The only part of the diencephalon that can be seen on the intact brain is on the inferior surface of the hypothalmus. It includes the mammilary bodies and the infundibular stalk. PLATE 140

In a hemisected brain, the medial surface of the diencephalon can be seen. (The medial walls form the wall of the third ventricle.)

Anterior-Posterior it extends from the interventricular foramen to the posterior commissure.

The diencephalon borders on the cerebral cortex, the latreral ventricles and the efibers of the corpus collosum superiorly.

The diencephalon is divided into four parts: each of which conveniently includes the word thalmus "inner chamber".

1) epithalmus which includes the pineal gland (shaped rather like a pine cone) and the habenular nucleus is the oldest part of the diencephalon. It is concerned with diurnal and autonomic functions. The pineal gland is an endocrine structure and involved in reproductive cycles. It also secretes large amounts of melatonin at high rates during periods of darkness. PLATE 105 & 104

2) the plain old thalmus (egg shaped) - all other thalamic structures are related to it. It consists of numerous nuclei connected with other regions of the brain via both efferent and afferent connections.

3) the subthalmus which is ventral to the thalmus is important in motor functions and connects with the brainstem, basal ganglia and other thalmic structures. Although it is included as part of the diencephalon it is functionally related to the basal ganglia. Tumors in this area cause hemiballism,a motor disorder resulting in involuntary, violent, flinging movements that appear while awake but disappear with sleep.

4) the hypothalmus is located beneath the thalmus and is functionally a part of the autonomic nervous system

The thalmus has three important functions

a) it channels sensory information through specific nuclei associated with pain,

taste, temperature audition and vision to the primary sensory cortex

b) it integrates sensory motor information from the basal ganglia and the

cerebellum and then transmits it to the primary and premotor cortex.

C) it regulates functions of the associational cortex and mediates speech,

language and cognitive function.

Each of the nuclei has bidirectional fibers to specific cortical areas which form functional units.

The thalmus consists of three tiers of nuclei: medial, lateral, and anterior and each tier is composed of multiple nuclei. In addition are several other structures including the :

The internal medullary lamina is a thin,Y-shaped sheet of myelinated fibers that divides the thalmus into medial, dorsal, and lateral tiers.

The anterior nucleus mediates visceral and emotional information. Electrical stimulation and its ablation induce changes in blood pressure and emotional drives.

This nucleus consista of a thin invisable layer of nerve cells that is located between the external medullary lamina of the thalmus and the internal capsul. It consists of neurons similar to those in the brainstem reticular formation. It probably integrates and regulates thalamic neuronal activity. It regulates the regulator.

This is a complex of nuclei in the core of the internal medullary lamina. This system modulates the excitability and overall function of the cortex and the basal ganglia related to both sensory and to cognitive function. The intralaminar nuclei can evoke cortical recruiting response with the frontal, parietal, cingulate and orbital areas of the asssociation cortex.

This thing has two parts:

a. The Dorsomedial Nucleus has numerous connections with other thalamic nuclei and the rprefrontal cortex. It is important in emotional and personality development. It integrates visceral information with emotions, affect, thought processes and judgement. OH WOW!

Destruction of this nucleus can result in:

-lowering the threshold for rage

-memory loss in patients with Koraskoff’s syndrome (a personality disorder

caused by chronic alcohalism)

b. The Midline Nuclear Complex This nuclear complex receives information from the brainstem reticular formation and is important in visceral function.

This is a narrow strip of three nuclei located on the dorsolateral surface of the thalmus.

a. The lateral Dorsal Nucleus functions and connections are poorly understood but it probably contributes to visceral-sensory integration.

b. The Lateral Posterior Nucleus lies caudal to the lateral Dorsal Nucleus and its functions are also poorly understood. but it is connected to an area that is involved with multisensory integration and transcoding.

c. The Pulvinar is the largest thalamic nucleus and makes important contributions to language formulation and processing, lexical properties, reading, writing and other language functions.

This complex consits of five nuclei whose boundaries are not distinct and who share many overlapping fibers. This nuclear complex serves as the relay center for specific sensory and motor information.

a. The Ventral anterior nulceus is the smallest of the ventral nucleus and is innvolves in facilitation of skilled movements and initiating voluntary movements.

b. The Ventrolateral nuleus is important in the regulation of volitional movements a disruption of the efferent projection causes a motor abnormalities.

3. The Ventral Posterior Nucleus really consists of two areas (lateral and posterior) both of which serve as thalmic relay centers for somatosensory (both primitive and distinctive) sensation from the body and the face.

 

d. Lateral Geniculate Body This nucleus is the thalmic relay center for the sensation of vision.

4. Medial geniculate center is the thalamic relay for audition.

Small branches of the Posterior Cerebral Artery provide most of the blood supply to the Thalamus. These include the posterior choroidal artery and the ganglionic or penetrating arteries of the posterior cerebral and posterior communicating.

The Epithalamus includes the pineal gland ("pine cone" shaped) & the habenular nucleus.

Pineal Gland In mammals the pineal glad is an endocrine gland, involved in reproductive cycles, and has no neural output. YEAH !!

It secretes a derivative of seratonin, called melatonin, during darkness, suggesting a role in circadian rhythms and sleep induction.

Increasing periods of daylight causes a decrease in melatonin. Because melatonin decreases gonadal function, the decrease in melatonin causes an increase in that and spring has sprung. The effect upon some species with seasonal reproductive cyclesis much more pronounced than others.

The pineal gland is attached to the diencephalon by a stalk. At the base of the stalk on either side is the habenular nuclei.

The two nuclei are connected by the habenular commissure.

