Potassium plasma membrane conductivity

In the myocardium, automaticity is the ability of the cardiac muscle to depolarize spontaneously (i.e., without external electrical stimulation from the autonomic nervous system). This spontaneous depolarization is due to the plasma membrane within the heart that has reduced permeability to potassium (K+) but still allows passive transfer of calcium ions, allowing a net charge to build. Automaticity is most often demonstrated in the sinoatrial (SA) node, the so-called pacemaker cells. Abnormalities in automaticity result in rhythm changes. The mechanism of automaticity involves the pacemaker channels of the HCN (Hyperpolarization-activated, Cyclic Nucleotide-gated) family14 (e.g., If, "funny" current). These poorly selective cation channels conduct more current as the membrane potential becomes more negative, or hyperpolarized. They conduct both potassium and sodium ions. The activity of these channels in the SA node cells causes the membrane potential to slowly become more positive (depolarized) until, eventually, calcium channels are activated and an action potential is initiated. 

The striking compartmentalization of potassium in intracellular fluid, and of sodium in extracellular fluid, is a condition which is established and maintained by active transport across all plasma membranes. In the absence of active transport pumps, cotransporters and conductance channels, a directional, selective, rapid and regulated movement of potassium (or sodium) through the cell membranes would be impossible. The major molecular pathways of potassium permeation through plasma membranes are Na, K- ATPase, H-K-ATPase, Na-2C1-K-transporter, and potassium conductance channels (Peterson 1997). 

Sodium channels spontaneously close and potassium channels begin to open at about this time (see Figure 13.19B). Consequently, potassium ions flow outward, and so the membrane potential returns to a negative value. The resting level of —60 mV is restored in a few milliseconds as the K conductance decreases to the value characteristic of the unstimulated state. Only a very small proportion of the sodium and potassium ions in a nerve cell, of the order of one in a million, flows across the plasma membrane during the action potential. Clearly, the action potential is a very efficient means of signaling over large distances. 

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