CVR+in+class+Lesson+88-+Electrophysiology+of+the+heart

Presenter: Dr. Hector ZerpaInstitution: TAS St. George's University, School of Veterinary Medicine, Grenada, West Indies

Students will be able to:

Describe clinically relevant characteristics of the heart's electro-mechanic activity, including the timing and sequences of electrical impulses and how they correlate with muscle contractions.
Identify different types of cardiomyocytes, specifically distinguishing between contractile and autorhythmic cardiomyocytes and their respective roles in heart function.
Organize the genesis and conduction system based on pacemaker depolarization rates, with detailed knowledge of how these rates affect overall heart rhythm and function.
Discuss ionic bases of two types of action potentials in cardiomyocytes, highlighting the specific ion channels involved and their contributions to heart rhythm disturbances.
Describe how the autonomic nervous system regulates heart rate, including detailed pathways of neurotransmitters and their physiological effects.

Components:

Sinoatrial Node (SA Node): The primary pacemaker of the heart, initiating electrical impulses that dictate heart rhythm.(Right Atrium)
Atrioventricular Node (AV Node): Provides a delay, allowing for proper ventricular filling before contraction.(Right atrium)
Bundle of His: Transmits impulses from the AV node to the ventricles, crucial for synchronizing heartbeat.
Bundle branches (left and right): Distribute impulses to the left and right ventricles, facilitating uniform contraction.
Purkinje Fibers: Specialized fibers that ensure rapid conduction throughout the ventricles, enabling coordinated contraction.

Group I: Partial penetration – Describes pathways with less electrical resistance allowing slower conduction speeds.
Group II: Full penetration – Represents pathways providing maximal conductivity to ensure rapid response during critical heart events.

Functions:

The heart pumps blood through the circulatory system effectively, adapting to the body's varying demands during rest and activity.
Contractile phenotype cardiomyocytes account for 99% of heart cells, designed primarily to contract and generate force.
These cardiomyocytes form an electrical syncytium, ensuring that all heart cells can contract in unison via coordinated electrical and mechanical connections.

Action Potential:

Action potentials are initiated in specialized regions (e.g., SA Node) and are essential for facilitating synchronized contractions, which is critical for effective circulation.
Normal function relies on:
- Synchronization of contraction during systole (heart muscle contraction phase).
- Relaxation during diastole (heart muscle relaxation phase), allowing the heart chambers to fill with blood.

Phases of Action Potential:

Phase 0: Rapid depolarization occurs due to an influx of Na+ ions (↑ INa+), sharply increasing membrane potential.
Phase 1: Partial repolarization with a decrease of Na+ permeability and an increase of K+ ions (↑ IK+).
Phase 2: Plateau phase where balance is achieved between calcium influx (↑ ICa++) and potassium efflux (↑ IK+), sustaining the contraction.
Phase 3: Repolarization occurs through the efflux of K+ ions as K+ channels open, restoring the resting potential.
Phase 4: Resting membrane potential established, involving K+ "leak" channels that maintain baseline ionic distribution.

Refractory Period: This period is longer than in skeletal muscle fibers, preventing premature contractions and ensuring effective heartbeats.

Make up 1% of heart cells and are responsible for generating and propagating electrical impulses throughout the heart tissues.
origin of pacemaking activity
Function similarly to neurons but are specialized muscle cells adapted to create and transmit rhythmic electrical signals.
Regulated by autonomic nervous system fibers, responding to physiological changes in the body to adjust heart rate accordingly.

Structure:

Myofibrils are surrounded by a less dense sarcoplasmic reticulum compared to skeletal muscle, influencing calcium handling.

Calcium Requirement:

Extracellular Ca++ is crucial for successful muscle contraction, highlighting the importance of calcium ions in cardiac physiology.

Contraction Mechanism:

Initiated by an increase in intracellular Ca++ due to action potentials, leading to muscle contraction.

Relaxation:

Involves the reaccumulation of Ca++ by the sarcoplasmic reticulum, facilitated by proteins like SERCA (Sarcoplasmic Reticulum Ca2+ ATPase) and NCX (Na+/Ca2+ exchanger).

Chronotropism:

Heart rate is modulated by:
- Parasympathetic nervous system: Often decreases heart rate via the action of acetylcholine on heart tissues, enhancing resting tone.
- Sympathetic nervous system: Increases heart rate through norepinephrine release, promoting greater cardiac output. Multiple mechanisms exist, including changes in the pacemaker potential slope affecting heart rate dynamics.

Inotropism:

Refers to the force of heart muscle contractions:
- The heart cannot recruit additional muscle fibers for stronger contractions, as seen in skeletal muscle physiology, due to its syncytial structure.
- Long action potential duration helps prevent tetany, allowing for proper relaxation and filling between contractions.
- Sympathetic stimulation can increase intracellular Ca++ levels, leading to enhanced contractility. Positive Inotropy is triggered by β-adrenergic receptor activation, resulting in larger intracellular Ca++ transients that strengthen contractions.

Stretch and Metabolism:

Stretch: The Frank-Starling law describes how increased stretching of heart muscle fibers leads to an increase in contractile force, optimizing cardiac output based on venous return.
Metabolism: Cardiac muscle requires continuous ATP replenishment primarily through aerobic metabolism, highlighting the importance of oxygen availability.

Regulation by:

Autonomic Nervous System: Balances exciting and inhibiting influences, maintaining appropriate heart function in response to internal and external stimuli.
Electrolyte Concentrations: Fluctuations in Na+, K+, and Ca++ levels critically affect action potentials and overall heart rhythm.

Autonomic Nervous System Roles:

Parasympathetic: Primarily limits excitation and contractility, promoting rest and digestion states.
Sympathetic: Increases heart rate, conduction speed, excitability, and contraction force to prepare for fight-or-flight responses.

Control Mechanisms:
- Chronotropic: Adjustment of heart rate to meet physiological demands.
- Inotropic: Variability in force of contraction, crucial for managing systemic and pulmonary circulation. The SA node's functionality and the distribution via the Purkinje fibers underscore the importance of coordination in conduction and pacing of the heart.

Engagement with more detailed topics on electrocardiography (ECG) readings, understanding arrhythmias, and clinical applications of cardiovascular principles.