Dual mechanisms of twitching during sleep in neonatal rats.

Twitches of the limbs during REM sleep in adult mammals result from descending motor activation from the brainstem. In contrast, many spontaneous movements in embryos appear similar to REM-related twitches and result from the local firing of spinal motor neurons. To determine which mechanism produces twitches in neonates, we analyzed twitching in 5- and 8-day-old rat pups that had spinal cords transected in the lower thoracic region. This transection separated motor units controlling forelimb movements from motor units controlling hindlimb movements. Spinal transection did not significantly affect the amount of forelimb twitching. In contrast, the amount of hindlimb twitching in transected pups was reduced by only 35%–50%. Given that hindlimb twitching was not eliminated by spinal transection, it is concluded that there are 2 independent mechanisms producing twitches at these ages. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of botulinum neurotoxin type A on abducens motoneurons in the cat: alterations of the discharge pattern

The discharge characteristics that abducens motoneurons exhibit after paralysis of the lateral rectus muscle with botulinum neurotoxin type A were studied in the alert cat. Antidromically identified motoneurons were recorded during both spontaneous and vestibularly induced eye movements. A single injection of 0.3 ng/kg produced a complete paralysis of the lateral rectus muscle lasting for about 12-15 days, whereas after 3 ng/kg the paralysis was still complete at the longest time checked, three months. Motoneurons recorded under the effect of the low dose showed differences in their sensitivities to both eye position and velocity according to the direction of the previous and ongoing movements, respectively. These directional differences could be explained by post-saccadic adaptation of the non-injected eye in the appropriate direction for reducing ocular misalignment. Thus, backward and forward post-saccadic drifts accompanied on- and off-directed saccades, respectively. The magnitude of the drift was similar to the magnitude of changes in eye position sensitivity. The discharge of the high-dose-treated motoneurons could be described in a three-stage sequence. During the initial 10-12 days, motoneuronal discharge resembled the effects of axotomy, particularly in the loss of tonic signals and the presence of exponential-like decay of firing after saccades. In this stage, the conduction velocity of abducens motoneurons was reduced by 21.4%. The second stage was characterized by an overall reduction in firing rate towards a tonic firing at 15-70 spikes/s. Motoneurons remained almost unmodulated for all types of eye movement and thus eye position and velocity sensitivities were significantly reduced. Tonic firing ceased only when the animal became drowsy, but was restored by alerting stimuli. In addition, the inhibition of firing for off-directed saccades was more affected than the burst excitation during on-directed saccades, since in many cells pauses were almost negligible. These alterations could not be explained by adaptational changes in the movement of the non-injected eye. Finally, after 60 days the initial stages of recovery were observed. The present results indicate that the high dose of botulinum neurotoxin produces effects on the motoneuron not attributable to the functional disconnection alone, but to a direct effect of the neurotoxin in the motoneuron and/or its synaptic inputs.     

Rapid, corticosterone-induced disruption of medullary sensorimotor integration related to suppression of amplectic clasping in behaving roughskin newts (Taricha granulosa)

Endogenously secreted or injected corticosterone (CORT) rapidly suppresses courtship clasping in male roughskin newts (Taricha granulosa) by an action on a specific neuronal membrane receptor. Previous studies, using immobilized newts, showed that CORT administration rapidly depresses excitability of reticulospinal neurons and attenuates medullary neuronal responsiveness to clasp-triggering sensory stimuli. The present study used freely moving newts to examine clasping responses and concurrently record sensorimotor properties of 67 antidromically identified reticulospinal and other medullary reticular neurons before and after CORT injection. Before CORT, reticulospinal neurons fired in close association with onset and offset of clasps elicited by cloacal pressure. Reticulospinal neurons also showed firing correlates of nonclasping motor events, especially locomotion. Neuronal activity was typically reduced during clasping and elevated during locomotion. Medullary neurons that were not antidromically invaded (unidentified neurons) usually showed sensorimotor properties that resembled those of reticulospinal neurons. Intraperitoneal CORT (but not vehicle) reduced the probability and quality of hindlimb clasping in response to cloacal pressure, especially within 5-25 min of injection. Simultaneously, responses of reticulospinal and unidentified neurons to cloacal pressure and occurrence of clasping-related activity were attenuated or eliminated. CORT effects were relatively selective, altering clasping-related neuronal activity more strongly than activity associated with nonclasping motor events. The properties of CORT effects indicate that the hormone impairs clasping by depressing processing of clasp-triggering afferent activity and by disrupting the medullary control of clasping normally mediated by reticulospinal neurons. The rapid onset of these CORT effects implicates a neuronal membrane receptor rather than genomic action of the steroid.     

