How does the hypothalamus regulate the autonomic nervous system?
The hypothalamus performs the highest-level correction of the autonomic nervous system (ANS) activity. Its regions exert opposite effects on the body:
- Rostral (anterior) division: is responsible for parasympathetic (trophotropic) activity. Its stimulation promotes rest and recovery. Blood pressure (BP) and heart rate (HR) decrease, and tidal volume diminishes. Concurrently, GI motility, salivation, and bladder contractility increase. The pupils constrict and metabolism slows down.
- Caudal (posterior) division: triggers sympathetic (ergotropic) activity. It prepares the body for stress and physical exertion. BP, HR, and respiratory rate increase. Blood supply to skeletal muscles and lungs is enhanced, while capillary blood vessels in the gastrointestinal tract constrict. Motility is suppressed, urine is retained, and blood levels of glucose and adrenaline rise. Pupils and palpebral fissures dilate.
Neuroendocrine regulation and hypothalamic nuclei
The hypothalamus contains up to 50 pairs of nuclei and is functionally connected to the pituitary gland via the hypothalamo-hypophysial system. Two secretory systems are distinguished:
- Magnocellular nuclei (anterior hypothalamus): these include the supraoptic (above the optic chiasm) and paraventricular (in the lateral wall of the third ventricle) nuclei. They are formed by large neurosecretory cells that synthesize peptide hormones — antidiuretic hormone (ADH, vasopressin) and oxytocin. These hormones travel down axons to the posterior pituitary (neurohypophysis), where they enter the bloodstream.
- Parvocellular nuclei (medial hypothalamus): are located in the region of the tuber cinereum (tuber cinereum), for example, the arcuate nuclei. They produce hypophysiotropic hormones that control the anterior pituitary (adenohypophysis). Neurosecretion is released into the hypophysial portal system. These hormones include releasing hormones (releasing factors) (stimulate the synthesis and secretion of adenohypophyseal hormones, e.g., corticotropin-releasing hormone and thyrotropin-releasing hormone) and inhibiting hormones (inhibiting factors) (suppress the synthesis and secretion of glandotropic hormones, e.g., somatostatin and prolactin-inhibiting factor — dopamine).
Centers of biological motivations and emotions
The hypothalamus is the center of biological motivations: hunger and thirst, sexual drive, and it also contains temperature centers as well as sleep-wake centers.
- Regulation of eating behavior: the sensation of hunger is triggered by a drop in plasma glucose and gastric muscle contractions during food evacuation and stomach emptying. Appetite stimulants are produced in the hypothalamus: GABA, dopamine, $\beta$-endorphin, enkephalins. Appetite inhibitors that create a sense of satiety include serotonin, norepinephrine, cholecystokinin, and somatostatin.
- Emotions and the heart: stimulation of negative emotionogenic centers in the hypothalamus causes severe cardiac disturbances — arrhythmias and premature beats. Stimulation of positive emotionogenic centers causes similar, but less pronounced, changes. If the stimulation of negative centers is followed by the stimulation of positive ones, cardiac disturbances are virtually absent.
- Memory: the hypothalamic pacemaker plays a critical role in the formation of memory engrams. Learning and engram formation occur through the interaction of a motivational state and reinforcing factors. Complete disappearance of a memory engram is observed only when the activity of the hypothalamic pacemaker is suppressed.
Physiological features of neurons
Physiological features of hypothalamic neurons:
- Absence of the blood-brain barrier: ensures direct contact of neurons with the blood.
- High sensitivity to humoral factors: neurons respond to changes in the partial pressure of oxygen and carbon dioxide, environmental pH, sodium and potassium ion concentrations, as well as catecholamine levels.
- Presence of glucoreceptors: they are found in the ventrobasal and lateral nuclei.
Clinical significance
Knowledge of hypothalamic hormones is applied in pharmacology. For example, a stable octapeptide analog of somatostatin — octreotide — has been synthesized. It suppresses growth hormone secretion, as well as exocrine pancreatic secretion and gastrointestinal peptide secretion. The drug is used in endocrinology for the treatment of acromegaly and gigantism, in gastroenterology for acute pancreatitis, and to stop bleeding from esophageal varices.
Class and exam questions
How is the hypothalamus connected to the posterior pituitary?
The connection is mediated through the axons of magnocellular nuclei — the supraoptic and paraventricular nuclei. Vasopressin and oxytocin are transported via these axons into the neurohypophysis and released into the blood.
What factors induce the sensation of hunger at the hypothalamic level?
Hunger is triggered by a decrease in plasma glucose, gastric muscle contractions during food evacuation and stomach emptying, as well as the subjective sensation of an «empty stomach». The production of appetite stimulants increases in the hypothalamus: GABA, dopamine, β-endorphin, and enkephalins.
Why can the hypothalamus directly respond to blood composition?
The blood-brain barrier is absent in the hypothalamic region. This ensures direct contact of neurons with blood and high sensitivity to pO₂, pCO₂, pH, catecholamine content, and Na⁺ and K⁺ concentrations.
What else to study on this topic
- Pituitary gland
- Autonomic Nervous System
- Limbic system
- Thalamus
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