Hypothalamus - Anatomy & Physiology

()Map ENDOCRINE SYSTEM (Map)

Canine Brain Cross Section Courtesy of BioMed Image Archive

Hypothalamus

The hypothalamus is a small area in the ventral diencephalon of the forebrain, in the floor of the third ventricle, and is a functional link between the nervous and endocrine systems.

The hypothalamus controls most of the endocrine glands within the body, largely through stimulation of the Pituitary Gland by secretion of neurohormones. It is a vital regulator of homeostasis, including Thermoregulation.

Nuclei

Nuclei	Function
Paraventricular	Production of Oxytocin and ADH
Supraoptic	Production of Oxytocin and ADH
Suprachiasmatic	Circadian Rhythm Biological Clock
Lateral	Arousal
Arcuate	Energy
Mammilary	Wakefulness
Ventromedial	Feeding centre
Dorsomedial	Feeding, drinking and regulates body weight

Inputs

The hypothalamus recieves direct inputs via specific receptors. It also recieves indirect inputs from the blood and nerves.

Direct Inputs via Receptors:

• Thermoreceptors: Neurons in the anterior hypothalamus respond to heat by initiating peripheral vasodilation and sweating. Neurons in the posterior hypothalamus respond to cold by initiating shivering, piloerection and peripheral vasoconstriction. These mechanisms are part of the Thermoregulation system.
• Osmoreceptors: respond to an increase in blood osmolarity by releasing Antidiuretic Hormone (ADH) from the Supraoptic Nucleus which is then secreted by the posterior pituitary and acts to retain water at the kidneys. It also stimulates neurons in the thirst centre in the lateral hypothalamus in an attempt to increase water intake.
• Energy Balance: Neurons in the arcuate nucleus sense blood glucose and hormones. Leptin causes satiety, ghrelin stimulates appetite.

Indirect Inputs from bloodstream include information about:

• Temperature
• Osmotic Pressure
• Hormone concentrations - including Leptin, Cortisol, Oestrogens, Progesterone, Androgens, T3.

Indirect Neural Inputs:

• Uses Visceral and Somatic Sensory neurons, the Limbic System and the Reticular Activating System.

Outputs

1. Biological Clock - Light sensed by retina causes stimulation of neurons leading the the suprachiasmatic nucleus which stimulates the Pineal Gland as a result.
2. Secretory Neurons - ADH and Oxytocin are released by the Supraoptic and Paraventricular nuclei cell bodies. The axons descend into the Posterior Pituitary gland, where they terminate in blood vessels releasing the hormone directly into circulation. Thus the posterior pituitary acts as a storage site and is not a true endocrine gland.
3. Hypothalamic Hormones - the hypothalamus releases hormones which have an activating or inhibitory effect on their target organ, hence they are named Releasing or Inhibitory Hormones respectively.

Releasing Hormones:

• Thyrotropin Releasing Hormone (TRH)
• Growth Hormone Releasing Hormone (GH-RH)
• Gonadotrophin Releasing Hormone (GnRH)
• Corticotropin Releasing Hormone (CRH)
• Prolactin Releasing Hormone (PRL-RH)

Inhibitory Hormones:

• Growth Hormone Inhibiting Hormone aka Somatostatin (GH-IH)
• Gonadotrophin Inhibiting Hormone (GnIH)
• Dopamine (PRL-IH)

Negative Feedback

The Hypothalamus works along with the Pituitary and it's target glands by negative feedback.

Schematic Diagram of the Hypothalamic Pituitary Axis