Neurophysiology

The brain is an electrical organ, and the conduction of electrical impulses from one neuron to another is fundamental to brain activity. A variety of neuro-transmitters serve as chemical messengers between neurons, but transmission of impulses within each neuron is equally critical. The continuous process of neuronal communication depends on the integrity of white matter. Myelinated fibers are essential for the rapid and efficient propagation of neuronal information.

The basic physiologic phenomenon in neurons, whether in the central or peripheral nervous system, is the action potential. This is an electrical event in which the neuronal membrane is rapidly and transiently reversed from its resting potential of —70 mV by the influx of positively charged sodium ions. This ''all-or-none'' potential quickly ends with the efflux of potassium ions, and then follows a short time, the refractory period, during which no impulse can be

Figure 2 Major white matter tracts in the cerebral hemispheres. UF, U or arcuate fibers; SOFF, superior occipitofrontal fasciculus; IOFF, inferior occipitofrontal fasciculus; AF, arcuate fasciculus; C, cingulum, UnF, uncinate fasciculus; CC, corpus callosum (reprinted with permission from C. M. Filley, Neurobehavioral Anatomy, p. 188, Univ. Press of Colorado, Niwot, 1995).

Figure 2 Major white matter tracts in the cerebral hemispheres. UF, U or arcuate fibers; SOFF, superior occipitofrontal fasciculus; IOFF, inferior occipitofrontal fasciculus; AF, arcuate fasciculus; C, cingulum, UnF, uncinate fasciculus; CC, corpus callosum (reprinted with permission from C. M. Filley, Neurobehavioral Anatomy, p. 188, Univ. Press of Colorado, Niwot, 1995).

propagated. The resting membrane potential is restored by the action of the sodium-potassium pump, an energy-dependent process requiring the hydrolysis of adenosine triphosphate (ATP), in which sodium is extruded from the cell and potassium taken in.

The primary role of myelin is to increase the conduction velocity of neurons as they propagate action potentials. Myelin is laid down in such a fashion that short segments of the axon, called nodes of Ranvier (or simply nodes), are left unmyelinated (Fig. 1). These nodes contain high concentrations of sodium channels, and the action potential can "jump" sequen tially from one node to the next as the depolarizing current shunts the current rapidly forward along the axon (Fig. 3). This phenomenon, known as ''saltatory conduction,'' greatly increases neuronal conduction velocity.

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