Our previous paper (Granit & Rutledge, 1960) showed that recurrent inhibition silences the discharge of a motoneurone which is feebly supported by excitatory drive, even in the face of constant depolarizing pressure as defined in that work. One answer was also provided thereby to the general question of why sense organs and interneurones fire at frequencies which are in excess of immediate apparent needs. In the present study every precaution was taken to maintain afferent excitation in excess of that barely needed to keep up a given rate of reflex firing to electrical stimulation of muscular afferents. This is because the main question here concerns the relationship between normal firing frequency Fn of a motoneurone and its rate of discharge Fi under recurrent inhibition. F, was now found to be a basically linear function of Fn. This finding proved to be of methodological interest in work on the physiological significance of recurrent inhibition. Holmgren & Merton (1954), making use of an analogy derived from electronics, suggested that recurrent inhibition has a stabilizing effect on the discharge from motoneurones. Defining stabilization for our purpose as the integrated net effect of one or several processes engaged in holding the rate of firing within relatively narrow limits, we have also tried below to scrutinize this concept as a biological proposition. (It is not our intention to elaborate an electronic analogy which may or may not be valid.) This means that special attention will be given to the factors which determine the upper and lower limits of the frequency range. Thus limitation of discharge frequency is possibly a more accurate term than 'stabilization'.We are not aware of any previous work concerned with the relation between Fn and Fi. The general problem of 'frequency limitation' may,
After long-term electrical stimulation of the brain, which presumably produced increased neuronal use, histological studies were made of neocortical neurons involved in transcallosal and extracallosal systems. Adult cats with implanted electrodes received 20 trains (2 set each) of electrical stimulation to the suprasylvian gyrus daily for several weeks. In four cats, brain stimulation was paired with foreleg shock (trained), in two, 'it was not (untrained). Cortical tissues ipsilateral and contralateral to the stimulated side were prepared with a modified Golgi-Cox method. In cortex contralateral to the stimulated side, apical dendrites of layers II and III pyramidal cells had significantly more branchings in terminal regions, greater lengths, and terminated nearer the pia than they did on the stimulated side. There were also more spines on oblique, vertical, and terminal portions of apical dendrites. Increases in oblique and vertical spine counts appeared to be more related to training than to just brain stimulation. Qualitatively, apical dendritic terminals in contralateral cortex had fine branchings, filamentous bare twigs, especially long spines, convolutions with close packing of spines, acute angles of terminals reflecting from the pia, and a general appearance of increased density of apical dendrites near the pia. The observed changes in neuronal structure described in these experiments are interpreted as evidence that increased use of specific pathways to the cerebral cortex produces postsynaptic growth in some cortical neurons. 209
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