The brain region lies beneath the thalamus and forms the floor of the third cerebral ventricle. It is an integral part of the brain.
A small cone-shaped structure projects downward from the brain, ending in the pituitary stalk (infundibular), a tubular connection to the pituitary gland.
The hypothalamus contains a control center for many autonomic nervous system functions and has effects on the endocrine system due to its complex interaction with the pituitary gland.
Anatomy of the hypothalamus
The hypothalamus and the pituitary gland are connected by nerve and chemical pathways. The posterior part of the hypothalamus, called the median eminence, contains the nerve endings of many neurosecretory cells, which descend through the infundibular stem to the pituitary gland.
Important structures adjacent to the median eminence of the hypothalamus include the mammillary bodies, the third ventricle, and the optic chiasm (a part of the visual system). On the hypothalamus is the thalamus.
Like the rest of the brain, the hypothalamus consists of interconnected neurons nourished by an abundant supply of blood. To understand the hypothalamic function, it is necessary to define the various forms of neurosecretion.
First, neurotransmission occurs throughout the brain and is the process by which a nerve cell communicates with another through a synapse, a small space between the ends (nerve terminals) of neurons.
The nerve terminals are often called presynaptic or postsynaptic about the direction in which an impulse travels, with the presynaptic neuron transmitting an inspiration to the postsynaptic neuron.
The transmission of an electrical impulse requires the secretion of a chemical substance that diffuses through the synapse from one neuron’s presynaptic membrane to another neuron’s postsynaptic membrane. The chemical that is secreted is called the neurotransmitter.
Hypothalamic regulation of hormone secretion:
The process of synthesis and secretion of neurotransmitters is similar to the synthesis of protein hormones, with the exception that neurotransmitters are contained in neurosecretory granules that are produced in the cell body and migrate through the axon (a projection of the neuron) to the terminal nerve, from where they are discharged into the synaptic space.
There are four classic neurotransmitters: epinephrine, norepinephrine, serotonin, and acetylcholine. Many additional neurotransmitters have been discovered, of which a vital group is neuropeptides.
Neuropeptides function not only as neurotransmitters but also as neuromodulators. As neuromodulators, they do not act directly as neurotransmitters, but they increase or decrease the action of neurotransmitters.
Well-known examples are opioids (e.g., Encephalins) because they are endogenous peptides (produced in the human body) with a high affinity for receptors that bind opiate drugs, such as morphine and heroin.
The brain and the entire central nervous system consist of an interconnected network of neurons. The secretion of neurotransmitters and specific neuropeptides provides an organized and directed function.
The connection of the hypothalamus to many other regions of the brain, including the cerebral cortex, allows intellectual and functional signals and external signals, including physical and emotional stresses, to be channeled into the hypothalamus into the endocrine system.
These signals can exert their effects throughout the body from the endocrine system.
The hypothalamus produces and secretes neurotransmitters and neuropeptides and several neurohormones that alter the function of the anterior pituitary gland and two hormones, vasopressin (antidiuretic hormone) and oxytocin, which act on distant target organs.
Neurons that produce and secrete neurohormones are actual endocrine cells because they have hormones that are incorporated into secretory granules that are then transported through the axons and stored in nerve terminals located in the median eminence or the posterior pituitary gland.
In response to neural stimuli, the contents of the secretory granules are extruded from the nerve terminals to a capillary network.
In the case of hormones that affect pituitary function, the content of the secretory granules is transported through the pituitary-portal circulation and administered directly to the anterior pituitary gland.
These hypothalamic neurohormones are known as releasing hormones because their primary function is to stimulate the secretion of hormones that originate in the anterior pituitary gland.
For example, certain releasing hormones secreted by the hypothalamus trigger the release of the anterior pituitary from substances such as adrenocorticotropic hormone and luteinizing hormone.
Hypothalamic neurohormones consist of simple peptides that vary from only three amino acids (thyrotropin-releasing hormone) to 44 amino acids (growth hormone-releasing hormone).
A hypothalamic hormone, somatostatin, has an inhibitory action, inhibiting mainly growth hormone secretion, although it can also inhibit the secretion of other hormones.
The neurotransmitter dopamine, produced in the hypothalamus, also has an inhibitory action, inhibiting prolactin secretion from the anterior pituitary hormone.
The cell bodies of the neurons that produce these neurohormones are not evenly distributed throughout the hypothalamus. Instead, they are grouped into paired groups of cell bodies known as nuclei.
A classic model for neurohormonal activity is the posterior lobe of the pituitary gland (neurohypophysis). Its secretory products, vasopressin, and oxytocin, are produced and packaged into neurosecretory granules in specific groups of nerve cells in the hypothalamus (the supraoptic nuclei and the paraventricular nuclei).
The granules are transported through the axons that extend through the infundibular stem and end in and form the posterior lobe of the pituitary gland. The secretory granules are extruded into a capillary network that feeds directly into the general circulation in response to nerve signals.
In addition to regulating the release of pituitary hormones, this also influences weight and caloric intake regulation, establishing a stable “set point” for individual weight gain.