Posterior Pituitary Gland

Posterior Pituitary

The pituitary glandEndocrine gland at the brain’s base controlling many hormones. can be considered as two different organs. There is a specific line, called the pars intermedialis, that separates the anteriorThe front of the body or toward the front when standing in the anatomical position. and posterior pituitary glands. To identify the anterior pituitary, look for the optic chiasma, where the optic nerves cross. The anterior pituitary is abbreviated as APG, and the posterior pituitary is PPG.

The posterior pituitary is connected to the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions. via neuronsThe functional cells of the nervous system that transmit signals. that run down through the infundibulumThe stalk connecting the hypothalamus to the pituitary gland.. There are more than these two neurons running in the infundibulum, but let’s just focus on these two.  This one is the supraoptic nucleusThe control center of the cell that contains DNA and directs cellular activities. and this is the paraventricular nucleus, both named for their locations. 

These neurons make the only two hormones secreted by the posterior pituitary into blood.  These hormones are made in the cell bodies, up here in the hypothalamus. They are then transported anterograde down to the axon terminals. There, they wait for the signal to enter the blood.  You might be thinking that this sounds a lot like a neurotransmitterChemicals that transmit signals across synapses.. However, remember that neurotransmittersChemicals released by neurons to transmit signals across a synapse. are secreted only into a synapseThe junction between two neurons where communication occurs.. In contrast, hormones are secreted into blood.  What is important is that the posterior pituitary doesn’t make any hormones; it just stores hormones made by the hypothalamus.

PPG Histology

The posterior and anterior pituitary glands differ in histology. The posterior pituitary appears less stained due to its neuron content and myelin. In contrast, the anterior pituitary cellsThe basic structural and functional units of life. take up more stain. In the posterior pituitary, small purple dots represent the nucleiClusters of neurons in the CNS responsible for processing information. of neuroglia. The neurons themselves, called pituicytes, are specific to this region.

Antidiuretic Hormone

ADH, or anti-diuretic hormone, conserves waterThe universal solvent essential for life. in the body. As its name suggests, it prevents diuresis (excessive urination). It was previously called vasopressin because it increases blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange.. Conserving water increases the pressure in your blood vessels, similar to adding more water to a balloon. ADH is triggered by several factors. Pain is sensed by nociceptorsPain receptors that respond to tissue damage or potentially harmful stimuli.. The saltiness of your blood is sensed by osmoreceptorsHypothalamic cells detecting blood osmolarity to regulate ADH.. Your blood pressure is sensed by baroreceptors. All these factors could contribute to the triggering of ADH. 

ADH is a life-saving mechanism for the conservation of water in the body. This is crucial because, as land mammals, we are always dehydrated.  That’s just a factA statement based on direct observation that is repeatedly confirmed..  Let’s consider your blood’s saltiness as an example. This topic will be revisited frequently in this class.  When your blood becomes salty, osmoreceptors in the hypothalamus detect that change. More precisely, when your blood’s osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments. or colloid osmotic pressureThe pressure exerted by proteins (mainly albumin) in the blood that pulls water into the capillaries increases, these sensors respond.  The supraoptic nucleus sends an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. down its axon to tell the posterior pituitary to release ADH.

ADH then circulates through the bloodstream, targeting specific organs. The kidney is the only organ with receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter for ADH, making it the effector organ. In response to ADH, the kidney conserves water, increasing blood volume and pressure.

ADH is also associated with the phenomenon known as “breaking the seal.” For example, after consuming alcohol, you might hear warnings about not “breaking the seal” by urinating. This idea relates to turning off ADH, which leads to increased urination. When ADH is inhibited, the body stops conserving water, resulting in frequent trips to the bathroom.

Oxytocin

Oxytocin is released in exactly the same way.  Oxytocin is made in the cell bodyThe central part of a neuron containing the nucleus and organelles. of the paraventricular nucleus in the hypothalamus.  To trigger oxytocin(OXT): Stimulates uterine contractions and milk ejection., a few different things must happen simultaneously. They all involve chemoreceptors.  There comes a point where the baby starts to made hormones that are sensed by the hypothalamus.  So, the hypothalamus is like, “OK, ready to eject one baby.” Or more.  The hypothalamus sends an action potential down the paraventricular axon and triggers the release of oxytocin.  Oxytocin then circulates and attaches to receptors found on the myometriumThe thick muscular layer of the uterus or the contractile layer of the uterusThe muscular organ where a fertilized egg implants and develops. and….it contracts.  Eject one baby.  Or more.