GENERAL ZOOLOGY 



Resting 

 + + + + + 



Membrane 



of 

 nerve fiber 



Na+ 



Na"^ 



Recovery 

 + + + 



Activity 



+ + 



Na+ 







I I 

 I I 



+ + + + + 

 Impulse 



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Excitation 

 - + + + + 



+ 



Na"^ 



-4^ 



K+ 



Na'* 



Na"^ 



Resting 

 + + + + + 



Na^ 



4' 



Na'^ 



K^ 



Fig. 4.21. Diagram showing movement of sodium (Na ) and potassium (K ) ions across succes- 

 sive regions of the membrane of a nerve fiber as a nerve impulse is conducted. Direction and 

 relative rates of diffusion are indicated by broken arrows, active transport by unbroken arrows. 



nervous system. Conduction of an impulse by a nerve fiber is correlated with 

 electrical changes, the magnitude and duration of which can be measured. 

 The electrical properties of a nerve fiber result from its associated ions. 

 When the fibec is not conducting, its interior is electronegative with respect 

 to its surface (Fig. 4.21). Let us assume that a stimulus has been received 

 in a receptor. This activity in the vicinity of a sensory nerve fiber increases 

 the permeability of its membrane. Its electrical properties are changed by a 

 rapid influx of positively charged sodium ions (Na^), and a nerve impulse 

 starts toward the central nervous system. If the metabolic stores of the nerve 

 fiber are adequate, the current generated by the movement of the ions stim- 

 ulates the adjacent area of the fiber where similar changes then occur. In 

 this way, the impulse is conducted or propagated along the entire length of 

 the fiber and into all its branches. In the wake of the propagated impulse, 

 positively charged potassium ions (K"*") move out of the fiber, restoring the 

 electrical characteristics of its surface. Also in the restitution phase, the 

 excessive sodium ions are removed from the interior of the fiber by active 

 transport, and it is ready to conduct another impulse, provided its metabolic 

 reserves are still adequate. During the restitution phase the fiber is incapable 

 of responding to a stimulus; it is said to be refractory. The refractory period 

 results in a discontinuity in the propagation of impulses along a nerve fiber. 

 In other words, nerve conduction is pulse-like. 



An interesting feature of nerve conduction is that the direction in which 

 the impulse moves in the nerve fiber is not reversible under physiological 

 conditions. Afferent neurons always conduct impulses toward the central 

 nervous system; efferent neurons always conduct impulses away from the 

 central nervous system. However, it can be shown that a nerve fiber is 

 capable of conducting an impulse in both directions from the site of an 

 experimental stimulus. The normal unidirectional movement of nerve im- 

 pulses is determined not by the nature of the nerve fiber but by the nature 

 of the synapse. Impulses can move across a synapse in only one direction. 



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