82 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



gating toward B. Simultaneously, another impulse 

 starting at B should travel toward A. Then, the im- 

 pulses are bound to undergo a collision at a point 

 about half way between the two sites of stimulation 

 of the axon. After such a head-on collision, it should 

 be impossible for the two impulses to travel further 

 since the region where the impulse from A is heading 

 is freshly traversed by the impulse from B and is conse- 

 quently incapable of carrying another impulse. The 

 same thing can be said of the region on the other side 

 of the site of collision. 



In the bottom record of figure 6, the two stimu- 

 lating shocks are delivered in such a way that the two 

 impulses collide exactly at the site of recording. This 

 is accomplished by adjusting the delays of the two 

 shocks after the start of the sweep of the oscillograph 

 beam in such a manner that the respon.se to shock A 

 alone appears at the same spot on the oscillograph 

 screen as the response to shock B alone. Delivery of 

 two shocks under these conditions elicits, as can be 

 seen in the figure, only one response which has 

 almost the same configuration as the respon.se to one 

 shock. The shock response interval is known to be 

 slightly reduced by collision. A further discussion on 

 this topic may be found elsewhere (120). 



d) Multiplication of impulses at the branching point of a 

 nerve fiber. Histological studies indicate that vertebrate 

 motor nerve fibers frequently undergo dichotomy or 

 ramification at nodes of Ranvier, one mother fiber 

 giving rise to two (or more) daughter fibers [cf. e.g. 

 Eccles & Sherrington (26)]. During the course 

 of isolating single nerve fibers innervating the toad 

 gastrocnemius muscle, such branching motor fibers 

 are sometimes encountered. It has been shown 

 in such preparations that the muscle tension developed 

 by stimulation of the mother fiber (with two daughter 

 fibers intact) is nearly twice as great as the tension 

 observed after severing one of the daughter fibers. 

 Obviously, this indicates that the iinpulse travelling 

 down the mother fiber invades the two daughter 

 fibers. By this process of successive dichotomy, an 

 impulse travelling along a motor nerve fiber multi- 

 plies itself before it reaches a large number of muscle 

 fibers. 



Sensory nerve fibers generally dichotomize as they 

 approach their peripheral endings. They also branch 

 off many collaterals in the spinal cord. It is generally 

 believed that impulses multiply themselves at the.se 

 bifurcating points. In the squid axons, multiplication 

 of impulses at bifurcation points has also been ob- 

 served. 



e) Interaction between nerve fibers. When a group 



of fibers in a nerve trunk carries nerve impulses, it 

 never happens, under ordinary experimental condi- 

 tions, that these impulses are transmitted to the other 

 surrounding nerve fibers. This can be shown bv the 

 following simple observation. 



The gastrocnemius muscle of the toad or frog is 

 innervated by a small nerve twig branching off from 

 the large tibial nerve which innervates also plantar 

 muscles and the skin of the foot. Stimulation of the 

 tibial nerve at a point distal to the exit of the muscle 

 nerve to the gastrocnemius does not evoke any po- 

 tential variation in the muscle nerve nor any contrac- 

 tion in the muscle. Such a stimulus sets up a ' volley 

 of impulses' in the majority of the fibers in the tibial 

 nerve, but these impulses do not spread to the nerve 

 fibers entering the muscle. 



It has been found, however, that there is a very 

 weak, barely detectable interaction between the 

 nerve fibers in a common nerve trunk. Otani (96) 

 found that, when the peroneal branch of the sciatic 

 nerve carries a volley of impulses, the threshold for 

 the fibers from the tibial branch undergoes a transient 

 change. This observation was confirmed and ex- 

 panded by several investigators, notably by Marrazzi 

 & Lorente de No (85). This result is now interpreted 

 on a purely electrical basis: when a group of fibers 

 carries impulses, the fluid in the intercellular space is 

 traversed by action currents developed by these active 

 fibers. If a stimulating current pulse is delivered in 

 this region of nerve, the effect of the stimulus is 

 modified when it is superposed on or antagonized by 

 the action currents. The maximum change in thresh- 

 old is of the order of 10 per cent. 



If the mechanism of interaction between nerve 

 fibers is electrical in nature, it would be expected that 

 the interaction should be greatly enhanced by re- 

 ducing the shunting effect of the fluid medium around 

 the nerve fiber. Katz & Schmitt (73) have shown 

 that this is actually the case. 



The diagram at the top of figure 7 illustrates their 

 experimental arrangement. Two nerve fibers of the 

 crab were immersed in a pool of mineral oil. Fiber I 

 was stimulated with electrodes A and B and its re- 

 sponse was observed by means of the recording elec- 

 trodes D and E in the figure. At about the time of 

 arrival of an impulse from B at the site of recording, 

 testing current pulses were delivered through elec- 

 trodes C and D to determine changes in threshold of 

 fiber II at D. The triphasic curve at the bottom of 

 figure 7 is the time course of the threshold changes 

 observed. Katz & Schmitt explained these results, 

 with good reason, as due to the flow of the action cur- 



