping as heart fills and ejects blood.

Cardiac contraction functions as twitch cycles due to action potentials.
Two important phases: Systole (contraction) and Diastole (relaxation) are integral to the cardiac cycle.

Absolute refractory period: Phase when no new action potentials can be generated.
- Prevents tetanic contraction of the heart muscle.
Relative refractory period: The timeframe where another action potential can be generated but only with a strong enough stimulus.

Calcium plays a critical role in heart muscle contraction and regulation of action potentials.
Blood tests for cardiac troponin can indicate myocardial infarction, confirming heart cell death.

Defined as all electrical and physical events during one heartbeat:
- Chambers fill during diastole (relaxation) and contract during systole.
- Blood moves through the heart driven by pressure gradients established during these phases.
Ventricular Filling Stage: Atria and ventricles are in diastole allowing blood flow into the ventricles.
Misconceptions around how atrial contraction doesn't solely push blood into ventricles; it's a continual flow due to relaxation, not just muscular contraction.

Understanding the structure and function of the heart is essential for recognizing how disruptions lead to vascular disease and heart attacks.
Familiarity with physiological and electrical rhythms in the heart informs about treatment options and intervention strategies for various cardiac issues.