Journal of Neurophysiology

Hentall, I. D.; Zorman, G.; Kansky, S.; Fields, H. L.

doi: N/Apmid: 6726321

Abstract This report describes how the threshold for extracellular, electrical stimulation of cell bodies in the rat's rostromedial medulla depends on the distance to the stimulating electrode. A monopolar microelectrode both delivered current pulses near medullospinal neurons and, after decay of the stimulus artifact, detected whether an orthodromic spike had occurred by collision of that spike with a suitably timed antidromic spike initiated at the thoracic spinal cord. The liminal current and the height of antidromic spikes were noted at a series of vertical electrode positions. Regression analysis was performed to determine whether threshold and the inverse of peak-to-peak spike height varied more as the radial distance or its square. The square relationship provided a much better fit for threshold and a marginally better fit for the inverse of spike height. The spatial decline in excitability (K2) averaged 859 microA/mm2, falling within the range of values found for fibers and cell bodies in other studies. The constant of spatial decline in spike height (C2) in millivolts per square millimeter was positively correlated with K2. Both C2 and K2 were negatively correlated with conduction velocity. From threshold distance curves fitted by regression analysis, the mean separation of sites of spike maxima and threshold minima along each electrode path was 16 micron; the estimated distance from these sites to, respectively, the loci of spike generation and spike excitation were positively correlated and similar. The variation of C2 and K2 with conduction velocity may be due either to an influence of the size and shape of the dendritic tree on the spatial decrement of excitability and spike height or to a confounding in the studied equations of the space-independent effect of the size of a cell body on spike height and excitability. Copyright © 1984 the American Physiological Society

Gruberg, E. R.; Grasse, K. L.

doi: N/Apmid: 6610025

Abstract The basal optic projection in the frog Rana pipiens has been investigated by single-unit extracellular recording and horseradish peroxidase (HRP) histochemistry. We approached the projection from the ventral side of the brain and recorded single units in the basal optic projection proper as well as in the adjacent dorsomedial region (jointly called the basal optic complex). We found a) units responsive to stimuli moving in a vertical direction, b) an approximately equal number of units responsive to stimuli moving in a horizontal direction, and c) a smaller number of units responsive to changes in ambient light and moving stimuli without direction selectivity. Directional units display significant maintained activity and usually decrease their firing rate in response to stimulus motion in a direction opposite to that which elicits the maximal increase in firing rate. Receptive-field sizes for directional units ranged from 10 to 60 degrees. All units displayed vigorous excitatory response to a wide variety of moving stimuli within the velocity range of 0.2-10 degree/s. HRP histochemistry shows that in addition to the retina, the basal optic complex is connected to three principal areas: the ipsilateral tegmental griseum centrale, the ipsilateral dorsal ventrolateral nucleus of the anterior thalamus, and the ipsilateral posterior thalamic nucleus. In addition, a pathway was observed consisting of two groups of cells that send axons to the ipsilateral rostroventral medulla. This pathway originates a) in cells whose somata lie within the dorsomedial aspect of the basal optic complex (BOC); and b) in cells whose somata lie immediately outside the BOC in the adjacent gray, with apical dendrites extending into the BOC. Some of these fibers continue to the level of the spinal cord. Injection of HRP into the rostroventral medulla led to retrograde labeling of cells of the BOC. Copyright © 1984 the American Physiological Society

Hablitz, J. J.

doi: N/Apmid: 6327932

Abstract Picrotoxin-(PTX) induced epileptiform activity was studied in guinea pig hippocampal slices maintained in vitro, using intra- and extracellular recording techniques. The observed pattern of spontaneous and evoked epileptiform activity was quite complex. Spontaneous epileptiform events originated in the CA3 region and subsequently spread or propagated to CA1. Activation of CA1 could then reactivate CA3. This reverberation of activity was seen also following stimulation of the mossy fiber afferents from the dentate gyrus to CA3. Stimulation of fibers in the stratum radiatum of the CA1 region could trigger, at short latency, epileptiform activity that either was localized in CA1 or also occurred in CA3, with a late secondary discharge in CA1. This is attributed to a backfiring of the Schaffer collaterals and illustrates the ability of a variety of CA3 inputs to trigger epileptiform activity. Bath-applied PTX, at concentrations of 50-200 microM, had no apparent effect on the resting membrane potential or input resistance of the CA3 cells tested. Depolarizing current pulses elicited characteristic endogenous-burst responses that were not altered by PTX. Synaptic activity evoked by mossy fiber stimulation was altered markedly by PTX. The pattern of observed changes indicated that PTX reduced inhibitory postsynaptic potential (IPSP) amplitudes, resulting in the appearance of repetitive (presumably recurrent) excitatory inputs. Paroxysmal depolarizing shifts ( PDSs ) were generated by the coalescence of these excitatory inputs. Two types of spontaneous bursting were observed after PTX application. The first type was nonepileptiform , all or none in nature, and its frequency was voltage dependent. The second type of spontaneous burst was the PDS. It was epileptiform in character because it was associated with the synchronous discharge of many neurons. It was graded in nature, and its frequency was voltage independent. The graded nature of the PDS was demonstrated by varying the duration and intensity of the orthodromic stimulation. Trains of stimulation could produce PDSs that lasted 500-800 ms. A refractory period was observed following a PDS. By varying the strength of the orthodromic stimulation, it was possible to demonstrate that for the intervals tested this was a relative, not absolute, refractory period. Intracellular recordings in CA3 neurons indicated that each spontaneous PDS was followed by an afterhyperpolarization (AHP). Copyright © 1984 the American Physiological Society

