Home > Conscious > Chapter 2 > 2.2. Nerve Cell Membrane



Figure 2-6. Schematic drawing of a nerve cell membrane. [Source: OpenStax]

Like other cells, a neuron contains a membrane separating the intracellular materials from the extracelluar fluid (Figure 2-6). The nerve cell membrane is formed by a phospholipid bilayer, embedded with a variety of ion channels and other proteins. Many channels are ion-selective. They allow a particular type of ions to pass through while excluding others. For instance, the sodium channels are permeable to Na+ while excluding K+, whereas the potassium channels select K+ over Na+. The generation of nerve impulses involve two classes of ion channels: voltage-gated sodium channels and voltage-gated potassium channels.

Voltage is the electric potential difference between two points. Cell membrane voltage is defined as the intracellular potential minus the extracellular potential. Usually the potential of the extracellular fluid is set to zero, then the cell membrane voltage is equal to the intracellular potential, called the "membrane potential" or "membrane voltage". In the resting state, the membrane potential is approximately -70 mV (Figure 2-7). Depolarization means that the membrane potential is higher than the resting value, and hyperpolarization refers to lower membrane potential than the resting value.


Figure 2-7. Membrane voltage (membrane potential). The electric potential of the extracelluar fluid is usually set to zero, so that the cell membrane voltage is equal to the intracellular potential, called the membrane potential. In the resting state, the membrane potential is about -70 mV. [Source: OpenStax]

The concentration of Na+ ions in the extracellular fluid is significantly higher than in the cytosol. The concentration of K+ ions, however, is higher in the cytosol than in the extracellular fluid. This concentration difference is due to the Na+ / K+ -ATPase (also known as Na+ / K+ pump), which is an enzyme that can move three Na+ ions out of the cell and simultaneously two K+ ions into the cell (Figure 2-8). This specific ion concentration difference is essential for normal cellular operations.


Figure 2-8. The Na+ / K+ pump is responsible for maintaining the concentration differences of Na+ and K+ ions in the extracellular fluid and cytosol. [Source: Wikipedia]