Electrical impedance changes in many sites of brain in paradoxical sleep, anesthesia, and activity

The electrical impedance of a part of brain approximately 1-mm dimensions was measured with a four-electrode, very low current method in 61 male rats. The testing frequency was usually only 1000 Hz, and only the magnitude of impedance was measured. Impedance increased in paradoxical sleep in 42 of the 61 sites and decreased at three sites in the pons. The greatest changes were in subiculum and presubiculum with changes usually more than 10% and up to 25-30%. Intermediate changes of 2-10% were found in parasubiculum and entorhinal cortex. Most other changes were less than 4%, and there is a suggestion of greater changes in the pretectal area. All sites with changes greater than 4% were within 1 mm of a pial or ependymal surface. During anesthesia with pentobarbital in 23 rats impedance increased in two, eight showed no change, and 13 decreased. During unrestrained spontaneous activity in a small familiar cage the impedance usually became either more variable or decreased generally to a maximum of 1-10%, or both, but at a single site the response was not always the same. No tests beyond simple observation were used, and with this limited basis no clearer relation of impedance to behavior than simply to motor activity was apparent. In a change from quiet arousal to slow-wave sleep, or vice versa, there were no impedance changes. But at all sites activity usually had an effect. This was particularly marked in entorhinal cortex--parasubiculum and brain stem. Almost no other impedance changes were seen than of these three types. Impedance changes in brain are thus widespread and occur frequently in the usual behavior of rats. These results generally corroborate and expand the results of the Adey-Kado group with a different method with some advantages.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32751/1/0000120.pd

Specific impedance of cerebral cortex during spreading depression, and an analysis of neuronal, neuroglial, and interstitial contributions

The specific impedance of cerebral cortex of rabbits anesthetized with urethane was measured during spreading cortical depression (SD) at frequencies from 5 to 50,000 cycle/sec. During SD the amplitude of impedance increased at all frequencies, the maximum occurring progressively later at lower frequencies for those less than 5,000 cycle/sec. The phase angle increased at 5,000 and 50,000 cycle/sec with the same time course as the changes in amplitude of impedance. At 50 cycle/sec the phase angle initially decreased, sometimes to zero, and then increased to greater than the pre-SD state, with the maximum occurring later than the maximum in amplitude of impedance at this frequency. At 500 cycle/sec the phase angle initially decreased or did not change. It then increased, the maximum occurring later than the maximum of the amplitude of impedance at 500 cycle/sec, but earlier than the maximum phase angle at 50 cycle/sec. These data are interpreted in terms of a previously published analysis of specific impedance of cerebral cortex. Three processes must occur during SD. Early in SD the size of the interstitial space of cortex must decrease and the membrane resistance of neurons must decrease. Later the membrane resistance of neuroglia must increase. The significance of these processes is discussed with relevance to SD and cortical processes in general. It is suggested that the membrane change of neuroglia is similar to that of "anomalous rectification" of skeletal muscle.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32151/1/0000205.pd

Electrical impedance in the subicular area of rats during paradoxical sleep

A four-electrode method of chronically measuring impedance of less than 1 mm3 parts of brain in unrestrained rats is described. The method is usually good from about 1.5 to 3,000 cycle/sec for the absolute value of phase angle, and relative changes in magnitude of impedance can be measured from about 1.5 to 10,000 cycle/sec. Testing current of 2 x 10-8 amp is used, which produces a signal of about 20 [mu]v. Stability for hours and days is sometimes, but not always, achieved. In the subicular area, there is invariably an increase in magnitude of impedance of up to 25% during paradoxical sleep. In the same animal, this increase is the same during all episodes of paradoxical sleep lasting longer than 1 min, and is the same at 32 to 10,000 cycle/sec. The phase angle at 100 to 1,000 cycle/sec becomes 1 or 2 deg more negative during paradoxical sleep. There are no changes or very small changes in Ammon's horn and the fascia dentata. Because these data are fairly complete, the mechanisms which might possibly be responsible for these impedance changes can be stated specifically in semiquantitative form. Among several possibilities, a single one cannot be chosen on the basis of these impedance data alone, but from other considerations, a decreased size of interstitial space seems by far the most likely.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33399/1/0000799.pd

Studies on single neurons in dorsal hippocampal formation and septum in unrestrained rats , : Part I. Behavioral correlates and firing repertoires

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33782/1/0000036.pd

Which elements are excited in electrical stimulation of mammalian central nervous system: A review

1. (1) There are data on the amount of current necessary to stimulate a myelinated fiber or cell body and/or its axon a given distance away from a monopolar electrode over the entire range of practical interest for intracranial stimulation. Data do not exist for other electrode configurations.2. (2) Currents from a monopolar cathode of more than 8 times threshold may block action potentials in axons. Therefore, only axons lying in a shell around the electrode are stimulated. Elements very close to the electrode may not be stimulated. Close to an electrode small diameter axons may be stimulated and larger ones may not be.3. (3) Most, and perhaps all, CNS myelinated fibers have chronaxies of 50-100 [mu]sec. When gray matter is stimulated, the chronaxie is often 200-700 [mu]sec. It is not clear what is being stimulated in this case. Current-duration relations should be determined for many more responses.4. (4) There are no current-distance or current-duration data for central finely myelinated or unmyelinated fibers.5. (5) It takes less cathodal current than anodal to stimulate a myelinated fiber passing by a monopolar electrode. When a monopolar electrode is near a cell body, on the opposite side from the axon, often the lowest threshold is anodal, but sometimes cathodal. Stimulation of a neuron near its cell body is not well understood but in many cases the axon is probably stimulated.6. (6) Orientation of cell body and axons with respect to current flow is important. For an axon it is the component of the voltage gradient parallel to the fiber that is important.7. (7) The pia has a significant resistance and capacitance. Gray matter, white matter, and cerebrospinal fluid have different resistivities, which affect patterns of current flow.8. (8) More is known about stimulation of mammalian CNS than most workers are aware of. Much of what is unknown seems solvable with current methods.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/21946/1/0000353.pd

