The hypothalamus is named according to its position below the thalamus. This organ is the body’s coordinative hub, impacting the autonomic nervous system and regulating hormones. The location of the hypothalamus is relevant to the complex way it operates in connection to its surrounding structures.
The hypothalamus is situated beneath the thalamus and above the pituitary gland. It’s cone-shaped and approximately 0.4 to 0.5 inches in diameter. It’s the brain’s core, and this tiny epicenter plays a vital role. The hypothalamus is tasked with regulating hormones and homeostasis.
The hypothalamus exhibits an incredibly intricate circuitry system more than any other brain structure. As with the rest of the brain, the hypothalamus is networked with neural interconnections. However, unlike the brain, the hypothalamus has extensive non-neural communication pathways.
Where Is The Hypothalamus Located?
The hypothalamus belongs to the central nervous system and is within the forebrain region (prosencephalon). It lies deep within the brain, sits above the brain stem, and directly beneath the front part of the thalamus. The hypothalamus is on both sides of the base of the third cerebral ventricle.
Cerebral ventricles are sites within the cerebrum that are filled with cerebrospinal fluid (CSF). Ventricles connect to the liquid in the spine. They’re an interconnecting system found within the brain parenchyma.
The brain parenchyma is a connective tissue in the brain composed of two distinct cell types- neurons and glial. These cells are specialized for cognition and body control. The remaining brain tissue serves a supportive and structural role. This tissue is called the stroma.
The forebrain is the most extensive section of the brain, and the hypothalamus makes up a part of this structure. Other aspects of the forebrain feature the telencephalon and the diencephalon. The forebrain consists of the whole cerebrum and an arrangement of vital cranial structures below it- the pineal gland, the hypothalamus, the thalamus, and the limbic system.
The pituitary gland is nestled snugly with the hypothalamus and is attached by a small stalk (infundibulum). These two brain organs are deeply connected. The pituitary gland is a tiny endocrine secretory organ situated at the base of the brain, beneath the hypothalamus. It’s referred to as the master gland as it emits several essential hormones and is responsible for the operation of other endocrine glands.
The hypothalamus is positioned directly within the two tracts of the optic nerve. It resides where the optic nerves cross and meet. As light lands on the retina of the eye, the pupils contract, and in the absence of light, the pupils dilate and regain their resting state. These parasympathetic responses govern the visual system.
The hypothalamus makes use of the visual system input and regulates pupillary reflexes. When the retina responds to high quantities of light, the hypothalamus activates the parasympathetic response. The hypothalamus activates the sympathetic response if low light levels stimulate the retina.
Why Is The Location Of The Hypothalamus Important?
The hypothalamus administers its essential function via an immensely complex arrangement of interconnecting network interactions with its surrounding cranial structures. The hypothalamus is an advanced sensory integration and motor output site that governs homeostasis by regulating endocrine, autonomic, and somatic behavior.
The Hypothalamus-Homeostasis And Hormones
The hypothalamus’s crucial functions are maintaining homeostasis and administering and managing hormones. The body’s biology functions effectively in a consistent state of maintained balance.
Blood pressure, body temperature, and nutrition contribute to internal regulation and homeostasis. The hypothalamus facilitates these by informing the relevant organs.
If the body is imbalanced, the hypothalamus performs in two ways to regain homeostasis. The hypothalamus’ mechanisms include the autonomic nervous system and control hormone release from the pituitary gland.
Autonomic Nervous System And Hypothalamic Circuitry Control
The hypothalamus performs as a command center for autonomic processes and governing neurohormones. It also determines various facets of motivation. The main integrational organ of the autonomic nervous system is the hypothalamus.
The hypothalamus influences the processors of the autonomic nervous system. The autonomic nervous system is so named for the automatic functions performed throughout the body and operates independently of conscious control. Examples include hormone secretions, gland function- perspiration, heart rate, respiration, and the digestive system.
There are three main routes from which the hypothalamus manages the central autonomic network. These are the dorsal longitudinal fasciculus, the medial forebrain bundle, and the mammillotegmental tract.
The Hypothalamus’ Circuitry- Pathways And Connections
The hierarchy of the hypothalamus is structured in loops from the brainstem to the spinal cord. This connection oversees rapid short-term regulation of the autonomic nervous system. The hypothalamic-brainstem-spinal cord pathways result in extended metabolic and reproductive regulation.
The limbic system-hypothalamic-brainstem-spinal cord loops result in anticipatory autonomic regulation. Output from the hypothalamus trails two optic tracts: the dorsal longitudinal fasciculus (DLF) and the medial forebrain bundle (MFB).
