Journal of Neurophysiology

Edwards, D. H.; Mulloney, B.

doi: N/Apmid: 3035112

Abstract The passive integrative properties of two crayfish abdominal motoneurons, the fast flexor inhibitor (FI) and a posterior, ipsilateral fast flexor excitor (FE), were studied electrophysiologically and through simulations with multicompartment models of their electrotonic structures. Responses of the models to simulated giant neuron input were quite similar to the motoneurons' responses to giant neuron stimulation, which suggests that differences in the electrotonic structures and the sites of synaptic input to the two cells can account in large part for differences in their responses to a common input. A full action potential created in the initial axon compartment of the FI model produced attenuated potentials in the adjacent integrating segment compartment and contralateral soma compartment. These potentials are similar in amplitude and time course to attenuated antidromic action potentials recorded in the corresponding regions of the FI neuron. A location of the spike initiation zone of the FI at the initial axon segment is consistent with this result. The responses of FI to ipsi- and contralateral inputs are different. Shock of a single abdominal second root produced a larger, faster rising excitatory postsynaptic potential in the ipsilateral FI soma than in the contralateral soma. Second root shock also caused the contralateral FI to produce an action potential either alone or before the ipsilateral FI neuron. Responses of the FI model to ipsilateral and contralateral inputs differ in the same way as the cell's responses. Inputs to the FI model that are ipsilateral to the soma compartment produce larger responses there than do contralateral inputs. Conversely, those contralateral inputs produce larger responses in the initial axon compartment than do ipsilateral inputs. This difference results from the long integrating segment that connects the soma compartment to the initial axon compartment. These results can account for the FI responses to lateralized inputs. Unlike the responses of FIs, the soma responses of contralaterally homologous FEs to ipsilateral and contralateral second root shocks were similar in waveform and amplitude, with the ipsilateral root producing the larger response. This result is consistent with theoretical results from the FE model simulations. We conclude that a smaller size, larger input resistance and shorter membrane time constant allow the FE to respond to giant neuron input before the FI, and so help to achieve the proper timing of flexor contraction and relaxation during a tailflip.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1987 the American Physiological Society

Libersat, F.; Clarac, F.; Zill, S.

doi: N/Apmid: 3585482

Abstract The activities of individual force-sensitive mechanoreceptors of the dactyl (terminal leg segment) of the crab, Carcinus maenas, have been recorded during free walking. These receptors have also been mechanically and electrically stimulated in freely moving animals to directly evaluate their function in locomotion. All force-sensitive mechanoreceptors fired during the stance phase of walking and were silent during swing. Receptor discharges showed regular phase relationships to bursts in motor neurons of leg muscles. Crabs walk laterally and use the legs of one side either in trailing to actively push the animal to the opposite side, or in leading, to less forcefully pull the animal in that direction. Individual force-sensitive mechanoreceptors differed in their patterns of activity during trailing or leading according to their location on the dactyl. Units of proximal receptors fired more vigorously when used in trailing than in leading. Discharges in trailing were also increased by loading of the animal. In contrast, distal receptors near the dactyl tip fired equally intensely during walking in either direction. Proximal receptors thus encode forces and loads applied to the leg. Distal receptors do not encode loads but can signal leg contact and, potentially, exteroceptive vibrations. Sensory stimulation of force-sensitive mechanoreceptors was produced during walking by a device that imposed continuous mechanical bending of the dactyl and by electrical stimulation of dactyl nerves. Intra- and inter-segmental reflexes were evaluated by myographic recordings from leg muscles. Continuous mechanical deformation of the dactyl increased the activity of the levator and decreased firing in the depressor muscles of the homonymous leg during walking. The same stimulus produced enhanced activity in depressor muscles of adjacent legs. The latter effect was not due to simple mechanical coupling resulting from reflexes in the stimulated leg. These reflexes can function to limit forces applied to a leg and provide compensatory adjustments in other legs. Brief low-threshold electrical stimuli applied to nerves in which the activities of force-sensitive mechanoreceptors were recorded produced reflex effects similar to those obtained by mechanical stimulation. These stimuli also reset the rhythm of motor neuron bursting in both homonymous and adjacent legs during walking. These studies confirm the importance of force-sensitive mechanoreceptors in adapting walking patterns and in determining leg coordination in locomotion. Copyright © 1987 the American Physiological Society

Epping, W. J.; Eggermont, J. J.

