Sensory and motor components of reproductive behavior: pathways and plasticity.

Reproductive behavior in most mammalian species consists of a highly stereotyped pattern of movements, is elicited by specific sensory stimuli and is sex steroid dependent. The present paper describes a concept of the pathways in the midbrain, brainstem and spinal cord which control the receptive posture of the female cat. The midbrain periaqueductal gray (PAG), which is an important structure in the Emotional Motor System (EMS), receives direct input from a distinct group of neurons in the dorsal horn of the lumbosacral cord. This cell group overlaps with the location of pelvic and to lesser extent, pudendal nerve primary afferents, which convey information from the pelvic viscera and sex organs to the central nervous system. The PAG, in turn, controls various motor components of female receptive behavior using different pathways. For example, immobility, which is one of the characteristics of receptive behavior, might be mediated by a diffuse pathway from the PAG, via the ventral part of the medial medullary tegmentum, to all parts of the spinal ventral horn. More specific components, such as hindlimb treading, lateral deviation of the tail and elevation of the lower back, are thought to be controlled by a circumscribed projection from the PAG to the nucleus retroambiguus (NRA). The NRA is a group of interneurons at the transition between brainstem and spinal cord and projects directly to distinct lumbosacral motoneuronal cell groups, which innervate muscles that are likely to be involved in the female receptive posture. Estrogen induces axonal sprouting of the NRA-lumbosacral pathway in adult female cats, which explains why female cats only display receptive behavior when estrogen levels are high.