"i;i 



II VNOHOOK OF I'lIYSIOI.nOY 



NKl'ROI'HYSIOI.OGY III 



i- rM iiwnvi considerations. Whatever t lie specific 

 attributes of neuron populations ma\ be, other than 

 those imbedded in a fixed structure, they can affect 

 function only by influencing the physiological proper- 

 ties at the neuronal and the synaptic levels. Changes 

 in neuron thresholds and in their spontaneous fluctua- 

 tions can dominate the decision as to discharge or no 

 discharge ol the neuron; and this is largely determined 

 l>\ the potential picture, across and along the mem- 

 brane. The membrane state, including that of trans- 

 mitter or receptor regions, also contributes to the 

 effectiveness of transmission across a junction and to 

 the ease of fixation the leaving of an enduring 

 change- -as a result of such junctional activity. 

 There is reason to believe that most conditions alter- 

 ing the neural machine do not act on the nerve fibers 

 primarily; these are ordinarily little affected as 

 compared with the perikarya and the synaptic por- 

 tions of the mechanisms. Only as neurons alter their 

 Bring patterns and as synapses pass or stop the im- 

 pulses reaching them is immediate behavior affected; 

 and only as the places of such activity are altered is 

 subsequent behavior changed. Perhaps subjective 

 experience can arise from changes in potential fields 

 and metabolic processes involving cells, aside from 

 the How of impulses; but the answer is at present bc- 

 yond us. 



Organelles and Fum lion 



The organelles of neurons serve the same basic 

 functions as those of other cells and, far from bags of 

 enzymes, are highly organized for their particular 

 functions as are neurons in their nets and masses. 

 I he genes and chromosomes are there to carry an- 

 cestral wisdom, the nuclear and surface membranes 

 to bound molecular space, the mitochondria to 

 control energy flow, and the microsomal particles to 

 preside oxer synthetic processes. But special condi- 

 tions and functions obtain for these in neurons, and 

 there are even special structures present (Abood). 



Nucleoli are solid protein and oversize (lower) 



related to the rapid protein synthesis by neurons. 



Iln vesicles at prejunctional endings may contain 



transmittei molecules and the discharge of these 

 has been related to physiological properties, especially 

 .it tin end plate (Fatt). [In some situations vesicles 

 are postjunctional ( jHi i, whi< h seen is to raise doubts 

 Some particulate materials are practically pure 

 mules (von Buler, Ortmann, fiillarp) 

 Membranes have highly specialized sites, usually 

 regul.nl and finely folded, at which chemical altera 



lions are transduced into potentials, largely by elec- 

 tron shifts at the intramolecular level (Fatt, Wald). 

 The thousand-layered zone of retinal rods, acting 

 like .1 photomultiplier tube, and the similarly piled-up 

 end plates or electroplaques (Grundfest), making a 

 voltaic pile, demonstrate the rich organizational 

 resources of protoplasm, as its synthetic and accumu- 

 lative ability is shown by the 40 per cent concentra- 

 tion of visual pigment in rods, the solid protein 

 nucleoli, and the nearly pure catechol amine gran- 

 ules in orthosympathetic neurons or their deriva- 

 tives. 



Related to special receptor sites, some neuron 

 regions seem adapted to maintain a relatively steady 

 potential level of variable magnitude, as in certain 

 dendrites; other regions depolarize explosively and, 

 after an all-or-none propagation, regain the initial 

 level. Thus, within a single neuron, there is a clear 

 separation of information storage and information 

 transmission. Finally, different membrane regions are 

 differentially sensitive to particular molecules, to 

 different ion concentrations and ratios, to the rate of 

 energy flow and creatine phosphate concentration, 

 etc., as already noted. 



Molecules and Memory 



Not only are neurons specialized for short-term 

 handling of information, they are also involved in 

 long-term storage. Whatever the locus and mecha- 

 nism of such engrain formation, the trace must in- 

 volve patterned changes of molecular arrangements 

 in organelles or of atoms in molecules, or both. 

 The number of bits held in memory strongly suggests 

 the involvement of protein or nuclein molecules, 

 since only polymers of such multiple units are sitlh- 

 cientlv rich in patterns to hold all the information. 

 It still remains, however, to demonstrate that ex- 

 perience, carried in nerve impulse trains and cell 

 potentials, can alter the char. icier of these molecules. 



[The 'specification' of an innervating nerve fiber to 

 lit the newly innervated muscle, noted by Sperry 

 (268), perhaps comes close to this. 2 ] A reasonable 

 transducing and storage mechanism has yet to be 



•Work recently reported by Morrell (913) promises .i 

 definite answei regarding the quantitative involvement of RNA 

 in storing experience. It some cortex area is made ictaJ with 

 alumina 1 ream, the equivalent crossed area, it supplied l>v both 

 callosal (specific) and deep (nonspecific) connections for a 

 week, will develop .1 l«i" threshold and become ictal. Ii then 

 fires, even when isolated .is .1 slab, l>..th spontaneous!) and 

 when triggered b) activit) in adjoining cortex. Neurons in 



