The Hypothalamus and Pituitary Gland
Objective 3
Describe the anatomical and physiological relationships between the pituitary (which includes the adenohypophysis and the neurohypophysis) and the hypothalamus. Describe hypothalamic-pituitary hormone pathways. Differentiate between releasing hormones, inhibiting hormones, and tropic hormones.
The Hypothalamus and Pituitary Gland
The hypothalamus and the pituitary gland, located just inferior to the hypothalamus, can be thought of as the “command center” of the endocrine system. Messages between the endocrine and the nervous system are coordinated through this command center. Hormones produced and released here may directly produce responses in target tissue. Other hormones may regulate the production and release of hormones in other glands.
Many of these hormone pathways act like a relay system, with one hormone targeting the release of the next, much like handing off a baton in a relay race.
We’ve already looked at the hypothalamus, part of the diencephalon of the brain. The hypothalamus has both neural and endocrine functions. The pituitary gland (or hypophysis) is a pea-shaped gland that hangs like a boxing bag from the hypothalamus by a stalk, the infundibulum. The origin of the term hypophysis is Greek, referring to its position and growth under the brain. The pituitary is also commonly referenced as the master gland, but the truth is that it has a master, the hypothalamus. The hypothalamus is the connection between the nervous and endocrine systems.
The Posterior Pituitary (Neurohypophysis)
The posterior pituitary is nervous tissue; hence, it’s named the neurohypophysis. It consists of special axons and axon terminals from neurosecretory neurons. The posterior pituitary does not synthesize any hormones. The hormones from the posterior pituitary are produced by neurosecretory cells of the hypothalamus. These hormones are transported by axoplasmic transport within axons to be stored in the axon terminals.
When these hypothalamic neurons send signals, hormone is released from the axon terminals housed in the posterior pituitary. In synaptic signaling, a peptide neurotransmitter is released into a synaptic cleft. In endocrine signaling, a peptide hormone is released directly into the bloodstream.
The Anterior Pituitary (Adenohypophysis)
The pituitary has two lobes that are named based on their position and functional relationship to the hypothalamus. The anterior lobe of the pituitary, also called the adenohypophysis, is about 75% of the total pituitary weight. The second lobe is the posterior pituitary or neurohypophysis.
The terms adeno- and neuro- are very important to understand their relationships with the hypothalamus. “Adeno–” means “gland”, so the relationship of the hypothalamus and anterior pituitary is a gland-to-gland relationship. The hypothalamus connects to the adenohypophysis through a vascular system, the hypophyseal portal veins. The hypothalamus produces hormones that will tra