Amygdala

When TRH or thyroid releasing hormone is made by the hypothalamus, it triggers TSH or thyroid stimulating hormone to be released by the pituitary gland. This hormone travels to the thyroid gland so that it is stimulated to make thyroid hormones for cellular metabolism. When these levels get too high, there is a negative feedback loop back to the hypothalamus, where TRH is shut off and levels of all resultant hormones drop. This is what creates metabolic homeostasis is the body.

A similar pattern happen in the reproductive system. GnRH or gonadotropin releasing hormone gets released in a pulsatile fashion in both males and females. When it does this, it triggers both LH or luteinizing hormone an FSH or follicle simulating hormone to be released by the anterior pituitary gland. What happens to them after that depends on whether the person is male or female but both hormones are crucial to the development of egg and sperm cells and both regulate the hormonal milieu in the reproductive organs, helping to regulate the menstrual cycle in females.

Another hormone system with hypothalamic input is that related to growth hormone releasing hormone or GHRH by the pituitary gland, which helps to release Growth hormone or GH by the anterior pituitar gland. This promotes metabolism and tissue growth but is inhibited by somatostatin, which is also made by the hypothalamus. The entire process is self-regulatory in nature.

There is also a mamillary nucleus in the hypothalamus that relates to the motional circuits of the brain. It has an effect on exploratory behavior and on the formation of new memories. If these are damaged in some way, as happens in alcohol-induced Wernicke-Korsakoff syndrome, there will be significant anterograde and some retrograde memory losses.

AMYGDALA

The amygdala is a deep brain structure that is a major feature of the limbic system or emotional center of the brain. It is considered the main receptor organ for the perception of fear. There are many other aspects of the limbic system besides the amygdala that we will discuss later in its own chapter. The amygdala itself is small and almond shaped. Here are several nuclei divided into five different groupings. These are

the cortical-like nuclei, the basolateral nuclei, the extended amygdala, and other amygdaloid nuclei. Figure 15 describes what the amygdala looks like:

Figure 15.

The amygdala has many functions as you will soon learn. It is important in the development of the fear response, in the development, and in what happens to the brain during sleep deprivation. There are many neuropsychiatric diseases that are directly or indirectly related to abnormalities or unusual responses from the amygdala.