English: Simplified diagram of the neurobiological circuits of the lordosis sexual reflex, specific to female mammals. Lordosis is indispensable to the achievement of copulation. This complex motor sexual reflex is hardwired in the spinal cord and receives modulatory afferents from the forebrain.

The lordosis sexual reflex is mainly hardwired in the spinal cord, at the level of the lumbar and sacral vertebrae (L1, L2, L5, L6 and S1). In the brain, several regions modulate the lordosis reflex. The vestibular nuclei and the cerebellum, via the vestibular tract, send information which makes it possible to coordinate the lordosis reflex with postural balance. More importantly, the ventromedial hypothalamus sends projections that inhibit the reflex at the spinal level. For this reason, in general, the lordosis reflex is not functional. Sex hormones control reproduction and coordinate sexual activity with the physiological state. Schematically, at the breeding season, and when an ovum is available, hormones (especially estrogen) simultaneously induce ovulation and oestrus (heat). Under the action of estrogen in the hypothalamus, the lordosis reflex is uninhibited. The female is ready for copulation and fertilization. During the copulation, when a male approaches the female, male pheromones (part 1 of the above diagram) are detected by the olfactory circuits (part 2). The pheromonal signals stimulate, among other things, the hypothalamus, which facilitates the lordosis reflex. Then when the male mounts the female (part 3), tactile stimuli on the flanks, the perineum and the rump of the female are transmitted via the sensory nerves in the spinal cord. In the spinal cord, they are integrated with the information coming from the brain, and then, in general, a nerve impulse is transmitted to the muscles via the motor nerves. The contraction of the longissimus and transverso-spinalis muscles causes the curve of vertebral column (part 4). The lordosis position which results from it makes it possible to present properly the vagina to the male (part 5), facilitating penile intromission. Then, during intromission, tactile and deep sensations from the genital area and clitoris accentuate the lordosis reflex (part 6). It is thus observed that the physiological and neurobiological organization of the lordosis reflex is specifically adapted to heterosexual copulation.

Caption: a) Median preoptic nucleus; b) Anterior hypothalamic nucleus; c) Ventromedial hypothalamic nucleus; d) Midbrain reticular formation; e) Vestibulo-spinal tract; f) Reticulo-spinal tract; g) Dorsal roots L1, L2, L5, L6 et S1. NB: the neural circuit is bilateral. The diagram is simplified for readability.

Diagram adapted from: Gonzalez-Flores O., Beyer C., Lima-Hernandez F.J., Gomora-Arrati P., Gomez-Camarillo M.A., Hoffman K., Etgen A.M. Facilitation of estrous behavior by vaginal cervical stimulation in female rats involves alpha1-adrenergic receptor activation of the nitric oxide pathway. Behavioural Brain Research, 176(2):237-243, 2007. --- Haga S., Hattori T., Sato T., Sato K., Matsuda S., Kobayakawa R., Sakano H., Yoshihara Y., Kikusui T., Touhara K. The male mouse pheromone ESP1 enhances female sexual receptive behaviour through a specific vomeronasal receptor. Nature, 466(7302):118-122, 2010. --- Kow L.M., Florea C., Schwanzel-Fukuda M., Devidze N., Kami K.H., Lee A., Zhou J., Maclaughlin D., Donahoe P., Pfaff D. Development of a sexually differentiated behavior [lordosis] and its underlying CNS arousal functions. Curr. Top. Dev. Biol., 79:37-59, 2007. --- Knobil E., Neill J.D. (Eds). The physiology of reproduction. Academic Press, 3nd edition, 2005. --- Wunsch S. Comprendre les origines de la sexualité humaine. Neurosciences, éthologie, anthropologie. L'Esprit du Temps, 2014.

Image: Vector_diagram_of_laboratory_mouse_(black_and_white) – from Gwilz