Interdependence of Spatial Properties and Projection Patterns of Medial Vestibulospinal Tract Neurons in the Cat

S. I. Perlmutter , Y. Iwamoto , J. F. Baker , and B. W. Peterson Department of Physiology, Northwestern University School of Medicine, Chicago, Illinois 60611 Perlmutter, S. I., Y. Iwamoto, J. F. Baker, and B. W. Peterson. Interdependence of spatial properties and projection patterns of medial vestibulospinal tract neurons in the cat. J. Neurophysiol. 79: 270-284, 1998. Activity of vestibular nucleus neurons with axons in the ipsi- or contralateral medial vestibulospinal tract was studied in decerebrate cats during sinusoidal, whole-body rotations in many planes in three-dimensional space. Antidromic activation of axon collaterals distinguished between neurons projecting only to neck segments from those with collaterals to C 6 and/or oculomotor nucleus. Secondary neurons were identified by monosynaptic activation after labyrinth stimulation. A three-dimensional maximum activation direction vector (MAD) summarized the spatial properties of 151 of 169 neurons. The majority of secondary neurons (71%) terminated above the C 6 segment. Of these, 43% had ascending collaterals to the oculomotor nucleus (VOC neurons), and 57% did not (VC neurons). The majority of VOC and VC neurons projected contralaterally and ipsilaterally, respectively. Most C 6 -projecting neurons could not be activated from oculomotor nucleus (V-C 6 neurons) and projected primarily ipsilaterally. All VO-C 6 neurons projected contralaterally. The distributions of MADs for secondary neurons with different projection patterns were different. Most VOC (84%) and contralaterally projecting VC (91%) neurons had MADs close to the activation vector of a semicircular canal pair, compared with 54% of ipsilaterally projecting VC (i-VC) and 39% of V-C 6 neurons. Many i-VC (44%) and V-C 6 (48%) neurons had responses suggesting convergent input from horizontal and vertical canal pairs. Horizontal and vertical gains were comparable for some, making it difficult to assign a primary canal input. MADs consistent with vertical-vertical canal pair convergence were less common. Type II yaw or type II roll responses were seen for 22% of the i-VC neurons, 68% of the V-C 6 neurons, and no VOC cells. VO-C 6 neurons had spatial properties between those of VOC and V-C 6 neurons. These results suggest that secondary VOC neurons convey semicircular canal pair signals to both ocular and neck motor centers, perhaps linking eye and head movements. Secondary VC and V-C 6 neurons carry more processed signals, possibly to drive