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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) Spikes of neurons in the medial and descending vestibular nuclei were recorded extracellularly and their responses to stimulation of the interstitial nucleus of Cajal (INC) were studied in cerebellectomized cats under chloralose anesthesia. Stimuli applied in the ipsilateral INC excited 37% of neurons that did not exhibit spontaneous activity. About 84% of spontaneously discharging neurons were influenced by the INC; typical responses were excitation (35%), inhibition (22%) and excitation followed by inhibition (27%). Of the neurons that were excited, 24% fired monosynaptically. Such monosynaptic activation was evoked by stimulating the INC and midbrain medial longitudinal fasciculus (MLF), but was not evoked by stimulating the lateral midbrain reticular formation. Polysynaptic excitation or inhibition was evoked more widely, but the lowest threshold points were within the INC. Stimulation of the contralateral INC also evoked polysynaptic excitation or inhibition. However, the frequency of occurrence of the evoked responses was significantly smaller compared to the ipsilateral responses. (2) Intracellular recordings revealed that some medial and lateral vestibular neurons received monosynaptic excitatory postsynaptic potentials (EPSPs), others received polysynaptic EPSPs or inhibitory postsynaptic potentials (IPSPs) from the ipsilateral INC. The minimum latency for the IPSPs suggests that the pathway is at least disynaptic. No significant collision was observed between monosynaptic EPSPs evoked by the ipsilateral INC and contralateral vestibular nuclei. Acute lesions that damaged the pontine MLF and part of the reticular formation did not abolish monosynaptic responses of vestibular neurons by the INC. Depth threshold curves for mono- or polysynaptic responses drawn before and after the lesions were virtually similar. Antidromic thresholds of interstitio-vestibular fibers evoked from the pontine MLF showed that a great majority of these fibers run outside the MLF at the pontine level. These results control for vestibular axon reflexes, since vestibulo-interstitial fibers ascend within the MLF (cf. Gacek 1971). (3) Responses to stimulation of the INC were not different among different types of canal responding neurons; vertical and horizontal canal responding neurons received similar effects. However, canal responding neurons that received excitation from the contralateral vestibular nerve were activated more frequently by the INC than those that received inhibition (χ2 test, p〈0.01). Qualitatively similar results were obtained from vestibular neurons that had different projection sites; vestibulospinal, contralateral INC-projecting and contralateral vestibular nuclei-projecting neurons received similar effects. (4) Vestibulo-collic reflexes, studied with EMG, were modified by preceding INC stimulation. Intracellular recordings from some neck motoneurons showed that disynaptic EPSPs evoked by stimulation of the contralateral vestibular nerve were modified by preceding INC stimulation applied ipsilateral to the stimulated vestibular nerve. INC stimulation alone did not evoke any response in these motoneurons, suggesting that the interaction of the labyrinthine and interstitial effects occurred at least in part at the vestibular nuclei. (5) Some medial and descending vestibular neurons showed multiple branching, projecting to the contralateral INC, C1 or contralateral vestibular nuclei. About 34% of neurons that projected to the contralateral INC were also antidromically activated from the C1; some of them received vertical canal inputs.

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.