Recovery of locomotion after ventral and ventrolateral spinal lesions in the cat. I. Deficits and adaptive mechanisms

The recovery of treadmill locomotion of eight adult cats, subjected to chronic ventral and ventrolateral spinal lesions at low thoracic levels (T11 or T13), preserving at least one dorsolateral funiculus and the dorsal columns, was documented daily using electromyographic (EMG) and kinematic methods. The data show that all cats eventually recovered quadrupedal voluntary locomotion despite extensive damage to important pathways (such as the reticulospinal and the vestibulospinal) as verified by injection of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) caudal to the site of lesion. Initially (in the early period after the spinal lesion), all the cats suffered from pronounced locomotor and postural deficits, and they could not support their hindquarters or walk with their hindlimbs. Gradually, during the recovery period, they regained quadrupedal walking, although their locomotion was wobbly and inconsistent, and they suffered from poor lateral stability. EMG and kinematic data analyses showed a tendency for an increase in the variability of the step cycle duration but no major changes in the step cycle structure or in the intralimb coupling of the joints. However, the homolateral fore- and hindlimb coupling was highly perturbed in cats with the largest lesions. Although the general alternating pattern of extensor and flexors was maintained, there were various changes in the duration and amplitude of the EMG bursts as well as a lack of amplitude modulation during walking uphill or downhill on the treadmill. In cats with larger lesions, the forelimbs also seem to take a greater propulsive role than usual as revealed by a consistent increase of the activity of the triceps. In cats with smaller lesions, these deficits were transient, but, for the most extensively lesioned cats, they were pronounced and lasted long term postlesion even after reaching a more or less stable locomotor behavior (plateau period). It is concluded that recovery of quadrupedal locomotion is possible even after a massive lesion to ventral and ventrolateral quadrants, severing the vestibulospinal pathway and causing severe, although incomplete, damage to the reticulospinal tract. The quick recovery in the less lesioned cats can be attributed to remaining pathways normally implicated in locomotor function. However, in the most extensively lesioned cats, the long period of recovery and the pronounced deficits during the plateau period may indicate that the compensation, attributed to remaining reticulospinal pathways, is not sufficient and that other pathways in the dorsolateral funiculi, such as the corticospinal, can sustain and adapt, up to a certain extent, the voluntary quadrupedal walking.     

Reticulospinal systems mediate atonia with short and long latencies

The pontomedullary region is responsible for both the tonic and phasic reduction of muscle activity in rapid-eye-movement sleep and contributes to the control of muscle tone in waking. This study focused on determining the time course of activity in the pontomedullary systems mediating atonia. Short-train stimulations (3 0.2-ms pulses at 330 Hz) of the pons and medulla suppressed neck and hindlimb muscle activity in decerebrate cats. We identified two distinct phases of suppression, early and late. The anatomic sites that produced each suppression were intermixed. We estimated the dividing value of the conduction velocity for reticulospinal projections responsible for early and late phases of hindlimb muscle tone suppression to be 22.8 m/s. In the medial medulla, 238 reticulospinal units, which send axons to the L1 level of the spinal cord, were identified. Pontine stimulation that suppressed hindlimb muscle tone increased the firing rate of 138 units (type I). Sixteen type I units showed a delayed response to the pontine stimulation with a latency of 10 ms or longer (type Id), whereas 122 type I units exhibited an earlier response (type Ie). Seven type Ie units had an axonal conduction velocity of <22.8 m/s, whereas the remaining 115 conducted at faster than 22.8 m/s. Early and late hindlimb muscle tone suppressions were hypothesized to be mediated through fast and slow conducting type Ie reticulospinal units. The activity of type Id neurons may contribute to the cessation of the early-phase suppression as well as to the induction, maintenance, or cessation of the late-phase suppression.     

