Library

Notes: Summary 1. Interstitiospinal neurons were activated by antidromic stimulation of the ventromedial funiculus of the spinal cord at C1 and C4 in cerebellectomized cats under chloralose anesthesia. 46% of these neurons responded only at C1 (N cells) and the remaining 54% responded at C4 also (D cells). There is no topographical difference in the location of N and D cells. Conduction velocities of N cells were significantly slower than those of D cells. 2. Stimulation of the contralateral whole vestibular nerve evoked firing of 31% of both N and D cells; some responded early enough to suggest disynaptic connections, many responded late. Stimulation of the ipsilateral whole vestibular nerve evoked firing of several cells, one spontaneously discharging D cell was inhibited. 3. Stimulation of the contralateral individual semicircular canal nerves evoked firing of 33% of N cells and 13% of D cells. Most of these responses were late. N cells responded not only to the vertical canals but also to the horizontal canal, whereas D cells responded to the horizontal canal, but seldom to the vertical ones. Most canal responding neurons received specific input, only two N cells received convergent input from both the anterior and horizontal canals. Stimulation of the ipsilateral canals did not evoke excitation of any cells tested; one D cell was inhibited by stimulation of the horizontal canal nerve. 4. Stimulation of the rostral medial vestibular nucleus evoked characteristic negative field potentials centered in the contralateral interstitial nucleus of Cajal (INC). Approximately 60% of both N and D cells received excitation from the contralateral vestibular nuclei. About 17% of these responding neurons received monosynaptic excitation, most frequently from the rostral medial nucleus. Stimulation of the ipsilateral vestibular nuclei evoked firing of 12% of both N and D cells. 5. Twenty-nine neurons were fired antidromically by weak stimuli applied to the ipsilateral vestibular nuclei. Twenty-seven of the 29 were activated only from C1 and were found in the INC (10 cells) and in the reticular formation dorsal to the INC (19 cells). Measurement of the spread of the effect of stimulus current and comparison of latencies to stimulation of the vestibular nuclei and C1 indicated that these neurons have axon collaterals going to the ipsilateral vestibular nuclei. Only one of them received excitation from the contralateral posterior canal, others did not respond to the labyrinth. Some were activated by stimulation of the vestibular nuclei.

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 Recent studies have shown that the interstitial nucleus of Cajal (INC) in the midbrain reticular formation is involved in the conversion of vertical semicircular canal signals into eye position during vertical vestibuloocular reflexes. Secondary vestibulo-ocular relay neurons related to the vertical canals, which constitute the majority of output neurons sending signals from the vestibular nuclei directly to the oculomotor nuclei, have been shown to project axon collaterals to the region within and near the INC. To understand how the INC is involved in the signal conversion, latencies of response of neurons in the INC region to electrical stimulaton of the vestibular nerve were examined in alert cats. The responses of 96 cells whose activity was clearly modulated by sinusoidal pitch rotation (at 0.31 Hz) were analyzed. These included 41 cells whose activity was closely correlated with vertical eye movement (38 burst-tonic and 3 tonic neurons), and 55 other cells (called pitch cells as previously). Twenty nine of the 96 cells (30%) were activated at disynaptic latencies following single shock stimulation of the contralateral vestibular nerve. Disynaptically activated cells were significantly more frequent for pitch cells than for eye movement-related cells (25/55 = 45% vs 4/41 = 10%; p 〈 0.001, Chi-square test). Conversely, cells that did not receive short-latency activation (〈 6 ms) were more frequent among eye movement-related cells than pitch cells (26/41 = 63% vs 13/55 = 24%; p 〈 0.001, Chi-square test). Pitch cells showed significantly less phase lag (re head acceleration) than eye movement-related cells during sinusoidal pitch rotation (mean ± SD 124° ± 17° vs 138° ± 14°. p 〈 0.01, t-test). These results suggest that 1) cells in the INC region other than burst-tonic and tonic neurons mainly receive direct inputs from secondary vestibulo-ocular relay neurons, and that 2) vertical canal signals reach eye movement-related neurons mainly polysynaptically.

