NEUROPHYSIOLOGY 1 AN INTEGRATION 



'935 



suggested. The steady turnover of neuron protoplasm, 

 up to several times daily, may be related to such 

 learning ability. The outsize nucleus, the presence of 

 large and variable amounts of ribonucleic acid 

 (RNA) in the cytoplasm, contributing to the Nissl 

 substance, the fast peripheral flow of enzymes and 

 protoplasm, and the high metabolism of neural 

 somata are all related to an unusually rapid rate of 

 synthesis of polynucleotides and proteins. 



If so, one can vaguely imagine that the flow of 

 impulses and electric fields could, like frequencv 

 modulation, alter the order and frequency of laying 

 down of nucleotide and amino acid moieties and so 

 control their pattern in the molecular tape that is 

 formed (145), or the configuration of the entire 

 molecule (Yuwiler, A., personal communication). 

 How such a coded molecular tape would, in turn, 

 control the synaptic and other properties of a neuron 

 so that impulses will enter and leave it in appropriate 

 relationships has not been faced, nor even tied to 

 the changes seen in microsomes (Abood; 79), end 

 knobs or apical dendrites (Galambos & Morgan; 

 260) on activation. The close relation between outer 

 membrane, endoplasmic reticulum and other cyto- 

 plasmic particulates may contain clues to the answer. 

 Nor has the problem of long-enduring memories, 

 far beyond the presumptive life of any particular 

 molecular inhabitant of a neuron, been effectively 

 handled (286). Indeed, the apparent storage of 

 learned behavior patterns in the essentially nonneural 

 tail of a planarian, which is able to regenerate .1 new 

 head so that the re-formed animal performs as well 

 as an intact trained one (198-200), almost demands 

 molecular templates and raises profound problems 

 as to the manner and locus of storage. (Learning 

 processes and mechanisms are considered further in a 

 later section.) 



DYNAMIC: ORGANIZATION 



Neuron Properties 



threshold and excitation. The neuron is the in- 

 tegrating element of the nervous system. It receives 

 multiple messages via its synaptic scale, is influenced 

 by the physical and chemical surround, and is modi- 

 fied by its own past experience; and to all these it 

 equates an output which may be as simple as a single 



such a locus, 'remembering' their over-activation, seemed, in 

 preliminary observations, to have accumulated RNA under 

 the perikaryon membrane. Further, Kreps (.163) is reported to 

 have shown an altered RNA turnover on conditioning. 



discharge-no discharge or as complex as an impulse 

 train with wide variance in the number and timing 

 of pulses — or even as two simultaneous trains differing 

 in pattern in two axons of a single cell body (36). 

 A unit fires when the excitation state at an appropriate 

 locus, presumably the proximal axon segment (Eccles, 

 Frank), exceeds threshold. Excitation is ordinarily 

 associated with brief and transient changes, mainlv 

 induced by synaptic activity and occurring as more 

 or less quantized increments that add in time and 

 space. Threshold is ordinarily a maintained state, 

 involving a slowly reversible or essentially irreversible 

 shift in properties 



Between the extreme cases of, say, a single supra- 

 threshold presynaptic impulse, on the one hand, 

 and a threshold level maintained by the intra- and 

 extracellular potassium ratio, and the attendant 

 membrane potential, on the other, there are probabl) 

 all possible gradations Nevertheless, it makes for 

 clarity to keep the distinction sharp and to examine 

 separately the control of threshold and the control 

 of excitation. It also seems useful to recognize the 

 shorter and the more enduring threshold shifts. A 

 shift in dendrite potential, induced by excitatory or 

 inhibitory terminals, constitutes a short threshold 

 change more than an actual excitation change (Bart- 

 lev). Thresholds are concerned primarily with 

 the storage of information, temporary or enduring; 

 excitation, with its transmission 



threshold control. The axon segment will fire 

 when its membrane potential falls to a critical level; 

 .is the pre-existing level is low or high, so is the 

 threshold. Both the maintained level and the quick 

 displacement of it can be influenced by surrounding 

 chemicals, by direct current potentials and the asso- 

 ciated Steady currents, and l>\ nerve impulses with 

 the accompanying synaptic potentials and edd) 

 currents. An instructive example of the interaction of 

 chemicals and impulses on neuron thresholds is af- 

 forded by the exaggerated sex behavior induced l>\ 

 pyriform lesions when sex hormones are present, 

 not in their absence (Gloor). A high external potas- 

 sium-to-calcium ratio can lower the axon segment 

 membrane potential so as to give repeated firing, 

 but this is unlikeK in the range of normal variation. 

 Sufficient impulse bombardment of an apical den- 

 drite can produce the explosive increase in membrane 

 permeability of an active response, and the resulting 

 eddy current may fire the axon, but present evidence 

 indicates that this is the exception rather than the 

 rule (Eccles, French). Ordinarily the threshold of 



