Effect of a spider toxin (JSTX) on excitatory postsynaptic current at neuromuscular synapse of spiny lobsterMiwa, A.; Kawai, N.; Saito, M.; Pan-Hou, H.; Yoshioka, M.
doi: N/Apmid: 3655870
Abstract 1. We studied the blocking properties of a spider (Nephila clavata) toxin (JSTX) purified from venom on the spiny lobster neuromuscular junction. 2. When a small amount of JSTX was applied to the neuromuscular junction, the excitatory postsynaptic potential (EPSP) was partially suppressed. The amplitude of EPSPs remained at a steady level for several hours during the washing of the preparation, showing that the action of JSTX is irreversible. 3. We recorded the excitatory postsynaptic current (EPSC) from synaptic site using a macro-patch electrode. The amplitude of EPSC increased linearly with hyperpolarization of the membrane potential in the presence and absence of JSTX. 4. The decay phase time constant of EPSC and spontaneous EPSC was decreased by hyperpolarizing the membrane potential both in the absence and in the presence of JSTX. The relationship between the decay time constant and the membrane potential was not modified by JSTX. 5. It is suggested that JSTX irreversibly blocks EPSC by acting on the site that is apart from the ionic channel of the glutamate receptor molecule. Copyright © 1987 the American Physiological Society
Eye movements induced by pontine stimulation: interaction with visually triggered saccadesSparks, D. L.; Mays, L. E.; Porter, J. D.
doi: N/Apmid: 3655869
Abstract 1. Rhesus monkeys were trained to look to brief visual targets presented in an otherwise darkened room. On some trials, after the visual target was extinguished but before a saccade to it could be initiated, the eyes were driven to another orbital position by microstimulation of the paramedian pontine reticular formation. If, as current models of the saccadic system suggest, a copy of the motor command is used as a feedback signal of eye position, failure to compensate for stimulation-induced movements would indicate that stimulation occurred at a site beyond the point from which the eye position signal was derived. 2. Animals compensated for perturbations of eye position induced by stimulation of most pontine sites by making saccades that directed gaze to the position of the visual target. With stimulation at other pontine sites, compensatory saccades did not occur. 3. Pontine stimulation sometimes triggered, prematurely, impending visually directed saccades. The direction and amplitude of the premature movement depended upon the location of the briefly presented visual target. The amplitude of the premature movement was also a function of the interval between the stimulation train and the impending saccade. These data suggest that input signals for the horizontal and vertical pulse/step generators develop gradually during the presaccadic interval. Saccade trigger signals need to be delayed until the formation of these signals is completed. 4. The implications of these findings for models of the saccadic system are discussed. Robinson's local feedback model of the saccadic system can explain compensation for pontine stimulation-induced changes in eye position but cannot easily account for the failure to compensate for perturbations in eye position produced by stimulation at other sites. Modified versions of Robinson's model, which assume that the input signal to the pulse/step generator is the desired displacement of the eye, can account for both compensation and the failure to compensate since two separate neural integrators are employed. However, these models ignore kinematic arguments that commands to the extraocular muscles must specify the absolute position of the eye in the orbit rather than a relative movement from a previous position. Copyright © 1987 the American Physiological Society
Evidence that periodontal pressoreceptors provide positive feedback to jaw closing muscles during masticationLavigne, G.; Kim, J. S.; Valiquette, C.; Lund, J. P.
