The intermedius nucleus of the medulla: a potential site for the integration of cervical information and the generation of autonomic responses.

The intermedius nucleus of the medulla (InM) is a small perihypoglossal brainstem nucleus, which receives afferent information from the neck musculature and also descending inputs from the vestibular nuclei, the gustatory portion of the nucleus of the solitary tract (NTS) and cortical areas involved in movements of the tongue. The InM sends monosynaptic projections to both the NTS and the hypoglossal nucleus. It is likely that the InM acts to integrate information from the head and neck and relays this information on to the NTS where suitable autonomic responses can be generated, and also to the hypoglossal nucleus to influence movements of the tongue and upper airways. Central to the integratory role of the InM is its neurochemical diversity. Neurones within the InM utilise the amino acid transmitters glutamate, GABA and glycine. A proportion of these excitatory and inhibitory neurones also use nitric oxide as a neurotransmitter. Peptidergic transmitters have also been found within InM neurones, although as yet the extent of the pattern of co-localisation between peptidergic and amino acid transmitters in neurones has not been established. The calcium binding proteins calretinin and parvalbumin are found within the InM in partially overlapping populations. Parvalbumin and calretinin appear to have complementary distributions within the InM, with parvalbumin being predominantly found within GABAergic neurones and calretinin being predominantly found within glutamatergic neurones. Neurones in the InM receive inputs from glutamatergic sensory afferents. This glutamatergic transmission is conducted through both NMDA and AMPA ionotropic glutamate receptors. In summary the InM contains a mixed pool of neurones including glutamatergic and GABAergic in addition to peptidergic neurones. Neurones within the InM receive inputs from the upper cervical region, descending inputs from brain regions involved in tongue movements and those involved in the coordination of the autonomic nervous system. Outputs from the InM to the NTS and hypoglossal nucleus suggest a possible role in the coordination of tongue movements and autonomic responses to changes in posture.