Saltatory conduction

The glial cells support the neurons physically. Certain glial cells (oligoden-droglial cells) synthesize myelin, a fatty insulation layer wrapped around the axons. Myelin is necessary for the so-called saltatory conduction of electrical... 

Signal travels by local current flow or by saltatory conduction... 

Saltatory conduction results in a significant increase in the velocity of conduction of the nerve impulse down the axon compared to that of local current flow in an unmyelinated axon (see Table 4.2). The speed of conduction is... 

Figure 4.4 Saltatory conduction. Transmission of electrical impulses in a myelinated axon occurs by way of saltatory conduction. Composed primarily of lipid, the myelin sheath insulates the axon and prevents generation of membrane potentials. Membrane potentials occur only at gaps in the myelin sheath, referred to as the nodes of Ranvier. Therefore, transmission of the impulse, or generation of action potentials, occurs only at the nodes.

Myelin affects axonal structure. The presence of a myelin sheath affects the structure of the axon that it surrounds [5], presumably optimizing its properties for transmission of action potentials by saltatory conduction. Generally, one of the effects of myelin is to increase axonal diameter by inducing biochemical changes in components of the axonal cytoskeleton such as neurofilaments (see Ch. 8). The effects of myelin on axonal structure imply... 

Conduction of the action potential in myelinated axons is called saltatory conduction. Because ion flux only occurs at the nodes of Ranvier, the action potential jumps, in effect, from node to node. This provides two advantages, speeding the rate of conduction and reducing the metabolic cost of an action potential, because energy-dependent ion transporters are not needed along myelinated segments. 

The axon is effectively insulated from the surrounding medium by the myelin sheets except for special regions, the nodes of Ranvier, which lie at 1-to 2-mm intervals along the nerve. The nerve impulse in effect jumps from one nerve to the next. This saltatory conduction occurs much more rapidly (up to 100 m / s) than conduction in unmyelinated axons. It depends upon Na+ and K+ channels that are concentrated in the nodes of Ranvier. 

Figure 16.2 Saltatory conduction. Myelin acts as an insulator to prevent current loss as the action potential travels down the axon. Sodium and potassium channels are clustered at the Nodes of Ranvier, where there is no myelin. Action potentials jump from one node to the next, reducing the overall membrane area involved in conduction, and speeding up electrical transmission.

Hirota N, Kaji R, Bostock H, Shindo K, Kawasaki T, Mizutani K, Oka N, Kohara N, Saida T, Kimura J (1997) The physiological effect of anti-GMl antibodies on saltatory conduction and transmembrane currents in single motor axons. Brain 120 2159-2169. 

Figure 20-6 n Representation of the transmission of a nerve impulse along a neuron fiber by saltatory conduction. 

Conduction velocity of an action potential can be increased dramatically by the presence of myelin, a fatty sheath that surrounds the axon and that is interrupted into gaps every millimeter or so at the nodes of Ranvier. Myelin is elaborated by Schwann cells in the peripheral nervous system and by oligodendrocytes in the central nervous system (the biochemistry of myelin will be discussed later in the article). The presence of myelin will dramatically alter the mode and velocity of conduction of the action potential in the axon. As in unmyelinated nerves, the action potential is still transmitted from one section of the axon to another by the presence of local circuit currents. However, the fatty sheath of myelin has poor conduction properties and therefore acts as an insulator. Hence, the local circuit currents jump from one gap to another at the nodes of Ranvier and the rate of conduction is enhanced as local circuit currents travel faster than the action potential itself This process of discontinuous conduction is known as saltatory conduction. Numerous diseases involving myelin deficiency have been described clinically. As one might predict, demyelinating diseases have profound effects on neuronal conduction and on the well-being of the patient. A few of these conditions will be described briefly in the upcoming section on myelin biochemistry. 

There are breaks in the sheath, between supporting cells, called Nodes ofRanvier. These allow for currents to flow to support propagation of the action potential. The action potential, and associated currents, proceed by jumping from one node to the next (called saltatory conduction) at a rate of up to 150 m/s. 

Fig. 15. Possible interactions at the level of divalent cation bridges. L is phospholipid ligand is divalent cation M is monovalent cation A is monovalent anion. Actual bridge occurs in case (a) only. This can be represented as shown, or either as L-D-L or L"D L. In all other cases the D-bridge is severed. Electrical repulsion oc -curs in cases (d) (excess D++) and (e) (removal of by depolarizing potential E"). E is either applied by electrode or transmitted through saltatory conduction.

Fine Structure of Nerves. The conducting elements of the nervous system are the long nerve fibers, most of whieh are clad in relatively thick myelin sheaths (myelin-containing nerves). The sheath is discontinuous at the rings of Ranvier and only a thin membrane separates the neuroplasm from its surroundings at these points. Neural excitation leaps from one such ring to the next (saltatory conduction). 

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