Diencephalon

Question 1

The diencephalon is the part of the brain between the cerebrum above and midbrain below. It extends from the ______(a)_______ to posterior commissure. The ______(b)______ divides the diencephalon into two parts — a dorsal part (pars dorsalis) and a ventral part (pars ventralis).

Answer 1

(a) interventricular foramen (of Monro)
(b) hypothalamic sulcus

Further notes:
The interventricular foramen, also known as the foramen of Monro, is a part of the brain’s ventricular system and serves as a connection between the third ventricle and the lateral ventricles.

Question 2

With reference to the diencephalon:
(a) Pars dorsalis consists of: (HINT: 3 parts)
(b) Pars ventralis consists of: (HINT: 2 parts)

Answer 2

(a) thalamus, metathalamus and epithalamus
(b) hypothalamus and subthalamus
[Diagram 1] [Diagram 2]

Question 3

The cavity of the diencephalon is the _____ ventricle.

Answer 3

3rd

Question 4

The superior surface of thalamus is covered by a thin layer of white matter called the ____(a)____ and its lateral surface, by a similar layer called the ____(b)____. Internally a ‘Y’ shaped bundle of white matter called ____(c)____ divides the grey matter of thalamus into three major groups of nuclei: anterior, medial and lateral.

Answer 4

(a) stratum zonale
(b) external medullary lamina
(c) internal medullary lamina

Question 5

Describe the location and relations of the thalamus. (Hint: How is the thalamus related to the following: lateral ventricle, caudate nucleus, 3rd ventricle, hypothalamus, midbrain, internal capsule?)

Answer 5

• The superior surface of the thalamus lies in contact with the floor of the lateral ventricle, postero-inferior to the caudate nucleus.
• Forms the lateral wall of the upper part of the 3rd ventricle.
• It is located supero-posterior to the hypothalamnus and superior to the midbrain.
• The internal capsule lies lateral to it.
• [Diagram 1] [Diagram 2] [Diagram 3] [Diagram 4] [Diagram 5]

Question 6

Functional classification of nuclei of thalamus
Identify the thalamic nuclei classified under the:
1. Motor relay group
2. Sensory relay group
3. Sensory modulator group

Answer 6

1. Motor relay group: ventral anterior nucleus, ventral lateral nucleus
2. Sensory relay group: ventral posterolateral nucleus, ventral posteromedial nucleus, medial geniculate body, lateral geniculate body
3. Sensory modulator group [regulation of sensory input]: lateral dorsal nucleus, lateral posterior nucleus, pulvinar nucleus
4. [Diagram: Thalamic nuclei (1)] [Diagram: Thalamic nuclei (2)]

Question 7

Functional classification of nuclei of thalamus
Identify the thalamic nuclei classified under the:
1. Limbic group
2. Nonspecific thalamic group

Answer 7

1. Limbic group: anterior nucleus, medial dorsal nucleus
2. Nonspecific thalamic group: intralaminar nuclei, midline nuclei, reticular nucleus
3. [Diagram: Thalamic nuclei (1)] [Diagram: Thalamic nuclei (2)]

Further notes:
~ The reticular nucleus on the lateral aspect of thalamus (Thalamic Reticular Nucleus - TRN) consists of nerve cells which connect with intralaminar nucleus and plays an important role in gating the impulses to and from the cerebral cortex. Recently it has been proposed that this nucleus may have a role in autism. This nucleus is also considered as part of ventral thalalmus because of its proximity to zona incerta.
~ The intralaminar nuclei are embedded within the
internal medullary lamina. There are several nuclei in this group. The most important of these is the centromedian nucleus.
~ The midline nuclei consist of scattered cells that lie between the medial part of the thalamus and the ependyma of the third ventricle. Several nuclei are recognized.
~ The medial and lateral geniculate bodies (traditionally described under metathalamus) are now included as part of the thalamus.
~ Functionally, the centromedian nucleus (CM) participates in sensorimotor coordination, cognition (e.g. attention, arousal), and pain processing. The role of CM as ‘gate control’ function by propagating only salient stimuli during attention-demanding tasks has been proposed.