doi: N/Apmid: 3495645

Abstract With a dual-electrode configuration separable few-unit activity was recorded both on one electrode as well as on two electrodes in the auditory midbrain of the grassfrog to a large variety of stimuli. Activity recorded on one electrode was separated by a pattern recognition technique through the use of features of the action potential waveform. Functional connections between units were established on basis of cross-correlation histograms of pairs of simultaneously recorded units. A hierarchical scheme was adopted to describe the various manifestations of neural correlation. If a peak or trough was observed in the simultaneous cross-correlation histogram, irrespective of stimulus conditions, this was called neural synchrony. If this peak or trough was not equal to its shift predictor estimating the stimulus contribution, neural correlation was considered to be present. About 60% of the pairs exhibited neural synchrony, mostly due to shared stimulus influences, independent of mutual distance of units. About 15% of the unit pairs showed neural correlation indicating a functional neural connection. Neural correlation was observed only in units with a distance smaller than 300 micron. The majority (approximately 85%) of the cases showing neural correlation could be ascribed to neural shared input. Unidirectional excitation was observed only in unit pairs recorded on the same electrode. Unidirectional inhibition could not be demonstrated. The dependency of occurrence of neural correlation on unit distance has implications for models of the functional organization of the auditory midbrain. About half of the neurally correlated pairs showed stimulus dependencies of their functional connections. Together with the observed lack of stimulus invariance of single-unit spectrotemporal sensitivities this indicates a dynamic stimulus dependency of functional neuronal organization in the auditory midbrain of the grassfrog. Neuron pairs with a large overlap of their spectrotemporal sensitivities on average had neurally correlated activities more often than pairs with a smaller amount of overlap. In comparison to single-unit coding, ensemble coding by populations of neurons may show an enhanced selectivity to stimulus characteristics. Copyright © 1987 the American Physiological Society

Cannon, S. C.; Robinson, D. A.

doi: N/Apmid: 3585473

Abstract Eye movement were recorded from four juvenile rhesus monkeys (Macaca mulatta) before and after the injection of neurotoxins (kainate or ibotenate) in the region of the medial vestibular and prepositus hypoglossi nuclei, an area hypothesized to be the locus of the neural integrator for horizontal eye movement commands. Eye movements were measured in the head-restrained animal by the magnetic field/eye-coil method. The monkeys were trained to follow visual targets. A chamber implanted over a trephine hole in the skull permitted recordings to be made in the brain stem with metal microelectrodes. The abducens nuclei were located and used as a reference point for subsequent neurotoxin injections through cannulas. The effects of these lesions on fixation, vestibuloocular and optokinetic responses, and smooth pursuit were compared with predicted oculomotor anomalies caused by a loss of the neural integrator. Kainate and ibotenate did not create permanent lesions in this region of the brain stem. All the eye movements returned toward normal over the course of a few days to 2 wk. Histological examination revealed that the cannula tips were mainly located between the vestibular and prepositus hypoglossi nuclei, in their rostral 2 mm, bordered rostrally by the abducens nuclei. Dense gliosis clearly demarcated the cannula tracks, but for most injections there were no surrounding regions of neuronal loss. Thus the eye movement disorders were due to a reversible, not a permanent, lesion. The time constant for the neural integrator was determined from the velocity of the centripetal drift of the eyes just after an eccentric saccade in total darkness. For intact animals this time constant was greater than 20 s. Shortly after bilateral injections of neurotoxin, the time constant began to decrease and reached a minimum of 200 ms; every horizontal saccade was followed by a rapid centripetal drift with a time constant of approximately 200 ms. For vertical eye movements, in this acute phase, the time constant was approximately 2.5 s. The vestibuloocular reflex (VOR) was drastically changed by the lesions. A step of constant head velocity in total darkness evoked a step change in eye position rather than in velocity. In the absence of the neural integrator, the step velocity command from the canal afferents was not integrated to produce a ramp of eye position (normal slow phases); rather this signal was relayed directly to the motoneurons and caused a step in eye position. The per- and postrotatory decay of the head velocity signal was decreased to 5-6 s indicating that vestibular velocity storage was also impaired.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1987 the American Physiological Society

Yoshimura, M.; Polosa, C.; Nishi, S.