They receive on major input, the stria medullaris thalami . These fibers originate in various parts of the limbic system so the pathway through the habenula is one way the limbic system can influence the brainstem reticular formation.

It gives off one major input bundle the habenulointerpeduncular tract or the fasciculus retroflexus.

The hypothalmus lies below and anterior to the thalmus. It forms the floor and the lower lateral walls of the third ventricle.

The hypothalmus weighs only about 4 grams.

It is a very important nodal point in the pathways concerned with autonomic, endocrine, emotional, and somatic functions.

It plays a major role in governing the endocrine system, the secretions of glands, and many maintainence functions such as body temperture, water balance and thirst, hunger and satiety.

Longitudinally the hypothalmus can be divided into anterior, tuberal and posterior regions as seen in Fig 10-15 on page 265.

Because of its association with the endocrine system the hypothalmus plays an essential role in reproductive behavior. Recent and PRELIMINARY research has revealed a difference in the size of on area of the hypothalmus between men and women. A similar difference has been found between male heterosexuals and homosexuals, suggesting that the region may be involved in determining sexual orientation.

The hypothalmus is one of the primary regulators of homeostatis . It regulates heart rate, endocrine secretions, digestion and responses to stress.

Stimulation can cause

vasodilation

feeding behavior

aggressiveness

The connections of the hypothalmus fall into three main categories

1) interconnections with various connections in the limbic system

2) interconnections with various viscera and somatic nuclei, both sensory and motor, of the brainstem and the spinal cord.

3) outputs that influence the pituitary gland

GHRH GH (somatotropin) Promotes body growth

Growth hormone

releasing hormone

GHIHa GH Inhibits growth

Growth hormone

inhibiting hormone

(somatostatin)

TRH Thyroid simulating Indirectly controls thyroid

(thyroid releasing hormone (TSH)

hormone)

GnRH Controls release of regulates reproductive

(gonadotrophin controls release of cycling, ganete

releasing hormone) gonadotrophins production, and secretion

FSH (follicle stimulating of sex hormones

hormone) and

LH (Lutenizing hormone)

PHR (prolactin Prolactin Milk secretion

releasing hormone)

The infundibular stalk is located in almost the middle of the Circle of Willis and the inferior aspectof the hypothalmus is more or less surrounded by the circle.

The arterial suply of the hypothalmus comes form the ganglionic arteries arising from arteries in and adjacent to the circle of Willis.

Many hypothalmic centers have been described that are concerned with feeding, drinking temperature regulation, gut motility, sexual activity and other functions too numerous to mention.

In most cases, fragments of behavior patterns elicited by stimulating a hypothalmic location can also be elicited by stimulating a brainstem site.

This leads us to suspect that the hypothalmic site may be triggers that serve to stimulate neural activity in other parts of the CNS.

Since the hypothalmus is so small, discrete leisions affeting individual functional area of the human hypothalmus are quite rare. especially sine they have to be bilateral in order to be completely disrupted. Dispite this clincial findings in humans with hypothalmic damage are consistent with what we might expect.

Examples:

Although there tends to be overlap, hypothalmic sites associated with the parasympathetic responses tend to be located anteriorly.

Hypothalmic sites associated with the sympathetic responses tend to be located posteriorly.

The hypothalmus acts as a thermostat, monitoring the temperature of blood passing through it.

Stimulation of the anterior hypothalmus and preoptic area induces panting (or sweating) in humans and cutaneous vasodilation, which causes body temperature to fall.

Stimulation of the posterior hypothalmus causes vasoconstriction and shivering resulting in the opposite effect.

Bilateral leisions of the anterior hypothalmus make the animal unable to dissapate heat in warm weather and may also cause diabetes insipidus.

Bilateral leisions of the posterior hypothalmus may make the animal unable to control heat or cold since the destruction involves not only the area concerned with the production and conservation of heat but also the fibers descending from the more anterior heat-dissapation area.

Here again two areas with opposing influences have been found. Bilateral destruction of the ventromedial nucleus the "satiety center", produces animals that overeat.

Destruction of the tuberal region the "feeding center" results in animals that will literally strave themselves to death.

The complex interconnections of the hypothalmus can be seen in cats who have experienced bilateral ventromedial leisions. Not only do they become fat and lazy but they also become extremely nasty. Hmmm.

Internal capsule

The internal capsule is divided into five parts based upon the relationship of each part to the lenticular nucleus:

1) The anterior limb is the part between the lenticular nucleus and the head of

the caudate nucleus.

2) The posterior limb is the portion between the lenticular nucleus and the

thalmus.

3) The genu is the portion at the junction of the anterior and the posterior limbs.

The genu is adjacent to the interventricular foramen.

4) The retrolenticular part is the portion posterior to the lenticlar nucleus.

5) The sublenticular portion in inferior to the lenticular nucleus.

Small brancehes of the Middle Cerebral Artery provide most of the blood supply to the Internal capsule.

There are two main sources to the internal capsule:

1) lateral striate arteries- these are a collection of fine ganglionic branches of the proximal portion of the middle cerebral artery. They supply most of the

anterior limb

genu and

posterior limb

The anterior limb and genu also receive part of their supply from the gangionic branches of the anterior cerebral and anterior communicating artery -particularly from a large one called the recurrent artery of Heubner= medial striate artery.

2) anterior choriodal artery - this artery supplies the inferior and posterior regions of the internal capsule.

Small strokes in the capsule can have major consequences.

Hemorrage of the lateral striate in the posterior limb can result in spastic paralysis and hemianesthesia on the contralateral side.

If the retrolenticular and sublenticular parts are also involved visual deficits would occur. Auditory radiations would be dammaged as well but because of the bilateral auditory pathway the damage would not be as severe.