Motomu Manabe

Distribution of medullary respiratory neurons in the rat

In Nembutal-anesthetized and spontaneously breathing rats, a total of 226 respiratory neurons were recorded in the medulla extending from the caudal end of the facial nucleus to 1 mm caudal to the obex. They were classified into inspiratory (I) and expiratory (E) neurons by their temporal relationships to diaphragm EMGs. One hundred and seventeen I and 108 E neurons were identified. I and E neurons were further classified into augmenting, decrementing, and other types based on their firing patterns. Almost all the respiratory neurons recorded were located around the nucleus ambiguus and the nucleus retroambigualis, corresponding to the ventral respiratory group (VRG) of the cat. On the other hand, only a few respiratory neurons were identified around the ventrolateral nucleus of the solitary tract, corresponding to the dorsal respiratory group of the cat. In the VRG, 3 subgroups were distinguished rostrocaudally. One group of E neurons was located ventrally to the rostral part of the nucleus ambiguus, presumably corresponding to the Bötzinger complex defined in the cat. Another group of E neurons extended caudally beyond the obex, from the caudal portion of the nucleus ambiguus through the nucleus retroambigualis. Between these two groups of E neurons, an assembly of predominantly I neurons existed in the vicinity of the nucleus ambiguus. These characteristics of distributions were basically similar to those of the VRG of the cat.

Efferent projections of inspiratory neurons of the ventral respiratory group. A dual labeling study in the rat

The efferent projections of the medullary respiratory neurons of the rat were studied using an anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). In Nembutal-anesthetized rats, PHA-L was iontophoretically applied to (1) the area of inspiratory neurons of the ventral respiratory group (VRG) around the nucleus ambiguus, or (2) the area ventrolateral to the solitary tract. In addition, a fluorescence retrograde tracer, Fast blue (FB), was injected into the cervical phrenic nerve several days after the PHA-L injection. When PHA-L was injected into the area of predominantly inspiratory neurons of VRG, dense PHA-L-labeled axons were observed bilaterally in the spinal cord: the ipsilateral projections were noticeably denser than the contralateral ones. Fine axonal branches were distributed around a column of the phrenic motoneurons and boutons were observed on the somata of the FB-labeled motoneurons, suggesting monosynaptic connections between VRG inspiratory neurons and phrenic motoneurons. On the other hand, when PHA-L was injected into the area ventrolateral to the solitary tract, only a few descending axons to the spinal cord were seen bilaterally. No contacts between the PHA-L-labeled axons and the FB-labeled phrenic motoneurons were observed. The brainstem projections of the VRG were found bilaterally in the nuclei ambigui, Cajals interstitial nuclei of the solitary nucleus, the solitary nuclei, the hypoglossal nuclei, the Kölliker-Fuses nuclei, and the subcoeruleus areas.

Excitation and inhibition of medullary inspiratory neurons by two types of burst inspiratory neurons in the cat

In Nembutal-anesthetized and artificially ventilated cats, we studied the connectivity of burst inspiratory (I) neurons in the Bötzinger complex and the ventral respiratory group (VRG) with spike-triggered averaging methods. Burst I neurons exhibited tonic (I-TON) or decrementing (I-DEC) firing patterns. Spikes of I-TON neurons induced monosynaptic EPSPs in intracellularly recorded I neurons of both the VRG and the dorsal respiratory group (DRG). Spikes of I-DEC neurons induced monosynaptic inhibitory postsynaptic potentials (IPSPs) in both VRG and DRG I neurons.