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Notes: Summary Responses of neck motoneurons to electrical stimulation of the pontomedullary reticular formation were recorded intracellularly in cerebellectomized cats anesthetized with chloralose. Stimulation of nucleus reticularis (n.r.) ventralis and the dorsal part of n.r. gigantocellularis evoked short latency, monosynaptic inhibitory postsynaptic potentials (IPSPs) in the majority of motoneurons supplying the ipsilateral splenius, biventer cervicis and complexus muscles and in 25% of motoneurons projecting in the ipsilateral spinal accessory nerve. Monosynaptic IPSPs were also evoked by stimulating the medial longitudinal fasciculus (MLF) but lesion and collision experiments indicated that these IPSPs were independent of those evoked by reticular stimulation. Monosynaptic IPSPs were also occasionally observed following stimulation of the contralateral reticular formation, especially of the dorsal part of n.r. gigantocellularis. Monosynaptic excitatory postsynaptic potentials (EPSPs) were evoked in all classes of neck motoneurons studied by stimulation of n.r. pontis caudalis, gigantocellularis and ventralis. Each reticular nucleus appeared to contribute to this excitation. The excitation was bilateral but large monosynaptic EPSPs were most often seen in motoneurons ipsilateral to the stimulus site. Data indicated that pontine EPSPs were mediated by ventromedial reticulospinal fibers while medullary EPSPs were mediated by ventrolateral reticulospinal fibers. Neck motoneurons thus receive at least three distinct direct reticulospinal inputs, two excitatory and one inhibitory.

Notes: Summary Responses of motoneurons supplying muscles of the forelimbs, hindlimbs, back, and neck to stimulation of the medial pontomedullary reticular formation were studied with intracellular recording in cere-bellectomized cats under chloralose anesthesia. Stimulation of the midline or of a reticular region consisting of nucleus reticularis (n.r.) pontis caudalis and the dorsorostral part of n.r. gigantocellularis produced monosynaptic excitation of ipsilateral motoneurons supplying axial muscles and flexor and extensor muscles in both proximal and distal parts of the limbs. This widespread excitation appears to have been produced by rapidly conducting medial reticulospinal fibers. Stimulation of a second region consisting of n.r. ventralis and the ventrocaudal part of n. r. gigantocellularis produced monosynaptic excitation of ipsilateral neck and back motoneurons but only longer latency, apparently multisynaptic excitation of limb motoneurons. Collision tests indicated that this monosynaptic excitation did not involve fibers descending along the midline. It therefore appears to have been produced by lateral reticulospinal fibers. Reticular stimulation also produced short latency, monosynaptic inhibition of neck motoneurons, long latency, apparently polysynaptic inhibition of limb motoneurons and intermediate latency inhibition of back motoneurons. The latencies and properties of inhibitory responses of back motoneurons indicated that they were produced either disynaptically by fast fibers or monosynaptically by slower fibers. The data indicate that the medial pontomedullary reticular formation can be divided into a number of different zones each with a distinct pattern of connections with somatic motoneurons. These include the dorsorostrally located medial reticulospinal projection area, from which direct excitation of a wide variety of motoneurons can be evoked, the ventrocaudally located lateral reticulospinal projection area from which direct excitation of neck and back and direct inhibition of neck motoneurons can be evoked and the dorsal strip of n.r. gigantocellularis which has direct excitatory and inhibitory actions only on neck motoneurons.