Studies on single neurons in dorsal hippocampal formation and septum in unrestrained rats , : Part II. Hippocampal slow waves and theta cell firing during bar pressing and other behaviors

Hippocampal slow waves and firing of theta cells were investigated during voluntary and automatic behaviors of the rat, including bar pressing on continuous reinforcement and fixed-ratio 50. Voluntary behaviors (walking, orienting, postural adjustments, approaching food or water, following the experimenter's hand, jumping, and exploring) were accompanied by theta in the slow waves and fast, rhythmical firing in the theta cells. For a given cell the rates of firing were similar for all voluntary behaviors. Automatic behaviors (eating, drinking, teeth chattering, grooming, vomiting, and yawning) were accompanied by irregular slow-wave activity and slow, irregular firing in the theta cells. For a given cell the rates of firing were similar for all automatic behaviors. Electrode placement within the hippocampus was critical with regard to how much slow-wave theta could be recorded during voluntary behaviors, whereas theta cells throughout the hippocampus were identical in the form of their firing. Bar pressing on both continuous reinforcement and fixed-ratio 50 gave clearly non-theta responses in both units and slow waves. Some well-learned voluntary behaviors can become automatic and are not in the theta mode.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33784/1/0000038.pd

The specific impedance of the dorsal columns of cat: An anisotropic medium

Low frequency, nonstimulating current was passed from a small electrode on the surface of the dorsal columns in the cervical cord of cats. A glass microelectrode was used to record the voltage at distances of 0.5 to 3 mm from the current electrode. The voltage fell off more rapidly in depth and across the dorsal columns than it did longitudinally--the resistance was lower in the longitudinal direction. Accordingly, the dorsal columns are anisotropic. An approximate equation is presented which describes the data fairly well and which is consistent with the anatomy. From this equation, the resistivity in the longitudinal direction was 138 to 212 ohm-cm and in the transverse direction, 1,211 ohm-cm. These values are shown to be consistent with the view that the anisotropy is primarily due to current flowing longitudinally in axons. The frequency dependence of the specific impedance was also measured. Some features of this frequency dependence have no clear explanation, but some of them are consistent with a nodal membrane having a time constant of roughly 50 [mu]sec.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32035/1/0000078.pd

Generation of theta rhythm in medial entorhinal cortex of freely moving rats

A regular slow wave theta rhythm can be recorded in the medial entorhinal cortex (MEC) of freely moving rats during voluntary behaviors and paradoxical sleep. Electrode penetrations normal to the cortical layers proceeding from the deeper to the more superficial layers reveal a continuous theta rhythm in layers IV-III (deep MEC theta rhythm) with an amplitude maximum in layer III, a null between the outer one-third of layer III and the inner one-half of layer I, and a continuous phase-reversed theta rhythm in layers II-I (superficial MEC theta rhythm) with an amplitude maximum there. Deep MEC theta rhythm is similar in phase and wave shape to CA1 theta rhythm; superficial MEC theta rhythm is similar in phase to DG theta rhythm. Laminar profiles throughout MEC show that the theta rhythm is generated there; it is not volume conducted from hippocampus.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23244/1/0000177.pd

Potassium accumulation in interstitial space during epileptiform seizures

The K42 surface efflux from the cat neocortex and rat hippocampus was studied after preloading with K42 from the surface. This K42 efflux increases from two to nine times during or after all electrically or Metrazol-induced seizures and "spontaneous" seizures. The K42 influex does not change during a seizure, so this increased efflux is not due to change in the diffusion constant or water flow from brain. The time course of the efflux is not compatable with the efflux simply idicating increased turnover of potassium from cells. We conclude there is an increase in the concentration of potassium in the interstitial space during seizures. A model is proposed in which this potassium accumulation is an important step in the regenerative, all-or-none-aspect of the initiation of seizures.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32795/1/0000168.pd

Prevention of supersensitivity in partially isolated cerebral cortex

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.Abstract1. 1. Sur quinze chats, une partie du gyrus marginal du cortex cerebral est isolee par section sous-corticale a 3-4 mm de profondeur. Dans des experiences terminales sous chloralose, pratiquees 2-18 semaines plus tard, la stimulation electrique locale determine des post-decharges (chez 12 chats) d'une duree plus longue du cote sectionne que du cote sain.2. 2. Un autre groupe de dix-sept chats, chacun avec un gyrus marginal isole par section sous-corticale, recoit des stimulations quotidiennes (sous-liminaire pour les post-decharges) sur le cortex sectionne a partir d'une semaine apres la section (ce delai a ete de 6 semaine pour deux chats). La stimulation totale est d'environ 400 applications de 0,6 mA, 400 de O,8 mA et 200 de 1,0 mA. Des experiences terminales sous chloralose une semaine apres la fin de la stimulation (6 semaines pour un chat), montrent que quatorze chats ne presentent plus d'hypersensibilite du cortex sectionne.3. 3. Ces resultats suggerent que la stimulation electrique chronique peut prevenir le developpement de l'hypersensibilite.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33291/1/0000684.pd