456 - Multicellular Animals, Especially Man 



principal sensory and motor pathways (p. 

 463) in the nervous system. Likewise, a study 

 of axon regeneration, which follows the 

 channel of degeneration in the central nerv- 

 ous system, has yielded a great deal of con- 

 firmatory evidence. 



Recent studies indicate that the cell body 

 has other important functions. Neurosecre- 

 tory products, apparently, are synthesized 

 mainly in the centron. They are then carried 

 by a sort of "peristalsis" along the fibers to 

 the tips (p. 414), where storage and utiliza- 

 tion occur. But even more important, per- 

 haps, is the fact that the cell body, together 

 with its associated dendrites, is not just a 

 passive transmitter of such impulses that are 

 brought to it. These parts may profoundly 

 modify the pattern of such impulses, before 

 passing them on. This newly discovered fact 

 has had great impact upon current thinking 

 on the question of how the nervous system 

 achieves its complex integrative functions. 



Some nerve cells discharge impulses spon- 

 taneously in rhythmic fashion. This phe- 

 nomenon was demonstrated most clearly by 

 T. H. Bullock of the University of California, 

 working on the pacemaker neurons of the 

 heart of the lobster. An exceedingly fine 

 microelectrode (p. 428) inserted into the 

 centron of such a cell reveals an alternating 

 series of depolarizations and repolarizations 

 in the membrane potential of the dendritic 

 region of the cell. As depolarization reaches 

 a critical level, one or more spikelike action 

 potentials are fired, causing cardiac contrac- 

 tion. Immediately after the spike, however, 

 repolarization commences, and the next ac- 

 tion potential is not discharged until an- 

 other depolarization occurs in the changing 

 cycle of the "pacemaker potential." 



Impulses coming to a pacemaker neuron 

 across synaptic contacts from other neurons 

 do not directly lead to the firing of impulses 

 to the heart. They merely change the rate of 

 firing. Some fibers cause acceleration and 

 others deceleration; and presumably these 

 effects are mediated by two different synaptic 

 transmitter substances. A neuron does not 



necessarily fire off an exact carbon copy of 

 the pattern of impulses received by its den- 

 drites. 



The generator potential, previously dis- 

 cussed (p. 428), provides another example of 

 specialization and versatility among neuronal 

 cells and synaptic junctions. Some small neu- 

 rons in the central nervous system, appar- 

 ently, do not handle true nerve impulses — 

 conducted in all-or-none fashion and without 

 decrement — as do the axons of most nerve 

 cells. They conduct slow changes of potential 

 that suffer decrement during propagation. 

 Impulses arriving at a synapse may either 

 raise or lower the dendritic membrane po- 

 tential of the postsynaptic cell. Also they 

 may either excite or inhibit, depending upon 

 several factors, such as the intrinsic nature of 

 the presynaptic and the postsynaptic cells, 

 and local conditions, both past and present. 

 Presumably these activities are mediated by 

 synaptic transmitter substances. However, 

 much remains to be learned, although the 

 known mechanisms are sufficiently complex 

 and versatile to account for the great com- 

 plexity and versatility of the nervous system. 



"One-Way" Conductivity: The Law of 

 "Forward Direction." In the intact animal, 

 sensory impulses always travel inward, to- 

 ward the central nervous system, and motor 

 impulses always pass outward from the CNS. 

 However, this law of "forward direction" is 

 determined, not by the conductivity of the 

 neurons, but by the fact that synapses can 

 conduct in one direction only. An excised 

 nerve will conduct impulses equally well in 

 either direction, but in the body an impulse 

 traveling "backward" through a neuron is 

 always stopped when it reaches a synapse. In 

 other words, impulses may be transmitted 

 from axons to dendrons in successive neu- 

 rons, but not from dendrons to axons. 



The "two-way" conductivity of nerve fibers, 

 in contrast to the "one-way" conductivity of 

 the synapses, may be demonstrated by experi- 

 ments in which the nerves of an animal such 

 as a frog are exposed and stimulated directly. 

 The simplest method is to expose a motor 



