Hypothalamus

General characteristics and morphology

The hypothalamus is a crucial structure of the diencephalon, lying beneath the optic thalamus. Along with the hypophysis and pineal gland, it forms the highest level of control over the body's endocrine processes.

The structural and functional unit of the hypothalamus is the neurosecretory nucleus — a localized cluster of specialized neuron cell bodies. It is important to distinguish this concept from a cell nucleus (organelle). Histologically, neurosecretory cells are oval-shaped, containing pale cell nuclei and secretory granules in the cytoplasm.

All hypothalamic nuclei are subdivided into two major functional groups: magnocellular and parvocellular. Each of these groups has its own anatomical landmarks, targets, and secretes a specific set of hormones.

Magnocellular nuclei

This group of nuclei is located in the anterior hypothalamus and is formed by large neurosecretory cells. Their activity is regulated by cholinergic stimulation, as well as signals from the limbic system and hippocampus.

Main representatives:

• Supraoptic nuclei: lie directly above the optic chiasm (chiasma opticum).
• Paraventricular nuclei: located in the lateral walls of the third cerebral ventricle.

Function and hormone transport: The cells of these nuclei synthesize peptide hormones — antidiuretic hormone (ADH, also known as vasopressin) and oxytocin (or their polypeptide precursors). Hormones do not enter the blood immediately. They travel down the long axons of the cells directly into the posterior lobe of the hypophysis (neurohypophysis). There, the axons form axovasal synapses on capillaries. The posterior lobe of the hypophysis acts as a neurohemal organ — a site where hormones are ultimately secreted into the systemic circulation.

This anatomical and functional connection is called the hypothalamo-neurohypophysial system.

Parvocellular nuclei (tuber cinereum region)

The parvocellular nuclei are localized in the middle part of the hypothalamus — in the region of the tuber cinereum (tuber cinereum), which forms the floor of the third ventricle. These include the arcuate (infundibular), ventromedial, and dorsomedial nuclei. Together, they form the arcuate-ventromedial complex, which arches around the infundibulum of the brain (pituitary stalk).

Neurons of this complex respond to adrenergic and dopaminergic signals from higher CNS centers. They produce regulatory releasing factors:

1. Liberins — stimulate the production of tropic hormones in the hypophysis.
2. Statins — inhibit the production of tropic hormones.

Pathway of hormone excretion: The axons of parvocellular nuclei run toward the inferior surface of the hypothalamus into the median eminence, which serves as a neurohemal organ for this system. Here, the axons form contacts with the primary fenestrated capillaries of the hypophyseal portal system.

The blood-tissue barrier in this zone has specific features: the basement membrane is split into two layers. The hormone is released from the axon into the pericapillary space, passes through the outer layer, and enters the capillary lumen. Further via portal veins, the secretion reaches its main target — the anterior and intermediate lobes of the hypophysis (adenohypophysis). The stroma of the median eminence also contains specialized glial cells of ependymal origin — tanycytes. They contact synapses and presumably regulate the rate of hormone entry into the bloodstream. The totality of these structures is called the hypothalamo-adenohypophysial system.

Types of hypothalamic regulation

The hypothalamus controls the endocrine system via two main pathways:

• Transhypophyseal pathway: This is the primary pathway. In this case, the hypothalamus controls peripheral endocrine glands indirectly, through control over the adenohypophysis (using liberins and statins).
• Parahypophyseal pathway: Exerts a direct effect on the periphery, bypassing the hypophysis. Axons of certain hypothalamic nuclei go directly to the centers of the autonomic nervous system, providing direct neural control of endocrine glands.

Briefly: answers to topical questions

What else to study on this topic

• Structure of the blood-brain barrier in the median eminence
• Portal blood supply system of the hypophysis
• Cytological features of neurosecretory cells under a microscope
• Mechanism of axonal transport of hormones
• Interaction of the hypothalamus with the limbic system

Other topics in the section «Nervous system: brain»

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