doi: N/Apmid: 3585470

Abstract Intracellular recordings were performed in Cs-loaded sympathetic preganglionic neurons (SPNs) of the intermediolateral nucleus, identified by antidromic stimulation, in the slice of the T2 or T3 segment of the cat spinal cord. Loading the neurons with Cs resulted in broadening of the action potential, depression of the fast component of the afterhyperpolarization (AHP), and appearance of an afterdepolarization (ADP). A typical ADP in a Cs-loaded neuron had time to peak of 45-110 ms, half-decay time of 70-250 ms, and amplitude of 2-10 mV at membrane potentials between -60 and -70 mV and at a Ca and K concentration of 2.5 and 3.6 mM, respectively, in the superfusion medium. The ADP was associated with a decrease in neuron input resistance and increased in magnitude with hyperpolarization of the cell membrane. The relation between peak ADP amplitude and membrane potential was linear within the range of membrane potentials from -60 to -100 mV. The ADP was reversibly suppressed by the Ca-channel blocker cobalt (2 mM) or by low Ca Krebs solution (0.25 mM). Superfusion with BaCl2 (1.0 mM) or tetraethylammonium (TEA) (10-20 mM) caused an increase in amplitude of the ADP and an increase in action potential duration. Hyperpolarizing pulses, delivered during the course of the spike shoulder, resulted in a decrease of spike duration and ADP amplitude. The ADP was not affected by tetrodotoxin, at a dose blocking the Na-spike, and was enhanced, in association with an increase in action potential duration, when NaCl in the Krebs solution was replaced with choline chloride. Increasing intracellular Cl concentration or decreasing extracellular Cl concentration had no effect on the ADP. Changes in external K concentration from 3.6 to 10 or 0.36 mM increased and decreased, respectively, the amplitude of the ADP. In the absence of Cs, and ADP, with similar time course to that recorded in Cs-loaded SPNs, was recorded when CaCl2 was replaced by BaCl or NaCl was replaced by TEAC1. It is concluded that the SPN afterpotential includes a Ca-dependent inward current, in addition to the already described fast and slow outward K currents of the AHP. Copyright © 1987 the American Physiological Society

Dursteler, M. R.; Wurtz, R. H.; Newsome, W. T.

doi: N/Apmid: 3585468

Abstract Ibotenic acid lesions of the middle temporal visual area (MT) have previously been shown to impair a monkey's ability to initiate smooth pursuit eye movements to targets moving in the extrafoveal visual field (30). This is a retinotopic deficit: pursuit is impaired in all directions within the affected portion of the contralateral visual field. In the present experiments we analyzed the effects of lesions of the foveal representation of MT on the maintenance of foveal pursuit. Injections of ibotenic acid were directed toward the representation of the fovea within MT but spread into extrafoveal regions of MT and adjacent visual areas within the superior temporal sulcus. Chemical lesions of the foveal representation produced a directional deficit in the maintenance of pursuit: the monkey failed to match eye speed to target speed when pursuing a target that moved toward the side of the brain with the lesion. This deficit was evident regardless of the part of the visual field in which target motion began, and pursuit at higher target speeds was more severely affected. The directional deficit was qualitatively similar to pursuit deficits observed in human patients following large parietal-occipital lesions. Extension of the lesions into extrafoveal regions of the contralateral visual field representation also resulted in retinotopic deficits for pursuit initiation: the monkey was unable to match the speed of its pursuit eye movement to that of a target or to adjust the amplitude of its saccade to compensate for target motion. The errors in pursuit speed and saccade amplitude for initiation of pursuit into the contralateral visual field were linearly related, which supports the hypothesis that both deficits arise from damage to the same underlying visual motion processing mechanism. The selectivity of the retinotopic deficit for motion information was also investigated by reducing retinal motion through the use of a stabilized image. After the lesion, the monkeys continued normal pursuit when a position error was present during stabilization, supporting the view that the deficit was related to loss of motion but not position information. Copyright © 1987 the American Physiological Society

Yoshimura, M.; Polosa, C.; Nishi, S.

