The potential for different serotonin (5-hydroxytryptamine [5-HT]) receptors that act at different sites to modulate hypoglossal motor neurone activity. In addition, modulation of 5-HT1A autoreceptors on dorsal raphe neurones may also indirectly affect hypoglossal motor neurones via effects on rapid eye movement sleep (Fig. 2). See text for more details. +, excitation; -, inhibition.