Modulation of oscillator interactions in the crab stomatogastric ganglion by crustacean cardioactive peptide

The modulation of the pyloric rhythm of the stomatogastric ganglion of the crab, Cancer borealis, by crustacean cardioactive peptide (CCAP) is described. CCAP activated pyloric rhythms in most silent preparations, and altered the phase relationships of pyloric motor neuron firing in all preparations. In CCAP, the pyloric rhythms were characterized by long lateral pyloric (LP) neuron bursts of action potentials. The threshold for CCAP action was approximately 10(-10) M, with increasing effects at higher CCAP concentrations. The changes in motor pattern evoked by CCAP produced significant changes in LP-innervated muscle movement. These movements were additionally potentiated by CCAP applications to isolated nerve-muscle preparations. Thus, enhanced motor neuron firing and increase of the gain of the neuromuscular junctions are likely to operate coordinately in response to hormonally released CCAP. High CCAP concentrations sometimes resulted in modification of the normal 1:1 alternation between the pyloric dilator (PD) and LP neurons to patterns of 2:1, 3:1, or 4:1 alternation. CCAP seems to activate slow intrinsic oscillations in the LP neuron, as well as enhance faster oscillations in the pacemaker group of PD/anterior burster (AB) neurons. Simulations of fast and slow oscillators with reciprocal inhibitory coupling suggest mechanisms that could account for the mode switch from 1:1 alternation to multiple PD bursts alternating with one LP neuron burst.     

Three types of reticular neurons involved in the spino-bulbo-spinal reflex of cats

Reticular neuron activity was recorded in 28 chloralosed cats in order to analyze the reflex arc of the spino-bulbo-spinal (SBS) reflex. Three types of reticular neurons, types I (input), II(output) and III (relay), were identified by unit discharges in response to stimulation of the sural nerve. (1) Type I (input) neurons received spinal ascending volleys monosynaptically and responded to stimulation of the sural nerve with spikes of low amplitude and short latency. Unit spikes, however, were not produced by stimulation of the superficial radial nerve and the sensorimotor cortex. These input neurons were located in the dorsocaudal part of the medial bulbar reticular formation. (2) Type II (output) neurons were part of the reticulospinal tract, which sends axons to the spinal cord, since these neurons exhibited antidromic spikes following stimulation of the ventrolateral funiculus of the spinal cord. Unit spikes were evoked by stimulation either to the sural or superficial radial nerves. These neurons were located in the ventrocaudal part of the medial bulbar reticular formation. (3) Type III neurons included relay neurons. Unit spikes were evoked by stimulation of the sural nerve, superficial radial nerve and sensorimotor cortex. However, unit discharges were not obtained by antidromic stimulation to the reticulospinal tract. These neurons were distributed widely in the brain stem, both in the bulb and pons. (4) Latency difference of unit discharges between input and output neurons was 3.5--5 msec, indicating the presence of interneurons (relays) between input and output neurons. Spikes of output neurons with 3.8--4.2 msec latency were observed following stimulation of the region where input neuron activity was found. We may conclude that three kinds of reticular neurons, input, relay and output, were involved in pathways of the SBS reflex.     

