Anatomy

The hypothalamus is split into three zones: The periventricular, the medial and the lateral zone.

Each zone consists of different nuclei where most of the nuclei are named after their location within the hypothalamus eg. periventricular nucleus

These nuclei have different functions involved with regulating inputs and outputs of endocrine and autonomic function. This is done by the nuclei communicating with other parts of the brain and body through electrical stimuli and chemical release.

Below is a lateral view of the hypothalamus illustrating all the different nuclei involved in these processes .

An illustration to show all the various nuclei located within the hypothalamus. Image courtesy of: www.flickr.com/photos/furmanborst/3468708975/

Pathways

The hypothalamus receives numerous inputs from nerves or other nuclei within the brain. It can then relay output signals to other parts of the body by balancing inhibitory and excitatory neurotransmitter release from individual nuclei in order to provide the substantial output required to return the body to its milieu interior.

Inputs

Reticular formation- is found within the brain stem where it relays information from the spinal cord about body temperature.

Solitary tract- receives visceral sensory information from the vagus nerve. This information is concerns with blood pressure and gut digestion.

The retina- relays sensory information to the optic nerve and furthermore onto the suprachiasmatic nucleus regulating circadian rythm.

Circumventricular organs- located next to the ventricles detect a change in osmolarity in the blood.

Outputs

The hypothalamus responds to these inputs by secreting hormones via two main pathways: the Endocrine system and the autonomic nervous system. 

The endocrine system is mediated by another structure linked to the hypothalamus called the pituitary gland which has been formentioned under the subheading of development.

The hypothalamus regulates the action of the pituitary by secreting neurotransmitters known as releasing hormones, which are hormones whose main purpose is to control the release of another hormone. This is done via two main pathways: The parvocellular and the magnocellular pathways.

 Parvocellular pathway

This pathway involves the secretion of a releasing hormone into the the adenohypophysis where it travels through the infundibulum via the median eminence, here it is released into the blood stream of the hypothamalamo-pituitary portal system (see fig at bottom of page). The blood vessels in this system then transport the neurotransmmitter to the anterior pituitary gland where it regulates the secretion of subsequent hormones into the systemic circulation which can furthermore be released into the rest of the body's blood stream.

Source taken from: Roberto Osti/The Scientific American

Corticotropin releasing hormone (CRH) is a preprohormone secreted from the suprachismatic nucleus in response to stress. It induces the production of ACTH (adreno cortico-thyroid hormone) in the anterior pituituitary gland (adenohypophysis) which is a more stable hormone released in the blood stream causing an increase in androgens. 

Thyrotropin-releasing hormone (TRH) is secreted from the paraventricular nucleus and also travels via the parvocellular pathway in much the same way as the CRH. This hormone stimulates the release of the more stable hormone: TSH and prolactin.

Thyroid hormones are involved in growth and development and increased metabolism where as prolactin is a hormone secreted to stimulate the mammary glands to fill with milk in response to an infant suckling.

Growth hormone releasing hormone (GhRh) is produced in the arcuate nucleus and is released into the blood stream to the anterior pituitary also by the parvocellular pathway. This hormone stimulates the production of growth hormone from the anterior pituitary.

Picture adapted from: Neuroscience exploring the brain

Magnocellular neurons

This pathway involves axons projecting from the periventricular nucleus and the supraoptic nucleus to the infundibulum via the median eminence directly into the posterior pituitary (neurohyphysis). The axons in this pathway are the largest of the neurosecretory cells within the hypothalamus.

Oxytocin is the hormone which causes the uterus to contract facilitating birth and controls the release of milk from the mammary glands after the hormone prolactin. After it has been released from the paraventricular and supraoptic nuclei from the hypothalamus, it is then stored in the axon termini within the posterior pituitary awaiting stimuli for its release into the blood circulation there.

Vasopressin is released and stored from the hypothalamus in much the same was as oxytocin. It's main function is to control the retention of water and regulates salt concentration at the same time. Salt concentration sensitive cells within the hypothalamus detect this change and secrete vasopressin preventing water from being excreted from the kidneys, thus providing another example where the hypothalamus is indirectly connected to an organ within the body in order to maintain milieu interior.

The Autonomic nervous system

 Involves regulating heart rate, sweating, digestion, breathing rate,vasoconstriction/vasodilation. This is governed by the neurons located in the periventricular zone  as well as medial and lateral hypothalamus which are connected via interneurons to the lateral medulla. It is the lateral medulla that contains the nuclei which control the autonomic nervous system. 

The parasympathetic nervous system's impulses derive from the vagal nuclei located within the medulla, on the other hand the sympathetic nervous system has a group of cells located in the medulla which then descend along the spinal cord, transmitting excitatory signal to the rest of the body.

A video can be viewed summarising the pathways on the left hand side named "How the body works: Anatomy of the hypothalamus"