Hess, R. F.; Baker, C. L.

doi: N/Apmid: 6726319

Abstract We have recorded electroretinographic (ERG) responses to grating patterns whose spatial, temporal, and contrast parameters were varied. The resultant evoked potential is dependent on spatial frequency and it exhibits a spatial band-pass characteristic. The peak frequency is dependent on retinal eccentricity. These findings are independent of temporal frequency, contrast, or mean luminance in the photopic range. These results suggest that the pattern ERG originates from a postreceptoral site. Copyright © 1984 the American Physiological Society

McKenna, T. M.; Light, A. R.; Whitsel, B. L.

doi: N/Apmid: 6726311

Abstract High-impedance micropipettes are used to record (both extra- and intracellularly) the electrical activity of neural elements located 550 micron or less from the pial surface of cerebral cortical areas 3a, 3b, 1, and 2 in unanesthetized cats. These elements are designated as "upper-layer SI units" and most frequently are sampled within the arm and forelimb digit sectors of areas 3b and 1. Mechanical stimulation of the skin is employed to determine the receptive field (RF) and response properties of the upper-layer units sampled. Single-shock electrical stimulation of the skin is used to obtain estimates of the minimal latency for eliciting spike discharge. Intracellular iontophoretic injection of horseradish peroxidase (HRP) is used to determine the laminar locations of the somata of the neural elements from which recordings are obtained. The receptive field (RF) and response properties of most upper-layer units sampled in areas 3b and 1 differ substantially from those of units recorded at depths greater than 550 micron from the pial surface in the same cortical fields. The members of one group of upper-layer area 3b and 1 units (U units) respond best to infrequently repeated (typically less than 0.5/s), slowly moving (1-5 cm/s) tactile stimuli. For the same units, repetitive application of slow-moving tactile stimuli to the RF typically leads to an enhancement of responsiveness accompanied by an elevation of spontaneous activity. In contrast, repetitive stimuli delivered at high velocity and at short interstimulus intervals lead to a decrease in unit responsiveness and to an absence of spontaneous activity. The members of a second group of upper-layer units (R units) respond best to moving stimuli delivered at higher velocities (5-20 cm/s), respond reliably at stimulus repetition rates well in excess of 0.5/s, and do not exhibit pronounced changes in responsiveness to repeated stimulation. The RFs of most upper-layer units (both R and U units) involve restricted regions on the contralateral upper limb, but the RFs of U units have poorly defined borders. In addition, the distribution of sensitivity within the RF of at least some U units is nonuniform and, frequently, discontinuous. Contralateral as well as ipsilateral body regions are included within the RFs for 12% of the upper-layer neurons sampled; the remainder (8%) have RFs restricted to the contralateral body.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1984 the American Physiological Society

Stanford, L. R.; Hartline, P. H.