The Dorsal Longitudinal Fasciculus And The Medial Forebrain Bundle
The hypothalamus is connected to the brainstem via the dorsal longitudinal fasciculus (DLF). The DLF is an expanse of white matter (myelinated axons) found within the brain stem. It transmits information via myelinated axons between neurons and has ascending and descending fibers.
The DLF is relevant to the hypothalamus. The DLF’s ascending fibers synapse onto the hypothalamus and communicate visceral data to the brain. The ascending fibers travel from the reticular formation and transmit information to the hypothalamus.
Descending nerves travel from the hypothalamus to different brain structures. The fibers descend through the brain stem periaqueductal gray matter. This region is positioned on the floor of the fourth ventricle. The descending nerve fibers are destined to features tasked with autonomic processes, managing pain, and cardiorespiratory movements.
The medial forebrain bundle (MFB) connects the hypothalamus to the cerebral cortex. The medial forebrain bundle is a neural pathway that consists of nerve fibers originating from the basal olfactory area, the periamygdaloid site, and the septal nuclei. In addition, the MFB belongs to the brain stem areas.
The MBF travels through the lateral hypothalamus and the basal forebrain from the rostral to the caudal, and is made of ascending and descending neural fibers. The MBF is one of the main routes that join the limbic forebrain, midbrain, and hindbrain. The MBF functions are focused on pleasure sensations.
The cerebral cortex is the peripheral layer of the brain. This area relates to the more sophisticated aspects of human reasoning and higher levels of mental aptitude. The cerebral cortex is predominately grey matter- which is neural tissue and is approximately 14 -16 billion neurons.
This brain’s outermost region (the cerebral cortex) is marked by thin yet voluminous folds that comprise half the weight of the brain’s total bulk. The cerebral cortex’s crinkled form has protuberances called gyri and deep creases named sulci.
The Hippocampus, The Amygdala, And The Thalamus
The hypothalamus is attached to the hippocampus via the fornix. The fornix is a collection of nerve fibers formed in a c-shape. This serves as the primary output tract of the hippocampus and is a part of the limbic system. The hippocampus is a highly intricate and sophisticated brain structure situated deep in the temporal lobe. It’s responsible for learning and memory.
The hypothalamus is joined to the amygdala via the stria terminalis. The stria terminalis or terminal stria travels from the bed nucleus of the stria terminalis (BNST) to one of the eleven nuclei of the hypothalamus- the ventromedial nucleus of the hypothalamus.
The stria terminalis is the main pathway between the hypothalamus and amygdala, facilitating reciprocal transmissions between the two structures. The amygdala is a constituent of the limbic system and is known for stimulating fear. It sends trigger responses to the hypothalamus like fight-flight, raising the heart rate and respiration to anticipate action.
The hypothalamus is connected to the thalamus through the mammillothalamic tract. Brain stem nuclei compose the mammillary bodies. These bodies reside on the inferior posterior aspect of the hypothalamus.
Mammillary bodies are tiny, circular bodies and feature in the limbic system. These bodies formulate memories as they are connected to the hippocampus and assist in maintaining direction. They are also responsible for emotions and motivation.
The Retina- The Retinohypothalamic Tract And Hypothalamus
The retinohypothalamic tract connects the hypothalamus to the retina. The retinohypothalamic tract transmits information, about light, from the retina to the suprachiasmatic nuclei of the hypothalamus. The suprachiasmatic nuclei transform this information and synchronize the body with the solar circadian rhythms.
The Pituitary And The Median Eminence
The pituitary connects to the hypothalamus via the median eminence. Situated at the base of the hypothalamus, the median eminence is the point where hypothalamic-releasing and hypothalamic-inhibiting hormones engage the portal capillary system.
The median eminence is a specialized neural structure that consists of nerve terminals and glial cells. However, it’s almost entirely absent of synapses. It’s a small bump situated in a depression of the middle-ventral hypothalamus (tuber cinereum). It doesn’t possess a blood-brain barrier and has highly permeable capillaries (circumventricular organ).
The Hypothalamus, The Pituitary, And Hormone Secretion
The hypothalamus reestablishes balance via regulating hormone release from the pituitary gland. The pituitary gland comprises two lobes: the anterior pituitary (front) and posterior pituitary (back). The hypothalamus emits elements into the bloodstream known as releasing hormones.
The Anterior Pituitary And Releasing Hormones
Releasing hormones reach the anterior pituitary from the hypothalamus. The anterior pituitary lobe activates the hormones synthesized within and then releases them into the bloodstream. The pituitary doesn’t synthesize and release these hormones unremittingly. Mainly, these hormones are released in bursts every three hours.
