Central nervous system control of ejaculation.

An overview is given of the regions in the spinal cord that are active during ejaculation. Motoneurons involved are the preganglionic sympathetic motoneurons in the upper lumbar spinal cord and the motoneurons in the nucleus of Onuf, located in the upper sacral cord. The first group is involved in the so-called emission phase of ejaculation, the last group in the expulsion phase. Both groups receive afferents from premotor interneurons in the so-called intermediomedial cell groups located at about the same level as the motoneurons themselves. A concept is put forward in which these premotor cell groups represent the central spinal pattern generators for ejaculation, one for the emission phase and one for the expulsion phase. Clinical observations in patients suffering from transection of the spinal cord indicate that the ejaculation motoneurons as well as their spinal central pattern generators are under strong influence of descending pathways originating in supraspinal parts of the brain. The various pathways possibly involved in ejaculation control are reviewed. Finally, the results of the brain activation of a PET-scan study in human males, ejaculating after penile stimulation by their female partner are discussed. Especially the ventral tegmental area and the cerebellum seem to be activated during ejaculation, while the amygdala region is deactivated. Apparently, a general lack of fear is necessary for ejaculation to occur.