Diencephalon

Question 1

Where is the diencephalon located?

Answer 1

• it sits on top of the brainstem and connects it to the cerebral hemispheres
• it is obscured by the cerebral hemispheres

Question 2

What are the 4 divisions of the diencephalon?

Answer 2

1. thalamus
2. epithalamus
3. hypothalamus
4. subthalamus

• both “hypo-“ and “sub-“ mean below
• the hypothalamus is located immediately below the thalamus and the subthalamus is located below and lateral to the thalamus

Question 3

What type information are the LGN and MGN of the thalamus important for?

Answer 3

Lateral geniculate nucleus (LGN):

• important structure in the relay of visual information
• connected to superior colliculus via the superior brachium

Medial geniculate nucleus (MGN):

• important structure in the relay of auditory information
• connected to inferior colliculus via the inferior brachium

Question 4

Label the following structures

Answer 4

• the mammillary bodies and infundibulum (pituitary stalk) are part of the hypothalamus
• the pineal gland is part of the epithalamus

Question 5

Which structures does the septum pellucidum run between?

What structures are located superior and inferior to the diencephalon?

Answer 5

• septum pellucidum is the wall between the corpus callosum and the fornix
• the diencephalon is wedged between the fornix superiorly and the midbrain inferiorly

Question 6

What are the following structures?

What is marked by the dotted line and ?

Answer 6

• ? is the roof of the thalamus, which is formed by the choroid plexus of IIIrd ventricle
• dotted line is the hypothalamic sulcus, which separates the thalamus from the hypothalamus

Question 7

What is located superior, anterior and inferior to the thalamus?

What is the interthalamic adhesion?

Answer 7

• the body of the fornix is located superior to the thalamus
• the choroid plexus of the IIIrd ventricle forms the roof of the thalamus
• the interventricular foramen and column of the fornix are located anterior to the thalamus
• the midbrain forms the inferior border of the diencephalon
• the inter-thalamic adhesion is a small bundle of fibres that connects the 2 thalami to each other

Question 8

What separates the thalamus from the hypothalamus?

What are the borders of the hypothalamus?

Answer 8

• the hypothalamic sulcus separates the thalamus from the hypothalamus
• the anterior border of the hypothalamus is formed by the lamina terminalis** and the **anterior commissure
• the infundibulum is located ventrally and is connected to the pituitary gland

Question 9

What is indicated by the *

Answer 9

• recesses of the IIIrd ventricle found in the space bounded by the hypothalamus on both sides

1. supraoptic recess
2. infundibular recess

• these are spaces within the IIIrd ventricle
• if there is an expanded IIIrd ventricle, these recesses can balloon out and compress structures surrounding them
  • e.g. optic chiasm, infundibulum

Question 10

What are the 2 components of the epithalamus?

Answer 10

habenula + pineal gland

Question 11

Label the structures and nuclei of the thalamus

Answer 11

• the thalamus is not one homogenous mass, but it is composed of many nuclear complexes

Question 12

What forms the medial and lateral borders of the thalamus?

Answer 12

• the medial border is the IIIrd ventricle
  
  • the thalamus has a free border medially
• the lateral border is the internal capsule

Question 13

Label the coronal section

What is located medial, lateral and anterior to the thalamus?

Answer 13

• the medial border of the thalamus is a “free border” formed by the III^rd ventricle
• the lateral border is formed by the internal capsule
  
  • this is continuous with the cerebral peduncles at the level of the midbrain
• the body of the fornix and choroid plexus are located superiorly

Question 14

What are the borders of the diencephalon?

Answer 14

Question 15

What is the massa intermedia?

Answer 15

• another name for the interthalamic adhesion that connects the thalami to each other
• the thalami are bilateral egg-shaped masses of grey matter that sit on top of the brainstem

Question 16

Why are the thalami referred to as the “receptionist of the brain”?

Answer 16

• they receive information, process it and pass it to the appropriate location in the brain
• they are relay nuclei to the cerebral cortex that are involved in processing sensory impulses
• it is also involved in motor processing through its connection with the basal ganglia and cerebellum
  
  • ​they do not directly connect to the corticospinal tract

Question 17

What is the only sense not processed by the thalamus?

Answer 17

olfaction

Question 18

What are the fibres that travel between the thalamus and the cortex?

Answer 18

• thalamocortical fibres travel from the thalamus to the cerebral cortex
• corticothalamic projections travel from the cortex to the thalamus

Question 19

What is the internal medullary lamina and how does it segregate the thalamus?

Answer 19

• it is a Y-shaped layer of myelinated fibres
• it divides most of the thalamus into nuclear groups

1. anterior nuclear group
2. medial nuclear group
3. lateral nuclear group (further subdivided)

Question 20

What are the thalamic nuclei that are only visible in coronal section?

Answer 20

midline thalamic nuclei (MTN):

• these are adjacent to the IIIrd ventricle

intralaminar nuclei:

• these are embedded within the internal medullary lamina

reticular nucleus:

• this is located lateral to the external medullary lamina

Question 21

What does the anterior nuclear group communicate with?