doi: N/Apmid: 3035111

Abstract Sympathetic preganglionic neurons of the intermediolateral nucleus were identified by antidromic stimulation in the slice of the T2 or T3 segment of the cat spinal cord. In normal Krebs solution, the action potential of these neurons had a shoulder on the repolarization phase and was followed by a long-lasting afterhyperpolarization (AHP). The AHP had a fast and a slow component. Superfusion of the slice with noradrenaline (NA), 10-50 microM, resulted in depression of the shoulder on the repolarization phase of the action potential, in the appearance of an afterdepolarization (ADP), which was absent in control conditions, and in depression of the slow component of the AHP. These effects were present whether the membrane potential of the sympathetic preganglionic neurons was decreased, increased, or not changed by NA. A typical ADP had time to peak of 50 ms and decay time of 200-500 ms; the amplitude was variable and large ADPs could be suprathreshold, causing repetitive firing. The amplitude and duration of the ADP increased with NA concentration. The appearance of the ADP seemed to be independent of the depressant effect of NA on the slow AHP. The ADP was associated with a decrease in neuron input resistance and was voltage dependent, being depressed in nonlinear fashion by membrane hyperpolarization. The ADP decreased in amplitude or disappeared within a range of membrane potentials from -70 to -90 mV. The ADP was reversibly suppressed by the Ca-channel blocker cobalt (2 mM), by low Ca Krebs (0.25 mM), and by iontophoretic injection of ethyleneglycol-bis(B-aminoethyl-ether)-N,N'-tetraacetic acid into the cell. Increasing Ca concentration from 2.5 to 10.0 mM had no effect. The ADP was unaffected by tetrodotoxin, at a concentration blocking the Na spike, but was suppressed in Na-free medium, even when the Ca spike was prolonged by tetraethylammonium 20 mM. Changes in external K concentration from 3.6 to 2.5 or 10.0 mM did not change the ADP. Increasing intracellular Cl concentration or decreasing extracellular Cl concentration had no effect on the ADP. It is concluded that the ADP, evoked by NA, is due to an increase in membrane conductance involving Na and Ca ions, possibly a Ca-activated Na conductance. The ADP provides a mechanism with which NA may modulate sympathetic preganglionic neuron responsiveness to excitatory synaptic inputs. Copyright © 1987 the American Physiological Society

Libersat, F.; Zill, S.; Clarac, F.

doi: N/Apmid: 3585481

Abstract This paper examines the responses and reflex effects of force-sensitive mechanoreceptors of the most distal leg segment, the dactyl, of the leg of the crab, Carcinus maenas. The goals of these studies are to establish the potential activities and functions of these receptors in posture and locomotion. The responses of force-sensitive mechanoreceptors to imposed mechanical stimuli depended upon their location on the dactyl. A distal group of receptors is located on a specialized region, the dactyl tip, which is composed solely of epicuticle. Another group of receptors is distributed throughout more proximal regions of the dactyl where the cuticle is completely calcified. Both groups of receptors showed vigorous responses to imposed bending forces. When bending forces were applied as step functions at the dactyl, tip distal receptors showed only phasic responses to all levels of force application. Receptors located at more proximal positions on the dactyl showed phasic responses to low levels of step applied forces and phasicotonic discharges at higher levels of force. Increasing levels of force produced a sigmoid increase in the tonic firing of these units. When bending forces were applied using ramp functions, receptors of the distal group responded with an intense initial discharge followed by firing at a constant rate throughout both force application and release. This response was not related to the velocity of force application. In contrast, receptors located more proximally responded directionally to force application and release. Proximal receptors also effectively encoded the velocity of force application. Responses of these two groups of receptors also differed when vibrations were applied at the dactyl tip: proximal receptors only followed vibrational stimuli up to 50 Hz, whereas distal receptors showed 1:1 responses at vibrations as high as 95 Hz. Mechanoreceptors of the dactyl also responded intensely to bending forces resulting from resisted contractions of the animal's own muscles. No responses were obtained from unresisted movements of the leg. Stimulation of force-sensitive mechanoreceptors of the dactyl produced intra- and interleg reflex discharges in motor neurons to leg muscles. Mechanical bending of the dactyl or electrical stimulation of dactyl nerves in which force-sensitive mechanoreceptors were recorded produced strong tonic excitation of motors neurons to the levator muscles of the same leg.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1987 the American Physiological Society

Rudomin, P.; Solodkin, M.; Jimenez, I.

