STS E Book - Cardiovascular Physiology Flashcards
The two primary ion exchangers in the cardiomyocyte are the Na-Ca pump and the Na-H pump. What are their roles?
The Na-Ca pump exchanges 3 Na for 1 Ca, decreasing the electrical gradient across the membrane and to remove excess intracellular Ca.
The Na-H pump exchanges 1 Na for 1 H+, which although electrically neutral, serves to prevent intracellular acidification.
What are the 5 phases of the action potential?
2 types of action potentials - slow and fast. Slow-response are in nodal tissue. Fast-response are in atrial and ventricular myocytes.
Phases are 0-4.
0 - Rapid depolarization d/t net positive inflow (Na channel activation in response to AP arriving from neighboring cardiomyocytes).
1 - Slow repolarization. Na channels are inactivated, decreasing Na influx. K channels open to allow potassium efflux.
2 - Plateau of membrane potential. Steady influx/efflux of charge, progression towards repolarization. K leaves the cell, Ca influx occurs, triggering release of additional Ca from sarcoplasmic reticulum, converting electrical energy to mechanical energy (Ca interaction with tropomyosin complex). Chloride channels also activated by Ca influx.
3 - Rapid REpolarization. Closure of L-type Ca channels and continued opening of K channels results in a net positive current. D/t fall of intracell Ca, contractile apparatus can relax.
4 - Pacemaker potential phase. Membrane potential slowly increases until a threshold potential is reached or depolarization is induced by an upstream AP.
Where does spontaneous depolarization occur in the heart?
Nodal tissue. SA node.
Describe depolarization pathway starting from SA node.
Spontaneous depolarization in SA node.
Atrial cardiomyocytes are stimulated, signal is further carried forward via 3 pathways to the AV node.
AV node activation -> propagation via bundle of His to the Purkinje fibers within the endocardium of the ventricles.
What happens if there is no depolarization signal upstream from the AV node and Purkinje fibers? How does the SA node regulate this?
The AV node and Purkinje fibers are able to spontaneously depilarize on their own as well.
This spontaneous activity is suppressed by the SA node b/c SA is intrinsically faster.
Describe the coordination of the depolarization in the SA and AV nodes. IE Why is there a delay in the AV node?
SA signal moves quickly through atria allowing for synchronized atrial contraction.
There is a delay of signal through the AV node that allows for completion of atrial contraction before the coordinated ventricular contraction.
***Intercalated discs are important in this coordinated contraction. They are located at the end of each cardiomyocyte. Gap junctions exists within these intercalated discs that allow for ions to flow between cells, propagating changes in electrical potential within the cell. ATP and cyclic-AMP kinases regulate permeability through gap junctions (alter conduction depending on energy available).
What is the primary contractile unit of the cardiomyocyte?
Describe their filaments?
These units connect at the z-line to form what?
Sarcomere.
Connect at the z-line to form myofibrils.
Thin filaments made of actin are bound to the z-line. Thick filaments are made of myosin that possess ATPase that can bind to actin.
Troponins regulate the interaction of actin and myosin.
The interaction between actin and myosin, and the resulting conformational change that produces contractions in the heart are mediated by what element? How?
Calcium. An influx of calcium occurs during depolarization of the cell membrane, resulting in additional calcium release from the sarcoplasmic reticulum. The inc in intracell calcium results in binding to troponin, unblocking actin-myosin binding sites (troponin I). A cross-bridge is formed, ATPase on myosin is activated -> conformational change -> z-lines drawn together.
Intracell Ca levels fall, troponin-I rebinds and inhibits actin-myosin bridge -> relaxation.