Notes: Summary 1. Eye movement responses were examined in alert cats during sinusoidal vertical linear acceleration. Stimulus frequencies of 0.20–0.85 Hz with a constant amplitude of 10.5 cm (corresponding to 0.02–0.31 g) were used. A random visual pattern was presented to give sinusoidal vertical optokinetic stimuli of similar amplitude and frequency to the up-down motion of the cat. 2. Sinusoidal linear acceleration in the presence of a stationary visual pattern produced robust eye movement responses with near compensatory phase at all stimulus frequencies tested. With both eyes covered, a vertical linear vestibulo-ocular reflex (LVOR) was frequently produced at a stimulus strength corresponding to 0.04–0.31 g. The evoked LVOR was always small, and the overall mean response phase values advanced by as much as 70 ° at frequencies below 0.56 Hz, indicating that the otolith signals activated by sinusoidal linear acceleration were not, by themselves, converted into compensatory eye position signals under these experimental conditions. 3. Optokinetic stimulation alone produced more lag of response phase as stimulus frequency increased, and the gain of evoked eye movement responses was smaller at higher stimulus frequencies compared to the gain during linear acceleration in the light. Bilateral labyrinthectomies resulted in a significant change of the eye movement responses during linear acceleration when visual inputs were allowed: there was more phase lag at higher stimulus frequencies and a decreased gain at all frequencies tested. These results indicate that the interaction of otolith and visual inputs produces robust eye movement responses with near compensatory phase during sinusoidal linear acceleration in the light.

Notes: Summary (1) Discharge characteristics of neurons in the region of the interstitial nucleus of Cajal (INC) were studied in alert cats during spontaneous or visually induced eye movement and sinusoidal vertical (pitch) rotation. Activity of a majority of cells (n = 68) was closely related to vertical eye position with or without bursting activity during on-direction saccades. They were called vertical burst-tonic (n = 62) and tonic (n = 6) neurons. Mean discharge rates for individual cells when the eye was near the primary position ranged from 35 to 133 (mean 75) spikes/s with a coefficient of variation (CV) ranging from 0.04 to 0.29 (mean 0.15). Average rate position curves were linear for the great majority of these cells with a mean slope of 3.9 ± 1.2 SD spikes/s/deg. (2) The burst index was defined as the difference in discharge rate between maximal rate during an on-direction saccade and the tonic rate after the saccade. The values of mean burst index for individual cells ranged from 8 to 352 (mean 135) spikes/s. Tonic neurons had a burst index lower than 60 spikes/s and were distributed in the lower end of the continuous histogram, suggesting that burst-tonic and tonic neurons may be a continuous group with varying degrees of burst components. During off-direction saccades, a pause was not always observed, although discharge rate consistently decreased and pauses were seen when saccades were made further in the off-direction toward recruitment thresholds. Significant positive correlation was observed between average discharge rate during off- as well as on-direction saccades and tonic discharge rate after saccades for individual cells, which was not due to cats making saccades mainly from the primary position. (3) During pitch rotation at 0.11 Hz (±10 deg), burst-tonic and tonic neurons had mean phase lag and gain of 128 (±13 SD) deg and 4.2 (±1.7 SD) spikes/s/deg/s2 relative to head acceleration. During pitch rotation of a wide frequency range (0.044–0.495 Hz), the values of phase lag were mostly constant (120–140 deg), while simultaneously recorded vertical VOR showed the mean phase lag of 178 deg. Vertical eye position sensitivity and pitch gain (re head position) showed significant positive correlation. (4) Comparison of the discharge characteristics of vertical burst-tonic and tonic neurons with those of secondary vestibulo-ocular neurons (Perlmutter et al. 1988) and extraocular motoneurons (Delgado-Garcia et al. 1986) in alert cats suggests that signals carried by burst-tonic and tonic neurons are partially processed signals in vertical VOR and saccades, and different from oculomotor signals. (5) The INC region also contained many cells that did not belong to the above groups but whose activity was clearly modulated by pitch rotation (called pitch cells for the present study, n = 44). Many (n = 23) showed some correlation with vestibular quick phases, and some (n = 12) with visually elicited eye movement, although they showed significantly lower and more irregular discharge rates than burst-tonic and tonic neurons (mean discharge rate when the eye was near the primary position 34, range 3–91, spikes/s; mean CV 0.61, range 0.15–1.7). During pitch rotation they showed the mean phase lag and gain of 119(±26 SD) deg and 3.2(±2.1 SD) spikes/s/deg/s2. Some cells showed a much lower phase lag of about 90 deg. (6) More than half the burst-tonic, tonic and pitch cells tested were antidromically activated by stimuli applied to the ponto-medullary medial longitudinal fasciculus at the level of abducens nucleus, while none of them were activated from the inferior olive, suggesting that vertical eye position signals carried by some burst-tonic and tonic neurons are carried to the lower brainstem.

