Introduction to Cardiac Electrical Activity and Cardiovascular Physiology

This course transitions from the study of the nervous system and the control of body movement to the cardiovascular system, emphasizing the integration between these systems.
A central theme is the collaboration between the nervous system and the cardiovascular system to regulate cardiac function.
The lecture sequence follows a specific progression:
- Lectures 1 and 2: Focus on electrical events, including the generation of electrical signals in the heart and measurement via Electrocardiogram (ECG).
- Lectures 3 and 4: Focus on mechanical events, specifically cardiac output and pressure-volume loops.
- Final Lectures: Integration of concepts within the context of exercise, examining oxygen demand, changes in autonomic drive, and various exercise modalities.
The primary focus is understanding how electrical activity is generated in the heart, how the signal propagates through cardiac tissue, and the subsequent path to mechanical contraction.

Explain the maintenance of ion gradients across cardiac cell membranes.
Identify and describe the function of specific ion channels within the cardiac cell membrane that open and close to facilitate changes in membrane potential.
Compare and contrast the characteristics of action potentials in different cell types:
- Sinoatrial (SA) node cells (autorhythmic/pacemaker cells).
- Ventricular myocytes (contractile cells).
Explain the mechanisms by which the autonomic nervous system modulates pacemaker firing.
Detail the process of Excitation-Contraction Coupling: the conversion of a cardiac myocyte action potential into force production.
Apply these foundational concepts to clinical and practical scenarios, including:
- Interpretation of ECGs.
- Physiological responses to exercise.
- Understanding arrhythmias.
- Pharmacological modulation of cardiac behavior.

Foundations in cardiac physiology are essential for various health and medical fields, including clinical practice, medical research, therapeutic development, and diagnostic technology innovation.
Cardiovascular disease (CVD) remains the leading cause of mortality globally.
In Australia, cardiovascular disease has resulted in an estimated loss of $685,000$ years of healthy life.
Dietary habits are a critical factor in CVD development and serve as a primary target for intervention.
Dyslipidemia, specifically elevated levels of low-density lipoprotein (LDL) cholesterol, is a major metabolic risk factor for CVD.
Diet quality is a confirmed determinant for the risk of developing cardiovascular outcomes, including heart attacks, and is vital for prevention.
While death rates from CVD are declining in Australia due to improved prevention and treatment, it still represents a significant health service burden.

The physiological process of a heartbeat consists of four interconnected steps:
1. Generation of an electrical signal by specialized autorhythmic cells located in specific regions of the heart.
2. Conduction of that signal to the cardiac muscle cells.
3. Response of the muscle cells with their own electrical signal (action potential).
4. Triggering of the mechanical contraction by the electrical response.
The heartbeat initiates as an electrical event and culminates in a mechanical event.

Cardiac electrical activity is governed by the selective opening and closing of ion channels, which leads to changes in membrane potential.
Different types of cardiac cells express distinct combinations of ion channels, resulting in unique action potential shapes.
Ion Gradients and Charge Distribution:
- Sodium ( $Na^+$ ) and Calcium ( $Ca^{2+}$ ) concentrations are higher outside the cell than inside.
- Potassium ( $K^+$ ) concentration is higher inside the cell than outside.
Depolarization and Excitability:
- When $Na^+$ or $Ca^{2+}$ channels open, positively charged ions move into the cell down their concentration gradients.
- This makes the interior of the cell more positive (depolarization), moving it toward a more excitable state and potentially reaching the threshold for an action potential.
- $Na^+$ channels in the heart can be categorized based on their patterns as "fast" or "slow."
Repolarization:
- When $K^+$ channels open, positively charged ions leave the cell.
- This makes the cell interior more negative, leading to repolarization and making the cell less likely to be excited.

Conducting (Autorhythmic or Pacemaker) Cells:
- These cells do not generate forceful contractions.
- Their primary function is to initiate and conduct action potentials throughout the heart.
- Locations include the Sinoatrial (SA) node, the Atrioventricular (AV) node, the Bundle of His (including left and right bundle branches), and the Purkinje fibers.
Contractile Cells:
- These cells comprise the majority of the myocardium (the heart muscle).
- Their primary function is to perform the mechanical work of the heart by generating force.
- They do not typically initiate their own action potentials; instead, they are stimulated electrically via gap junctions by neighboring cells.
Action Potential Profiles:
- SA node cells and ventricular muscle cells have distinctly different action potential profiles, which are fundamental to their specific functions.

Mini Quiz Question 1: If several sodium ( $Na^+$ ) channels within the membrane of a cardiac myocyte are opened, what will be the impact on the cell membrane potential?
Mini Quiz Question 2: Which statement best describes autorhythmic or conducting cells of the heart?
- Options for study: Consider their ability to generate force versus their role in initiating and conducting signals.