doi: N/Apmid: 6726312

Abstract The spatial and temporal characteristics of the infrared responses of single neurons in the nucleus of the lateral descending trigeminal tract (LTTD) of the rattlesnake were investigated. The LTTD is the sole projection site of trigeminal neurons that innervate the thermoreceptive pit organ. In contrast to the responses of the primary infrared neurons, which have phasic and tonic components, the neurons in the LTTD respond strictly phasically to a sustained infrared stimulus. During an excitatory stimulus, the transient burst is followed by suppression of firing or by reduction of the new rate below the rate that would have occurred in the absence of stimulation. The phasic character of the responses may enable these neurons to encode more accurately changes in the pattern of infrared stimuli. Neurons in the LTTD show adaptation within limited regions of their receptive fields, while responses in other regions remain undiminished. This indicates that each LTTD neuron receives input from a population of primary infrared neurons. LTTD neurons respond to infrared stimuli of intensity less than 0.01 mW/cm2, which is below the threshold reported for primary afferent neurons; this also suggests convergence of a number of primary infrared afferents onto each LTTD neuron. LTTD neurons have smaller excitatory receptive fields than do the primary afferent neurons in the infrared system, indicating that spatial sharpening also occurs in this nucleus. Receptive fields of LTTD neurons may have inhibitory areas flanking the excitatory area. Introduction of a stimulus into the inhibitory area results in depression of the background discharge; thus, the inhibition is due to an active process, not to rebound from excitation. Inhibition can also be demonstrated by simultaneous stimulation of the excitatory and inhibitory receptive-field areas, resulting in a decreased excitatory response. We suggest that convergence of antagonistic excitatory and inhibitory inputs can explain the time course of LTTD responses to infrared stimulation and the architecture of LTTD receptive fields. Such excitatory and inhibitory interaction, similar to that postulated for the responses of some vertebrate retinal ganglion cells, could function to provide the basis for directional selectivity, motion sensitivity, and border enhancement in the infrared system. Unlike the visual system, however, in the infrared system excitatory-inhibitory interactions allow the construction of small excitatory receptive fields in the LTTD from the larger receptive fields of the primary afferent neurons, resulting in a highly evolved trigeminal system with visionlike function. Copyright © 1984 the American Physiological Society

Hentall, I. D.; Zorman, G.; Kansky, S.; Fields, H. L.

doi: N/Apmid: 6726322

Abstract The tail-flick reflex elicited by noxious heat in lightly anesthetized rats is known to be prevented by trains of low-amplitude current pulses passed through a monopolar microelectrode in the rostromedial medulla ( RMM ). The effect of the distance from such an electrode on the threshold of cell bodies was described in the preceding paper (11). This paper estimates the density of cell bodies in the RMM and, subsequently, estimates the number of cell bodies excited by the aforementioned pulses, a figure whose upper bound is between 30 and 75. The mean chronaxy for suppression of tail flick was found to be 162 microS. Correspondingly, for activation of spikes in somata of the RMM , it was found to be 170 microS. The axons belonging to these somata, located in the spinal lateral columns, had mean chronaxies of 360 microS. These comparisons favor the idea that cell bodies in the RMM , not axons, mediate the suppression of tail flick. Other evidence for this conclusion is given in the text. Resting activity in the RMM was found to average 6.33 Hz. Thus if the inhibitory process depends only on the instantaneous sum of activity in the many thousands of RMM neurons, all nocifensive reflexes should be continuously suppressed. But since this is not so, the relative timing of spikes in the population may also be critical. The synchronizing effect of electrical stimulation then explains the low number of cells needed to prevent the reflex. Copyright © 1984 the American Physiological Society

Clemo, H. R.; Stein, B. E.

doi: N/Apmid: 6726314

Abstract Using electrophysiological techniques, the present study demonstrated that substantial direct somatosensory cortical influences on the superior colliculus (SC) originate from three areas: a) SIV, b) para-SIV (the cortex adjacent to SIV but deeper in the anterior ectosylvian sulcus (AES) and for which no topography has yet been described), and c) the rostral suprasylvian sulcus. Influences also appeared to originate from SI and SII, but these may have been indirect. Detailed examination of the AES revealed that these corticotectal projections are topographically organized, and stimulation of a given cortical locus was observed to affect only those cells in the SC whose receptive fields overlapped those of cells at the stimulation site. A similar receptive-field register was found between the suprasylvian sulcus and the SC. Within this topographic pattern, considerable convergence was evident and an individual SC cell could be influenced from a surprisingly large cortical area. This was particularly evident within the representation of the forelimb. Thus, an SC cell with a receptive field covering the forelimb and paw could receive convergent input from many cortical cells with receptive fields covering all or restricted portions of this body region. Considerable corticotectal divergence also was observed within this general topographic scheme. For example, a given corticotectal site representing the digits sent projections to many different SC cells that included the digits within their receptive fields. These data are more consistent with a block-to-block than a point-to-point corticotectal projection. Somatosensory corticotectal projections excited only those SC cells that could also be activated by peripheral somatosensory stimuli. Similarly, the caudal AES, which contains auditory cells, excited only those SC cells activated also by peripheral auditory stimuli. Yet convergent influences from both auditory and somatosensory regions of the AES were observed in the SC cells that could be activated by both auditory and somatosensory stimuli. These data indicate that the AES is a major source of excitatory input to cells of the deep laminae of the SC. Since it is these deep laminae cells that project to premotor regions of the brain stem and the spinal cord, it is reasonable to suppose that the AES has a significant impact on the output signals of the SC that initiate the orientation responses to peripheral sensory stimulation. Copyright © 1984 the American Physiological Society