Azimuth coding in primary auditory cortex of the cat. I. Spike synchrony versus spike count representations

The neural representation of sound azimuth in auditory cortex most often is considered to be average firing rate, and azimuth tuning curves based thereupon appear to be rather broad. Coincident firings of simultaneously recorded neurons could provide an improved representation of sound azimuth compared with that contained in the firing rate in either of the units. In the present study, a comparison was made between local field potentials and several measures based on unit firing rate and coincident firing with respect to their azimuth-tuning curve bandwidth. Noise bursts, covering a 60-dB intensity range, were presented from nine speakers arranged in a semicircular array with a radius of 55 cm in the animal's frontal half field. At threshold intensities, all local field potential (LFP) recordings showed preferences for contralateral azimuths. Multiunit recordings showed in 74% a threshold for contralateral azimuths, in 16% for frontal azimuths, and in only 5% showed an ipsilateral threshold. The remaining 5% were not spatially tuned. Representations for directionally sensitive units based on coincident firings provided significantly sharper tuning (50-60 degrees bandwidth at 25 dB above the lowest threshold) than those based on firing rate (bandwidths of 80-90 degrees). The ability to predict sound azimuth from the directional information contained in the neural population activity was simulated by combining the responses of the 102 single units. Peak firing rates and coincident firings with LFPs at the preferred azimuth for each unit were used to construct a population vector. At stimulus levels of >/=40 dB SPL, the prediction function was sigmoidal with the predicted frontal azimuth coinciding with the frontal speaker position. Sound azimuths >45 degrees from the midline all resulted in predicted values of -90 or 90 degrees, respectively. No differences were observed in the performance of the prediction based on firing rate or coincident firings for these intensities. This suggests that although coincident firings produce narrower azimuth tuning curves, the information contained in the overall neural population does not increase compared with that contained in a firing rate representation. The relatively poor performance of the population vector further suggests that primary auditory cortex does not code sound azimuth by a globally distributed measure of peak firing rate or coincident firing.     

Ionic mechanisms involved in the spontaneous firing of tegmental pedunculopontine nucleus neurons of the rat

We have previously defined three types of tegmental pedunculopontine nuclei neurons based on their electrophysiological characteristics: Type I neurons characterized by low-threshold Ca2+ spikes, Type II neurons which displayed a transient outward current (A-current), and Type III neurons having neither low-threshold spikes nor A-current [Kang Y. and Kitai S. T. (1990) Brain Res. 535, 79-95]. In this report, ionic mechanisms underlying repetitive firing of Type I (n=15) and Type II (n=69) neurons were studied in in vitro slice preparations. Type I neurons did not fire rhythmically but their spontaneous firing frequency ranged from 0 to 19.5 spikes/s (mean 9.7 spikes/s). The spontaneous firing of Type II neurons was rhythmic, with a mean frequency of 9.6 spikes/s (range 3.5-16.0 spikes/s). Choline acetyltransferase immunohistochemistry combined with biocytin labeling indicated that none of the Type I neurons were immunopositive to choline acetyltransferase, while 60% (42 of 69) of Type II neurons were immunopositive. There was no apparent difference in the electrophysiological membrane properties of immunopositive and immunonegative Type II neurons. At membrane potentials subthreshold for Na+ spikes (-50 mV), spontaneous membrane oscillations (11.6 Hz) were observed: these underlie the spontaneous repetitive firing of Type I neurons. The subthreshold membrane oscillation was tetrodotoxin sensitive but was not affected by Ca2+-free medium. A similar tetrodotoxin-sensitive subthreshold membrane oscillation (10.5 Hz) was also observed in Type II neurons. However, in Type II neurons a membrane oscillation was also observed at higher membrane potentials (-50 mV). This high-threshold oscillation was insensitive to tetrodotoxin and Na+-free medium, but was eliminated in Ca2+-free conditions. The amplitude and frequency of the high-threshold oscillation was increased upon membrane depolarization. At the most prominent oscillatory level (around -40 mV), the high-threshold oscillation had a mean frequency of 8.8 Hz. The high-threshold Ca2+ spike was triggered from the peak potential (-35 to -30mV) of the high-threshold oscillation. Application of tetraethylammonium chloride (< 5 mM) increased the amplitude of the high-threshold oscillation, while nifedipine greatly attenuated the high-threshold oscillation without changing the shape of the high-threshold Ca2+ spike. Application of Cd2+ eliminated both the high-threshold oscillation and the high-threshold Ca2+ spike, and omega-conotoxin reduced the size of the high-threshold Ca2+ spike without affecting the frequency of the high-threshold oscillation. Nickel did not have any effect on either the high-threshold oscillation or the high-threshold Ca2+ spike. These data suggest an involvement of N- and L-type Ca2+ channels in the generation of the high-threshold oscillation and the high-threshold Ca2+ spike, respectively. The results indicate that a persistent Na+ conductance plays a crucial role in the subthreshold membrane oscillation, which underlies spontaneous repetitive firing in Type I neurons. On the other hand, in addition to a persistent Na+ conductance for subthreshold membrane oscillation, a voltage-dependent Ca2+ conductance with Ca2+-dependent K+ conductance (for the high-threshold oscillation) may be responsible for rhythmic firing of Type II neurons.     

