Ependymal lining of the spinal canal, a. Photomicrograph of the center region of the spinal cord stained with toluidine blue. The arrow points to the central canal. x20. b. At higher magnification, ependymal cells, which line the central canal, can be seen to consist of a single layer of columnar cells. x340. (Courtesy of Dr. George D. Pappas.) c. Transmission electron micrograph showing a portion of the apical region of two columnar ependymal cells. They are joined by a junctional complex (JC) that separates the lumen of the canal from the lateral intercellular space. The apical surface of the ependymal cells has both cilia (C) and microvilli (M). Basal bodies (BB) and a Golgi apparatus (G) within the apical cytoplasm are also visible, x20,000. (Courtesy of Dr. Paul Reier.)
ated 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). Following 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 due 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 elec-
nerve fibers oligodendrocyte
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