Hormones released by the anterior pituitary from signals belonging to the hypothalamus include growth hormone (GH) or somatotropin. This hormone governs height, bone length, and muscle growth. In addition, it regulates body composition, fluids, sugar and fat metabolism, and cardiovascular function.
The luteinizing hormone (LH), or lutropin, and follicle-stimulating hormone (FSH), are both gonadotropins. The gonadotropin-releasing hormone directs the pituitary gland to release the necessary hormones to assist the functioning of the sexual organs.
The corticotropin-releasing hormone (CRH) assists metabolic regulation and strengthens the immune response. This hormone functions with the pituitary gland and adrenal gland to release steroids.
In addition, the anterior pituitary produces and releases adrenocorticotropic hormone (ACTH) in its corticotropin-producing cells (corticotrophs). ACTH is responsible for the production of cortisol and stress response.
The thyroid-stimulating hormone (TSH) or thyrotropin actives the metabolism in virtually all tissues belonging to the human body. The hormone prolactin or lactotropin is responsible for milk production.
The Posterior Pituitary And Non-Releasing Hormones
The hypothalamus is also tasked with producing non-releasing hormones. Hormones like oxytocin and vasopressin- are directed from the hypothalamus to the posterior pituitary. Vasopressin or antidiuretic hormone (ADH) regulates the amount of water absorbed by the kidneys as they filter waste from the blood.
The ADH is also called arginine vasopressin (AVP). The effects of this hormone include water retention, which involves blood dilution, increased blood pressure and volume, and lower blood osmolality.
If the body’s balance isn’t resumed, the brain will keep sending signals eliciting thirst. If there is a medical condition, symptoms include excessive thirst, frequent urination, high blood sodium levels, and dehydration.
Oxytocin facilitates several bodily processes, including regulating sleep cycles and moderating body temperature. This chemical messenger is also responsible for human behavior- like trust, compassion, and social bonding.
Oxytocin and its role in the brain are complex. Positive-feedback mechanisms regulate this hormone. Currently, research is focused on determining the function of oxytocin on addiction, depression, and post-traumatic stress.
Oxytocin stimulates the uterine muscles and increases prostaglandins production. Prostaglandins assist uterine contraction. Sometimes mothers are presented with oxytocin to assist and speed the birthing process. When the baby is born, oxytocin directs milk in the duct and increases bonding.
What Is The Limbic System?
The limbic system is comprised of a sophisticated network of several components, including the thalamus and hypothalamus. It operates from a region below the center of each cerebral hemisphere on the top of the brainstem. It’s situated within the cerebrum, directly below the temporal lobes, and deep beneath the cerebral cortex.
The limbic system is concerned with emotion, sexual arousal, learning, and memory formation. The limbic system is a coordination between the hippocampus, the amygdala, the hypothalamus, and part of the thalamus. The limbic system is also involved with the endocrine and autonomic nervous systems and varying stress responses.
The limbic system is also responsible for survival responses like fear (fight and flight) and anger. In addition, other survival responses involve pleasure, like eating and sex. The limbic system impacts the peripheral nervous system and the endocrine system.
The hippocampus and amygdala play a pivotal role in memory regarding the limbic system. These significant structures are tasked with storing memories and selective memory storage. It’s believed that storage is determined by the amount of emotional response attributed to it.
How The Hypothalamus Is Connected To The Limbic System
A portion of the forebrain, the diencephalon (mentioned above), is included in the limbic system. The diencephalon is found below the cerebral hemispheres, consisting of the hypothalamus and thalamus. The fornix, a band of white matter axons, connects the hippocampus to the hypothalamus.
The fornix is situated in the mesial aspect of the cerebral hemispheres and serves as the interface between various facets of the limbic system. It plays a prime role in cognition and episodic recall. White matter protects the fornix to some degree.
White matter resides in the subcortical regions- in the deeper tissues of the brain. It consists of nerve fibers (axons) which are an expansion of nerve cells (neurons). A significant number of these nerve fibers are covered in myelin sheathe.
These fibers act as a direct output tract of the hippocampus. The fornix connects to the mammillary bodies of the hypothalamus. From there, it links to the anterior nuclei of the thalamus. There remains doubt as to the full extent of the fornix regarding its role in the limbic system. It’s understood that the fornix supports memory recall more so than memory recollection.
Conclusion
The hypothalamus gets its name from its position below and in front of the thalamus. Its located directly above the pituitary gland. It lies above the brain stem and on both sides of the base of the third cerebral ventricle.
The location of the hypothalamus is relevant as it’s part of an immense and complex interworking of systems and networks. It plays a crucial role in homeostasis and hormones governing how the body maintains inner balance. The hypothalamus also enacts a pivotal role in the limbic system.
References
https://en.m.wikipedia.org/wiki/Hypothalamus
