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Diencephalon

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DIENCEPHALON ANATOMY

Vital structures of the brain such as the thalamus, hypothalamus, epithalamus, and the pituitary gland are all housed within the autonomic region within the diencephalon. As a second subdivision of the forebrain, the diencephalon is neighbored by the cerebral hemispheres of the telencephalon. The thin cavity running along the midline within the diencephalon is known as the third ventricle. The vital structures within the diencephalon control various functions and are required for basic survival.

THALAMUS



Approximately 4/5 of the diencephalon is made up of the thalamus. Its appearance is a large ovular mass of gray matter. Though it appears as one organ, in reality the thalamus is paired. The thalamus in its entirety lies directly below the lateral ventricle, with each half extending into the appropriate hemisphere.

The thalamus is responsible for the internal “relay center” for sensory interpretation between the brain and the sensory receptors, with the exception of those related to smell. Intensely specialized areas of nuclei relay the necessary impulses which enter the brain directly to the exact location within the cerebral cortex.

DIENCEPHALON DIAGRAM

Diencephalon
Image: Diencephalon




The thalamus is capable of a small amount of sensory interpretation. The cerebral cortex is responsible for refined tactile interpretation as well as the response to painful stimuli. The crude interpretation possible in the thalamus is not sufficient for the necessary enhanced tactile interpretation. The thalamus is responsible for a crude and immature interpretation of sensory awareness such as intense pain. It most likely contributes to the body’s reaction to intense and immediate pain while the brain’s cerebral cortex has time to take over to fine tuned reactions to pain. Psychological shock that often accompanies phenomenal pain is contributed by via the thalamus.

HYPOTHALAMUS



Just below the thalamus lies the hypothalamus. This would be the furthermost inferior section of the diencephalon. Forming a section of the lateral walls as well as the floor of the third ventricle, the hypothalamus houses numerous masses of nuclei which is interconnected to various places along the central nervous system. The hypothalamus is quite small yet is still able to perform vital functions which are responsible for the regulation (both directly and indirectly) of visceral organs. The hypothalalmus is also responsible for emotional and instinctual reactive processes.

THALAMUS DIAGRAM

Thalamus
Image: Thalamus


By accelerating and decelerating specific visceral functions, the hypothalamus is considered an autonomic nervous center. Part of its general function is the release of numerous varying hormones, including two which are released from the posterior pituitary. One of the regulation responsibilities of the hypothalamus is cardiac regulation. While it does not control whether or not the cardiac muscle beats, it does command acceleration and deceleration as it related to both emotional and physical stimuli. The posterior hypothalamus sends out impulses to increase arterial blood flow, which causes heart rate acceleration. The anterior section sends out the impulses responsible for deceleration of the arterial flow. The pulses do not go straight to the cardiac muscle but rather head to the cardiovascular centers of the medulla oblongata.

HYPOTHALAMUS FUNCTIONS

The hypothalamus is also responsible for body temperature regulation. There are masses of highly specialized nuclei that are housed in the anterior section of the hypothalamus which can sense changes in the body’s internal temperature. The hypothalamus sends out the necessary impulses to entice the body to sweat and lower the body’s temperature when the sensors determine a high blood temperature. Heat preservation tactics are encouraged by different impulses which encourage the body to shiver, constrict the cutaneous, and cease sweating.

The hypothalamus is also responsible for the body’s water and electrolyte balance. Within the blood, the body has specialized receptors known as osmoreceptors which can detect electrolyte imbalances as well as indications of a lack of hydration. This is known as monitoring the osmotic levels within the blood. An anti-diuretic hormone can be triggered via the posterior pituitary gland when impulses from the hypothalamus are sent out. The body is then also triggered to become thirsty.

HYPOTHALAMUS DIAGRAM

Hypothalamus
Image: Hypothalamus


Hunger is also triggered by the hypothalamus. While it is responsible for the control over gastrointestinal activity, the hypothalamus houses a feeding center, which which is a specialized segment responsible for the constant monitoring of the blood glucose, fatty acids, and amino acid levels throughout the body. Hunger is a sensation that the hypothalamus triggers when the monitored blood levels are too low, and alternatively, the mid section of the hypothalamus controls the feeding center when the body has been satiated. Sensory impulses from the GI tract also determines the hypothalamus reaction to peristaltic action and the regulation of glandular secretions. The hypothalamus is able to tell the body when it has rested enough and when it requires rest. The hypothalamus communicates constantly with other various portions of the brain to determine the level of appropriate wakefulness or sleepiness, depending on the body’s needs.

Within the superior portion of the hypothalamus resides the sexual sensation center. Tactile receptors in the genitalia respond to the physical sensations of sexual practice. The hypothalamus then responds with the appropriate neural activity to produce sexual pleasure and orgasms.

Certain sections and masses of nuclei within the hypothalamus are responsible for the emotions the human mind reacts with. Fear, anger, pain, and joy are in part due to the responses generated via the hypothalamus. Neurosecretory chemicals are responsible for the control of various endocrine functions. Impulses from the hypothalamus are sent to the anterior or posterior portions of the pituitary gland to release various hormones in response to the body’s needs.
Pituitary Gland
Image: Pituitary Gland


EPITHALAMUS



Along the posterior section of the diencephalon lies the epithalamus. This causes the formation of a fine roof over the third ventricle. The thin roof is then lined with the vascular choroid plexus which is responsible for the production of cerebrospinal fluid. Named for its appearance, which is similar to that of a pine cone, the pineal gland is a small tissue mass which projects outwards from the posterior section of the epithalamus. While scientific studies have yet to confirm all of its functions, it is thought that this has a neuroendocrine function.

Positioned inferior to the lineal gland, the posterior commissure is a tract of commissural fibers which are responsible for the connection of the midbrain’s right and left colliculi.

PITUITARY GLAND



The cerebral hypophysis is also known as the little pea shaped gland known commonly as the pituitary gland. Situated along the inferior aspect of the diencephalon, the pituitary stalk attaches the gland to the hypothalamus. The sella turcica of the sphenoid bone provides additional support. Structurally, the pituitary gland is segregated into two various sections, the anterior portion and the posterior portion, which are known as adenohypophysis and neurohypophysis, respectively. The basic function of the pituitary gland is the produce the appropriate chemicals and hormones necessary for endocrine function.

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