Severance of the pituitary stalk disconnects the posterior lobe of the pituitary from the hypothalamus. As a result, antidiuretic hormone cannot be transported from its hypothalamic origin to the pituitary, and its secretion cannot be regulated. Without ADH, the kidneys reabsorb less water than normal, and urine output is therefore greatly increased.
(a) Hydrocephalus can result because an enlarged pineal gland compresses the cerebral aqueduct, so cerebrospinal fluid accumulates in the lateral and third ventricles and cannot drain into the fourth ventricle, where it would normally exit the ventricular system. (b) The pineal gland overhangs the midbrain, which is the origin of the oculomotor nerve. As a growing pineal tumor puts pressure on this nerve or its midbrain nucleus, it blocks output in the autonomic fibers of this nerve that normally control accommodation and pupillary responses to light. The eye cannot focus, and the pupil appears dilated and unresponsive to light. (c) The oculomotor nerve also contains somatic motor fibers that control most eye movements. Compression of the nerve or its nucleus therefore also blocks somatic signals and paralyzes most of the extrinsic muscles of the eye. The eye is directed downward and laterally, and the person experiences double vision.
The kidneys produce most erythropoietin (EPO), a hormone that stimulates erythrocyte production. Kidney failure results in an EPO deficiency, which in turn results in reduced erythrocyte production and therefore in anemia. Dialysis patients are therefore routinely given EPO. The kidneys also perform the final stage in the synthesis of calcitriol (vitamin D), a hormone that promotes absorption of dietary calcium. In the absence of calcitriol, a person suffers hypocalcemia. Vitamin D can be given orally to compensate for this.
The adrenal cortical hormones are steroids, while those of the pancreatic islets are peptides. Electron microscopy of adrenal spongiocytes will therefore reveal an abundance of the steroid-synthesizing organelle, smooth endoplasmic reticulum, whereas the islet cells will show an abundance of the protein-synthesizing organelle, rough endoplasmic reticulum.
Some hormones are synthesized and secreted by neurons, such as oxytocin and antidiuretic hormone, produced by neurons of the paraventricular and supraoptic nuclei of the hypothalamus. Some chemicals function as both hormones and neurotransmitters. Prolactin-inhibiting hormone, for example, is identical to the neurotransmitter dopamine; antidiuretic hormone is the same as the neurotransmitter, vasopressin; and serotonin is regarded as a hormone when it is secreted by the pineal gland and as a neurotransmitter when secreted by neurons of the brain.
