DIRECTION-SPECIFIC DIFFERENCES IN THE MAGNITUDE OF ABDUCENS NERVE RESPONSES DURING OFF-VERTICAL AXIS ROTATION ARE A BASIC PROPERTY OF THE UTRICULO-OCULAR REFLEX IN FROGS
C. Pantle et al., DIRECTION-SPECIFIC DIFFERENCES IN THE MAGNITUDE OF ABDUCENS NERVE RESPONSES DURING OFF-VERTICAL AXIS ROTATION ARE A BASIC PROPERTY OF THE UTRICULO-OCULAR REFLEX IN FROGS, Experimental Brain Research, 106(1), 1995, pp. 28-38
Abducens nerve multiunit responses were recorded in darkness from dece
rebrated frogs during steps of angular velocity about an axis tilted w
ith respect to the earth vertical (off-vertical axis rotation, OVAR).
Thereby, a rotating gravity vector activated utricular hair cells and
modulated the abducens nerve discharge sinusoidally as a function of h
ead position in space. As expected, a bias velocity response component
and nystagmus-related changes in neural activity were absent, since f
rogs do not possess a functioning velocity storage mechanism. Response
s increased as a function of the tilt angle and of the velocity and di
rection of the platform rotation. OVAR in the direction of the recorde
d abducens nerve (clockwise for the right and counterclockwise for the
left abducens nerve) evoked significantly smaller responses than rota
tion in the opposite direction. The possible origin of these direction
-specific response properties was further studied after lesioning vari
ous structures assumed to modify utriculo-ocular reflexes. Each of the
se lesions (ipsilateral hemilabyrinthectomy, cerebellectomy, contralat
eral canal nerve sections) had a specific effect on the recorded respo
nse properties, but none of them, nor combinations thereof, abolished
the direction-specific characteristics of the responses as long as the
contralateral utricular nerve branch remained intact. Our results dem
onstrate that direction-specificity is a proper ty of the basic utricu
lo-ocular reflex that is independent of the velocity storage mechanism
in the brainstem, of the intervestibular commissural system, of the i
nhibitory control by the cerebellum and of the central convergence of
utricular and horizontal canal inputs. A simple, unidirectional intera
ction between central utricular neurons with adjacent functional polar
ization vectors is suggested as the basic element for the observed dir
ection specificity.