Microscopic Anatomy of the Heart

Pericardium

Outer layer; tough and dense; protects the heart and maintains the heart's position in the mediastinum; attached to the great vessels and the diaphragm

Serous pericardium

Inner layer divided into 2 layers with a fluid-filled cavity between them

Parietal pericardium

Fused to the inner side of the fibrous pericardium; made of simple squamous epithelium

Visceral pericardium (epicardium)

Bound to the heart and is part of the heart wall; located next to the organ

Epicardium

Outermost layer, also known as the visceral layer of pericardium

Myocardium

Middle layer made of cardiac muscle cells; the muscle wall of the left ventricle is thicker than the right ventricle

Endocardium

Innermost layer that lines the chambers; one cell layer thick, made of simple squamous endothelium

Contractile cells

Make up 99% of all cells; pass impulses responsible for contractions from one cell to another; shorter than conducting cells

Conducting Cells

Form the heart's conduction system; generate and conduct action potentials; have autorhythmicity and are smaller than contractile cells

Autorhythmicity

Ability to initiate electrical potential at a fixed rate that spreads rapidly from cell to cell to trigger contraction

Voltage-gated NA Channel

Found in excitable cell membranes; opens in response to a membrane potential increase to about -55mV, allowing sodium influx and depolarization

Voltage-gated K channel

Found in excitable cell membranes; responsible for returning the cell from a depolarized state to a resting polarized state by allowing potassium efflux

Membrane potential

Difference in electrical charge across a cell membrane; one side is more negative or positive than the other side

Action potential

Occurs when charges across the cell membrane change

Resting membrane potential

When a cell is at rest; inside is more negative than outside (-30 to -90 mV) due to the Na-K pump maintaining the gradient

Model Depolarization-Repolarization

Process where the membrane goes from resting potential to depolarization, then repolarization, returning to regular resting potential

Plateau phase

Part of the unique electrical pattern of cardiac muscle cells; refractory period that allows full relaxation before another contraction

Stable resting membrane potentials

Atria: -80 mV, Ventricles: -90 mV

Spontaneous depolarization

Occurs in conducting cells as they depolarize as soon as they return to a negative membrane potential, giving the heart its autorhythmicity

Phase 1 (of conducting cells)

Membrane potential reaches -40 mV; VG calcium ion channels open, causing further depolarization until reaching +5 mV

Phase 2 (of conducting cells)

Membrane potential reaches +5 mV; VG K channels open, leading to repolarization

Phase 3 (of conducting cells)

Membrane potential reaches -60 mV; K channels close and Na channels open, causing depolarization again