INTERDEPENDENCE OF SPATIAL PROPERTIES AND PROJECTION PATTERNS OF MEDIAL VESTIBULOSPINAL TRACT NEURONS IN THE CAT

Interdependence of spatial properties and projection patterns of media l vestibulospinal tract neurons in the cat. J. Neurophysiol. 79: 270-2 84, 1998. Activity of vestibular nucleus neurons with axons in the ips i-or contralateral medial vestibulospinal tract was studied in decereb rate cats during sinusoidal, whole-body rotations in many planes in th ree-dimensional space. Antidromic activation of axon collaterals disti nguished between neurons projecting only to neck segments from those w ith collaterals to C-6 and/or oculomotor nucleus. Secondary neurons we re identified by monosynaptic activation after labyrinth stimulation. A three-dimensional maximum activation direction vector (MAD) summariz ed the spatial properties of 151 of 169 neurons. The majority of secon dary 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 c ontralaterally and ipsilerally, respectively. Most C-6-projecting neur ons could not be activated from oculomotor nucleus (V-C-6 neurons) and projected primarily ipsilaterally. All VO-C-6 neurons projected contr alaterally. The distributions of MADs for secondary neurons with diffe rent projection patterns were different. Most VOC(84%) and contralater ally projecting VC (91%) neurons had MADs close to the activation vect or of a semicircular canal pair, compared with 54% of ipsilaterally pr ojecting 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 horizont al and vertical canal pairs. Horizontal and vertical gains were compar able for some, making it difficult to assign a primary canal input. MA Ds consistent with vertical-vertical canal pair convergence were less common. Type II yaw or type IT 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 neur ons had spatial properties between those of VOC and V-C-6 neurons. The se results suggest that secondary VOC neurons convey semicircular cana l pair signals to both ocular and neck motor centers, perhaps linking eye and head movements. Secondary VC and V-C-6 neurons carry more proc essed signals, possibly to drive neck and forelimb reflexes more selec tively. Two groups of secondary i-VC neurons exhibited vertical-horizo ntal canal convergence similar to that present on neck muscles. The ve rtical-vertical canal convergence present on many neck muscles, howeve r, was not present on medial vestibulospinal neurons. Spatial transfor mations achieved by the vestibulocollic reflex may occur in part on se condary neurons but further combination of canal signals must take pla ce to generate compensatory muscle activity.