Gap Junctions

(CELL-TO-CELL CONDUCTION)

In the heart, cardiac muscle cells (myocytes) are connected end to end by structures known as intercalated disks. These are irregular transverse thickenings of the sarcolemma, within which are desmosomes that hold the cells together and to which the myofibrils are attached. Adjacent to the intercalated disks are the gap junctions, which allow muscle action potentials to spread from one myocyte to the next. More specifically, the disks join the cells together by both mechanical attachment and protein channels. The firm mechanical connections are created between the adjacent cell membranes by

Fig. 10. The comparative time-courses of membrane potentials and ion permeabilities that would typically occur in a fast-response (left; e.g., ventricular myocyte) and a slow-response cell (right; e.g., a nodal myocyte). Modified from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed., 2003.

Fig. 10. The comparative time-courses of membrane potentials and ion permeabilities that would typically occur in a fast-response (left; e.g., ventricular myocyte) and a slow-response cell (right; e.g., a nodal myocyte). Modified from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed., 2003.

Fig. 11. Shown are several cardiac myocytes in different states of excitation. The depolarization that occurred in the cell on the left causes depolarization of the adjacent cell through cell-to-cell conduction via the gap junctions (nexus). Eventually, all adjoining cells will depolarize. An action potential initiated in any of these cells will be conducted from cell to cell in either direction.

Fig. 11. Shown are several cardiac myocytes in different states of excitation. The depolarization that occurred in the cell on the left causes depolarization of the adjacent cell through cell-to-cell conduction via the gap junctions (nexus). Eventually, all adjoining cells will depolarize. An action potential initiated in any of these cells will be conducted from cell to cell in either direction.

Fig. 12. Shown are the predominant conduction pathways in the heart and the relative time in milliseconds that cells in these various regions become activated following an initial depolarization within the sinoatrial (SA) node. To the right are typical action potential waveforms that would be recorded from myocytes in these specific locations. The SA and atrioventricular (AV) nodal cells have similar shaped action potentials. The nonpacemaker atrial cells elicit action potentials that have shapes somewhat between the slow-response (nodal) and fast-response cells (e.g., ventricular myocytes). The ventricular cells elicit fast-response-type action potentials; however, their durations vary in length. Because of the rapid excitation within the Purkinje fiber system, the initiation of depolarization of the ventricular myocytes occurs within 30 to 40 ms and is recorded as the QRS complex in the electrocardiogram.

Fig. 12. Shown are the predominant conduction pathways in the heart and the relative time in milliseconds that cells in these various regions become activated following an initial depolarization within the sinoatrial (SA) node. To the right are typical action potential waveforms that would be recorded from myocytes in these specific locations. The SA and atrioventricular (AV) nodal cells have similar shaped action potentials. The nonpacemaker atrial cells elicit action potentials that have shapes somewhat between the slow-response (nodal) and fast-response cells (e.g., ventricular myocytes). The ventricular cells elicit fast-response-type action potentials; however, their durations vary in length. Because of the rapid excitation within the Purkinje fiber system, the initiation of depolarization of the ventricular myocytes occurs within 30 to 40 ms and is recorded as the QRS complex in the electrocardiogram.

proteins called adherins in the desmosome structures. The electrical connections (low-resistance pathways, gap junctions) between the myocytes are via the channels formed by the protein connexin. These channels allow ion movements between cells (Fig. 11).

As noted, not all cells elicit the same types of action potentials, even though excitation is propagated from cell to cell via their interconnections (gap junctions). The action potentials elicited in the sinoatrial nodal cells are of the slow-response type and those in the remainder of the atria have a more rapid depolarization rate (Fig. 12). Although there is a significant temporal displacement in the action potentials elicited by the myocytes of the two nodes (sinoatrial and atrioventricular), their action potential morphologies are similar.

It takes approx 30 ms for excitation to spread between the sinoatrial and atrioventricular nodes, and total atrial activation occurs over a period of approx 70-90 ms (Fig.12). The speed at which an action potential propagates through a region of cardiac tissue is called the conduction velocity (Fig. 5). The conduction velocity varies considerably in the heart and is directly dependent on the diameter of the myocyte. For example, action potential conduction is greatly slowed as it passes through the atrioventricular node. This is because of the small diameter of these nodal cells, the tortuosity of the cellular pathway (2), and the slow rate of rise of their elicited action potentials. This delay is important to allow adequate time for ventricular filling.

Action potentials in the Purkinje fibers are of the fast-response type (Fig. 12); that is, there are rapid depolarization rates that are partly caused by their large diameters. This feature allows the Purkinje system to transfer depolarization to the majority of cells in the ventricular myocardium nearly in unison. Because of the high conduction velocity in these cells that span the myocardium, there is a minimal delay in the time of onset of these cells. It is important to note that the ventricular cells that are last to depolarize have shorter duration action potentials (shorter Ca2+ current) and thus are the first to repo-larize. The ventricular myocardium repolarizes within the time period represented by the T wave in the electrocardiogram.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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