Compensation of postural reactions to fall in the vestibular neurectomized monkey. Role of the reamining labyrinthine afferences.

The electromyographic (EMG) responses from soleus and tibialis anterior muscles and the monosynaptic H- and T-reflex responses from soleus muscles were recorded bilaterally from conscious baboon while unexpectedly dropping it with unrestricted vision. These responses were recorded either after unilateral vestibular neurectomy (U.N. Baboons) or after bilateral neurectomy performed in one stage (B.N. 1 baboons) and in two stages (B.N. 2 baboons). A positive correlation was found between modifications and development of EMG responses and reflex data. In the U.N. baboons, some differences were observed when comparing data from the H- and T-reflex methods, suggesting that recovery of normal responses to fall is achieved both by means of direct influences on alpha-motoneurons and via the gamma-loop. In the U.N. baboons postural reactions to fall developed in three distinct periods. The first or critical stage showed asymmetrical EMG and reflex responses with increased responses from contralateral soleus muscle and decreased responses from ipsilateral soleus. Opposite effects were recorded from tibialis anterior flexor muscles. The second or acute stage which began around 4 to 7 days after surgery exhibited symmetrical, but very reduced, responses when compared to the control in soleus muscles, and symmetrical, but increased, responses from tibialis anterior muscles. This stage lasted until about the end of the second postoperative week and was followed by the third or compensatory stage during which EMG as well as reflex responses developed towards the control pattern in all tested muscles. Almost normal responses were recorded on both sides 3 weeks after surgery. Only a partial recovery was found in the B.N. 1 baboons, indicating that the contralateral remaining labyrinthine afferences constitute a necessary condition for the full compensation of postural reactions to fall in the case of unilateral vestibular neurectomy. The Bechterew's compensation was obtained in the B.N. 2 baboons. These results are discussed in relation with the general organization of the vestibulospinal pathways and with those concerning development of the postoperative activity at the vestibular nuclei level. A model of vestibular compensation achieved by means of a multisensory substitution process is suggested.