Properties and axonal trajectories of posterior semicircular canal nerve-activated vestibulospinal neurons

We studied the axonal projections of vestibulospinal neurons activated from the posterior semicircular canal. The axonal projection level, axonal pathway, and location of the vestibulospinal neurons originating from the PC were investigated in seven decerebrated cats. Selective electrical stimulation was applied to the PC nerve, and extracellular recordings in the vestibular nuclei were performed. The properties of the PC nerve-activated vestibulospinal neurons were then studied. To estimate the neural pathway in the spinal cord, floating electrodes were placed at the ipsilateral (i) and contralateral (c) lateral vestibulospinal tract (LVST) and medial vestibulospinal tract (MVST) at the C1/C2 junction. To elucidate the projection level, floating electrodes were placed at i-LVST and MVST at the C3, T1, and L3 segments in the spinal cord. Collision block test between orthodromic inputs from the PC nerve and antidromic inputs from the spinal cord verified the existence of the vestibulospinal neurons in the vestibular nuclei. Most (44/47) of the PC nerve-activated vestibulospinal neurons responded to orthodromic stimulation to the PC nerve with a short (<1.4 ms) latency, indicating that they were second-order vestibulospinal neurons. The rest (3/47) responded with a longer (≥1.4 ms) latency, indicating the existence of polysynaptic connections. In 36/47 PC nerve-activated vestibulospinal neurons, the axonal pathway was histologically verified to lie in the spinal cord. The axons of 17/36 vestibulospinal neurons projected to the i-LVST, whereas 14 neurons projected to the MVST, and 5 to the c-LVST. The spinal segment levels of projection of these neurons elucidated that the axons of most (15/17) of vestibulospinal neurons passing through the i-LVST reached the L3 segment level; none (0/14) of the neurons passing through the MVST extended to the L3 segment level; most (13/14) of them did not descend lower than the C3 segment level. In relation to the latency and the pathway, 33/36 PC nerve-activated vestibulospinal neurons were second-order neurons, whereas the remaining three were polysynaptic neurons. Of these, 33 second-order vestibulospinal neurons, 16 passed through the i-LVST, while 13 and 4 descended through the MVST and c-LVST, respectively. The remaining three were polysynaptic neurons. Histological analysis showed that most of the PC nerve-activated vestibulospinal neurons were located within a specific area in the medial part of the lateral vestibular nucleus and the rostral part of the descending vestibular nucleus. In conclusion, it was suggested that PC nerve-activated vestibulospinal neurons that were located within a focal area of the vestibular nuclei have strong connections with the lower segments of the spinal cord and are related to postural stability that is maintained by the short latency vestibulospinal reflex.