Question 8

Medial Geniculate Body
The medial geniculate body is an oval elevation on the inferior aspect of the pulvinar of the thalamus, lateral to the superior colliculus. It is more prominent than the lateral geniculate body. The inferior brachium runs upward, laterally and forward from ____(a)____ colliculus of the midbrain to the medial geniculate body. The inferior brachium conveys ____(b)____ impulses to the medial geniculate body for onward transmission to the ____(c)____ area of the cerebral cortex, Brodmann numbers: ____(d)____ in the ____(e)____ gyri.

Answer 8

(a) inferior
(b) auditory
(c) primary auditory
(d) 41, 42
(e) transverse temporal gyri

Question 9

Lateral Geniculate Body
The lateral geniculate body is smaller than the medial geniculate body and connected to the ____(a)____ colliculus by the ____(b)____ brachium.
The fibres of superior brachium are concerned with the production of visual reflexes such as turning of head and eyes toward the sudden flash of light and constriction of pupil when light falls or is thrown on the retina. The lateral geniculate body receives ____(c)____ fibres of both the eyes (from temporal half of the retina of the same side and nasal half of the retina of the opposite side) through the ____(d)____ and gives rise to fibres of the ____(e)____ which convey visual impulses to the visual cortex of the occipital lobe.

Answer 9

(a) superior
(b) superior
(c) retinal
(d) optic tract
(e) optic radiation

Further notes:
The optic radiation, also known as the geniculocalcarine tract or the geniculostriate pathway, is a collection of axons that relay visual information from the lateral geniculate nucleus (LGN) of the thalamus to the primary visual cortex, which is located in the occipital lobe of the brain.

Question 10

What is the anatomical basis of thalamic syndrome of Dejerine-Roussy?
What are the characteristic features of thalamic syndrome?

Answer 10

Anatomical basis: Vascular occlusion of the thalamogeniculate branch of posterior cerebral artery which supplies the posterolateral part of thalamus.
This condition is characterised by:
* Pansensory loss contralateral to the side of lesion (due to involvement of ventral posterior nuclei)
* Thalamic pain: severe, persistent, paroxysmal and intolerable pain (hence this is also known as painful anaesthesia or anaesthesia dolorosa) (due to involvement of intralaminar and other non-specific nuclei)
* Transient hemiparesis (due to involvement of internal capsule)
* Hemiataxia and choreiform movements (due to involvement of pallidofugal fibres which intersect internal capsule and subthalamus)
* Homonymous hemianopia (due to involvement of lateral geniculate body)
* [Diagram: Blood supply of the thalamus]

Further notes:
~ In thalamic syndrome, the threshold for pain, touch, and temperature is decreased on the opposite side of the body (thalamic overreaction), but when the threshold is reached, the sensations are exaggerated, perverted, and disagreeable. For example, the prick of a pin may be felt as a severe burning sensation and music that is ordinarily pleasing may be disagreeable. Sometimes even light touch may produce excruciating pain, which may become intractable and fail to respond to powerful analgesics (pain-relieving) drugs. There may be emotional instability with spontaneous laughing and crying.
~ Homonymous hemianopia: this refers to a field loss deficit in the same halves of the visual field of each eye.

Question 11

The thalamus is a great integrating centre where information from numerous sources is brought together. Therefore, understanding its connections is vital. State the connections of the anterior nuclei.
(NB: Connections are simply about afferents and efferents.)

Answer 11

Afferents: from mamillary body through the mamillothalamic tract
Efferents: to cingulate gyrus through the anterior or frontal thalamic radiation

Further notes:
The anterior nucleus is a part of circuit of Papez for recent memory.

