Hypothalamus

Hypothalamus 3 Jobs

The hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions. is called the neuroendocrine organ, also known as the master control. It controlsVariables that remain constant to ensure a fair test. other endocrine organs.  Discussing the hypothalamus often requires mentioning other endocrine organs. You might not yet be familiar with these other organs.  It is highly recommended that you revisit this minilecture after covering hormones of the pituitary glands.  The hypothalamus has three main jobs. The hypothalamus makes its own hormones, which are then stored in the pituitary glandEndocrine gland at the brain’s base controlling many hormones.. It controls the pituitary gland by telling the anteriorThe front of the body or toward the front when standing in the anatomical position. when to release its hormones. It also signals the posterior to release its hormones, acting as a hormonal trigger. Lastly, the hypothalamus stimulates the adrenal medullaInner part of adrenal glands producing catecholamines (epinephrine, norepinephrine).. The adrenal glandPaired endocrine glands above the kidneys producing stress-related hormones. has two parts. The outside is called the cortex. The inside is referred to as the medulla. The hypothalamus has a direct connection with neuronsThe functional cells of the nervous system that transmit signals. to the medulla.

Hypothalamus Innervation

The hypothalamus makes its own hormones, separate from controlling the pituitary gland. Let’s do a little anatomyThe study of the structure of the human body. here. The pituitary gland has two lobes. It sits just posterior to the crossing of the optic nerves, which are called the optic chiasm. The hypothalamus contains many neurons. This includes the supraoptic neurons located above the optic nerve. It also includes the paraventricular neurons near the ventricle and neurosecretory neurons. The supraoptic and paraventricular neurons extend into the pituitary gland, while the neurosecretory neurons do not. A narrow piece of tissue called the infundibulumThe stalk connecting the hypothalamus to the pituitary gland. connects the hypothalamus to the pituitary gland. During a sheep brain dissection, the infundibulum can break, separating the pituitary gland.

The Hypothalamus Vascularization

Now, let’s discuss the arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr in this region.  There are a few portal systems in your body.  This is one of them called the hypophyseal portal systemBlood vessel network connecting hypothalamus to anterior pituitary.. A portal system is a direct connection of blood vessels (usually referred to as venulesSmall veins that collect blood from capillaries and transport it to larger veins.) between two organs. Another example is the hepatic portal system which is a direct connection between your small intestine and your liverA large organ that produces bile, detoxifies blood, and stores nutrients..  All the food that enters your small intestine is absorbed. It is then sent directly to the liver. After this, it is released into systemic circulation.  Here, we have a direct connection between the hypothalamus and the anterior pituitary. 

Blood comes into the hypothalamus via this superiorAbove or toward the upper part of the body. hypophyseal artery.  The superior hypophyseal artery runs through this stalk here called the infundibulum. It makes a direct connection between the hypothalamus and the anterior pituitary gland here in orange. When the hypothalamus releases hormones, they go right to the anterior pituitary.  Therefore, we can never measure the hormones in this portal system.  Blood leaves the anterior pituitary gland via these two veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge here.  This blood would carry and pituitary hormones that are to be distributed to systemic tissues.

The inferiorBelow or toward the lower part of the body. hypophyseal artery enters the posterior pituitary gland(PPG): Stores and releases oxytocin and ADH produced by the hypothalamus.. The blood enters the posterior pituitary, picks up the posterior pituitary gland’s hormones and then distributes them to systemic tissues. 

Job 1

Returning to the hypothalamus’s jobs, the first is to make its own hormones. The supraoptic nucleusThe control center of the cell that contains DNA and directs cellular activities. produces oxytocin(OXT): Stimulates uterine contractions and milk ejection., and the paraventricular nucleus produces ADH. These hormones travel anterograde down these axons to be stored in the posterior pituitary gland. The hypothalamus sends an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. to trigger their release into the capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues., similar to neurotransmitterChemicals that transmit signals across synapses. release.  These two hormones are discussed in more detail in the minilecture on the posterior pituitary gland.

Job 2

The second job of the hypothalamus is controlling the anterior pituitary gland. Unlike the posterior pituitary, the anterior uses a different mechanism. Neurosecretory cellsThe basic structural and functional units of life. in the hypothalamus release hormones into the hypophyseal portal system.  These hormones affect only the APG. This is because the hypophyseal portal system is a one-way communication between the hypothalamus and the anterior pituitary gland. There are releasing hormonesHypothalamic hormones that regulate pituitary gland secretions. that prompting the anterior pituitary gland to release its own hormones. The hypothalamus can release inhibitory hormones to prevent the anterior pituitary gland from releasing its own hormones.  All these hormones I have listed here in abbreviation are releasing hormones.  Some of these don’t have a corresponding inhibiting hormone, which is discussed in another minilecture. These hormones are discussed in more detail in the minilecture on the anterior pituitary gland.

Job 3

Finally, the hypothalamus has a direct connection to the adrenal medulla. It connects via neurons of the sympathetic divisionPart of the autonomic nervous system that prepares the body for stress and activity. of the autonomic nervous systemThe part of the peripheral nervous system that controls involuntary functions such as heart rate, di.  When the hypothalamus detects a threat, it sends an action potential to the medulla. This triggers the release of epinephrineadrenaline): Fight-or-flight hormone from the adrenal medulla. and norepinephrineA neurotransmitter involved in attention, arousal, and the fight-or-flight response. into the bloodstream. These hormones have systemic effects, initiating the “fight or flight” response by causing various organs in the body to react.  This feedback loop is discussed in more detail in the Adrenal Medulla mini-lecture.