FEEDBACK MECHANISM OF HORMONE
The hormones are responsible for regulating the internal body temperature. A feedback mechanism
tends to accelerate or inhibit the hormonal secretion. This loop is usually the negative feedback
mechanism among most of the hormones.
NEGATIVE FEEDBACK MECHANISM
1. It occurs when the original effect of the stimulus is reduced by the output.
2. Negative feedback mechanism normalizes the things when they start becoming too extreme. For eg.,
the thyroid gland is regulated by a negative feedback mechanism.
3.The hormone secreted by the hypothalamus stimulates the pituitary gland to release thyroid stimulating
hormone.
4. In turn, the thyroid stimulating hormone triggers the thyroid gland to release its hormones.
5. When the thyroid level increases, the hormones stop the secretion of TRH by the hypothalamus and
secretion of TSH by the pituitary gland by the feedback mechanism.
6. The secretion of hormones by the thyroid gland is inhibited without the stimulation of TSH.
7. The thyroid hormone levels fall very low.
8. The insulin secretion by the pancreas is also controlled by the negative feedback mechanism.
POSITIVE FEEDBACK MECHANISM
1. t occurs when the original effect of the stimulus is enhanced by the output.
2. he conditions become extreme in this mechanism.
3. Milk secreted by a mother’s mammary glands is one of the examples of positive feedback mechanism.
4. While suckling, the nerve sends signals from the nipple to stimulate the pituitary gland to secrete
prolactin. 5. In turn, prolactin triggers the mammary glands to secrete milk.
6. As the baby suckles more milk, more prolactin is secreted and more milk is produced.
STRUCTURE OF HYPOTHELAMUS
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.
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.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.
The lateral zone of the hypothalamus contains the following nuclei:
1.Part of the preoptic nucleus 2.Part of suprachiasmatic nucleus 3.Lateral nucleus
4.Tuberomammillary nucleus 5.Lateral tubular nuclei
The medial zone of the hypothalamus contains the following nuclei:
1.Part of the lateral nucleus 2.Part of suprachiasmatic nucleus 3.Anterior nucleus 4.Paraventricular
nucleus 5.Dorsomedial nucleus 6.Ventromedial nucleus 7.Infundibular nucleus 8.Posterior nucleus.
Structure of pineal gland: Fig:
1.The epiphysis cerebri or pineal gland is a reddish-grey approximately 5 – 8 mm long
2. Pine cone-like structure that is located in the diencephalic part of the prosencephalon(forebrain).
3.The gland was formed as an outward growth of the roof of the third ventricle. Therefore, the gland rests
between the posterior aspects of the thalami as it projects posteriorly from the wall of the third ventricle.
4. Its attachment to either half of the brain is by the Habenular commissure and trigone superiorly, and the
posterior commissure inferiorly.
5.The Habenular and posterior commissures are a part of the pineal stalk. The Habenular commissure is a
part of the superior lamina of the stalk, while the posterior commissure is a part of the inferior lamina.
6.The space between the laminae is known as the pineal recess.
7. It communicates anteriorly with the hypothalamic sulcus and the third ventricle.
8. It has two essential components one is hormone-producing cells called Pinealocytes and the other is
supporting cells that transmit information called glial cells.
8.The pineal gland often appears calcified in x-rays, which is usually due to fluoride, calcium, and
phosphorus deposits that build up with age.
Histology :-
The pineal gland is encased by pia mater and lobulated by its connective tissue septae that projects into
the gland. Within the epiphysis cerebri, there are pinealocytes and neuroglia
cells.The pinealocytes account for approximately 95% of the cellular content of the gland. They are
irregularly shaped with peripheral processes, and lightly staining large round nuclei. Pinealocytes are
primarily concerned with the photo-regulated production of melatonin. This hormone works with the
body’s circadian rhythm (which is controlled by the suprachiasmatic nucleus of the hypothalamus) to
regulate the cycle of sleep and wakefulness. Additionally, some researchers believe that melatonin may
alter sexual development in humans, contribute to thermoregulation, and cellular metabolism. There are
also the corpora arenacea (brain sand) bodies present within the gland. Calcification of these bodies is a
common occurrence with increasing age. As a result, they appear as radiographic opacities on plain film
radiography and can, therefore, be used as landmarks.
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Function of Pineal gland
Secretion Of Melatonin :-
This gland secretes the hormone melatonin which regulates the circadian rhythm of the body and also
certain reproductive hormones. The secretion of this hormone depends upon the amount of light a person
is exposed to. This hormone is produced in large amounts when it is dark. The presence of light inhibits
the secretion of melatonin which controls our circadian rhythms.
Cardiovascular Health :-The melatonin secretion has a positive impact on the heart and blood
pressure.It may also be used for the treatment of cardiovascular diseases.
Reproduction :-Melatonin inhibits the secretion of reproductive hormones from the anterior pituitary,
which are responsible for the development and functioning of reproductive organs.
Hormones Secreted by Pineal Gland– Melatonin
The pineal gland synthesises melatonin and serotonin, hence they are also called as the Pineal Gland
Hormone. The pineal gland also produces neurosteroids.
Serotonin is the precursor of melatonin. Serotonin is acetylated and methylated to yield melatonin within
the pineal gland. The light exposure to the eyes affects the synthesis and secretion of melatonin.
Two melatonin receptors have been found in mammals- Mel1A and Mel1B. These are G-protein coupled
cell surface receptors. pMelatonin affects circadian rhythm. Our circadian rhythm is a 24-hour biological
cycle, characterised by the sleep-wake patterns. The daylight and darkness regulate our circadian rhythms.
The secretion of melatonin is stopped on exposure to light which in turn controls the circadian rhythm.
The secretion of melatonin is high during dark and low during daylight. This influences our reaction to
photoperiod.
The secretion of gonadotropins from the anterior Pituitary gland is blocked by melatonin thereby affecting
reproduction. These hormones aid in the development of ovaries and testes.