The vigor of metaphor in clinical practice

Multiple- and single-unit neuronal activities were recorded from cerebellar cortex (Larsell's lobule HVI and adjacent ansiform cortex) and the cerebellar interpositus nucleus during forward (CS-US), backward (US-CS), and explicitly unpaired classical eyeblink conditioning in several rabbits. Whereas learning-related activity was observed in the interpositus nucleus only during forward pairing of the conditioning stimuli, a variety of patterns of learning-related neuronal firings were observed in cerebellar cortex during forward, backward, and even unpaired presentations of the conditioning stimuli. These data suggest that the cerebellar cortex and the deep cerebellar nuclei play different roles during classical eyeblink conditioning.     

Brachially innervated ectopic hindlimbs in the chick embryo. I. Limb motility and motor system anatomy during the development of embryonic behavior

The functional status of brachially innervated hindlimbs, produced by transplanting hindlimb buds of chick embryos in place of forelimb buds, was quantified by analyzing the number and temporal distribution of spontaneous limb movements. Brachially innervated hindlimbs exhibited normal motility until E10 but thereafter became significantly less active than normal limbs and the limb movements were more randomly distributed. Contrary to the findings with axolotls and frogs, functional interaction between brachial motoneurons and hindlimb muscles cannot be sustained in the chick embryo. Dysfunction is first detectable at E10 and progresses to near total immobility by E20 and is associated with joint ankylosis and muscular atrophy. Although brachially innervated hindlimbs were virtually immobile by the time of hatching (E21), they produced strong movements in response to electrical stimulation of their spinal nerves, suggesting a central rather than peripheral defect in the motor system. The extent of motoneuron death in the brachial spinal cord was not significantly altered by the substitution of the forelimb bud with the hindlimb bud, but the timing of motoneuron loss was appropriate for the lumbar rather than brachial spinal cord, indicating that the rate of motoneuron death was dictated by the limb. Measurements of nuclear area indicated that motoneuron size was normal during the motoneuron death period (E6-E10) but the nuclei of motoneurons innervating grafted hindlimbs subsequently became significantly larger than those of normal brachial motoneurons. Although the muscle mass of the grafted hindlimb at E18 was significantly less than that of the normal hindlimb (and similar to that of a normal forelimb), electronmicroscopic examination of the grafted hindlimbs and brachial spinal cords of E20 embryos revealed normal myofiber and neuromuscular junction ultrastructure and a small increase in the number of axosomatic synapses on cross-sections of motoneurons innervating grafted hindlimbs compared to motoneurons innervating normal forelimbs. The anatomical data indicate that, rather than being associated with degenerative changes, the motor system of the brachial hindlimb of late-stage embryos is intact, but inactive.     

Locomotor-like movements evoked by leg muscle vibration in humans

We attempted to elicit automatic stepping in healthy humans using appropriate afferent stimulation. It was found that continuous leg muscle vibration produced rhythmic locomotor-like stepping movements of the suspended leg, persisting up to the end of stimulation and sometimes outlasting it by a few cycles. Air-stepping elicited by vibration did not differ from the intentional stepping under the same conditions, and involved movements in hip and knee joints with reciprocal electromyogram (EMG) bursts in corresponding flexor and extensor muscles. The phase shift between evoked hip and knee movements could be positive or negative, corresponding to 'backward' or 'forward' locomotion. Such an essential feature of natural human locomotion as alternating movements of two legs, was also present in vibratory-evoked leg movements under appropriate conditions. It is suggested that vibration evokes locomotor-like movements because vibratory-induced afferent input sets into active state the central structures responsible for stepping generation.     

