Hypothalamus

Hypothalamus is a very important part of the central nervous system present in the forebrain. It controls the firing of the autonomic nervous system as well as the functioning of the endocrine system. Thus, it plays a central role in controlling all the essential processes of life.

In this article, we will discuss the location, anatomical features, functions and important pathologies of the hypothalamus. This will greatly increase your knowledge about hypothalamus.

Anatomical Features

In this section, we will discuss the location,
structure, input and output fibers and blood supply of hypothalamus.

Location

According to the typical division of the brain into the forebrain, midbrain, and hindbrain, the hypothalamus is a part of the forebrain. It is considered to be a part of the diencephalon.

Hypothalamus is located just below the thalamus and forms the floor and the lower part of the lateral walls of the third ventricle. Anteriorly, it extends up to the optic chiasma and posteriorly it is continuous with the tegmentum of midbrain.

Structure

The structure of the hypothalamus is composed of a cluster of neurons that are arranged into nuclei. These nuclei send and receive fibers to other parts of the brain. For the purpose of understanding, the nuclei are divided into two groups; medial zone and lateral zone.

Lateral zone

The lateral zone of the hypothalamus contains the following
nuclei:

• Part of the preoptic nucleus
• Part of suprachiasmatic nucleus
• Lateral nucleus
• Tuberomammillary nucleus
• Lateral tubular nuclei

Medial zone

 The medial zone
of the hypothalamus contains the following nuclei:

• Part of the lateral nucleus
• Part of suprachiasmatic nucleus
• Anterior nucleus
• Paraventricular nucleus
• Dorsomedial nucleus
• Ventromedial nucleus
• Infundibular nucleus
• Posterior nucleus

The nuclei such as the preoptic nucleus, suprachiasmatic nucleus and the mamillary nuclei are present in both zones of the hypothalamus. The functions of these nuclei will be discussed in the subsequent section of this article.

Communications of Hypothalamus

Hypothalamus communicates with the rest of the body
via three routes:

• Bloodstream or Endocrine
  connections
• Nervous connections
• Cerebrospinal fluid

A little detail of these routes is given below.

Nervous connections

The nervous connections can be divided into afferent
and efferent fibers.

Afferent fibers

Hypothalamus receives afferent fibers carrying somatic
and visceral sensations as well as from special senses. Following are the
important afferents of hypothalamus:

• Somatic
  and visceral afferents via lemniscal afferent fibers and nucleus
  of tractus solitarius, that reach the hypothalamus via reticular formation
• Visual
  afferents from the optic chiasma reach the suprachiasmatic nucleus
• Olfactory
  afferents are received through medial forebrain bundle
• Auditory
  afferents though not identified completely but are influenced by the
  hypothalamus
• Hippocampo-hypothalamic
  afferents reach via fornix to mamillary bodies
• Tegmental
  fibers from midbrain
• Thalamo-hypothalamic
  fibers from the midline and dorsomedial nuclei of the thalamus
• Amygdalo-hypothalamic
  fibers from the amygdaloid complex reach the hypothalamus via stria terminalis

Efferent fibers

The efferent connections of hypothalamic nuclei are
also complex and numerous. Here, we will mention some important connections.

• To
  brain stem and spinal cord: The hypothalamic nuclei send
  efferent fibers to nuclei present in the brainstem and spinal cord. In this
  way, they control the autonomic nervous system.
• Mammillothalamic
  Tract: This tract consists of fibers arising in the mamillary
  body and terminating in the anterior nucleus of thalamus.
• Mammillotegmental
  Tract: These fibers terminate in the reticular formation,
  present in the tegmentum of the midbrain.
• Limbic
  System: The nuclei in the hypothalamus also send efferent
  fibers to the various nuclei of the limbic system.

Endocrine Connections

Hypothalamus uses the bloodstream to communicate with the pituitary gland. These connections of the hypothalamus are called the bloodstream or endocrine connections.

The cells of the pituitary gland release hormones in response to the regulating factors or hormones released by the hypothalamus. These regulatory factors reach the pituitary gland via the hypophyseal portal system of veins.

Hypothalamus also uses nervous connections to communicate with the pituitary gland. The cell bodies of these neurons are present in the hypothalamus and the axonal terminals in the posterior pituitary gland.

These neurons synthesize the oxytocin and vasopressin hormones, which are stored in the axonal terminals in the posterior pituitary and are released on demand.

Blood Supply of Hypothalamus

Hypothalamus receives blood mainly from the hypophyseal artery, a branch of the anterior cerebral artery. All the blood from the hypothalamus is drained into the hypothalamohypophyseal system of veins and distributed to the pituitary gland. From the pituitary gland, the blood is drained via the hypophyseal vein.

Functions of Hypothalamus

As said earlier, the hypothalamus controls almost every process of life. Here, we will discuss some important functions of the hypothalamus.

Autonomic Control

The most important function of the hypothalamus is to integrate the endocrine system and the autonomic nervous system. Hypothalamus acts as a higher center for controlling the autonomic functions of the brain stem and spinal cord.