Where is it located and what does it form the posterior boundary of?

Answer 21

• the anterior nuclear group communicates with the limbic system
• it relays information related to emotion, memory and learning
• it is located in the more narrow end of the thalamus and forms the posterior boundary of the interventricular foramen

Question 22

What is the role of the medial nuclear group?

Answer 22

• the medial group contains the medial dorsal nucleus
• it receives and relays information related to emotional behaviour

Question 23

What are the roles of the ventral anterior (VA) and ventral lateral (VL) nuclei of the lateral nuclear group?

Answer 23

• they are important for motor relay
• they communicate with motor structures such as the supplementary motor area, premotor area, cerebellum and basal ganglia

Question 24

What are the roles of the ventral posterior medial (VPM) and ventral posterior lateral (VPL) nuclei of the lateral nuclear group?

Answer 24

• they both have a role in sensory relay
• VPM receives sensory information from the face
• VPL receives sensory information from the body via the dorsal column pathway and spinothalamic tract

Question 25

What is the role of the lateral posterior (LP) and pulvinar nuclei of the lateral nuclear group?

Answer 25

* they both have an **_integrative function_** **_Lateral posterior (LP):_** * receives information from multiple parts of the brain and puts it together **_Pulvinar nucleus:_** * receives information from **all the association areas** and relays it to different parts of the thalamus and brain * has a role in **cognitive function associated with _visual and auditory stimuli_** * forms the **wide end** of the thalamus

Question 26

Where are the reticular nuclei of the thalamus found? What is significant about the communications of these nuclei?

Answer 26

* they are located between the **_external medullary lamina_** and the **_internal capsule_** * they are the only thalamic nuclei that **_DO NOT connect with the cerebral cortex_** * they still communicate with the cerebral cortex, but there are no thalamocortical fibres projecting directly to the cortex

Question 27

What type of neurones are found within the reticular nucleus? What are the 2 main roles of this nucleus?

Answer 27

* it contains a **thin sheet of _inhibitory neurones_** that project deep into the rest of the thalamus * the reticular nucleus has a role in: 1. dampening down **ascending sensory pathways** during **sleep** 2. modulating the **state of alertness**

Question 28

How is the reticular nucleus able to dampen down ascending sensory pathways during sleep?

Answer 28

* if someone touches your arm whilst sleeping, this information still **travels to the thalamus** via the **_dorsal column pathway_** * usually, **_thalamocortical fibres_** project from the **thalamus** to the **somatosensory cortex** to make us aware of touch information * when asleep, **_corticothalamic fibres_** travel **to the thalamus** to make it aware that sleep is occurring * the corticothalamic fibres connect to **_inhibitory interneurones_** in the **_reticular nucleus_** * the inhibitory interneurones increase their activity to **dampen down sensory information** and **stop it from coming into awareness**

Question 29

what are the 3 surgical approaches to the thalamus?

Answer 29

1. interhemispheric transcallosal 2. transcortical transventricular 3. pterional transylvanian transinsular

Question 30

What is involved in the transcortical transventricular approach to the thalamus? Why is this approach less preferable?

Answer 30

* involves **_cutting through the cortex_** to reach the **_ventricles_** to approach tumours in this vicinity * as it involves cutting through the cortex, there will be **side effects** (depending on which part of the cortex has been cut - usually **seizures**) * as **a lot of white matter is cut through** to reach the ventricles, it is associated with a **_higher rate of complications_**

Question 31

What is involved in the pterional transylvanian-transinsular approach to the thalamus and when might this be used? What is the associated risk?

Answer 31

* used to treat more **_lateral tumours_** located **_close to the insula_** * a window is made by **cutting through the pterion, dura and meninges** * the **lateral fissure is opened up** and a **cut is made through the insula** to reach the tumour * risk of **_disturbing the fibres of the internal capsule_**

Question 32

What is involved in the interhemispheric transcallosal approach to the thalamus? When can this be used and why is it a preferred approach?

Answer 32

* the hemispheres are pushed apart and a **cut is made through the _corpus callosum**_ to reach the _**lateral ventricles_ superiorly** * the IIIrd ventricle and thalamus can be reached **_via connecting channels_** from the lateral ventricles * **no cutting through the cortex** so less risk of complications * only suitable for **tumours that can be reached through the ventricles** * *these are tumours located more medially within the thalamus*

Question 33

What are the 3 most common complications of thalamic surgery and what injury causes them?

Answer 33

**_contralateral motor dysfunction:_** * **due to injury to the adjacent internal capsule** **_visual field defects:_** * due to injury to the optic pathways related to the **posterior thalamus** (**pulvinar & LGN**) **_aphasia:_** * can occur during **left thalamic tumour resection**

Question 34

What are the following borders of the thalamus?

Answer 34

Question 35

Where is the "critical surface" of the thalamus and why is it important? What are the 3 "free" surfaces of the thalamus?

