Pulvinar

The pulvinar is a caudal nucleus of the thalamus. The pulvinar complex is extensively and reciprocally interconnected in a visuotopic manner with many visual cortical areas. The pulvinar is usually divided into oral, inferior, lateral, and medial subnuclei. The lateral and inferior pulvinar are widely connected to visual cortical association areas; the oral pulvinar is linked with somatosensory cortical association areas; the medial pulvinar has widespread connections with cingulate, posterior parietal, and prefrontal cortical areas.

The number of fibers in the cortical, in contrast to the subcortical, input suggests that the response properties of pulvinar neurons may more mimic the former than the latter. Many cells are directionally and orientationally selective and have multiple discharge centers that are characteristic of prestriate but not tectal neurons.

The most dramatic differences between pulvinar and cortical neurons are receptive field size and breadth of orientation and directional tuning. Receptive field sizes of striate neurons are approximately 1º arc in width. Those of pulvinar cells average about 10º and may be as large as 60º. For the time being, the function of the pulvinar is uncertain. The coarse structure of the receptive field suggests that it does not analyze stimulus form.

Median Eminence

The median eminence is the region of the brain which lies below the third cerebral ventricle and behind the optic chiasm. It is bounded laterally by the arcuate nucleus, rostral to the neural stalk that connects the posterior pituitary gland to the hypothalamus.

The median eminence consists of two zones, an internal zone and an external zone. The internal zone is made up of axons of the magnocellular neurosecretory neurons of the supraoptic nucleus and paraventricular nucleus which project to the posterior lobe of the pituitary gland. The external zone is composed of the convoluted blood vessels of the hypothalamo-pituitary portal system, and the nerve endings of many neuroendocrine neurons, such as the dopamine neurons of the arcuate nucleus and somatostatin neurons from the periventricular neucleus. These portal vessels transport releasing factors released by neuroendocrine neurones of the hypothalamus to the anterior pituitary gland.

The median eminence is of great physiological importance, as it is integral to the hypophyseal portal system, which connects the hypothalamus with the anterior lobe of the pituitary gland. It is in this structure that the secretions of the hypothalamus (releasing and inhibiting regulatory hormones) collect before entering the portal system.