Notes: Summary 1. A total of 43 neurons that showed a close correlation with vertical eye movement with a burst-tonic or tonic type response during spontaneous saccades, were recorded within, and in the close vicinity of, the interstitial nucleus of Cajal (INC) in alert cats. Neuronal responses to sinusoidal vertical linear acceleration (0.2–0.85 Hz, amplitude 10.5 cm) and optokinetic stimuli (0.1–1.0 Hz, amplitude 10.5 cm), were examined. 2. All 43 eye movement-related neurons responded to sinusoidal vertical linear acceleration in the presence of a stationary visual pattern in correlation to robust eye movement responses with compensatory phase. Phase and gain values (re stimulus position) of response of individual cells were independent of the stimulus frequencies tested. Of these, 33 cells were examined during linear acceleration without visual input. Most cells (27/33) did not respond even when a weak linear vestibulo-ocular reflex was present (6/27). The remaining 6 cells (6/33) responded to linear acceleration. Their mean phase values advanced by 80 ° and gain dropped by 55% compared to the responses with visual inputs. 3. Twenty eight of the 43 cells were examined during vertical optokinetic stimuli. The activity of all 28 cells was modulated in correlation to eye movement responses. Response phase showed more lag, and gain decreased as stimulus frequencies increased, similar to optokinetic eye movement responses. 4. The close correlation between the activity of eye movement-related neurons in the INC region and robust eye movements during linear acceleration with visual inputs and optokinetic stimuli suggest that these neurons are involved in some aspect of vertical eye position generation during such stimuli.

Notes: Summary Neurons that project to the spinal cord were located in the mesencephalic reticular formation outside the interstitial nucleus of Cajal in cerebellectomized cats under chloralose anesthesia. Of these neurons 40% responded only at C1 (reticulospinal N cells) and the remaining 60% responded at C4 also (reticulospinal D cells). Conduction velocities of N cells were significantly slower than those of D cells. N cells and D cells responded similarly to stimulation of the whole vestibular nerves and vestibular nuclei. However, they differ in semicircular canal inputs; N cells were more responsive to canal stimulation. Comparison of properties between mesencephalic reticulospinal and interstitiospinal neurons (Fukushima et al. 1980) showed that many reticulospinal and interstitiospinal neurons have similar properties, suggesting that functionally similar neurons may be found distributed over more than one anatomically defined cell group.

Notes: Summary Neurons were recorded extracellularly in the mesencephalic reticular formation outside the interstitial nucleus of Cajal in cerebellectomized cats anesthetized with α chloralose. Reticulospinal neurons were identified by antidromic stimulation of the upper cervical segments. Stimulation in the deep layers of the ipsilateral superior colliculus evoked firing in 36% of reticulospinal neurons. For many neurons thresholds for activation were high in the intermediate tectal layers and declined as the electrodes entered the underlying tegmentum. However, low threshold points were found above the deep fiber layer within the superior colliculus for some cells. Stimulation of the contralateral superior colliculus excited 10% of neurons and thresholds for activation were high above the deep fiber layer for all neurons. Stimulation of the ipsilateral and contralateral pericruciate cortex excited 39 and 21% of neurons, respectively. The lowest threshold area was found in the frontal eye fields. Sixteen percent of neurons received excitation from neck muscle afferents (C2 biventer-cervicis) bilaterally. Comparison of responses between mesencephalic reticulospinal neurons and interstitiospinal neurons (Fukushima et al. 1981) showed that responses of the two groups of neurons were similar when the pericruciate cortex and neck muscle afferents were stimulated. However, a difference was observed in tectal responses, since low threshold points were rarely observed above the deep fiber layer for interstitiospinal neurons.