Figure 1: A schematic drawing summarising the gastrin-releasing peptide (GRP) system, which controls male sexual functions in the lumbar spinal cord. A sexually dimorphic spinal cord system of GRP-containing neurones in the lumbar spinal cord (at the L3–L4 level) − ‘the ejaculation centre’ − projects axons to the autonomic centre [e.g., sacral parasympathetic nucleus (SPN)] and to the somatic centre [e.g., spinal nucleus of the bulbocavernosus (SNB)] in the lower lumbar spinal cord. These centres regulate penile reflexes and trigger ejaculation via GRP receptor-mediated mechanisms.