Changes in posturo-kinetic limb responses to cortical stimulation following unilateral neck deafferentation in the cat

Abstract

Unilateral neck deafferentation produces in cats a postural asymmetry, characterized by an increase in the extensor activity of the ipsilateral limbs and a decreased activity in the contralateral limbs; moreover, the placing reactions are severely depressed in the ipsilateral limbs. The present experiments were performed to investigate the influence of unilateral section of the cervical dorsal roots C1-C2 or C1-C3 on the postural adjustments occurring during flexion limb movements induced by cortical stimulation in chronically implanted cats. Only weak stimulations were used and motor activity was mainly isometric, to facilitate quantification of responses analyzed by measuring changes in vertical force exerted at the level of each limb in the up-right position of the animal. The results obtained were the following. The general motor behaviour of the animal was not affected by the lesion. In particular, stimulation of the motor cortex evoked not only flexion movement of a contralateral limb but also an initial postural adjustment consisting of a diagonal force pattern. The response threshold of the moving limb increased on the hypertonic side and decreased on the hypotonic side. Moreover, the changes in threshold of the performing limb following unilateral neck deafferentation were coupled with parallel changes in threshold of the limbs involved in the postural adjustment. There were great quantitative modifications both of the movement and the postural adjustment after the neck deafferentation. In fact weight shift latencies increased when the performing limb was ipsilateral to the hypertonic side, while the weight shifts themselves decreased in amplitude and speed. The opposite occurred when the performing limb was ipsilateral to the hypotonic side. Parallel with these changes, the slope of the response curve of the moving limb as well as of the limbs involved in the postural adjustment decreased following stimulation of the motor cortex at different stimulus intensities if the performing limb was ipsilateral to the hypertonic side, but increased if the performing limb was ipsilateral to the hypotonic side. These changes compensated after chronic neck deafferentation. In summary, the results indicate that well coordinated posturo-kinetic responses were still elicited by cortical stimulation in spite of the prominent changes in posture and reflex movements induced by unilateral neck deafferentation. In particular, changes in threshold, latency, amplitude and speed of the force changes under the limb making a movement were coupled with parallel alterations of the force changes under the limbs undergoing postural adjustment.(ABSTRACT TRUNCATED AT 400 WORDS)