The stimulation of the posterior and lateral nuclei of
the hypothalamus has been shown to cause a sympathetic response. On the other
hand, the stimulation of the anterior nucleus and the preoptic area influences
parasympathetic responses in the body.

Endocrine Control

Hypothalamus produces releasing factors or inhibitory factors for controlling the hormones released by the pituitary gland. These factors include:

• Growth hormone-releasing hormone
  and inhibiting hormone also called somatostatin
• Prolactin releasing hormone and
  inhibiting hormone
• Corticotropin-releasing hormone
• Thyrotropin-releasing hormone
• Luteinizing hormone-releasing
  hormone

These factors promote or inhibit the release of hormones from the anterior pituitary.  The release of these factors from the hypothalamus is controlled by positive and negative feedback mechanisms depending on the levels of a particular hormone in blood.

Secretion of Hormones

Hypothalamus not only secretes the regulating factors but also secretes two important hormones; vasopressin and oxytocin. Although these hormones are released from the posterior pituitary, they are actually produced by the neurons in the hypothalamus and are stored in the axonal endings present in the posterior pituitary.

Temperature Regulation

Temperature regulation is another important function of the hypothalamus.

The anterior part of the hypothalamus controls
processes that dissipate heat from the body. Its stimulation causes dilation of
blood vessels and sweating, which causes a decrease in body temperature.

Contrary to this, stimulation of the posterior part of
the hypothalamus results in vasoconstriction of the skin blood vessels and
inhibition of sweating resulting in conservation of body temperature.

Regulation of Food and Water Intake

Hypothalamus is also the site of the hunger center and satiety center in the brain.

The stimulation of the lateral region of the hypothalamus
stimulates hunger and results in the intake of food. This region is termed as
hunger center.

On the other hand, stimulation of the medial region of
the hypothalamus inhibits eating and results in reduced food intake. This is termed as the satiety
center of the brain.

Controlling Emotions and Behavior

Being a part of the limbic system hypothalamus also controls the emotions and behavior of a person. It is believed that the hypothalamus integrates all the afferent information from other areas of the brain and brings about the physical expression of emotion.

Stimulation of the lateral area of the hypothalamus is
associated with the feelings of rage whereas the stimulation of the medial area
results in feelings of passivity.

Clinical Significance

As stated earlier, the hypothalamus controls almost all the processes in the body. The lesions of the hypothalamus may be due to inflammation, tumor or vascular disorder. It may also be compressed by the tumors of the surrounding region. Any lesion of the hypothalamus can result in the following clinical disorders.

Obesity and Wasting

Severe obesity can occur in case of hypothalamic
lesions associated with genital hypoplasia and atrophy.

Sexual disorders

In children, hypothalamic lesions can cause sexual
retardation. After puberty, they can result in impotence or amenorrhea.

Hyperthermia and Hypothermia

Depending on the part of hypothalamus damaged by a lesion, hypothalamic lesions can result in hypothermia or hyperthermia.

Diabetes Insipidus

It results from the lesion of the suprachiasmatic nucleus present in the hypothalamus. It may also be due to the destruction of nervous pathways from the hypothalamus to the posterior pituitary.

Emotional disturbance

Hypothalamic lesions can also cause emotional disturbances that can result in unexplained attacks of weeping, laughter, uncontrollable rage, excessive maniac outbursts and depressive reactions etc.

Conclusion/Summary

Hypothalamus is an important part of forebrain located just below the thalamus.

It consists of groups of neurons that are divided into
two-zone, lateral zone and medial zone.

Both zones have their own nuclei. However, nuclei like
the suprachiasmatic nucleus and preoptic nucleus overlap both the zones.

Hypothalamus receives afferent fibers from almost all regions of the body and brain. It receives somatic and visceral sensory afferents, visual and auditory afferents, afferent fibers related to smell, emotions and feelings.

It sends efferent fibers to the brainstem and spinal
cord, thalamus, tegmentum, and the limbic system. It is also considered to be a
part of the limbic system.

Hypothalamus communicates to the rest of the body via
nervous and endocrine connections and cerebrospinal fluid.

The important functions performed by hypothalamus
include:

• Control of autonomic nervous system
• Control of hormones released by the
  anterior pituitary
• Secretion and synthesis of
  Vasopressin and Oxytocin
• Control of body temperature
• Regulation of food and water intake

Hypothalamic lesions can result in following clinical
disorders:

• Weight loss and wasting
• Sexual disorders
• Emotional disorders
• Diabetes insipidus
• Hypothermia or hyperthermia

References

1. Dr. Boeree, C. George. "The Emotional Nervous System". The Limbic System. Retrieved 18 April 2016.
2.  "NCI Dictionary of Cancer Terms". National Cancer Institute.
3. Melmed, S; Polonsky, KS; Larsen, PR; Kronenberg, HM (2011). Williams Textbook of Endocrinology (12th ed.). Saunders. p. 107. ISBN 978-1437703245