Biological Chemistry - Berezov, T. T., & Korovkin, B. F. 1998

Nervous Tissue
Chemical Basis of the Generation and Conduction of Nerve Impulses

Let us examine the chemical basis for the generation and maintenance of bioelectric potentials (resting and action potentials). Most researchers agree that the electrical polarization of Cells is driven by the uneven distribution of K⁺ and Na⁺ ions across The Cell membrane. The membrane exhibits selective permeability: it is highly permeable to K⁺ ions and significantly less so to Na⁺ ions. Furthermore, Nerve Cells employ a mechanism that keeps intracellular sodium levels low against the concentration gradient. This mechanism is known as the sodium pump.

Under specific conditions, membrane permeability to Na⁺ ions increases dramatically.

At rest, the inner surface of the cell membrane carries a negative electrical charge relative to the outer surface. This occurs because the number of Na⁺ ions pumped out of the cell by the sodium pump is not entirely offset by the influx of K⁺ ions. As a result, a fraction of sodium cations is held by an inner layer of counterions (anions) on the outer surface of the cell membrane. Consequently, an electrical potential difference (transmembrane electrical potential difference) is maintained across the membranes bounding nerve cells; these membranes are electrically excitable.

Upon excitation triggered by a given stimulus, the permeability of the nerve cell membrane (axon) changes selectively: it increases markedly for Na⁺ ions (by approximately 500-fold) while remaining unchanged for K⁺ ions. As a result, Na⁺ ions rush into the cell. The compensatory outward flow of K⁺ ions is somewhat delayed. This leads to The Development of a negative charge on the outer surface of the cell membrane. The inner surface of the membrane becomes positively charged; the cell membrane (specifically the axon membrane, i.e., the nerve fiber) undergoes charge reversal, generating an Action Potential, or spike. The duration of the spike does not exceed 1 ms. It consists of a rising phase, a peak, and a falling phase. The falling phase (potential drop) is associated with the growing dominance of K⁺ efflux over Na⁺ influx, returning the Membrane Potential to baseline. Following impulse conduction, the resting state is restored within the cell. During this period, the Na⁺ ions that entered the neuron upon excitation are exchanged for K⁺ ions. This transfer occurs against the concentration gradient, since the extracellular environment surrounding Neurons contains a much higher concentration of Na⁺ ions than the cell interior immediately following excitation. As noted, the movement of Na⁺ ions against the concentration gradient is driven by the sodium pump, which requires ATP energy to function. Ultimately, all of this restores the initial intracellular concentrations of potassium and sodium cations within the cell (axon), preparing the nerve to receive the next excitation impulse. Notably, myelin sheaths formed by Schwann cells envelop nerve fibers and act as electrical insulators. This insulating layer covers the majority of nerve fibers and significantly accelerates the Propagation of the electrical wave (signal); ions enter and exit the cell exclusively at the nodes where the insulator is absent. As previously mentioned, the myelin membrane is composed of Phospholipids—specifically sphingomyelin—along with Cholesterol, Proteins, and glycosphingolipids. Certain conditions, such as multiple sclerosis, are characterized by demyelination and impaired Nerve Impulse Conduction. Another equally vital process in neural tissue is the transmission of nerve impulses from one nerve cell to another or to effector organ cells.