Journal of Neurophysiology, Vol 54, Issue 4 1006-1025, Copyright © 1985 by APS
Central compensation of vestibular deficits. IV. Responses of lateral vestibular neurons to neck rotation after labyrinth deafferentation
C. Xerri, S. Gianni, D. Manzoni and O. Pompeiano
The response characteristics of neurons located in the lateral vestibular
nucleus (LVN) to neck rotation at 0.026 Hz, 10 degrees peak displacement,
have been investigated in precollicular decerebrate cats submitted to
ipsilateral acute (aVN) or chronic vestibular neurectomy (cVN). On the
whole, 105 units were tested after aVN (i.e., during the first
postoperative hours) and 132 units after cVN (i.e., after full compensation
of the postural and locomotor deficits). The neurons were histologically
located either in the rostroventral (rvLVN) or the dorsocaudal part (dcLVN)
of Deiters' nucleus, which are known to project mainly to the cervical and
the lumbosacral cord, respectively. Moreover, 55 units in the former group
and 66 units in the latter group were identified as vestibulospinal neurons
projecting to lumbosacral segments of the spinal cord. The responses of
these 237 LVN neurons to the neck input were then compared with those of
120 LVN neurons recorded previously in decerebrate cats with intact
labyrinths. Whereas 58.3% of the LVN units recorded in control experiments
were responsive to neck rotation, 69.5% of the units were affected by this
stimulation at the acute stage of the neurectomy and 74.2% at the chronic
stage. This increase in responsive units after aVN and cVN with respect to
the controls was found exclusively in the dcLVN. The mean discharge rate of
the responsive LVN neurons decreased from 40.7 +/- 48.9 (SD) imp/s in
control experiments to 22.1 +/- 15.8 (SD) imp/s after a VN. Similar value
was also obtained after cVN [25.0 +/- 17.2 (SD) imp/s], suggesting that
compensation of the postural deficits elicited by the vestibular neurectomy
results from a redistribution of the excitatory drive within different
populations of LVN neurons. Indeed, the relation found in control
experiments, i.e., that the faster the conduction velocity of
vestibulospinal axons the lower was the unit discharge at rest, was lost
after aVN, due to a decrease in resting discharge of the slow units. The
mean discharge rate of the slow units, however, recovered after cVN, so
that the negative correlation between resting discharge rate and axonal
conduction velocity was reestablished. The average gain and sensitivity of
the first harmonic response of the LVN neurons to neck rotation recorded
after aVN and cVN were comparable to those obtained in preparations with
the vestibular nerves intact.(ABSTRACT TRUNCATED AT 400 WORDS)
