Responses of ponto-medullary reticular neurons to cortical, tectal and cutaneous stimuli

Responses of neurons in the medial ponto-medullary reticular formation to cortical, tectal and cutaneous stimuli were studied in cerebellectomized cats under chloralose-urethan or pentobarbital anesthesia. Reticulospinal neurons were identified by their antidromic responses to stimulation of the cervical and lumbar spinal cord. The reticulospinal axons of neurons located in the caudal part of n.r. gigantocellularis tended to have slower conduction velocities than axons of neurons in more rostral regions. Stimuli applied to the deep layers of the superior colliculus and to the pericruciate cortex or cerebral peduncles evoked monosynaptic EPSPs in many reticular neurons including reticulospinal neurons. Cutaneous stimuli evoked polysynaptic EPSPs or IPSPs. A prolonged period of depression that appeared to be caused by disfacilitation often followed the initial PSPs. Investigation of the origin of tecto-reticular excitation by means of movable stimulating electrodes revealed that excitation of contralateral reticular neurons was evoked by activation of elements within the tectum whereas excitation produced in ipsilateral reticular neurons may have involved activation of laterally situated, sub-tectal elements. The properties of reticular responses to tectal stimuli suggest that reticular neurons may act as relays between the tectum and neck motoneurons and possibly other spinal motoneurons as well. Investigation of the convergence of different inputs upon individual reticular neurons revealed a significant positive correlation between responses to stimulation of different tectal points and between responses to stimulation of ipsi- and contralateral pericruciate cortex but no correlation between responses to stimuli of different types.