doi: N/Apmid: 3655872
Abstract 1. Mastication was produced by stimulation of the right motor-sensory cortex of urethan-anesthetized rabbits with 15-s trains of shocks (1-ms duration) at 50 Hz. Movements of the lower jaw and jaw muscle electromyograms (EMGs) were recorded on magnetic tape for later computer analysis. 2. The stimulus site was chosen, and stimulus intensity adjusted, so that stereotyped movements were produced that included a wide swing of the mandible to the left side during jaw closure. 3. Control trials were alternated with trials in which a steel ball (2 mm diam) was thrust between the anterior molar teeth on the left side and left in place for several seconds. 4. When the obstruction was first introduced, a jaw opening reflex was sometimes evoked if the ball struck the buccal surface of the advancing mandibular molar teeth. Thereafter, when the ball was crushed between the occlusal surfaces of the teeth, no jaw opening reflex was seen. 5. Instead, the amplitude and duration of all the jaw closing EMGs increased, beginning at least 12 ms after contact with the ball. This caused a prolongation of the slow closing (SC) phase of the cycle that, coupled with a delay in the start of activity in the digastric muscle (jaw opener), prolonged the cycle by more than 60 ms. 6. During the SC phase of the obstructed trials, the medially directed grinding stroke was exaggerated because of an increase in the contraction of the contralateral zygomaticomandibular and anterior temporal muscles. 7. After collecting data, the sensory nerves to the maxillary and mandibular anterior molar teeth were cut to eliminate feedback from the periodontal pressoreceptors. Control and obstructed trials were repeated. 8. Following denervation, the obstructed cycles were of shorter duration. The mandible still moved to the right during SC in some animals, but the increase in closer muscle EMG activity was much reduced. 9. We conclude that periodontal receptors provide positive feedback to the jaw closing muscles during mastication. This is supplemented by input from other receptors, probably muscle spindles. In addition, an increase in periodontal feedback prolongs the SC phase and the early phases of the opening stroke. Copyright © 1987 the American Physiological Society
Use-dependent block of excitatory amino acid currents in cultured neurons by ketamineMacDonald, J. F.; Miljkovic, Z.; Pennefather, P.
doi: N/Apmid: 2443623
Abstract 1. Mouse hippocampal neurons grown in dissociated cell culture were patch clamped using a whole cell voltage clamp (discontinuous switching clamp) technique. The currents generated by pressure applications of excitatory amino acids were studied over a wide range of holding potentials, and current-voltage curves were plotted. Excitatory amino acids that activated the N-methyl-D-aspartic acid (NMDA) receptor demonstrated some degree of desensitization with repeated applications, whereas the currents observed in response to kainic acid (KAI) did not. Desensitization could be minimized by keeping the frequency of application sufficiently low (i.e., less than 0.1 Hz). 2. The short-acting dissociative anaesthetic, ketamine (2–50 microM), selectively blocked L-aspartic acid (L-Asp), NMDA, and L-glutamic acid (L-Glu) currents while sparing those in response to KAI. Therefore, ketamine is a relatively selective blocker of the NMDA response versus that (those) activated by KAI. 3. The block by ketamine of excitatory amino acid currents is highly voltage dependent. Concentrations of ketamine that had little effect on outward current responses at depolarized potentials were quite effective at blocking inward current responses at hyperpolarized potentials. In contrast, DL-2-amino-5-phosphonovaleric acid (APV) was equally effective at blocking both inward and outward currents (voltage independent). The voltage dependence of ketamine (a positively charged molecule) could be accounted for if ketamine blocked the NMDA response by binding to a site that experienced 55% of the membrane field. 4. In the presence of ketamine, peak inward currents evoked by repeated applications of NMDA, L-Asp, or L-Glu progressively declined to a steady-state level of block (use-dependent block). This decrement occurred at frequencies much lower than those that were employed to demonstrate desensitization (in the absence of ketamine). Moving the membrane potential to depolarized values did not, in itself, relieve the ketamine block. However, if the appropriate excitatory amino acid (L-Asp, NMDA, L-Glu) was applied during the period of depolarization, a relief of the block could be demonstrated. No recovery from the blockade occurred with periods of rest (no amino acid application) as long as 5 min. Furthermore, no recovery was observed even when ketamine was washed out of the bathing solution until the appropriate agonist was applied. Thus recovery from blockade, like development of blockade, was use dependent.(ABSTRACT TRUNCATED AT 400 WORDS) Copyright © 1987 the American Physiological Society
In-parallel and in-series behavior of human muscle spindle endingsBurke, D.; Aniss, A. M.; Gandevia, S. C.
