A nerve impulse is conducted along an axon much as a flame travels along the fuse of a firecracker. This electrochemical process involves the generation of an action potential, a wave of membrane depolarization that is initiated at the initial segment of the axon hillock. Its membrane contains a large number of voltage-gated Na and K channels. In response to a stimulus, voltage-gated Na channels in the initial segment of the axon membrane open, causing an influx of Na into the axoplasm. This influx of Na briefly reverses (depolarizes) the negative membrane potential of the resting membrane (70 mV) to positive (30 mV). After depolarization, the voltage-gated Na channels close and voltage-gated K channels open. K rapidly exits the axon, returning the membrane to its resting potential. Depolarization of one part of the membrane sends electrical current to neighboring portions of unstimulated membrane, which is still positively charged. This local current stimulates adjacent portions of the axon’s membrane and repeats depolarization along the membrane. The entire process takes less than one thousandth of a second. After a very brief (refractory) period, the neuron can repeat the process of generating an action potential once again.
Rapid conduction of the action potential is attributable to the nodes of Ranvier.
Myelinated axons conduct impulses more rapidly than unmyelinated axons. Physiologists describe the nerve impulse as “jumping” from node to node along the myelinated axon. This process is called saltatory (L. saltus, to jump) or discontinuous conduction. In myelinated nerves, the myelin sheath around the nerve does not conduct an electric current and forms an insulating layer around the axon. However, the voltage reversal can only occur at the nodes of Ranvier, where the axolemma lacks a myelin sheath. Here, the axolemma is exposed to extracellular fluids and possesses a high concentration of voltage-gated Na and K channels (see Figs. 12.13 and 12.20). Because of this, the voltage reversal (and, thus, the impulse) jumps as current flows from one node of Ranvier to the
FIGURE 12.17 • Protoplasmic astrocyte in the gray matter of the brain. a. This schematic drawing shows the foot processes of the protoplasmic astrocyte terminating on a blood vessel and the axonal process of a nerve cell. The foot processes terminating on the blood vessel contribute to the blood–brain barrier. The bare regions of the vessel as shown in the drawing would be covered by processes of neighboring astrocytes, thus forming the overall barrier. b. This laser-scanning confocal image of protoplasmic astrocyte in the gray matter of the dentate gyrus was visualized by intracellular labeling method. In lightly fixed tissue slices, selected astrocytes were impaled and iontophoretically injected with fluorescent dye (AlexaFluor 568) using pulses of negative current. Note the density and spatial distribution of cell processes. 480. (Reprinted with permission from Bushong EA, Martone ME, Ellisman MH. Examination of the relationship between astrocyte morphology and laminar boundaries in the molecular layer of adult dentate gyrus. J Comp Neurol 2003;462:241–251.)
FIGURE 12.18 • Fibrous astrocytes in the white matter of the brain. a. Schematic drawing of a fibrous astrocyte in the white mater a bb of the brain. b. Photomicrograph of the white matter of the brain, showing the extensive radiating cytoplasmic processes for which astrocytes are named. They are best visualized, as shown here, with immunostaining methods that use antibodies against GFAP. 220. (Reprinted with permission from Fuller GN, Burger PC. Central nervous system. In: Sternberg SS, ed. Histology for Pathologists. Philadelphia: Lippincott-Raven, 1997.)