Parts of Thalamus
Metathalamus (Part of thalamus)
The metathalamus consists of the medial and lateral geniculate bodies which are situated on each side of the midbrain, below the thalamus.
The epithalamus occupies the caudal part of the roof of the diencephalon and consists of
The right and left habenular nuclei, each situated beneath the floor of the corresponding habenular trigone.
The pineal body or epiphysis cerebri
The habenular commissure
The posterior commissure.
The pineal gland controls endocrine functions that depend on the length of the day. In lower vertebrates, the pineal gland contains photoreceptors and directly measures the length of a day by the amount and duration of light it receives. In mammals (and birds) there are no photoreceptors in the pineal gland, but it does receive input from the visual system.
In the dark the pineal gland secretes melatonin. As days get longer in the spring melatonin secretion decreases, which, in many mammals, induces fertility in both males and females. Recent evidence also suggests that melatonin levels affect immune responses
The nuclei of the habenula receive input from the strip medullary of the thalamus and project out through the habenula interpeduncular tract. This is a rarely discussed pathway that seems to connect the limbic system and the reticular formation of the brainstem. This connection suggests a role in mediating the influence of the limbic system on arousal states.
The pineal gland is composed of two types of cells. pinealocytes and neuroglial cells, with a rich network of blood vessels and sympathetic fibers. The vessels and nerves enter the gland through the connective tissue septa which partly separate the lobules. Sympathetic ganglion cells may be present.
Calcareous concretions are constantly present in the pineal after the 17th year of life and may form aggregations. Spaces or cysts may also be present.
The pineal body has for long been regarded as a vestigial organ of no importance. Recent investigations have shown that it is an endocrine gland of great importance. It produces hormones that may have an important regulatory influence on many other endocrines organs> the best-known hormone is melatonin which causes changes in skin color in some species. The synthesis and discharge of melatonin are remarkably influenced by exposure of the animal to light. Increase secretion occurs during dark periods.
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