Chapter 12/14 Bio360; monday 03/18

When threshold is reached at the SA node (an autorhythmic cell), what channels open causing further depolarization of the membrane?

Fast calcium channels. Yes, unlike nerve cells or cardiac muscle cells, fast calcium channels are responsible for the depolarization phase of the autorhythmic cell action potential. When the fast calcium channels open, calcium rushes into the cell making it less negative (or more positive).

One of the changes that occurs in the pacemaker potential (unstable resting membrane potential) in the SA node (an autorhythmic cell) is a decreased efflux of what ion?

Potassium. Yes, if there is a decreased efflux of potassium while there is a normal influx of sodium, the inside of the cell would become less negative. Thus, threshold would be reached. The ability of these autorhythmic cells to spontaneously depolarize is what results in the pacemaker potential.

Which of the following is a similarity between autorhythmic cells and contractile cells in the heart? They have similar size and similar numbers in the heart. Both generate the contractile force of the heart. Both generate action potentials spontaneously. Both are myocardial cells.

The one that is the similarity between autorhythmic cells and contractile cells in the heart is : Both are myocardial cells. - Cardiac muscle cells in myocardium consist of contractile cells in walls of atria and ventricles and autorhythmic cells in SA node, AV node. - There are more number of contratile cells than autorhythmic cells. - Contractile cells generate contractile force of the heart. - Autorhythmic cells generate action potentials spontaneously. Hence, the only similarity between two types of cells is they both are myocardial cells. Option D

Action potentials generated by the autorhythmic cells spread to the contractile cells through what structures in the membrane?

Yes, action potentials generated by the autorhythmic cells spread waves of depolarization to contractile cells through GAP JUNCTIONS. If the depolarization causes the contractile cells to reach threshold, they will in turn generate an action potential.

Repolarization of an autorhythmic cell is due to the opening of which channels?

Yes, opening of voltage-gated potassium channels causes positive potassium ions to move out of the cell. This efflux of potassium causes the cell to become more negative inside thus, repolarizing the cell.

In order to cause cardiac muscle contraction, the contractile cells must also depolarize. What causes the depolarization of the contractile cells?

Yes, the flow of positive ions from the autorhythmic cells (or adjacent cells) brings the membrane to threshold initiating depolarization of the contractile cell.