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Basic aNiAppliedAspects o!VestibulDr Fwnctio" I.C. Hwang, N.G. Daunton and V.I. Wilsoo (Eds.)© Hong Kong University Press, Hoog Koog, 1988 VESTIBULAR CONTROL OF THE CAT FORELIMB V.J. Wilson and R.H. Schor* The Rockefeller University, 1230 York Avenue, New York, NY 10021, U.SA. Abstract In response to tilt, the vestibular system produces reflex responses in the forelimb extensor muscles of the decerebrate cat. We have examined the spatial and temporal properties of the reflex, as well as the responses of central vestibular neurons and spinal intemeurons. The major spatial characteristic of the reflex is that roll (lateral) tilt produces much larger responses than does pitch (fore-aft) tilt. This spatial emphasis of roll arises over successive stages: it is already present in the vestibular nuclei and becomes more prominent at the level of spinal intemeurons. The temporal characteristics, or response dynamics, of the reflex suggest a combined canal and otolith input. Both second-order vestibular neurons and cervical intemeurons have similar dynamics. When the otolith input is isolated by using a canal-plugged preparation, reflex response dynamics consist of a gain increase and large phase lag as stimulus frequency increases to 1 Hz. This response is seen in many second-order vestibular neurons, but is rare among spinal intemeurons. Many reflex properties could be produced by a pathway in which the lateral vestibulospinal tract acts on motoneurons via spinal intemeurons, but the responses of the canal-plugged cat require the presence of additional connections. These could involve direct connections between vestibulospinal axons and forelimb motoneurons, or a parallel pathway through the reticular formation. For some time we have been studying the vestibulospinal reflex acting on the forelimb of the decerebrate cal This important postural reflex is a useful model for sensorimotor transformation because the relevant signals can be studied at many levels, from periphery to muscle. Although in normal function vestibulospinal reflexes interact with other inputs of exteroceptive or proprioceptive origin, in this brief review we will look at the vestibulo- * The present address of Dr. R.H. Schor is Department of Otolaryngology, University of Pittsburgh, 230 Lothrop Street, Pittsburgh, PA 15213, U.S.A. Keywords: cat, vestibulospinal reflexes, vestibular nuclei, spinal intemeurons, spatial and dynamic analysis . 74 Wilson and Schor forelimb reflex in isolation and address its spatial and dynamic properties and its neural substrate, emphasizing work in our own laboratory on the reflex evoked by tilt in vertical planes. From the time it was first described by Magnus (1924), experiments concentrated on the tonic vestibulospinal reflex, which is evoked by stimulation of the otolith organs. The accepted description of this reflex has been that of Roberts (1978): for example, when the whole body is rolled to one side, the limbs on the ear down side extend; when it is pitched nose down, the forelimbs extend and hindlimbs flex. This response pattern implies that the same muscles can be activated by both roll and pitch. Using a stimulus that combines these directions of rotation ('wobble', see Schor et al., 1984) we have recently examined how various directions of tilt activate the vestibulo-forelimb reflex and have determined the orientation of the 'response vector', i.e. the direction of tilt which gives the best response, for several forelimb and shoulder extensor muscles (Wilson et al., 1986). The vectors were all very close to roll, and, because reflex gain falls off as the cosine of the angle between the vector and the stimulus, the response to pitch was therefore relatively weak. In addition, there was no obvious pattern in the response to pitch: some extensor muscles were excited by nose up, some by nose down, tilt. The direction of the response vectors was the same for 0> A. R. Q) 10 ~ c .2 -0 :; "0 0 ~ 0'2. c 6 Q) "0 cV If) 0 .c a.. 0 ~ 90 f= o INFRASPINATUS J 01 1.0 B. L. MEDIAL C. R. LONG 10 10 ~~ o 0 -90 90 o 0.1 1.0 0.1 10 FREQUENCY. Hz Fig. 1. Dynamics and vector orientation of vestibular reflex in a shoulder muscle (infraspinatus) and two heads of triceps brachii. Each Bode plot and plot of vector orientation is based on a single frequency series. Gain and phase are plotted with respect to position. Note that a vector orientation of· 0 indicates maximal reflex sensitivity to right, -180 to left ear down roll; 90 and -90 indicate maximal sensitivity to nose down and nose up pitch, respectively. (From...

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