Cardiac Muscle and Cardiac Conduction System p1 wk2

Lecture Topics:
- Cardiac Muscle and Cardiac Conduction System
- Electrocardiogram (Lecture 2)
- Cardiac Cycle (Lecture 3)
- Control of Blood Flow and Blood Pressure (Lecture 4)

Describe the functional characteristics of cardiac muscle tissue and the cardiac conduction system.
Outline the phases of an action potential in autorhythmic cardiac muscle fibers.
Outline the phases of an action potential in contractile cardiac muscle fibers.
Discuss how excitation-contraction coupling occurs in cardiac muscle fibers.
Explain the significance of the long refractory period in cardiac muscle.

Also known as cardiac muscle fibers, these terms are used interchangeably.
Characteristics:
- Size and Shape: Tend to be shorter and smaller than skeletal muscle cells.
- Nucleus: Generally contain one nucleus.
- Mitochondria: Abundant, providing ATP essential for contraction and relaxation.
- Structure: Form a branching network known as a functional syncytium.
Striations: Under microscope, cardiac muscle appears striated, similar to skeletal muscle, due to the orderly arrangement of actin and myosin, which forms the contractile apparatus.
Comparison with Smooth Muscle: Smooth muscle cells do not have striations.

Sarcolemma: The cell membrane of cardiac muscle cells.
Transverse Tubules (T Tubules):
- Tubular invaginations of the sarcolemma.
- Lumen is continuous with extracellular fluid, crucial for action potential transmission into the cell.
Myofilaments and Cytoplasmic Reticulum:
- Myofilaments are surrounded by a network of sarcoplasmic reticulum, similar to endoplasmic reticulum in other cells.
- Sarcoplasmic reticulum stores calcium ions critical for contraction.

Actin and Myosin Arrangement:
- Actin filaments connect to the Z line; myosin filaments connect to the M line.
- Z Line: The region where actin and myosin filaments overlap.
- Sarcomere: The segment between consecutive Z lines, typically 1.6 to 2.2 microns in length in human heart tissue.
Excitation-Contraction Coupling: The interaction between actin and myosin during excitation causes the sarcomere to shorten as they slide past each other.

Structure: Specialized cell membranes where individual myocytes connect.
Functions:
- Structural Attachment: Desmosomes (composed of cadherins) glue cells together.
- Electrical Connection: Gap junctions formed by connexin proteins allow electrical impulses to travel between cells, enabling the myocardium to function synchronously.

Definition: The heart can contract spontaneously without nervous system input.
Types of Cardiac Cells:
- Autorhythmic Cells (about 1%): Spontaneously generate action potentials, thus creating intrinsic electrical activity of the heart (located in SA node, AV node, bundle of His, bundle branches, Purkinje fibers).
- Contractile Fibers (about 99%): Do not generate action potentials but contract in response to stimulation from autorhythmic cells.
Primary Pacemaker: SA node is the primary pacemaker, generating action potentials faster than others.

Cardiac Myocyte vs. Neuron:
- Cardiac myocytes have a resting membrane potential of approximately -90 mV, primarily due to high potassium permeability, allowing potassium to exit.
- Neurons have a resting membrane potential around -70 mV, as they have higher permeability to sodium and calcium ions, resulting in more positive charge inside than cardiac cells.
Ionic Movement:
- Potassium moves out via open potassium channels down its concentration gradient.
- Sodium and calcium move in via open sodium/calcium channels down their concentration gradients.

Neuron Action Potential: Rapid duration of 1-2 milliseconds.
Cardiac Myocyte Action Potential: Prolonged duration of 200-400 milliseconds, related to ionic conductance differences.

Break point before continuation into further detailed discussions in the next part.