Lecture 10: Cardiac Muscle Insights

Lecture 10: Cardiac Muscle

• Topic: Hearts/Cardiac Muscle
• Readings: 561, 339-339, 329-330, 672-675, 694-697, Box 25.1
• Date: January 27th

Types of Circulatory Pumps

• Specialized Muscular Chambers:
  • Contain valves that ensure one-way blood flow.
  • Found in most adult hearts.
• Contractile Elements:
  • Not part of the circulatory system.
  • Aid in venous return to the heart.
  • Examples include peristaltic contractions in some invertebrates and embryonic vertebrates.

Invertebrate Hearts

• Example (e.g.): Many arthropods
• Neurogenic:
  • Definition needed (see page 3).
  • Posterior cardiac ganglion nerves stimulate spontaneous rhythmic depolarization.
  • Anterior axons innervate the cardiac muscle.

Crab Heart

• Suspension: Suspended in a pericardial sinus by ligaments.
• Hemolymph Pooling: Incoming hemolymph from gills collects in the sinus.
• Hemolymph Entry: Hemolymph enters the heart through ostia.
• Diastole/Systole: "Sucked in" during relaxation (diastole) and contraction (systole), respectively.
• Ostia Valves: Heart contraction closes one-way ostia valves, which regulate blood flow.
• Blood Force: Blood is forced out through arteries.
• Ligament Elasticity: Ligaments stretch and store energy during heart contraction, essential for efficient blood expulsion. Then recoil, expanding the heart, and opening the ostia for inflow.

Vertebrate Cardiac Muscle

• Key Structures: Intercalated discs, gap junctions, and desmosomes are key structural elements that electrically couple cardiac cells.
• Importance: These structures enable efficient propagation of depolarization and synchronized contraction of cardiac muscle cells.
• Mitochondria: Mitochondria are an important component within cardiac muscle cells; responsible for energy production (ATP).

Vertebrate Cardiac Cell Types

• Pacemaker Cells:

  • Non-contractile cells that generate spontaneous action potentials (membrane potential changes).
  • Found in sinus venosus (fish) and SA node (mammals).

• Contractile Cells:

  • Located in atria and ventricles.

• Conducting Cells (Transmitting Cells):

  • Non-contractile cells that transmit action potentials.
  • Examples are the bundle of His, AV node, and Purkinje fibers.

Vertebrate Cardiac Muscle Contraction

• Myogenic: This type of heart contraction is self-excitable, meaning the heart generates contraction based on its own properties (no external nerves needed).
• Myogenic Mechanisms: Include a specific cell type called pacemakers that regulate rhythmicity of cardiac contraction (electrical signals).

Cardiac Action Potential

• Threshold: Factors or series of events that precede an action potential and push the membrane potential above the threshold.
• Depolarization/Repolarisation: Series of membrane potential fluctuations during contraction and relaxation (action potential steps).
• Plateau: An extended depolarization period that occurs during action potential (important time for contraction).
• Voltage-Gated Channels: Important ion-conducting channels for initiating and maintaining the plateau and subsequent repolarization.

Myogenic Mechanism

• Membrane Potential Changes: Cells experience cyclic changes in membrane potential (especially important in pacemaker cells).
• Funny Channels: These nonselective channels for ions play a key role in pacemaker potential (e.g. cell activity).
• Influx/Efflux: Na+ influx is more prominent than K+ efflux in the pacemaker cells, which contributes to the membrane potential fluctuation.

Cardiac Muscle Tetanus

• Plateau Phase Stimulation: Contraction and relaxation will not occur if stimulated at plateau phase (action potential); only during some of the phase.

Cardiac Muscle Cells: Electrical Conduction

• Gap Junctions: these junctions serve as low resistance pathways for the rapid movement of ions, facilitating the synchronized depolarization and contraction of cardiac muscle cells.
• Intercalated Discs: Gap junctions are concentrated in intercalated discs, which are specialized structures located between cardiac muscle cells.
• Depolarization Transmission: Rapid spread of electrical signal from one cardiac muscle to another.

Different Cells, Different Potentials

• Action Potentials: Each cell type exhibits characteristics features of the pacing and conduction.
• Timeframes: Timeframes during which the action potential and associated events occur in each cell.