Answer 35

* the critical surface is the **_ventrolateral border_** of the thalamus, which has relationships with the **internal capsule** and **subthalamic structures** * surgeons must take extra care when operating in this area * the **_anterior, posterior_** and **_medial surfaces_** of the thalamus are ***"free" borders***

Question 36

Where is the subthalamus located? What does it lie medial to?

Answer 36

* it is **_inferior to the thalamus**_ and _**lateral to the hypothalamus_** * it is located **medial** to the **internal capsule** and **cerebral peduncles**

Question 37

What is the role of the subthalamus? Why is its functional grouping different to its anatomical grouping?

Answer 37

* it has roles in **_initiating and coordinating smooth and controlled movements_** * it is anatomically grouped with the diencephalon, but functionally grouped with the **_basal ganglia_**

Question 38

What is the main component of the subthalamus?

Answer 38

**_subthalamic nucleus_** * this is associated with the **motor functions of the basal ganglia** and has roles in initiating smooth, controlled and coordinated movements

Question 39

What are the following structures and which component of the diencephalon are they part of?

Answer 39

* the **_habenula_** and **_pineal gland**_ make up the _**epithalamus_** * the **stria medullaris thalami** is a bundle of fibres that arches over the thalamus to **connect to the habenula**

Question 40

Where are the habenulas located? What underlies this structure?

Answer 40

* they are located on the **dorsomedial surface** of the **posterior thalamus** * underlying each habenula are the **_habenular nuclei_** * the habenular nuclei are a centre for the **_integration of smell_** * *they are involved in the link between smell and memory / emotions*

Question 42

Where does the stria medullaris thalami run to and from? What information is received by the habenula and what is its role?

Answer 42

* the **_septal (paraolfactory) area_** is located on the medial aspect of the hemispheres and receives **information about smell** after processing by the **primary olfactory cortex** * the septal area is also part of the **_limbic system_** * the stria medullaris thalami runs **_from the septal area to the habenula_** * the habenula receives information about **_smell, emotions and memory_** and projects to specific areas of the brainstem

Question 43

What nuclei can the habenula connect to to induce an autonomic response?

Answer 43

* the habenula integrates smell, memory and emotions and connects to the **_superior**_ and _**inferior salivatory nuclei_** * these send **GVE fibres** to the **salivatory glands** to stimulate salivation * salivation occurs in response to smell / the memory of a certain smell

Question 44

Where is the pineal gland located? What does it form the posterior aspect of?

Answer 44

* located **above the tectum of the midbrain** (superior colliculus) * located **below the splenium of the corpus callosum** * it forms the **_posterior aspect of the IIIrd ventricle_**

Question 45

What is the role of the pineal gland?

Answer 45

* it is an **endocrine gland** that releases **_melatonin**_ in response to _**levels of light_** * it has no direct connection to the CNS

Question 46

Describe the indirect pathway from the retina involving the pineal gland

Answer 46

* retina sends some information about the levels of light to the **_suprachiasmatic nucleus (SCN)_** of the **hypothalamus** * hypothalamus sends descending fibres through the thoracic spinal cord via the **_intermediolateral cell column_** (T1 - L2) * fibres pass into the **_sympathetic chain_** and ascend to reach the **_superior cervical ganglion_** * fibres pass from superior cervical ganglion into the **_nervus conarii**_, which travels to the _**pineal gland_** * the pineal gland decides how to **regulate levels of melatonin** in response to levels of light

Question 47

Why is the pathway from the retina to the pineal gland called the "indirect" pathway? Why is melatonin production important?

Answer 47

* it is an indirect pathway as the pineal gland does not have any direct connections to the CNS * melatonin production is important for the **regulation of circadian rhythms** (sleep-wake cycle)

Question 48

What are the boundaries of the hypothalamus?

Answer 48

Question 49

What is the role of the hypothalamus?

Answer 49

* it has a role in regulating the internal environment and **_maintaining homeostasis_**

Question 50

How is the hypothalamus involved in temperature regulation? Which part of the hypothalamus coordinates this?

Answer 50

* if there is an increase in blood temperature, the hypothalamus responds by **initiating heat loss mechanisms** (e.g. sweating) * this is performed by the **_anterior hypothalamus_** * if there is a decrease in blood temperature, the hypothalamus responds by **initiating heat maintenance mechanisms** (e.g. shivering) * this is performed by the **_posterior hypothalamus_**

Question 51

What is the result of a lesion to the anterior hypothalamus?

Answer 51

hyperthermia * the person feels warm all the time as they are unable to initiate heat loss mechanisms

Question 52

What are the 2 parasympathetic and 2 sympathetic nuclei of the hypothalamus? Other than autonomic events, what are the other roles of the hypothalamus?

Answer 52

**_Parasympathetic nuclei:_** * preoptic nucleus * anterior nucleus **_Sympathetic nuclei:_** * posterior nucleus * lateral nucleus **_Other functions:_** * also has roles in sexual and eating behaviour

Question 53

How can the thalamus be divided into zones?

Answer 53

* the column of the fornix descends through the hypothalamus to reach the mamillary bodies * this divides the hypothalamus into a **medial and lateral zone**