Brainstem projections to spinal motoneurons: an update.

1. The existence of direct projections to spinal motoneurons and interneurons from the raphe pallidus and obscurus, the adjoining ventral medial reticular formation and the locus coeruleus and subcoeruleus is now well substantiated by various anatomical techniques. 2. The spinal projections from the raphe nuclei and the adjoining medial reticular formation contain serotonergic and non-serotonergic fibres. These projections also contain various peptides, several of which are contained within the serotonergic fibres. Whether still other transmitter substances (e.g. acetylcholine) are present in the various descending brainstem projections to motoneurons remains to be determined. 3. The spinal projections from the locus coeruleus and subcoeruleus are mainly noradrenergic, but there also exists a non-noradrenergic spinal projection. 4. Pharmacological, physiological and behavioural studies indicate an overall facilitatory action of noradrenaline and serotonin (including several peptides) on motoneurons. This may lead to an enhanced susceptibility for excitatory inputs from other sources. 5. The brainstem areas in question receive an important projection from several components of the limbic system. This suggests that the emotional brain can exert a powerful influence on all regions of the spinal cord and may thus control both its sensory input and motor output.