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Last Session Overview

• Understood the differences in vertebrate heart structures and circulatory systems.

• Reviewed basic human heart anatomy.

• Identified three cell types in the human heart.

Gas Exchange / Hypoxia

Learning Goals

• Understand and describe:

  • Cardiac action potentials.

  • The importance of the plateau phase.

  • Transmission of electrical excitation over the heart.

Action Potentials

1. Definition: The signal used by neurons to send information along the axon.

2. All-or-none events: Action potentials occur fully or not at all.

3. Generation Elements:

   • Active transport produces asymmetric concentrations of ions.

   • Generates an electrochemical gradient, which provides energy.

   • This gradient drives ions across the membrane, causing a change in membrane potential (Vm).

Characteristics of Action Potentials

1. Threshold Current:

   • Just enough to achieve threshold potential, usually ranging from -30 to -50 mV.

2. Regenerative Nature:

   • Self-perpetuating; Vm becomes extremely positive (+10 to +50 mV).

3. After-Hyperpolarization:

   • When Vm falls below resting potential (Vrest).

Refractory Periods

1. Absolute Refractory Period:

   • A phase where no action potential can occur.

2. Relative Refractory Period:

   • An action potential can occur only if the trigger is more intense (analogous to flushing a toilet).

Ionic Basis of the Action Potential

Membrane States

• At Rest:

  • Membrane is permeable to K+ due to leak channels.

• Rising Phase:

  • Voltage-gated Na+ channels open, allowing Na+ to flow into the cell.

• After Na+ inactivation:

  • Membrane remains permeable to K+ briefly as voltage-gated K+ channels open.

Pacemaker Action Potentials

• Definition: Cells exhibit a more negative internal environment.

• Electrical Properties:

  • Pacemaker cells lack a stable resting potential.

  • Steady spontaneous depolarization occurs post-contraction, leading to a gradual rise in membrane potential.

  • K+ retention within cardiac cells increases positivity, bringing the membrane to threshold potential.

Modulation of Heart Rate

• Parasympathetic Innervation: Acetylcholine from the vagus nerve decreases heart rate by slowing pacemaker potential.

• Sympathetic Innervation: Epinephrine accelerates pacemaker potential, increasing heart rate.

• Catecholamines: Norepinephrine causes rapid depolarization and raises heart rate.

Cardiac Action Potentials

1. Rapid due to a significant increase in Na+ conductance (leading to positivity).

2. Delayed Repolarization:

   • Prolongs action potential and contraction, ensuring full contraction of heart chambers before relaxation and refilling.

3. Calcium Conductance:

   • Maintains a high Ca2+ influx, prolonging the plateau phase.

   • Delay in K+ conductance further contributes to this phase.

Calcium and the Sarcoplasmic Reticulum

• In lower vertebrates, Ca2+ enters through the plasma membrane.

• In birds and mammals, the larger cell volume limits sufficient Ca2+ entry, relying on the sarcoplasmic reticulum.

Termination of the Plateau Phase

• Occurs due to a reduction in Ca2+ conductance and an increase in K+ conductance.

Transmission of Excitation Over the Heart

1. Mechanism: Electrical activity passes between cardiac cells via gap junctions.

2. Impulse Direction: Transmission is generally unidirectional, ensuring impulses spread from the pacemaker region outward.

Spread of Electrical Activity

• Speed of excitation in the SA node and atria is about 0.8 m/s.

• From atria to ventricles via AV node, speed slows to 0.05 m/s, allowing complete filling of ventricles.

• Junctional fibers (0.1 m/s) connect to the Bundle of His, spreading excitation rapidly (4-5 m/s) through Purkinje Fibers for simultaneous ventricular contraction.

Functional Significance of Transmission

• Generates separate, synchronous contractions of the atria followed by the ventricles.

• Slowing through the AV node allows atrial contraction to complete, facilitating blood flow into ventricles.

Conducting System

Components

• Sinoatrial (S-A) Node: Pacemaker of the heart.

• Atrioventricular (A-V) Node: Delays impulse transmission to ventricles for optimal contraction timing.

• Bundle of His: Transmits impulses from A-V node to ventricles quickly.

Heartbeat Initiation and Spread

1. Depolarization begins at the S-A node, spreading quickly through atrial muscle.

2. A-V node delay allows atria to contract before ventricle activation.

3. Rapid spread through the ventricular myocardium leads to strong ventricular contraction.

Frank-Starling Mechanism

• Definition: Stretching of cardiac muscle increases the force of subsequent contractions.

• Importance: Allows the heart to match its output to its input, crucial for maintaining blood flow efficiency.