Question 12

State the following regarding the medial dorsal nucleus.
1) Connections
2) Functions
3) Effect of lesion

Answer 12

1. Connections:
   ~ Afferents: olfactory areas, piriform lobe, amygdala, hypothalamus and corpus striatum
   ~ Efferents: to prefrontal cortex through anterior or frontal thalamic radiation
2. Functions:
   ~ it is involved in controlling emotional states and has a role in determining the personality of the individual
   ~ concerned with integration of olfactory, visceral and somatic functions and in the mediation of visceral and somatic reflexes
3. decrease in anxiety, tension and aggression.
4. [Diagram: thalamic nuclei]

Question 13

State the connections of the:
a) ventral anterior nucleus
b) ventral lateral nucleus

Answer 13

a) Afferents: globus pallidus and substantia nigra particularly pars reticularis
Efferents: Premotor and supplemental motor cortices
b) Afferents: dentate nucleus and globus pallidus
Efferents: primary motor area

Question 14

State the afferents and efferents of ventral posterior nucleus (you already know this from tractology, believe in yourself and answer flawlessly😎).

Answer 14

Afferents: general sensory pathways [which tracts synapse at the VPL and VPM?]
Efferents: postcentral gyrus

Question 15

State the connections of the lateral group of thalamic nuclei.

Answer 15

1. Lateral dorsal
   Afferents: anterior and medial group of thalamic nuclei
   Efferents: cingulate gyrus, parahippocampal gyrus and hippocampus [this nucleus provides inputs to limbic cortical areas]
2. Lateral posterior
   Afferents: Ventral posterior groups of thalamic nuclei
   Efferents: Superior parietal lobule [somatosensory association area]
3. Pulvinar
   Afferents: LGB and MGB, superior colliculi
   Efferents: association areas of parietal, occipital and temporal lobes

Question 16

State the functions of the thalamus.

Answer 16

a) All sensory impulses, except the olfactory ones, terminate in the gray masses of the thalamus, from which they project to specific cortical areas by thalamocortical radiations. The nuclei of the dorsal thalamus serve as primary relay nuclei in various sensory pathways in which impulses are projected to specific regions of the cerebral cortex.
b) The thalamus is the chief sensory integrator of the neuraxis. Physiologically, the thalamus and related neuronal subsystems are concerned with high fidelity transmission of sensory information, input selection, output tuning, synchronization and desynchronization of cortical activity, parallel processing information, signal storing, and signal modification.
c) Some thalamic nuclei serve as integrative centres for motor functions, since they receive the principal efferent projections from deep cerebellar nuclei and the corpus striatum.
d) Specific parts of the thalamus play a dominant role in the maintenance and regulation of state of consciousness, alertness and attention.
e) The thalamus is concerned also with the emotional correlates that accompany most sensory experiences. For this, it has numerous connections with the limbic system.

Question 17

State the location and relations of the hypothalamus.

Answer 17

• The hypothalamus lies antero-inferior to the thalamus, from which it is separated by the hypothalamic sulcus.
• It projects into the interpeduncular fossa, by the tuber cinereum and the mamillary bodies.
• The optic chiasma, optic tracts and crus cerebri lie inferior to it.
• It lies in the lateral wall of the lateral ventricle.
• It is connected to the pituitary gland by the infundibular stalk
• [Diagram]

Question 18

Name the nuclei in the following regions of the hypothalamus:
1. preoptic region
2. supraoptic region
3. tuberal region
4. mamillary region

Answer 18

1. preoptic region: preoptic nucleus
2. supraoptic region: supraoptic nucleus, anterior nucleus, paraventricular nucleus, suprachiasmatic, lateral
3. tuberal region: arcuate (infundibular nucleus), ventromedial nucleus, dorsomedial nucleus, lateral tuberal, premamillary, tuberomamillary and lateral
4. mamillary region: posterior, mammillary nuclei (medial and lateral), lateral
5. [Diagram 1] [Diagram 2] [Diagram 3] [Diagram 4]

Further notes:
The preoptic region differs from the rest of the hypothalamus in being a derivative of the telencephalon. The lamina terminalis also belongs to the telencephalon.