doi: N/Apmid: 2958606
Abstract 1. The responses of 62 putative muscle spindle afferents innervating the pretibial flexor muscles of normal human subjects were studied during graded twitch contractions of the receptor-bearing muscle to search for possible in-series coupling between spindle endings and motor units. 2. The majority of afferents (n = 57) responded unequivocally in an in-parallel manner to the twitch contractions, regardless of contraction strength. There were two patterns of in-parallel response: afferents without background activity discharged during the relaxation phase of the twitch; afferents with a background discharge were transiently silenced during the contraction phase and resumed their discharge on the relaxation phase. 3. Evidence of in-series coupling was found for five afferents during submaximal twitch contractions, to which each afferent responded in a mixed “biphasic” manner, with increases in discharge during both the contraction and relaxation phases of the twitch. Background discharge, response to stretch, and response during isometric voluntary contractions suggested that four of the afferents innervated primary spindle endings and the fifth a secondary spindle ending. 4. It is argued that the five atypical spindle endings responded in an ambiguous manner during twitch contractions of the receptor-bearing muscle because there was an in-series mechanical coupling between motor units and the spindle. The incidence of demonstrable in-series responses has serious implications for the mechanisms of spindle activation during normal motor behavior, but has only minor implications for the use of the twitch test to identify muscle spindle endings. Copyright © 1987 the American Physiological Society
Linear mechanism of orientation tuning in the retina and lateral geniculate nucleus of the catSoodak, R. E.; Shapley, R. M.; Kaplan, E.
doi: N/Apmid: 3655866
Abstract 1. The orientation tuning of lateral geniculate nucleus (LGN) neurons and retinal ganglion cells (recorded as S potentials in the LGN) was investigated with drifting grating stimuli. 2. Results were compared with a quantitative model, in which receptive fields were constructed from linear, elliptical Gaussian center and surround subunits, and responses could be predicted to gratings of any spatial frequency at any orientation. 3. The orientation tuning of X and Y retinal ganglion cells and LGN neurons was shown to result from the linear mechanism of receptive-field elongation, as data from these cells could be well fit with this model. 4. The responses of LGN neurons and their input retinal ganglion cells were compared. The orientation tuning of LGN neurons was found to be a reflection of the tuning of their retinal inputs, showing that neither intrageniculate neural interactions nor the corticogeniculate projection play any role in LGN orientation selectivity. Copyright © 1987 the American Physiological Society
Timing of spike initiation in cochlear afferents: dependence on site of innervationRuggero, M. A.; Rich, N. C.
doi: N/Apmid: 3655874
Abstract 1. The phase of excitation of inner hair cells (IHCs) relative to basilar membrane motion has been estimated as a function of best frequency (BF) (or, equivalently, cochlear location) by recording responses to tones (100–1,000 Hz) from chinchilla cochlear afferent axons at their central exit from the internal auditory meatus. 2. The time of IHC excitation (i.e., the time of chemical transmitter release) was derived from the neural recordings at near-threshold levels by applying a correction for the latency of synaptic processes and the propagation time of action potentials. 3. The phase of basilar membrane motion at the appropriate innervation site was estimated on the basis of previously measured basilar membrane responses at a location close to the basal end of the cochlea and estimates of mechanical travel time from the basal end to the innervation site, derived from the neural latencies to intense rarefaction clicks, as a function of BF. 4. The derived near-threshold excitation of basal IHCs leads basilar membrane displacement toward scala tympani by approximately 40-60 degrees. 5. At BFs corresponding to midcochlear locations (2–6 kHz) there is an abrupt phase transition. The derived excitation for IHCs located at more apical locations (BFs large in relation to stimulus frequency) corresponds approximately to peak velocity of the basilar membrane toward scala vestibuli. 6. Although the derived response phases of apically located IHCs are consistent with intracellular recordings from IHCs, the derived near-threshold response phases of basal IHCs may be inconsistent with intracellular IHC recordings. 7. The foregoing results, based on responses of nearly 1,000 cochlear afferents to tones 100-1,000 Hz at near-threshold stimulus levels, amply confirm our previous conclusions that were based on a smaller sample of responses to very low frequency tones (less than or equal to 100 Hz): there is a spatial transition at midcochlear regions in the mode of excitation of IHCs, which does not seem to simply reflect the macromechanics of the basilar membrane. 8. It has been proposed that both the paradoxical response phases of high-BF afferents and the spatial phase transition arise from an influence of cochlear microphonics on the transmembrane potential of IHCs. The present results, which show that the spatial phase transition occurs for frequencies at least as high as 400 Hz, would appear to make such an electrical influence of outer hair cells on IHCs less likely. An alternative explanation might be that the phase transition has a mechanical basis, perhaps localized to micromechanical events in the subtectorial regio Copyright © 1987 the American Physiological Society