The function of the paramedian reticular nucleus in the control of heart rate in the cat.

1. Selective electrical stimulation of the paramedian reticular nucleus (PRN) in anaesthetized and decerebrate-anaesthetized cats elicited cardiac slowing which was shown to be due to inhibition of the sympathetic input to the heart.2. In decerebrate preparations stimulation of the PRN elicited cardioacceleration which was abolished by bilateral vagotomy.3. It is suggested that these changes in heart rate in opposite directions can be accounted for by an inhibitory influence of the PRN on both neural inputs to the heart. In the absence of vagal activity, demonstrated in anaesthetized and decerebrate-anaesthetized preparations, the response to stimulation of the PRN resulted in cardiac slowing. In contrast, when both sympathetic and parasympathetic pathways were active (decerebrate preparation) stimulation of the PRN resulted in cardio-acceleration, due to simultaneous inhibition of the two neural inputs to the heart.4. It is concluded that the two inhibitory functions of the PRN are probably mediated by two different populations of neurones and that the PRN plays an integrative role in the medullary control of heart rate.