Experiments are planned to study the neural substrate of tonic neck and vestibular reflexes to try and determine how an output with known reflex dynamics and appropriate spatial properties is produced in decerebrate cats. Emphasis will be on the spinal part of the reflex pathway, and a combination of electrophysiology and natural stimulation will be used. Tonic neck reflexes will be evoked by head rotation in labyrinthectomized cats or by body rotation with the head fixed in labyrinth-intact cats. Vestibular reflexes will be evoked by whole-body tilt. Combinations of sinusoidal pitch and roll tilt will be used to determine the most effective stimulus plane, i.e. the response vector of the muscle or neuron under investigation. At first, response vectors for neck and vestibular reflexes will be determined for a variety of limb muscles (EMG recording), and for different types of motoneurons (intracellular recording). Motoneuron recording will also enable us to study the dynamics of the depolarization evoked by neck rotation. Next will be a study of the connections of cervical and lumbar propriospinal neurons with identified neck response vectors. Intra-axonal HRP injection will be used to trace axonal branching, and spike triggered averaging to look for connections with motoneurons. In the latter case, recording will be from ventral roots or motoneurons. Neck and limb afferent input to propriospinal neurons will also be investigated. The role of Ia inhibitory neurons in the tonic neck reflex will be explored by looking for modulation of neurons with identified afferent input. For vestibulospinal reflexes the goal will be to find relay neurons between vestibular nuclei and motoneurons. Signals relayed to the cervical enlargement by C3-C4 propriospinal neurons and by reticulospinal neurons will be studied. In addition, we will study the responses of interneurons in C1-C4 that could be involved in pathways to neck motoneurons. Some work on vestibulospinal reflexes will be done on cats with inactivated semicircular canals, to study pure otolith effects.
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