Question 19

State the functions of the following hypothalamic nuclei:
(a) preoptic
(b) supraoptic
(c) suprachiasmatic
(d) anterior hypothalamic nucleus
(e) paraventricular nucleus

Answer 19

(a) preoptic nucleus: involved in the secretion of GnRH, which stimulates the release of LH and FSH
(b) supraoptic nucleus: secretion of antidiuretic hormone (vasopressin) and oxytocin
(c) suprachiasmatic nucleus: regulates circadian rhythms
(d) anterior hypothalamic nucleus: involved in cooling and parasympathetic responses
(e) paraventricular nucleus: secretion of oxytocin
[Diagram 1] [Diagram 2]: arrangement of hypothalamic nuclei

Question 20

State the functions of the following hypothalamic nuclei:
(a) medial and lateral nuclei
(b) arcuate nucleus
(c) posterior hypothalamic nucleus
(d) mammilary bodies

Answer 20

(a) medial and lateral nuclei: regulate eating. The lateral nucleus is the hunger centre, while the ventromedial nucleus is the satiety centre.
(b) arcuate nucleus: produces releasing hormones and inhibitory factors, which pass through hypophyseal-portal veins to reach the anterior pituitary gland
(c) posterior hypothalamic nucleus: involved in heating and sympathetic responses
(d) mammillary bodies: part of the Papez circuit involved in memory
(e) [Diagram 1] [Diagram 2]: arrangement of hypothalamic nuclei

Memory Hack:
1. Lateral Hypothalamic Nucleus; (Lateral = Let’s eat). Think of the LHN as the “Let’s eat centre”. The LHN is associated with hunger and feeding behaviour.
2. Ventromedial Hypothalamic Nucleus (Ventromedial = Very much full). The VMH is the “Very much full” centre. It is involved in satiety and fullness.

Question 21

Connections of the Hypothalamus
There are two types of hypothalamic input - neural and humoral. State the inputs.

Answer 21

Neural input:
☛ From the hippocampal formation via the formix.
☛ Amygdaloid nucleus (ventral amygdaloid path)
☛ Orbitalfrontal cortex
☛ Mid-line thalamic nuclei
☛ Retina
☛ Basal fore brain and septal region (via medial fore brain bundle)
☛ Reticular formation
☛ Cerebellum

Humoral input: This is vascular. Through this, various hypothalamic neurons get stimulated chemically (e.g. by glucose or hormones) or physically (e.g. by temperature)

Question 22

State the hypothalamic outputs.

Answer 22

Neural:
The main targets are:
☛ The anterior thalamic nuclei (mamillothalamic tract)
☛ Midbrain reticular formation (mamillotegmental tract)
☛ Amygdaloid nucleus (the ventral amygdaloid path)
☛ Brainstem and spinal cord autonomic centres (hypothalamo-bulbar/spinal)
☛ Cerebellum

Humoral: This influences the endocrine system directly, by secretion into the general circulation, and indirectly by secretion into the hypophyseal portal system.

Question 23

State the functions of the hypothalamus.

Answer 23

☛ regulation of eating and drinking behaviour (remember the two nuclei responsible?)
☛ regulation of sexual activity and reproduction (gonadotropic hormones are responsible)
☛ control of autonomic activity (remember the hypothalamic nuclei responsible for parasympathetic activity and sympathetic activity?)
☛ emotional behaviour (stimulation of lateral areas of the hypothalamus produces sensations of pleasure, while stimulation of medial areas produces pain or other unpleasant effects)
☛ control of endocrine activity
☛ response to stress
☛ temperature regulation (remember the hypothalamic nuclei that were heat-loss and heat-rise centres?)
☛ biological clock (associated with circadian rhythms [cycles of sleeping and waking]. Which hypothalamic nucleus is involved?)