Notes: Summary 1. Maximal activation directions of vertical burst-tonic and tonic neurons in the region of the interstitial nucleus of Cajal (INC) were examined in alert cats during vertical vestibulo-ocular reflex induced by sinusoidal rotation (at 0.11 Hz±10 deg, or 0.31 Hz±5 deg) in a variety of vertical planes using a null point analysis. The results were compared with the angles of anatomical and functional planes of vertical canals reported by Blanks et al. (1972) and Robinson (1982), and with the angles of vertical eye muscles measured in this study and by Ezure and Graf (1984). 2. Maximal activation directions of 23 cells (21 burst-tonic and 2 tonic neurons) were determined from their responses during rotation in 4 or more different vertical planes. All cells showed sinusoidal gain curves and virtually constant phase values except near the null regions, suggesting that their responses were evoked primarily by canal inputs. Phase values of 5 cells near the null regions depended on the rotation plane, suggesting additional otolith inputs. We used a measurement error range of ±10 deg for calculating the maximal activation directions from the null regions of individual cells and the values of error ranges of null calculation. Of the 23, the maximal activation directions of 7 cells were outside the measurement error ranges of vertical eye muscle angles and within the ranges of vertical canal angles (class A), those of 5 cells were within the ranges of eye muscle angles and outside the ranges of vertical canal angles (class B), and those of the remaining 11 cells were in the overlapping ranges for both angles (class C). Even if only the cells in which 5 or more measurement points were taken to determine maximal activation directions (n = 15), the results were similar. During vertical rotation with the head orientation +60 deg off the pitch plane, dissociation of cell activity and vertical compensatory eye movement was observed in 5 cells in class A or C that had null angles near +45 deg. These results suggest that the cells in class A and B carried individual vertical canal and oculomotor signals, respectively, although it is difficult to tell for the majority of cells (class C) which signals they reflected. Some cells in class A and C were antidromically activated from the medial longitudinal fasciculus at the level of abducens nucleus, suggesting that the signals carried by these cells may be sent to the lower brainstem. 3. Most burst-tonic neurons did not respond to horizontal rotation; significant responses were obtained in only 3 of 10 cells tested for which the gain was only 14–17% of their maximal vertical gain. There was no clear difference in gain or phase values of the responses to vertical rotation, or in eye position sensitivity (during spontaneous saccades) between cells whose responses coincided with individual vertical canal angles and those matching the angles of vertical recti muscles. The values of phase lag (re head acceleration during pitch rotation) and eye position sensitivity of these cells are still smaller compared to those of extraocular motoneurons reported by Delgado-Garcia et al. (1986), although they were larger than those of secondary vestibulo-ocular neurons (Perlmutter et al. 1988). All these results suggest that the signals carried by burst-tonic and tonic neurons in the INC region are different from oculomotor signals. 4. Similar analysis was done for comparison for 19 other cells that did not show close correlation with spontaneous eye movement but whose activity was clearly modulated by pitch rotation (pitch cells). More than a half (10/19) had maximal activation directions outside the measurement error ranges of individual vertical canal angles, and many shifted towards roll. Horizontal rotation produced responses with higher gain than burst-tonic neurons, suggesting a difference in the spatial response properties of burst-tonic and tonic neurons on one hand and pitch cells on the other.