Motor patterns and kinematics during backward walking in the pacific giant salamander: evidence for novel motor output

Kinematic and motor patterns during forward and backward walking in the salamander Dicamptodon tenebrosus were compared to determine whether the differences seen in mammals also apply to a lower vertebrate with sprawling posture and to measure the flexibility of motor output by tetrapod central pattern generators. During treadmill locomotion, electromyograms (EMGs) were recorded from hindlimb muscles of Dicamptodon while simultaneous high-speed video records documented movement of the body, thigh, and crus and allowed EMGs to be synchronized to limb movements. In forward locomotion, the trunk was lifted above the treadmill surface. The pelvic girdle and trunk underwent smooth side-to-side oscillations throughout the stride. At the beginning of the stance phase, the femur was protracted and the knee joint extended. The knee joint initially flexed in early stance and then extended as the foot pushed off in late stance, reaching maximum extension just before foot lift-off. The femur retracted steadily throughout the stance. In the swing phase, the femur rapidly protracted, and the leg was brought forward in an "overhand crawl" motion. In backward walking, the body frequently remained in contact with the treadmill surface. The pelvic girdle, trunk, and femur remained relatively still during stance phase, and most motion occurred at the knee joint. The knee joint extended throughout most of stance, as the body moved back, away from the stationary foot. The knee flexed during swing. Four of five angles showed significantly smaller ranges in backward than in forward walking. EMGs of forward walking showed that ventral muscles were coactive, beginning activity just before foot touchdown and ceasing during the middle of stance phase. Dorsal muscles were active primarily during swing. Backward locomotion showed a different pattern; all muscles except one showed primary activity during the swing phase. This pattern of muscle synergy in backward walking never was seen in forward locomotion. Also, several muscles demonstrated lower burst rectified integrated areas (RIA) or durations during backward locomotion. Multivariate statistical analysis of EMG onset and RIA completely separated forward and backward walking along the first principal component, based on higher RIAs, longer durations of muscle activity, and greater synergy between ventral muscles during early stance in forward walking. Backward walking in Dicamptodon uses a novel motor pattern not seen during forward walking in salamanders or during any other locomotor activity in previously studied tetrapods. The central neuronal mechanisms mediating locomotion in this primitive tetrapod are thus capable of considerable plasticity.     

Cerebellar complex spikes encode both destinations and errors in arm movements

Purkinje cells of the cerebellum discharge complex spikes, named after the complexity of their waveforms, with a frequency of approximately 1 Hz during arm movements. Despite the low frequency of firing, complex spikes have been proposed to contribute to the initiation of arm movements or to the gradual improvement of motor skills. Here we recorded the activity of Purkinje cells from the hemisphere of cerebellar lobules IV-VI while trained monkeys made short-lasting reaching movements (of approximately 200 milliseconds in duration) to touch a visual target that appeared at a random location on a tangent screen. We examined the relationship between complex-spike discharges and the absolute touch position, and between complex-spike discharges and relative errors in touching the screen. We used information theory to show that the complex spikes occurring at the beginning of the reach movement encode the absolute destination of the reach, and the complex spikes occurring at the end of the short-lasting movements encode the relative errors. Thus, complex spikes convey multiple types of information, consistent with the idea that they contribute both to the generation of movements and to the gradual, long-term improvement of these movements.     