Question 24

Below are various effects of lesion of hypothalamic nuclei. For each, state the hypothalamic nucleus involved:
a) Irregular menstrual cycle and loss of libido.
b) Hyperthermia
c) Hypothermia
d) Anorexia and emaciation
e) Obesity
f) Wernicke’s encephalopathy
g) Diabetes insipidus

Answer 24

a) Preoptic nucleus
b) Anterior nucleus
c) Posterior nucleus
d) Lateral nucleus
e) Ventromedial nucleus
f) Mamillary body
g) Supraoptic nucleus

Further notes:
☛ Kleine-Levin syndrome: Lesions of medial part of hypothalamus causes problems in satiety resulting in periods of somnolence followed by hyperphagia.
☛ Syndrome of Inappropriate ADH secretion (SIADDH): Lesions of supraoptic nucleus results in improper ADH secretion and diabetes insipidus.
☛ Wernicke encephalopathy (WE) is an acute neurological condition characterized by a clinical triad of ophthalmoparesis with nystagmus, ataxia, and confusion. This is a life-threatening illness caused by thiamine deficiency, which primarily affects the peripheral and central nervous systems. Click here for more information.

Question 25

State the blood supply to the thalamus.
(Artery, arises from, supplies what)

Answer 25

(1) Thalamogeniculate arteries from P2 portion of PCA supplies the ventrolateral region
(2) Tuberothalamic artery (polar artery) from the posterior communicating artery, supplies anteromedial and anterolateral regions
(3) Thalamosubthalamic arteries (paramedian thalamic arteries) from P1 portion of PCA, supplies the medial region of the thalamus
(4) Posterior choroidal arteries from P2 portion of PCA, supplies the posterior region of the thalamus, including the pulvinar
(5) [Diagram]

NB: All arteries supplying the thalamus are terminal arteries.

Question 26

Thalamic infarcts are localised. Discuss ventrolateral thalamic infarction.
(a) What artery is occluded?
(b) What syndrome is associated?

Answer 26

(a) thalamogeniculate artery
(b) Dejerine-Roussy (thalamic pain syndrome)

Question 27

Discuss anterior thalamic infarction.

Answer 27

occlusion of polar/tuberothalamic artery
clinical manifestations:
✓ altered consciousness
✓ disorientation
✓ personality disturbances
✓ visual field defects
✓ left sided thalamic aphasia
✓ right sided hemineglect, “alien hand”

Question 28

Discuss paramedian thalamic infarction.

Answer 28

• occlusion of paramedian/thalamic-subthalamic artery
• clinical manifestations:
  ~ somnolence
  ~ memory loss
  ~ mood disturbances
  ~ vertical gaze abnormalities
• can be bilateral if supplied by the artery of Percheron [NB: The artery of Percheron (AOP) is a relatively rare anatomic variant in which a solitary arterial trunk branches from the proximal segment of the posterior cerebral artery and provides arterial supply to the paramedian region of the thalami bilaterally and often to the rostral part of the midbrain]:
  ~ hypersomnolence
  ~ marked memory impairment

NOTE:
Somnolence, defined as a state of drowsiness or strong desire to fall asleep, may be characterized as either a benign symptom, such as a state of drowsiness prior to falling asleep or, most commonly, as a symptom of an underlying condition, such as a sleep disorder, anxiety, depression, or stress. This is different from hypersomnia.

Question 29

Discuss dorsal thalamic infarctions.

Answer 29

• occlusion of posterior choroidal artery
• clinically:
  ~ homonymous quadrantanopia
  ~ homonymous horizontal sectoranopias
  ~ may have aphasia or cardiac dysrythmias if pulvinar is affected

Question 30

State the position and contents of subthalamus.

Answer 30

Position: transition zone between diencephalon and tegmentum of midbrain
Contents:
- subthalamic nucleus
- parts of red nucleus
- substantia nigra