Non-reciprocal control of rhythmic activity in respiratory-modulated XII motoneurons

To elucidate the nature of phasic respiratory inputs to hypoglossal (XII) motoneurons, we assessed the phasic changes in membrane resistance (RN) in inspiratory- and expiratory-modulated motoneurons. In expiratory motoneurons, RN was consistently lower in inspiration than in expiration and intracellular injection of negative current reversed the phasic hyperpolarizing membrane potential wave in inspiration. In inspiratory motoneurons, RN changed less but also was minimal in inspiration. In these neurons, injection of a negative current sufficient to reverse a lingual nerve-evoked IPSP did not reverse the phasic expiratory hyperpolarization. Thus, respiratory-related inhibitory inputs are non-reciprocal in inspiratory and expiratory XII motoneurons, with inspiratory motoneurons probably lacking a somatic, amino acid-mediated inhibitory input in expiration.