Hypothalamus

Ashley Davidoff MD

Definition

The hypothalamus is located below the thalamus and above the and behind the pituitary gland and above the brainstem. It is approximately the size of an almond. It is composed of groups of nerve cells that are densely packed that function collectively as a single entity. The hypothalamus regulates internal homeostasis of water and electrolytes, hormonal function and body temperature. It is intimately related to the function of the autonomic nervous system and the function of the endocrine system through its connections with the pituitary gland. Lastly it is asociated with basic feelings of hunger, thirst, and rage.

The hypothalamus is part of the autonomic nervous system’s central controlling mechanism.

Structurally it is situated below the thalamus, on either side of the third ventricle and consists of periventricular nuclii, paraventricular nuclii, and medial and lateral nuclii.

From a functional standpoint it receives input from the the vagus nerve, reticular formation, retina, limbic and olfactory systems, processes the information and sends output to the autonomic nervous system via the vagus and descending tracts in the spinal cord and subsequently to the autonomic ganglia.

Relay reflexes for smell primitive

Regulator of homeostasis

Hormone control

ANS

Emotion aggression thirst hunger pain rhythms wakefulness

The hypothalamus also sends signals to the posterior pituitary causing the release of oxytocin and vasopressin, and to some extent causes teh anterior pituitary to release ACTH, and thyroid stimulating hormone.

Functionally it has similar traits to the limbic system in that it is responsible for some basic instinctual functions such as hunger thirst, response to pain, pleasure, sexual feelings, anger, aggression. Since it controls the autonomic nervous system, it will therefore also be involved in visceral responses including heart rate and pressure as described above.

The Thalamus (teal green))

Part of the Forebrain (Prosencephalon)

Member of the Group of Thalamic Structures (Diencephalon)

Member of the Group of Thalamic Structures

The basic and simplest classification of the brain into forebrain midbrain and hindbrain is shown in this diagram and advanced to a more complex tree using the embryological and evolutionary terminologies. The forebrain consists of the cerebrum also called the prosencephalon, which contains the more advanced form of the brain and the thalamic structures which contain more basic structures. The cerebrum (telencephalon) itself consists of two cerebral hemispheres and paired basal ganglial structures. Each cerebral hemisphere will have gray and white matter distributed in the frontal parietal temporal and occipital lobe, with the basal ganglia being part of the gray matter deep in the cerebral hemispheres. The most important thalamic structures arising from the diencephalons include the thalamus itself and the hypothalamus. The midbrain (mesencepaholon) consists of the tectum tegmentum and cerebral peduncles. The hindbrain has two major branch points based on the evolutionary development. The pons and cerebellum(part of the metencephalon) are grouped and the medulla (part of the myelencephalon is the second branch.

The Hypothalamus in the Context of the Limbic System

Organizing Concept Overlying Approxiamte Locations

The sagittal T2 weighted shows the limbic system which is a compound set of structures including the cingulate gyrus, (light green) cingulate cortex, hypothalamus (light blue) mamillary body (royal blue) fornix (darker green) hippocampus (darker green) amygdala (yellow) and thalamus (orange).

The Hypothalmus Deep in the Brain in Close Association with the Inferior Aspect of the 3rd Ventricle

This coronal section through the forebrain reveals the more superficial cortical structures that include the frontoparietal part of the cerebral hemispheres and the temporal pole(TP). In the deeper portions of the brain and centrally positioned the corpus callosum (unlabelled white matter (black) that crosses the midline is appreciated. The fornix (F) is also seen as a midline structure just below the corpus callosum. The foramina of Munro (unlabelled) lead into the 3rd ventricle (3V). The hypothalamus (Hyp) follows and then the optic chiasm (OC). Alongside these central deep structures and lateral to the central structures, starting superiorly, the lateral ventricles (LV) are seen as two symmetrically positioned white structures. The anterior nucleus (A) and stria medullaris thalami (SM) lie alongside the third ventricle. Lateral to these structures and starting superiorly the caudate nucleus body (Cau) ventral anterior nucleus (VAN) internal capsule (IC) globus pallidus(GP), putamen (Put) amygdala (Am) and uncinate fasciculus (UF) are noted.

Courtesy Department of Anatomy and Neurobiology at Boston University School of Medicine Dr. Jennifer Luebke , and Dr. Douglas Rosene 97346.C7.7L01

Hypothalamus in Coronal Projection

The hypothalamus is centered in the midline around the third ventricle. The coronal image of a T2 weighted MRI shows the third ventricle in the midline (white) immediately surrounded by a the thinnest layer of paraventricular component, which in turn is surrounded by a slighly thicker layer of periventricular component (orange followed laterally by a pair of medial nuclii and then a single larger lateral component. The image below is a magnified version to enable you to appreciate the two inner layers better.

Hypothalamus Magnified

The slightly magnified view of the coronal image of a T2 weighted MRI shown above again shows the third ventricle in the midline (white). The thinnest inner layer is a mere pencil thin yellow line (paraventricular layer) and is barely seen even in this magnified view. The periventricular layer surrounds it. (orange)

In the setting of acute pain the usual response of the hypothalamus and autonomic nervous system is to increase the sympathetic tone, (“fight or flight”) unless the stimulus is so intense that a vagal response is induced. With increased sympathetic tone the patient initially increases heart rate and pressure, and becomes pale, sweaty, associated with dilated pupils .

Applied Biology

Glioma of the Hypothalamus

The T1 weighted MRI in sagittal(a,b) and coronal (c,d) projections in a 2 year old patient show an 8mm nodule (green) in the hypothalamus. This was shown to be a glioma of the hypothalamus.

References

Saloman Excellent Detailed Slide Show review of Radiology of Hypothalamus and Basal Ganglia

Control Center