Lester T. Rutledge

Morphological changes in pyramidal cells of mammalian neocortex associated with increased use

Abstract 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 sec 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.

Intracellular recording of the convergence of input upon neurons in cat association cortex

Abstract Intracellular responses in anterior middle suprasylvian gyrus to peripheral stimuli (light flash, auditory click, and contralateral forepaw shock) and central stimuli (contralateral cortical and thalamic stimulation) were studied in cats anesthetized with chloralose. When cells discharged to peripheral and some central stimuli, spikes arose from asynchronous, summating, depolarizing postsynaptic potentials of 20- to 80-msec duration. Long-duration (30–300 msec) hyperpolarizing potentials also were evoked by one or more peripheral and central stimuli in over one-third of cells studied, and represent inhibitory activity at the recorded cell or at previous central synapses. In many instances, cortical inhibitory mechanisms were responsible for blocking effects during interactions between pairs of stimuli. However, when light flash was the conditioning stimulus, the block usually occurred before the recorded cell, either in cortex, or possibly in precortical areas of neuronal convergence. The data indicate that this portion of suprasylvian gyrus in the cat is unequally responsive to various stimuli, with different synaptic patterns of activity evoked by photic and other peripheral and central inputs.

A Multichannel Depth Probe Fabricated Using Electron-Beam Lithography

A multielectrode probe structure is described in which several thin-film metal electrodes are defined on the outer surface of a glass micropipette using electron-beam lithography. Electrode geometries are controlled to within one micron, resulting in electrode recording characteristics which are extremely well matched. Recording sites are 5 , um wide rings spaced 100 , um apart in depth. Analysis and characterization show the structure to be capable of accurately recording tissue potentials with a minimum of tissue damage. Use of these probes in current source-density (CSD) analysis of extraceliular current flow is described.

A STUDY OF PYRAMIDAL CELL AXON COLLATERALS IN INTACT AND PARTIALLY ISOLATED ADULT CEREBRAL CORTEX

Abstract A quantitative study was made of pyramidal cell axon collaterals in Golgi impregnated adult cat cerebral cortex. Cellular samples were obtained from 3 groups, intact, undercut and long-term electrically stimulated undercut cortex. Undercut tissue had cells with significantly fewer axon collaterals and fewer branches than did intact. Long-term electrically stimulated undercut tissue contained neurons which were not different from intact in these two measures. There was, therefore, a preservation of axonal morphology in stimulated undercut, adult cortex. Results do not support a theory of axonal proliferation to explain supersensitivity in partially isolated mature cortex. The preservation of axonal components and the preservation of dendritic spines, previously reported, indicates the prevention of some degenerative changes in certain cortical neurons brought about by long-term electrical stimulation.

Brain-stem and cortical interactions in the interhemispheric delayed response

Acute experiments on cats under chloralose anesthesia were designed to study the role of the brain stem in the transmission of the interhemispheric delayed response (IDR). Experiments on interaction of potentials recorded in brain stem and at cortex led to the conclusion that the IDR is relayed to the contralateral hemisphere via routes which pass through the general area of the mesencephalic reticular formation. Other brain-stem areas, not directly in the pathway, also have access to the cortical fields discharging the IDR. During passage through the mesencephalon and elaboration at the cortex the IDR comes under the influence of afferent activity. To a more or less considerable extent, depending on the time relationships, this interaction is reciprocal. Characteristics of the IDR were investigated and the findings confirmed in preparations with brains radically split longitudinally in the mid-line. An integrative function based upon cortical-brain-stem-cortical and afferent activity is suggested for the IDR system.

Prevention of supersensitivity in partially isolated cerebral cortex

Abstract 1. 1. A portion of the marginal gyrus of the cerebral cortex on each of fifteen cats was undercut 3–4 mm deep. In terminal experiments under chloralose, 2–18 weeks later, local electrical stimulation produced after-discharges (in 12 cats) which had a longer duration on the undercut side than on the intact side. 2. 2. Another group of seventeen cats, each with an undercut marginal gyrus, received daily electrical stimulation (subthreshold for after-discharges) of the undercut cortex starting 1 week after undercutting (6 weeks delay in two cats). Total stimulation was about 400 applications at 0.6 mA, 400 at 0.8 mA and 200 at 1.0. mA. In terminal experiments under chloralose 1 week after the end of stimulation (6 weeks for one cat), fourteen of these cats did not show supersensitivity of the undercut cortex. 3. 3. These results suggest that chronic electrical stimulation can prevent the development of supersensitivity.

