Nervous System 



363 



never yields muscular contraction (Gutmann, 

 '45; Weiss and Edds, '45). Cross unions 

 between somatic, sympathetic and parasym- 

 pathetic nerves that have been tried in 

 various combinations likewise are physio- 

 logically sterile. Cholinergic and adrenergic 

 nerves fail to achieve physiological innerva- 

 tion of each other's peripheries (Langley and 

 Anderson, '04; Dale, '35), not because of 

 lack of regenerative penetration, but because 

 of transmissive failure of the terminal junc- 

 tion. 



The fact of neuro-terminal selectivity is 

 proof of the existence of specific protoplasmic 

 differences both among the major classes of 

 neurons (sensory, motor, etc.) and among 

 the corresponding terminal tissues. Within 

 each class, however, transmissive junctions 

 can be made indiscriminately. Any motor 

 nerve shows functional affinity to any skeletal 

 muscle (Weiss, '37a; Weiss and Hoag, '46); 

 cross connections of different kinds of sensory 

 nerves have likewise been effected success- 

 fully (Anokhin, '35), and the paradoxical 

 sensations noted after irregular sensory nerve 

 regeneration in man (Stopford, '30) also 

 indicate interchangeability within the sen- 

 sory field. There are additional finer func- 

 tional selectivities, beyond those controlling 

 junction, but these are imposed upon the 

 connected neurons from their endings and 

 will be discussed later (see p. 384). 



Synaptic Connections. Natiurally, the ques- 

 tion arises whether specificities similar to 

 those observed in peripheral connections 

 govern the establishment of central synaptic 

 junctions. In the few instances thus far ex- 

 amined, intracentral neurons have shown a 

 remarkable lack of discrimination in making 

 terminal connections. Limb buds inserted 

 into gaps of the embryonic neural tube 

 receive effective motor innervation from 

 central fiber tracts (Nicholas, '33), and the 

 central gray matter of isolated fragments 

 of larval spinal cord or medulla oblongata 

 establishes fully functional connections with 

 both muscles and skin in the complete ab- 

 sence of primary motor and sensory neurons 

 (Weiss, '50a). The promiscuity of junctional 

 relations manifested in these cases contrasts 

 sharply with the acute selectivity of func- 

 tional response relations (see below, p. 384), 

 which throws doubt upon any theory ex- 

 plaining the latter purely in terms of specific 

 anatomical connections. The fact that a junc- 

 tion capable of transmitting an impulse has 

 been established does not explain when and 

 how it will be actuated in the coordinated 

 group activities of the centers, nor indeed 

 whether or not it will be used at all. Whether 



disuse entails eventual rupture of junctions 

 has not yet been clearly decided. 



AXON GROWTH 



Growth of Axon Caliber. While the elonga- 

 tion of the nerve fiber is essentially a phe- 

 nomenon of protoplasmic convection, it 

 proceeds pari passu with real growth, that 

 is, increase of the total protoplasmic mass 

 of the neuron, and is actually sustained by 

 the latter. This growth process continues 

 after the fiber has reached its final length 

 and can, in fact, best be studied during that 

 later period, when all further protoplasmic 

 gain accrues solely to the width of the fiber. 

 Since the eventual caliber of the axon, usu- 

 ally referred to as "fiber size," is of con- 

 siderable functional significance, as it deter- 

 mines velocity of impulse conduction, thres- 

 holds of excitability and susceptibility to 

 noxious agents, etc. (Erlanger and Gasser, 

 '37), a study of axonal growth offers both 

 physiological and developmental interest. 

 Nerve fiber caliber increases as animals 

 grow to mature size (Hursh, '39). 



Analytical information on axon growth 

 is mostly derived from recent experiments 

 dealing with the restoration of fiber diameter 

 in regenerated nerve fibers (Weiss and 

 Hiscoe, '48). These experiments are schemati- 

 cally summarized in Figure 135, which shows 

 a series of mature neurons in various stages 

 of normal (A-E) and modified (F-/) re- 

 generation. The nucleated cell body is at the 

 left, the peripheral end-organ at the right, 

 both connected by the neurilemmal tube that 

 envelops the fiber. From the proximal stump 

 of the severed fiber (fi), a thin axonal sprout 

 advances toward the periphery (C), effects 

 peripheral connection (D) and gradually 

 grows in width until it approximates its 

 old caliber and the width of the tube (£). 

 This recovery, however, can be significantly 

 impeded if one constricts the distal stump and 

 thereby reduces the diameter of all tubes at 

 a given spot. At first, regeneration proceeds 

 normally (compare D and F), but as soon 

 as the axon has reached the girth at which it 

 fills the narrow (constricted) part of the 

 tube, the portion lying distally to the con- 

 striction ceases to gain in width, while at 

 the proximal side of the constriction excess 

 axoplasm begins to pile up in configurations 

 such as are ordinarily assumed by a steadily 

 propelled column of plastic material sud- 

 denly faced with an obstruction ( G, H). If, 

 later, the constriction is released allowing 

 the tube to re-expand, the damraed-up mate- 



