<P> Neurons of the PAG are excited by endorphins and by opiate analgesics . It also plays a role in female copulatory behavior (see Lordosis behavior) via a pathway from the ventromedial nucleus of the hypothalamus . </P> <P> The PAG may be specifically involved in human maternal behavior . The PAG contains a high density of vasopressin and oxytocin receptors, and it has direct connections with the orbitofrontal cortex, which might mediate the role of the PAG in maternal love . The lateral orbitofrontal cortex is activated by pleasant visual, tactile, and olfactory stimuli . Its response depends on pleasantness rather than on intensity of stimulation . Here, its activity is likely to reflect one aspect of the pleasant emotions associated with motherly love . </P> <Ul> <Li> <P> Schematic representation of the chief ganglionic categories (I to V). </P> </Li> <Li> <P> Transverse section of mid-brain at level of inferior colliculi . </P> </Li> <Li> <P> Transverse section of mid-brain at level of superior colliculi . </P> </Li> <Li> <P> MRI section of human mid-brain showing periaqueductal gray </P> </Li> </Ul> <Li> <P> Schematic representation of the chief ganglionic categories (I to V). </P> </Li>

Where might a gray matter nucleus be located