Nociceptive integration in the rat spinal cord: role of non-linear membrane properties of deep dorsal horn neurons

Deep dorsal horn neurons (DHNs) involved in nociception can relay long-lasting inputs and generate prolonged afterdischarges believed to enhance the transfer of nociceptive responses to the brain. We addressed the role of neuronal membrane properties in shaping these responses, by recording lamina V DHNs in a slice preparation of the rat cervical spinal cord. Of 256 neurons, 102 produced accelerating discharges in response to depolarizing current pulses, whereas the other neurons showed spike frequency adaptation. Two mechanisms mediated the firing acceleration: a slow inactivation of a K+ current expressed upon activation of the neuron from hyperpolarized holding potentials, and the expression of a regenerative plateau potential activating around resting membrane potential. The increase in firing frequency was much stronger when sustained by the plateau potential (71 DHNs, 28%). A few neurons produced adaptation and both types of acceleration, in different membrane potential domains, showing that the firing pattern of a deep DHN is not a rigid characteristic. Plateau potentials could be elicited by stimulation of nociceptive primary afferent fibres. The bistability associated with plateau potentials permitted afterdischarges. Because plateau potentials had slow activation kinetics and were voltage-dependent, the neurons had non-linear input-output relationships in both the amplitude and time domains. Nociceptive primary afferent stimulation elicited intense and prolonged responses in plateau-generating DHNs, while brief bursts of spikes were evoked otherwise. These results indicate that in a population of deep DHNs, intense firing and prolonged afterdischarges in response to nociceptive stimulation depend on non-linear intrinsic membrane properties.     

Binocular rivalry: ambiguities resolved

For over 100 years, binocular rivalry was seen as the result of competition between the two eyes, involving reciprocal suppression of retinal inputs. Now it emerges that rivalry reflects alternating perceptual interpretations that are represented in the firing patterns of cells in the temporal visual cortex.     

Gaze-related activity of putative inhibitory burst neurons in the head-free cat

1. Previous studies in the cat have demonstrated that output neurons of the superior collicular as well as brain stem omnipause neurons have discharges that are best correlated, not with the trajectory of the eye in the head but, with the trajectory of the visual axis in space (gaze = eye-in-head + head-in-space) during rapid orienting coordinated eye and head movements. In this study, we describe the gaze-related activity of cat premotor "inhibitory burst neurons" (IBNs) identified on the basis of their position relative to the abducens nucleus. 2. The firing behavior of IBNs was studied during 1) saccades made with the head stationary, 2) active orienting combined eye-head gaze shifts, and 3) passive movements of the head on the body. IBN discharges were well correlated with the duration and amplitude of saccades made when the head was stationary. In both head-free paradigms, the behavior of cat IBNs differed from that of previously described primate "saccade bursters". The duration of their burst was better correlated with gaze than saccade duration, and the total number of spikes in a burst was well correlated with gaze amplitude and generally poorly correlated with saccade amplitude. The behavior of cat IBNs also differed from that of previously described primate "gaze bursters". The slope of the relationship between the total number of spikes and gaze amplitude observed during head-free gaze shifts was significantly lower than that observed during head-fixed saccades. 3. These studies suggest that cat IBNs do not fit into the categories of gaze-bursters or saccade-bursters that have been described in primate studies.(ABSTRACT TRUNCATED AT 250 WORDS)     

Representation of multiple kinematic parameters of the cat hindlimb in spinocerebellar activity

