A Neurophysiological Speculation Concerning Learning

A NEUROPHYSIOLOGICAL speculation CONCERNING LEARNING STEPHEN THESLEFF* Learning is generally considered dependent upon plastic changes in the nervous system. Such changes could be impulses circulating continuously in closed self-re-exciting chains ofneurones (i) or synaptic facilitation by usage (2). Attempts have also been made to explain memory and conditioning as the growth and development ofsynapses (3,4). This note describes a somewhat different type ofsynaptic change which may, in part, contribute to the plasticity ofthe nervous system. So far, this change has been demonstrated only at the neuromuscularjunction, but the similarity ofthe synaptic transmissionprocesses in muscle andinnerve suggest that it could also be present in nerve cells. In skeletal muscle it has been shown that the size ofthe postjunctional chemoreceptor area is controlled by influences from the motor nerve (5). The main features of this control can be summarized as follows: 1)A skeletal muscle cell at an embryonic stage ofinnervation, or when denervated, is uniformly sensitive to the chemical transmitter agent on its entire surface (6, 7). 2)When functional innervation is established, the chemoreceptor area decreases without a change in sensitivity. The decrease is initiated by motor-nerve activity and it occurs from the tendons toward the end-plate (6). 3)During normal nerve activity chemosensitivity is restricted to the end-plate region, and the size ofthe receptor area appears to be inversely correlated with the frequency ofrelease ofthe transmitter (8, 9). Ifthe size ofthe chemoreceptor area in a nerve cell is under presynaptic control in a manner similar to that observed in muscle, certainhypothetical predictions can be made regarding the changes in chemosensitivity that * Department ofPharmacology, University ofLund, Sweden. 293 would be produced by synaptic excitation. Three examples are schematically illustrated in Figure I. These diagrams show four neurones—a, b, c, and d—which are innervated by three other neurones—S1, J2, and J3. In the condition pictured in Diagram A, none of the cells J1, J2, or J3 has been previously stimulated, and consequently the surfaces ofthe four neurones are uniformly sensitive to the transmitter agent (shown by the heavy black line around the cells a, b, c, and d). In Diagram B, the cell J2 has been stimulated for some period oftime. This reduces the chemoreceptor area ofcells b and c and thereby restricts their excitability to the synapse from J2. So long as activity is maintained „ Q Q T ? ? ? ? Fig. ?.—Diagrams illustrating possible plastic changes in synaptic connections from J2, stimulation ofJ1 or J3 cannot elicit a response in b and c. Accordingly , these neurones are "permanently" set to respond only to a given input. Diagram C represents a situation inwhichJ2andJ3 have been stimulated in an overlapping time sequence. The receptor surface ofneurone c will now include both ofthe synapses, leaving the rest ofthe cell membrane unreactive to transmitter action. Cells b and d are limited to exclusive activation from J2 and J3, respectively. Thus the stimulus from J3, by being applied in close time relation to the stimulus from J2, has acquired the ability to elicit a response which in part is the same as that produced by a stimulus from J2. 294 Stephen Theslejf · A Neurophysiological Speculation concerning Learnin Perspectives in Biology and Medicine · Spring 1962 It would be possible to elaborate upon this type ofmechanism and to show that it could fulfill several ofthe properties postulated for a neurophysiological mechanism responsible for the establishment ofmemory and conditioning (2, 10). However, this would serve no useful purpose so long as it is not known whether this type ofsynaptic change does exist in nerve cells. Like any other hypothesis concerning the mechanism oflearning, the aforementioned one is speculative and thejustification for presenting it is that thought and experimental attempts at verification may be stimulated. REFERENCES i. A. Forbes. The foundations of experimental psychology. Worcester, Mass.: Clark University Press, 1929. 2.J. C. Eccles. The neurophysiological basis ofmind. Oxford: Clarendon Press, 1953. 3.S. R. Cajal. Histologie du système nerveux de l'homme et des vertébrés, Vol. I. Paris: Maloine, 1911. 4.D. O. Hebb. The organization of behavior. New York: John Wiley & Sons, 1949. 5.S. Thesleff. Physiol. Rev., 40:734, i960. 6.J. Diamond and R. Miledi. J...