Parts of Diencephalon and function of hypothalamus

i. Epithalamus:

a. It is the thin non-nervous roof of the diencephalon. It is fused anteriorly with the pia mater to form the anterior choroid plexus.

b. It is connected to pineal gland through a pineal stalk from its dorsal wall.

ii. Thalamus:

a. It is formed by lateral thick walls of diencephalon.

Thalami mainly contain grey matter.

b. The habenculor commissure connects two thalami. Different parts of the brain are interconnected by the RAS (Reticular Activating System) through the thalami.

C. It is called relay centre as it transmits all sensory impulses except those of olfactory (smell) to the cerebrum.

d. The narrow cavity of diencephalon is called 3rd ventricle or diocoel. It connects anteriorly to the two lateral ventricles by a single opening called Foramen of Monro and posteriorly to the 4th ventricle or metacoel through a narrow duct of Sylvius or iter.

iii. Hypothalamus: 

a. It is ectodermal in origin.

b. It forms the floor of the diencephalon.

C. It is richly supplied with blood vessels (Hypothalamo-hypophyseal portal vein) and helps in feedback mechanism for hormonal control. It maintains homeostasis (Homeostasis is the body's ability to maintain stable internal conditions despite external changes.), internal equilibrium of the body and involuntary behaviour control.

d. The hypothalamus also contains hypothalamic nuclei in its white matter with neuro-secretory cells involved in the production of hormones. 

e. The hypothalamus is a link between the nervous and the endocrine system.( The endocrine system is a network of glands that produce and release hormones into the bloodstream to regulate various bodily functions and maintain homeostasis. Hormones act as chemical messengers, influencing processes like metabolism, growth, reproduction, and more. Key glands include the pituitary, thyroid, adrenal, and pancreas.)

f. A complex neuronal circuit called the limbic system is formed by the hypothalamus, amygdala, parts of epithalamus and thalamus, hippocampus and other areas. It appears to be responsible for emotional reactions, motivational drives and memory.

g. The floor of the hypothalamus continues as a downward projection called hypophyseal stalk or infundibulum which connects it to the hypophysis (pituitary gland) both physically and functionally by secretion of neurotransmitters.

h. The inferior surface of hypothalamus also bears the optic chiasma (crossing of the two optic nerves) and a pair of mammillary bodies (unique to mammalian brain and responsible for recollective memory).

Functions of hypothalamus:

a. It regulates heart rate, respiration, blood pressure (B.P.), body temperature, water and electrolyte balance. a.

b. It has centres for hunger, thirst, sleep, fatigue, satiety centre, secretion of glands of stomach and intestine. It also produces neurohormones that stimulate the pituitary gland.

C. Major function of hypothalamus is maintaining homeostasis.

d. It controls the secretory activity of pituitary gland by releasing and inhibiting hormones.

The "hypothalamus" is a tiny but very important part of our brain that helps control many things like our body temperature, hunger, and thirst.

Now, when we say it's "ectodermal in origin," we mean that it comes from a particular layer of cells in our developing body when we're just tiny babies in our mother's womb. This layer is called the "ectoderm," and it's responsible for forming parts of our nervous system, including the brain.

So, in simple words, when we say the hypothalamus is "ectodermal in origin," we're saying that this crucial part of our brain originally comes from a specific layer of cells that eventually forms our nervous system.