RESTING MEMBRANE POTENTIALS

BLigand

vy

\y

Cell membrane

N___ J

Closed channel

Open channel

FIGURE4-18 Ion channels. (A) Nongated ion channel remains open, permitting free movement of ions across the membrane. (B) Ligand-gated channel is controlled by ligand binding to the receptor. (C) Voltage-gated channel is controlled by a change in membrane potential. (Rhoades R.A., Tanner G.A. [1996]. Medical physiology. Boston: Little, Brown)

Electrical potentials exist across the membranes of most cells in the body. Because these potentials occur at the level of the cell membrane, they are called membrane po­tentials. In excitable tissues, such as nerve or muscle cells, changes in the membrane potential are necessary for gen­eration and conduction of nerve impulses and muscle con­traction. In other types of cells, such as glandular cells, changes in the membrane potential contribute to hor­mone secretion and other functions.

Electrical Potentials

Electrical potential, measured in volts (V), describes the ability of separated electrical charges of opposite polarity (+ and −) to do work. The potential difference is the differ­ence between the separated charges. The terms potential dif-

 

ference and voltage are synonymous. Voltage is always mea­sured with respect to two points in a system. For example, the voltage in a car battery (6 or 12 V) is the potential dif­ference between the two battery terminals. Because the total amount of charge that can be separated by a biologic membrane is small, the potential differences are small and are measured in millivolts (1/1000 of a volt). Potential dif­ferences across the cell membrane can be measured by in­serting a very fine electrode into the cell and another into the extracellular fluid surrounding the cell and connecting the two electrodes to a voltmeter (Fig. 4-19). The move­ment of charge between two points is called current. It oc­curs when a potential difference has been established and a connection is made such that the charged particles can move between the two points.

 

CHAPTER 4 Cell and Tissue Characteristic

 

RE 4-19 Alignment of charge along the cell membrane. The electrical potential is negative on the inside of the cell membrane in relation to the outside.

Extracellular and intracellular fluids are electrolyte so­lutions containing approximately 150 to 160 mmol/L of positively charged ions and an equal concentration of neg­atively charged ions. These are the current-carrying ions responsible for generating and conducting membrane po­tentials. Usually, a small excess of positively charged ions exists at the outer surface of the cell membrane. This is rep­resented as positive charges on the outside of the mem­brane and is balanced by an equal number of negative charges on the inside of the membrane. Because of the ex­treme thinness of the cell membrane, the accumulation of these ions at the surfaces of the membrane contributes to the establishment of a resting membrane potential. Action potentials (discussed in Chapter 49) represent abrupt and pulselike changes in the membrane potential that are pro­pagated along a nerve or muscle fiber. They occur when the membrane potential reaches a threshold level, increas­ing membrane permeability and allowing charged ions such as sodium to move across the cell membrane.