Output-to-input approach to neural plasticity in vestibular pathways

Gradient of adaptability in four different motor systems performing the same learned motor task in cats

The ability of four different brainstem motoneuron pools to perform a newly acquired motor task was studied in alert cats. A classical conditioning of eyelid responses was carried out in (i). unoperated animals, and in animals with (ii). transection, 180 degrees rotation, and re-suture of the zygomatic facial nerve branch, (iii). a crossed anastomosis of the buccal to the zygomatic facial nerve branch and (iv). a hypoglossal-facial nerve anastomosis. Animals were conditioned with a delay paradigm using a tone (350 ms, 600 Hz, 90 dB) as conditioned stimulus, followed 250 ms later by an air puff (100 ms, 3 kg/cm2) as unconditioned stimulus. Animals with zygomatic nerve rotation performed conditioned responses (CRs) at control rate, with significantly larger amplitude, area and velocity, but a de-synchronized oscillatory pattern. Animals with buccal-zygomatic anastomosis acquired CRs at control rate, but these CRs had significantly smaller amplitude than those of controls and a de-synchronized pattern. Animals with a hypoglossal-facial anastomosis were unable to perform CRs. The trigeminal hyper-reflexia triggered by the axotomy was probably the origin of the large CRs after zygomatic nerve rotation. Trigeminal hyper-reflexia could also contribute to generation of the small CRs recorded after buccal-zygomatic anastomosis. Although trigeminal hyper-reflexia was also present following hypoglossal-facial anastomosis, hypoglossal motoneurons did not reach their firing threshold to perform CRs. In accordance with the embryonic origin of involved motoneurons, animals with buccal-zygomatic and hypoglossal-facial anastomoses moved the ipsilateral eyelid synchronously to mouth-related activities. It is suggested that there is a gradient of adaptability in motoneuron pools forced to perform new motor tasks through foreign muscles, which depends on their embryological origins and functional properties.

Relative contributions of eyelid and eye-retraction motor systems to reflex and classically conditioned blink responses in the rabbit

Early compensatory mechanisms between eyelid and eye-retraction motor systems following selective nerve and/or muscle lesions were studied in behaving rabbits. Reflex and conditioned eyelid responses were recorded in 1). controls and following 2). facial nerve section, 3). retractor bulbi muscle removal, and 4). facial nerve section and retractor bulbi muscle removal. Animals were classically conditioned with a delay paradigm by using a tone (350 ms, 600 Hz, 90 dB) as conditioned stimulus, followed 250 ms later by an air puff (100 ms, 3 kg/cm(2)) as unconditioned stimulus. Conditioned eyelid responses generated in the absence of the facial motor system (i.e., by the almost sole action of the retractor bulbi motor system) presented a wavy profile, due to the succession of eye-retraction movements. Learned eyelid responses generated in the absence of the eye-retraction motor system (i.e., by the almost exclusive action of the facial motor system) were similar to those of controls, but were reduced in amplitude and peak velocity. Finally, the isolated action of the extraocular recti muscle produced very small eyelid movements during both reflex and learned eyelid responses. Although each of these motor systems could act independently of the others, the motor result of their joint action did not coincide with the simple addition of their separate actions. Both facial and eye-retraction motor systems appear to be necessary for normal eyelid closure during blinking in rabbits. Central reorganization to compensate for loss of either of these systems may explain why the response of each system in isolation cannot be added linearly to obtain normal blink response magnitudes and profiles.

Neuronal premotor networks involved in eyelid responses: retrograde transneuronal tracing with rabies virus from the orbicularis oculi muscle in the rat

Retrograde transneuronal tracing with rabies virus from the right orbicularis oculi muscle was used to identify neural networks underlying spontaneous, reflex, and learned blinks. The kinetics of viral transfer was studied at sequential 12 hr intervals between 3 and 5 d after inoculation. Rabies virus immunolabeling was combined with the immunohistochemical detection of choline acetyltransferase expression in brainstem motoneurons or Fluoro-Ruby injections in the rubrospinal tract. Virus uptake involved exclusively orbicularis oculi motoneurons in the dorsolateral division of the facial nucleus. At 3-3.5 d, transneuronal transfer involved premotor interneurons of trigeminal, auditory, and vestibular reflex pathways (in medullary and pontine reticular formation, trigeminal nuclei, periolivary and ventral cochlear nuclei, and medial vestibular nuclei), motor pathways (dorsolateral quadrant of contralateral red nucleus and pararubral area), deep cerebellar nuclei (lateral portion of interpositus nucleus and dorsolateral hump ipsilaterally), limbic relays (parabrachial and Kölliker-Fuse nuclei), and oculomotor structures involved in eye-eyelid coordination (oculomotor nucleus, supraoculomotor area, and interstitial nucleus of Cajal). At 4 d, higher order neurons were revealed in trigeminal, auditory, vestibular, and deep cerebellar nuclei (medial, interpositus, and lateral), oculomotor and visual-related structures (Darkschewitsch, nucleus of the posterior commissure, deep layers of superior colliculus, and pretectal area), lateral hypothalamus, and cerebral cortex (particularly in parietal areas). At 4.5 and 5 d the labeling of higher order neurons occurred in hypothalamus, cerebral cortex, and blink-related areas of cerebellar cortex. These results provide a comprehensive picture of the premotor networks mediating reflex, voluntary, and limbic-related eyelid responses and highlight potential sites of motor learning in eyelid classical conditioning.