The method for measuring the membrane potential is simple in theory but often difficult in practice because of the small size of most of the fibers. Figure 5-2 shows a small pipette filled with an electrolyte solution. The pipette is impaled through the cell membrane to the interior of the fiber. Another electrode, called the “indifferent electrode,” is then placed in the extracellular fluid, and the potential difference between the inside and outside of the fiber is measured using an appropriate voltmeter. This voltmeter is a highly sophisticated electronic apparatus that is capable of measuring small voltages despite extremely high resistance to electrical flow through the tip of the micropipette, which has a lumen diameter usually less than 1 micrometer and a resistance more than a million ohms. For recording rapid changes in the mem brane potential during transmission of nerve impulses, the microelectrode is connected to an oscilloscope, as explained later in the chapter.

Fig1. Measurement of the membrane potential of the nerve  fiber using a microelectrode.

The lower part of Figure 2 shows the electrical potential that is measured at each point in or near the nerve fiber membrane, beginning at the left side of the figure and passing to the right. As long as the electrode is outside the nerve membrane, the recorded potential is zero, which is the potential of the extracellular fluid. Then, as the recording electrode passes through the voltage change area at the cell membrane (called the electrical dipole layer), the potential decreases abruptly to −90 millivolts. Moving across the center of the fiber, the potential remains at a steady −90-millivolt level but reverses back to zero the instant it passes through the membrane on the opposite side of the fiber.

Fig2. Distribution of positively and negatively charged ions in  the extracellular fluid surrounding a nerve fiber and in the fluid inside  the fiber. Note the alignment of negative charges along the inside  surface of the membrane and positive charges along the outside  surface. The lower panel displays the abrupt changes in membrane  potential that occur at the membranes on the two sides of the fiber.

To create a negative potential inside the membrane, only enough positive ions to develop the electrical dipole layer at the membrane itself must be transported out ward. All the remaining ions inside the nerve fiber can be both positive and negative, as shown in the upper panel of Figure 2. Therefore, transfer of an incredibly small number of ions through the membrane can establish the normal “resting potential” of −90 millivolts inside the nerve fiber, which means that only about 1/3,000,000 to 1/100,000,000 of the total positive charges inside the fiber must be transferred. Also, an equally small number of positive ions moving from outside to inside the fiber can reverse the potential from −90 millivolts to as much as +35 millivolts within as little as 1/10,000 of a second. Rapid shifting of ions in this manner causes the nerve signals discussed in subsequent sections of this chapter. 

مواضيع ذات صلة

BASIC PHYSICS OF MEMBRANE POTENTIALS

ACTIVE TRANSPORT” OF SUBSTANCES THROUGH MEMBRANES

The Cell Membrane Consists of A Lipid Bilayer With Cell Membrane Transport Proteins

Apoptosis—Programmed Cell Death

Cilia and Ciliary Movements

تركيب الخلية Structure of the cell

LYSOSOMES

السيطرة على انقسام الخلية

الظاهرة الأسموزية

طرق انتقال المواد من وإلى الخلية من خلال الغشاء

انتقال المواد الغذائية من وإلى الخلية عبر غشاء الخلية

خصائص البروتوبلازم