E

T7 I g K r . gCa 77 ENa +-EK +-ECa gtot gtot where R is the gas constant, T is the temperature (K), z is the valence of the ion (charge and magnitude), and F is the Faraday constant.

In Table 1, the concentrations of the ions (inside and outside) that play a role in the resting membrane potential of cardiac muscle cells are shown with their respective calculated equilibrium potentials. The measured membrane potential of a cardiac muscle cell is approx -90 mV, suggesting that it is primarily determined by either the chloride or potassium distribution. However, measurements of ion movements have shown that chloride is distributed passively across the cell membrane (following positive ion movement) and can therefore be ignored in such a calculation; this leaves potassium as the dominant ion species for control of the myocyte resting membrane potential.

More specifically, the membrane potentials of living cells depend on several other parameters, including the concentra where gNa is the membrane conductance for sodium, gK is the membrane conductance for potassium, gCa is the membrane conductance for calcium, gtot is the total membrane conductance, ENa is the equilibrium potential for sodium, EK is the equilibrium potential for potassium, and ECa is the equilibrium potential for calcium.

Evaluation of the Goldman-Hodgkin-Katz equation using the values in Table 1 and the conductance values for sodium, potassium, and calcium results in a membrane potential of -90 mV.

As noted, cells can have a variety of ion-selective channels in their membranes. The term gating refers to the trigger required for opening such a channel. More specifically, voltage-gated ion channels respond to changes in the local membrane potential of the cell, ligand-gated ion channels respond to specific circulating biochemical factors, spontaneously active ion channels have a random frequency of opening and closing, whereas leak channels seem to be constitutively open, although at a low level. In addition to the aforementioned classification, based on their control mechanisms, channels can also be classified by their ion selectivity or the direction of ion passage that such a channel facilitates (Fig. 13).

Fig. 12. "Typical" ionic distribution for cardiac cells. See text for further details.

Table 1

Internal and External Concentrations of Ion Species Involved in Determination of the Resting Potential of Cardiac Muscle Cells

Table 1

Internal and External Concentrations of Ion Species Involved in Determination of the Resting Potential of Cardiac Muscle Cells

Ion

[Inside]

[Outside]

Ratio of [Outside]/[Inside]

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.

Get My Free Ebook


Post a comment