6 



Survey of Neurobiology 



sion of excitation at the neuromyal junction 

 as well as at synapses in autonomic ganglia 

 and in the central nervous system. It also 

 is one that is being studied intensively by 

 a number of highly competent investiga- 

 tors. It can be said that the classic analyses 

 of the reflex arc, carried out by Sherrington 

 and his students, are being elaborated by 

 the use of still more potent techniques. The 

 old question of the nature of central ex- 

 citation and inhibition remains unan- 

 swered, but it should be noted that the at- 

 tack continues. Certainly the problem has 

 been well outlined and much useful infor- 

 mation has been obtained by recording po- 

 tential changes, both slow and rapid, from 

 gross electrodes on or in the cord and on 

 spinal roots, but the interpretation of some 

 of the results has been both difficult and 

 varied. Very recent studies by Eccles in 

 which potential changes within the somata 

 of individual spinal neurons were recorded 

 are apt to limit and reshape some of our 

 thinking about the intimate character of 

 synaptic processes. It is reasonable to sup- 

 pose that the extension of this technique 

 and the further testing, at the subcellular 

 level, of the hypothesis that basically both 

 excitation and inhibition are mediated by 

 chemical transmitters will lead to a sub- 

 stantial "break-through" in neurophysiol- 

 ogy- 



3. Studies of Single Units. Although 

 these have contributed greatly to those 

 branches of neurophysiology referred to in 

 paragraphs 1 and 2, they deserve separate 

 consideration. Directed toward an analy- 

 sis of the properties and activity of the in- 

 dividual neuron, such studies are the phy- 

 siological counterpart of those microscopic 

 investigations that led to the concept that 

 the neuron is the genetic and anatomical 

 unit of the nervous system. At the hands 

 of Adrian and Bronk and their students 

 and followers, the direct recording of the 

 electrical activity of single sensory and 

 motor fibers gave further evidence of the 

 all-or-none character of the response of 

 neurons, provided a picture of how sense 

 organs act in response to stimuli, demon- 

 strated a parallelism between the intensity 

 of an environmental stimulus and the num- 



ber of afferent impulses reaching the cen- 

 tral nervous system, and showed that fre- 

 quency of discharge in single motor neu- 

 rons is one way in which the magnitude of 

 an effector response is graded. The achieve- 

 ment of following the activity of single 

 units in the peripheral portions of the au- 

 ditory and visual systems indicated that 

 this method may be extended to the more 

 complex reaches of the nervous system. 

 It should be borne in mind that recent work 

 on cerebral mechanisms has been guided 

 and interpreted in terms of results obtained 

 by analyses of units in peripheral nerves 

 and in autonomic ganglia. 



Quite recently the electrical responses 

 of single thalamic neurons evoked by phy- 

 siological stimulation of cutaneous and 

 deep receptors of the limbs have been re- 

 corded. The results already obtained show 

 that the method employed is well adapted 

 for analyses of the functional organization 

 of the central mechanisms underlying sev- 

 eral somatic sensory modalities. Somewhat 

 similar techniques are being applied in 

 studies of units in the cerebellar and cere- 

 bral cortices. They will doubtless reveal 

 information that cannot possibly be ob- 

 tained by the relatively crude techniques of 

 electroencephalography. 



Several suggestions have been put for- 

 ward to the effect that activities within the 

 central nervous system are determined by 

 a mode of transmission that is extraneu- 

 ronal. While it would be undesirable to 

 ignore this possibility it seems wise to con- 

 clude that further advances in our under- 

 standing of cerebral functions will depend 

 primarily on the testing of hypotheses that 

 envisage central activity in terms of fiber 

 conduction and synaptic transmission. 



4. Transmission, Interaction and Control 

 of Reflexes. A definite advance was made 

 when Lloyd succeeded in dissecting the 

 segmental reflex discharge into its func- 

 tional components. The disclosure that 

 impulses in certain afferent fibers of mus- 

 cular origin activate a two-neuron arc 

 while impulses in others travel a path in- 

 volving interneurons led to work which 

 has given a better understanding of the 

 basic organization of spinal reflexes, of 