Dorsal spinocerebellar tract (DSCT) neurons have been shown to transmit signals related to hindlimb position and movement direction in the anesthetized cat. Because both parameters may be encoded by single neurons, we examined the extent to which their representations might occur sequentially or simultaneously by recording unit activity while the hindlimb was moved passively in the sagittal plane by a robot arm. A center-out/out-center paradigm moved the foot 2 cm from a given position radially to eight positions located 45 degrees apart, holding each position for 8 s. Another paradigm moved the foot along various paths to 20 positions distributed throughout most of the limb's workspace. With each paradigm, we could assess the activity related to foot position and the direction of movement to each position. Modulation of unit activity evoked by center-out/out-center movements was determined for each 1-s postmovement interval by use of a cosine tuning model that specified modulation amplitude and preferred direction. Of 125 units tested, 82.4% were significantly modulated (P < 0.05) according to this model. We assessed the relative contributions of position and movement by taking advantage of the fact that directional modulation following out-center movements to a common position could only be related to the movement, whereas that following the center-out movements related to both position and movement. The results suggested a simultaneous modulation by these two parameters. Each cell could be characterized by a similar preferred direction for position or movement modulation and the distribution of preferred directions across cells clustered significantly along an axis close to the limb axis. When the limb axis was rotated, the unit preferred directions rotated similarly, on average. Unexpectedly, we found the activity of more than half the cells to be modulated for > or = 8 s after out-center movements, implying a persistent movement-related activity well after a movement is completed. These findings were confirmed and extended with the second paradigm by using a multivariate regression model that included terms for position, movement, and their multiplicative interaction. The activity of 81.3% of the 97 neurons tested fit the model (R2 > 0.4, P < .0001); 31.6% were modulated exclusively by foot position, and 58.2% simultaneously by both position and movement, with significant interaction. We conclude from our results that DSCT neurons may be modulated simultaneously by limb position and movement, and their preferred directions tend to align with the limb axis. The modulation is interactive such that movement modulation amplitude depends on limb position, and many cells also retain a memory trace of recent movements. The results are discussed in terms of a possible role for the DSCT in encoding limb compliance.     

Emergence of spatio-temporal patterns in neuronal activity

This paper explores if dynamic modulation of coherent firing serves cortical functions. We recorded neuronal activity in the frontal cortex of behaving monkeys and found that temporal coincidences of spikes firing of different neurons can emerge within a fraction of a second in relation to the animal behavior. The temporal patterns of the correlation could not be predicted from the modulations of the neurons firing rate and finally, the patterns of correlation depend on the distance between neurons. These findings call for a revision of prevailing models of neural coding that solely rely on firing rates. The findings suggest that modification of neuronal interactions can serve as a mechanism by which neurons associate rapidly into a functional group in order to perform a specific computational task. Increased correlation between members of the groups, and decreased or negative correlation with others, enhance the ability to dissociate one group from concurrently activated competing groups. Such modulation of neuronal interactions allows each neuron to become a member of several different groups and participate in different computational tasks.     

Temporal firing patterns of Purkinje cells in the cerebellar ventral paraflocculus during ocular following responses in monkeys II. Complex spikes

Many theories of cerebellar motor learning propose that complex spikes (CS) provide essential error signals for learning and modulate parallel fiber inputs that generate simple spikes (SS). These theories, however, do not satisfactorily specify what modality is represented by CS or how information is conveyed by the ultra-low CS firing rate (1 Hz). To further examine the function of CS and the relationship between CS and SS in the cerebellum, CS and SS were recorded in the ventral paraflocculus (VPFL) of awake monkeys during ocular following responses (OFR). In addition, a new statistical method using a generalized linear model of firing probability based on a binomial distribution of the spike count was developed for analysis of the ultra-low CS firing rate. The results of the present study showed that the spatial coordinates of CS were aligned with those of SS and the speed-tuning properties of CS and SS were more linear for eye movement than retinal slip velocity, indicating that CS contain a motor component in addition to the sensory component identified in previous studies. The generalized linear model to reproduce firing probability confirmed these results, demonstrating that CS conveyed high-frequency information with its ultra-low firing frequency and conveyed both sensory and motor information. Although the temporal patterns of the CS were similar to those of the SS when the sign was reversed and magnitude was amplified approximately 50 times, the velocity/acceleration coefficient ratio of the eye movement model, an aspect of the CS temporal firing profile, was less than that of the SS, suggesting that CS were more sensory in nature than SS. A cross-correlation analysis of SS that are triggered by CS revealed that short-term modulation, that is, the brief pause in SS caused by CS, does not account for the reciprocal modulation of SS and CS. The results also showed that three major aspects of the CS and SS individual cell firing characteristics were negatively correlated on a cell-to-cell basis: the preferred direction of stimulus motion, the mean percent change in firing rate induced by upward stimulus motion, and patterns of temporal firing probability. These results suggest that CS may contribute to long-term interactions between parallel and climbing fiber inputs, such as long-term depression and/or potentiation.