Interactions of peripherally and centrally originating input to association cortex

Abstract Cortically recorded evoked potentials to various peripheral and central stimuli were studied in chloralose anesthetized cats. Summation, blocking interaction, and facilitation were investigated by systematic pairing of all possible combinations of two stimuli. 1. 1. From the same locus in association cortex responses were recorded to peripheral, photic, auditory, and somesthetic stimulation and to contralateral cortex and mesencephalic reticular formation (MR) stimulation. 2. 2. Distinct differences in amount of convergence for the various stimuli was the rule. Photically initiated input was the strongest, followed in order of strength by the interhemispheric delayed response (IDR), the response to MR stimulation, and the responses, frequently of equal strength, to auditory and somesthetic stimulation. 3. 3. True facilitation was observed in association cortex by activity originating in the MR. 4. 4. Data support an interpretation of association cortex integrative capacity, based upon plastic, dynamic function accompanying reception of neural discharges from peripheral and central systems.

Megimide and metrazol A comparison of their convulsant properties in man and cat

The report by Shaw et al. (1954) on the effectiveness of Megimide

Inhibition in penicillin-induced epileptic foci

A previous study indicated that the early surface negative component (associated with recurrent inhibition) of the evoked potential recorded from cat pericruciate cortex, subsequent to pyramidal tract stimulation, was altered after application of penicillin to the cortical surface (Van Duijn et al. 1973). This suggested that decreased effectiveness of recurrent inhibition might be the basis for epileptogenicity of penicillin. To verify that recurrent inhibition is functionally decreased in the penicillin epileptic focus and to assess alternative sites for penicillin action, this phenomenon was investigated at the cellular level. Neurons were recorded extracellularly and response to stimuli monitored throughout the transition from normal cortex to epileptogenic cortex. Stimuli employed were peduncular stimulation (to test recurrent inhibitory pathways), epicortical stimulation (to test inhibitory pathways, bypassing the recurrent collateral system), and forepaw footpad shock (to test the responsiveness ofneurons to afferent input). In normal cortex, PT cells were inhibited by peduncular or epicortical stimulation and excited by forepaw stimulation, with the excitation followed by a period of inhibition. In the penicillin focus, inhibition was not observed in response to any of the 3 stimuli, and the excitatory response to forepaw stimulation was maintained. The bursting non-PT cells, most likely candidates for interneurons, exhibited excitation in response to peduncular and epicortical stimulation, consistent with involvement in inhibitory pathways. Nonetheless, in the penicillin focus, excitatory response to peduncular and epicortical stimulation was maintained. Excitatory response to forepaw stimulation was also maintained in the penicillin focus. The results demonstrate a loss of effectiveness of recurrent inhibition measured at the PT cell body in the penicillin focus. Further, the reduction in inhibitory feedback occurs in conjunction with maintained or enhanced excitability of the neurons which are most likely candidates for inhibitory interneurons. Thus, penicillin is most likely exerting its effect at the inhibitory synapses onto PT cells in the cortex, thereby allowing excitatory input to have greater influence on neuronal firing.

Long-term status of pyramidal cell axon collaterals and apical dendritic spines in denervated cortex

Abstract Cortices of animals in 3 groups were denervated by undercutting at 4 or 40 days of age or at adulthood. Months later electrocortical seizures were studied in terminal experiments and cortical tissues prepared for light microscopic study. Four measures, assumed to be descriptive of or related to axon collateral proliferation, were made: number of axon collaterals, length of collaterals, number of collateral branches and number of apical dendritic spines. Data from adult intact cortex served as controls. Increases in number of collaterals and collateral length were observed, the best evidence being in the 40-day animals. However, since collateral branching decreased, the data do not support the concept of proliferation as described in young brains. If dendritic spines are valid indicators of synaptic contacts then the observed significant loss of spines argues against the maintenance of functional connections, if indeed they were established, following denervation at any age. There were no obvious relationships between morphological changes and duration of afterdischarges. A persistent alteration was frequently seen in cortex which had been undercut at 4 days of age. Lower lying pyramidal cell axons bifurcated with nonrecurrent spread parallel to the glia infiltrated site of the old fiber paths. This glial barrier appeared to have prevented centripetal axon growth.