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 Interstitiospinal neurons were activated by antidromic stimulation of the spinal cord ventromedial funiculus at C1 and C4 in cerebellectomized cats under chlor alose anesthesia. Neurons responding only to C1 were classified as N cells and those responding both to C1 and C4 were classified as D cells, as in previous experiments (Fukushima et al. 1980a). Vestibular branching interstitiospinal and reticulospinal neurons were also identified as in the previous experiments. Stimulation of the ipsilateral pericruciate cortex evoked firing in 31% of N cells, 41% of D cells and 35% of vestibular branching neurons, while stimulation of the contralateral cortex excited 6% of N cells, 29% of D cells and 14% of vestibular branching neurons. Response latencies ranged from 2 to 15 ms after the effective pulse. By measuring the thresholds of activation of these neurons while changing the depth of the stimulating electrodes, and by mapping the cortical areas, it was shown that the lowest threshold areas were in the frontal eye fields and the anterior sigmoid gyrus near the presylvian sulcus (Area 6). Stimulation of the latter area often evoked neck or shoulder muscle contraction. Stimulation in the deep layers of the ipsilateral superior colliculus evoked firing in about 20% of interstitiospinal neurons and about 42% of vestibular branching neurons, with typical latencies 2–3 ms after the effective pulse, while stimulation of the contralateral superior colliculus was rarely effective. N cells and D cells responded similarly. Thresholds for activation were high in the intermediate tectal layers and declined as the electrodes entered the underlying tegmentum. This suggests that the superior colliculus is not the main source of synaptic inputs to these neurons. Low threshold points were found above the deep fiber layer when stimulating electrodes were inserted into the pretectum. Stimulation of the C2 biventer cervicis nerve excited about 8% of N cells, 18% of D cells, and 15% of vestibular branching neurons bilaterally with typical latencies around 10 ms. Similar results were obtained when C2 splenius nerves were stimulated. The fibers responsible for such excitation are probably group II, since stimuli stronger than 1.8 times threshold of the lowest threshold fibers were needed to evoke excitation. Response decrement was often observed when stimuli were repeated at 1/s, while no such decrement was observed at the rate of 1/3 s. When the convergence of cortical and labyrinthine excitatory inputs was studied, 36% of interstitiospinal neurons received single inputs either from the pericruciate cortex or from the labyrinth, 22% of neurons received convergent excitation from both and the remaining 42% did not respond to either stimulus. Although vestibular branching neurons rarely received labyrinthine inputs, they frequently showed convergence of excitation to stimulation of the frontal cortex, superior colliculus and vestibular nuclei.

Notes: Summary Experiments were performed on cats anesthetized with a chloralose to locate neurons in and around the interstitial nucleus of Cajal (INC) that project to the vestibular nuclei, and to study labyrinthine inputs to these neurons. Neurons that project to the vestibular nuclei were identified by microstimulation confined to the vestibular nuclei on both sides. All neurons thus identified were activated antidromically from the ipsilateral (but not contralateral) vestibular nuclei. Vestibular projecting neurons were found in the INC and the reticular formation rostral, dorsal and caudal to the INC. About 23% of these neurons were vestibular branching spinal projecting neurons. The median conduction velocity of vestibular projecting neurons was estimated to be in the neighborhood of 12–16 m/s. Stimulation of the contralateral vestibular nerve evoked firing in 29% of neurons projecting to the vestibular nuclei, but not to the spinal cord. Interstitial neurons responded more frequently than reticular neurons (45% vs 11%, χ2 test, p 〈 0.001). By stimulation of individual semicircular canal nerves, it was shown that vestibular projecting neurons receive excitation from the contralateral vertical canals, but do not receive substantial inputs from the horizontal canal. Stimulation of the ipsilateral vestibular nerve excited 10% of neurons; suppression of activity was observed for six cells and four of the six were excited by stimulation of the contralateral vestibular nerve. Stimulation of ipsilateral individual semicircular canal nerves did not excite any cells tested; the activity of a few cells was suppressed by stimulation of the vertical canal nerves. One neuron received excitation from the contralateral anterior canal and suppression from the ipsilateral posterior canal. Vestibular branching spinal projecting neurons rarely received labyrinthine inputs as already reported (Fukushima et al. 1980a). These results suggest that vestibular projecting neurons may be involved in vertical vestibular reflexes.

Notes: Summary Attempts were made to study differences in the relative effectiveness of different size ranges of motor axons to Renshaw cells by differential blocking of larger fibers of the gastrocnemius nerve in cats anesthetized with Nembutal. 1. Differential blocking of larger fibers in the nerve was successfully obtained by applying trapezoid wave current to the nerve. 2. It was shown that more than half (58.1%) of the Renshaw cells receive homogeneous inputs from a motor axon collaterals, 25.8% of the cell receive collateral inputs from a certain group of fibers, and 12.5% of the Renshaw cells were activated by “γ range” fibers.