Muscle Electrophysiology
This histological feature, unique to cardiac muscle, contains gap junctions that allow the heart to function as a single functional syncytium.
What are intercalated discs?
This term describes the volume of blood remaining in the ventricle at the end of a contraction, usually about 50 ml.
What is End-Systolic Volume (ESV)?
This node is the "natural pacemaker" because it possesses the fastest intrinsic rate of spontaneous depolarization.
What is the Sinoatrial (SA) node?
This horseshoe-shaped cluster of cells in the visceral mesoderm is responsible for forming the entire left ventricle and portions of the atria.
What is the Primary Heart Field (PHF)?
These modified cardiac myocytes are found in the subendocardial layer, are larger than standard myocytes, and are rich in stored glycogen.
What are Purkinje fibers?
During Phase 0 of the ventricular action potential, the rapid upstroke is caused by the sudden opening of these voltage-gated channels.
What are fast sodium channels?
The second heart sound (S2) is a result of vibrations caused by the snapping shut of these two valves.
What are the semilunar valves (aortic and pulmonary)?
This autonomic neurotransmitter increases heart rate by increasing the "leakiness" of the SA node to sodium and calcium ions.
What is norepinephrine?
This "master gene" for heart development is induced by BMPs from the endoderm; its mutation often causes ASDs and conduction delays.
What is NKX2.5?
This is the thickest layer of the wall in an elastic artery, containing 40 to 70 layers of fenestrated elastic lamellae.
What is the tunica media?
This specific phase of the cardiac action potential is maintained by a balance between the inward movement of Ca2+ through L-type channels and the outward movement of K+.
What is the plateau (Phase 2)?
During this specific 0.03 to 0.06-second window, intraventricular pressure falls rapidly while the volume remains constant because all valves are closed.
What is isovolumic relaxation?
These specialized conducting fibers have a high density of gap junctions, allowing them to transmit impulses at 1.5 to 4.0 m/sec—roughly 150x faster than the AV node.
What are Purkinje fibers?
Known as the "master gene for left-sidedness," its expression is initiated by serotonin (5-HT) and Nodal to ensure proper heart looping.
What is PITX2?
In atherosclerosis, these cells are formed when macrophages in the tunica intima phagocytose oxidized LDL.
What are foam cells?
Unlike skeletal muscle, cardiac muscle cannot undergo wave summation or tetany because of this specific protective electrical property.
What is a prolonged refractory period?
This law (T = P x r) explains why a dilated failing heart (increased radius) must generate much higher wall tension to achieve the same systolic pressure.
What is the Law of Laplace?
This phenomenon occurs when a sudden AV block stops the heart, and the patient faints before the Purkinje system "escapes" and begins its own 15-40 bpm rhythm.
What is Stokes-Adams Syndrome?
This structure, derived from the proepicardial organ, is responsible for forming the smooth muscle and endothelial lining of the coronary arteries.
What is the epicardium (or visceral pericardium)?
This specialized layer of the heart valve acts as a shock absorber; it is rich in proteoglycans and is situated between the fibrosa and the ventricularis.
What is the spongiosa?
During the plateau, the heart prevents early repolarization by decreasing the membrane permeability of this ion fivefold, a phenomenon not seen in skeletal muscle.
What is potassium K+?
In a healthy heart, this component of work output is only 1%, but in aortic stenosis, it can exceed 50% of the total work due to high-velocity ejection.
What is the kinetic energy of blood flow?
Vagal stimulation causes hyperpolarization of the SA node (reaching -65 to -75 mV) by specifically increasing the membrane's permeability to this ion.
What is potassium (K+)?
Cells on the right side of the Secondary Heart Field (SHF) contribute to the left of the outflow tract, a complex laterality that ensures this specific anatomical feature of the great vessels.
What is the spiraling of the aorta and pulmonary artery?
This transcription factor is the "molecular switch" that prevents primary myocytes from becoming "working" muscle, instead forcing them into the conducting system lineage.
What is TBX3?