Fox, P. T.; Raichle, M. E.

doi: N/Apmid: 6610024

Abstract The purpose of this investigation was to determine the relationship between the repetition rate of a simple sensory stimulus and regional cerebral blood flow (rCBF) in the human brain. Positron emission tomography (PET), using intravenously administered H2(15)O as the diffusible blood-flow tracer, was employed for all CBF measurements. The use of H2(15)O with PET allowed eight CBF measurements to be made in rapid sequence under multiple stimulation conditions without removing the subject from the tomograph, thus minimizing changes in base-line CBF and in head position due to longer intervals between scans. Nine normal volunteers each underwent a series of eight H2(15)O PET measurements of CBF. Initial and final scans were made during visual deprivation. The six intervening scans were made during visual activation with patterned-flash stimuli given in random order at 1.0-, 3.9-, 7.8-, 15.5-, 33.1-, and 61-Hz repetition rates. In each subject the region of greatest rCBF increase was determined. Within this region the rCBF was determined for every test condition and then expressed as the percentage change from the value of the initial unstimulated scan (rCBF% delta). Anatomical localization of the region of greatest rCBF response was performed employing bony landmarks from a lateral skull radiograph, a template of the cranium created from a transmission attenuation scan and a stereotaxic atlas. In every subject, striate cortex rCBF% delta varied systematically with stimulus rate. Between 0 and 7.8 Hz, rCBF% delta was a linear function of stimulus repetition rate. The rCBF response peaked at 7.8 Hz and then declined. The rCBF% delta during visual stimulation was significantly greater than that during visual deprivation for every stimulus rate except 1.0 Hz. The anatomical localization of the region of peak rCBF response was determined for every subject to be the mesial occipital lobes along the calcarine fissure, primary visual cortex. We conclude that stimulus rate is a significant determinant of rCBF response in the visual cortex. Investigators of brain responses to selective activation procedures should be aware of the potential effects of stimulus rate on rCBF and other measurements of cerebral metabolism. For cerebral responses to selective activation to be meaningfully interpreted, the stimulus repetition rate must be taken into consideration. Response amplitude may be maximized by proper rate selection or be undetectable due to selection of too high or too low a repetition rate. Stimulus rate must be controlled for when responses to unlike stimuli or performance tasks are compared or ambiguities will be present as to whether response differences are Copyright © 1984 the American Physiological Society

Rose, J. D.; Bieber, S. L.

doi: N/Apmid: 6726310

Abstract Single midbrain neurons were examined for effects of ovarian hormone administration on responsiveness to an array of lordosis-controlling types of somatosensory stimuli in anesthetized, ovariectomized Syrian hamsters. Neuronal responses were recorded under four different hormonal treatment conditions: 1) estradiol benzoate (EB) followed by progesterone (P), 2) EB alone, 3) P alone, or 4) no hormone administration. Only the hamsters receiving both EB and P showed lordosis in a mating test immediately prior to preparation for recording. The joint administration of EB and P strongly facilitated unit responsiveness to lordosis-eliciting (e.g., lumbosacral tactile) forms of stimulation. The incidence of units showing sustained changes in firing of at least +/- 30% in response to these stimuli was highest in animals having received EB and P (69%) and lowest in those given P alone (37%), with the occurrence of responsive units significantly different across the four hormonal conditions. The magnitude of the median unit response to lordosis-trigger stimuli in hamsters given EB and P was significantly higher than unit responses in hamsters receiving either P alone or no hormone. Bilateral shoulders stimulation, a weak stimulus for lordosis elicitation, produced the most responses in units from animals given EB and P (42%) and the fewest in hamsters given EB alone (12%), with a significant difference in responsiveness across the hormonal conditions. The incidence of units responding exclusively to facial stimulation, which is strongly antagonistic to lordosis in behaving animals, was greatest in hamsters injected with P alone (22%) and least for cells from the animals given both EB and P (6%). The difference in unit responsiveness to this stimulus across the four hormone conditions was significant. Analysis of unit-response patterns by the multivariate technique of discriminant analysis revealed that neuronal responses to most stimuli, especially flanks, back, shoulders, and face, were differentially affected by the hormone treatments such that these response patterns could be used to identify the four hormonal conditions from which the neurons were sampled. In addition, the hormonal treatments were found to have influenced the incidence of accelerative or decelerative unit responses to somatic stimuli. Discriminant analysis of unit responses as a function of location of the cells within the midbrain showed that neurons in the tectum and central gray had response patterns highly distinguishable from each other as well as from those of cells in the central and ventromedial tegmental regions.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1984 the American Physiological Society