XVIII 



THE NERVE IMPULSE 



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(Readings: B. Katz, "The Nerve Impulse," Sci. Am. 187, No. 5, 55-64, Nov. 

 1952, Reprint No. 20. This excellent article contains what you really need to 

 know. Read also R. D. Keynes, "The Nerve Impulse and the Squid," Sci. Am. 

 199, No. 6, Dec. 1958, Reprint No. 58; Weisz, pp. 475^80; Villee, pp. 354-358.) 



One of the most important aspects of animal 

 evolution is the development of systems of rapid 

 intercommunication through nerve cells. Even 

 some of the protists, notably the ciliates, possess 

 intercommunicating systems of fibrillae that 

 seem to help to integrate the cell's motions, and 

 may be thought of as a sub-cellular nervous 

 system. The coelenterates possess a diffuse nerve 

 net. All the higher animals possess nervous 

 systems made up of discrete, intercommunicating 

 nerve cells. 



One nerve cell meets another at a boundary 

 called a synapse. Though a nerve cell conducts 

 impulses equally well in both directions, a 

 synapse transmits impulses only in one direction. 

 It is this characteristic that limits nervous trans- 

 mission to particular pathways and particular 

 lines of flow. In general we distinguish excita- 

 tions flowing into the central nervous system 

 from the receptors (afferent) from excitations 

 flowing out of the central nervous system 

 (efferent) toward the effectors (the muscles and 

 glands). 



* The instructor should consult Appendix B for 

 information about the electronic equipment. 



A nerve cell consists of a cell body containing 

 the nucleus and its cytoplasm, from which 

 springs a specialized, threadlike, conducting ele- 

 ment, the nerve fiber or axon. In a higher animal 

 the nerve cell bodies are all inside or just beside 

 the central nervous system — the brain and cen- 

 tral nerve cord. The nerves that one finds roam- 

 ing about the body are bundles of axons. 



The business of a nerve axon is to conduct an 

 excitation. If we dissect a nerve out of the body 

 together with the muscle that it innervates (for 

 example, the frog sciatic nerve and the gastro- 

 cnemius muscle), we can stimulate one end of the 

 nerve and know that it has transmitted an ex- 

 citation by the fact that a moment later the 

 muscle contracts. As the nerve conducts its 

 excitation, sufficiently delicate instruments can 

 measure the passage of an electrical change. 

 This electrical change, which invariably ac- 

 companies the nerve response, is the nerve im- 

 pulse, action potential, or action current. 



An electric current is a flow of electrons from 

 a region in which they are more concentrated to 

 a region of lower concentration: i.e., from a 

 more negatively to a more positively charged 

 region. By an odd historical convention, the 



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