doi: N/Apmid: 3585469

Abstract Spike-triggered averaging of dorsal and ventral root potentials was used in anesthetized cats to disclose possible synaptic connections of spinal interneurons in the intermediate nucleus with afferent fibers and/or motoneurons. With this method we have been able to document the existence of a distinct group of interneurons whose activity was associated with the recording of inhibitory potentials in the ventral roots (iVRPs), but not with negative dorsal root potentials (nDRPs). The iVRPs had mean durations of 60.8 +/- 22.1 ms and latencies between 1.7 and 5.1 ms relative to the onset of the interneuronal spikes. Within this group of neurons it was possible to characterize two categories depending on their responses to segmental inputs. Most type A interneurons were mono- or disynaptically activated by group I muscle afferents and polysynaptically by low threshold (1.08-1.69 X T) cutaneous fibers. Type B interneurons were instead polysynaptically activated by group II muscle and by cutaneous fibers with thresholds ranging from 1.02 to 3.1 X T. Whenever tested, both type A and B interneurons could be antidromically activated from Clarke's columns. There was a second group of interneurons whose activity was associated with the generation of both iVRPs and nDRPs. These potentials had mean durations of 107.5 +/- 35.6 and 131.5 +/- 32 ms, respectively, and onset latencies between 1.7 and 6.1 ms. The interneurons belonging to this group, which appear not to send axonal projections to Clarke's column, could be classified in three categories depending on their responses to peripheral inputs. Type C interneurons responded mono- or disynaptically to group I muscle volleys and polysynaptically to intermediate threshold (1.22-2.7 X T) cutaneous afferents. Type D interneurons were polysynaptically activated by group II muscle afferents (2.3-8.5 X T) and by intermediate threshold (1.4-3 X T) cutaneous fibers and type E interneurons only by group I muscle afferents with mono- or disynaptic latencies. A third group of interneurons produced nDRPs without iVRPs. The nDRPs had onset latencies varying from 1.9 to 6.2 ms and mean durations of 130.0 +/- 34.6 ms. These neurons (type F) showed spontaneous and evoked bursts of activity and were not antidromically activated from Clarke's column. They responded to stimulation of low- and intermediate-threshold cutaneous fibers (1.04-2.9 X T) with mono- and polysynaptic latencies, but not by group I muscle fibers. Type F interneurons appear to be located in more superficial layers than all the other interneurons.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1987 the American Physiological Society

Rhoades, R. W.; Belford, G. R.; Killackey, H. P.

doi: N/Apmid: 3585480

Abstract Single neurons were recorded from the ventral posteromedial thalamic nucleus (VPM) of urethan-anesthetized rats. Six of these animals were intact, 28 sustained kainic acid (KA) lesions of trigeminal nucleus principalis (PrV), and 9 received similar lesions of trigeminal subnucleus interpolaris (SpVi). Four animals sustained PrV lesions that were followed, at an interval of 1-3 mo, by KA injections into SpVi. Special attention was paid to the receptive-field characteristics of neurons that were sensitive to deflection of the mystacial vibrissae. In normal animals, we recorded a total of 167 VPM neurons, 85% (n = 142) of which were vibrissa sensitive. The remaining VPM cells were excited by either guard hair deflection (8.4%), indentation of the skin (0.6%), or deflection of either vibrissae or guard hairs (1.8%). Seven cells (4.2%) were unresponsive. The topography of the trigeminal representation in VPM was similar to that reported previously by Waite (59). Vibrissa-sensitive neurons in intact rats generally gave rapidly adapting responses (84.5%), and only 16.2% were directionally selective. The vast majority (80.3%) of the vibrissa-sensitive cells were activated by deflection of only one whisker (1.2 +/- 0.5, mean +/- SD); none were excited by deflection of more than four vibrissae. Injections of KA into SpVi of otherwise intact rats (n = 9) had no appreciable effect on the receptive-field characteristics of vibrissa-sensitive VPM neurons. Injections of KA into PrV markedly altered the receptive-field properties of VPM cells. Recordings were made from 45 VPM neurons over a period extending from 0 to 10 h after KA injections into PrV in five rats. Of these cells, 4.4% were excited by vibrissa deflection and the remainder were unresponsive. Additional recordings from SpVi and the superior colliculus of these same animals indicated that the neurotoxin probably did not damage interpolaris neurons or their axons. Recordings were made from 394 VPM cells in 22 rats that survived 1-6 days after KA lesions of PrV. These experiments demonstrated an increase in the number of thalamic cells that were responsive to peripheral stimulation over this period. By 6 days after the lesion (4 animals), 52.8% of the 73 VPM neurons we recorded were excited by somatosensory stimuli. Of these, 89.5% were activated by deflection of one or more mystacial vibrissae. The average number of whiskers that excited a given VPM cell in these rats was 6.3 +/- 2.0 (SD). Recordings were made from VPM in five rats that survived 30-90 days after KA injections in PrV.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1987 the American Physiological Society