Diencephalon Flashcards
Where is the diencephalon located?
- it sits on top of the brainstem and connects it to the cerebral hemispheres
- it is obscured by the cerebral hemispheres
What are the 4 divisions of the diencephalon?
- thalamus
- epithalamus
- hypothalamus
- 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
What type information are the LGN and MGN of the thalamus important for?
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
Label the following structures
- the mammillary bodies and infundibulum (pituitary stalk) are part of the hypothalamus
- the pineal gland is part of the epithalamus
Which structures does the septum pellucidum run between?
What structures are located superior and inferior to the diencephalon?
- septum pellucidum is the wall between the corpus callosum and the fornix
- the diencephalon is wedged between the fornix superiorly and the midbrain inferiorly
What are the following structures?
What is marked by the dotted line and ?
- ? 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
What is located superior, anterior and inferior to the thalamus?
What is the interthalamic adhesion?
- 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
What separates the thalamus from the hypothalamus?
What are the borders of the hypothalamus?
- 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
What is indicated by the *
- recesses of the IIIrd ventricle found in the space bounded by the hypothalamus on both sides
- supraoptic recess
- 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
What are the 2 components of the epithalamus?
habenula + pineal gland
Label the structures and nuclei of the thalamus
- the thalamus is not one homogenous mass, but it is composed of many nuclear complexes
What forms the medial and lateral borders of the thalamus?
- the medial border is the IIIrd ventricle
- the thalamus has a free border medially
- the lateral border is the internal capsule
Label the coronal section
What is located medial, lateral and anterior to the thalamus?
- the medial border of the thalamus is a “free border” formed by the IIIrd 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
What are the borders of the diencephalon?
What is the massa intermedia?
- 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
Why are the thalami referred to as the “receptionist of the brain”?
- 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
What is the only sense not processed by the thalamus?
olfaction
What are the fibres that travel between the thalamus and the cortex?
- thalamocortical fibres travel from the thalamus to the cerebral cortex
- corticothalamic projections travel from the cortex to the thalamus
What is the internal medullary lamina and how does it segregate the thalamus?
- it is a Y-shaped layer of myelinated fibres
- it divides most of the thalamus into nuclear groups
- anterior nuclear group
- medial nuclear group
- lateral nuclear group (further subdivided)
What are the thalamic nuclei that are only visible in coronal section?
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
What does the anterior nuclear group communicate with?
Where is it located and what does it form the posterior boundary of?
- 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
What is the role of the medial nuclear group?
- the medial group contains the medial dorsal nucleus
- it receives and relays information related to emotional behaviour
What are the roles of the ventral anterior (VA) and ventral lateral (VL) nuclei of the lateral nuclear group?
- they are important for motor relay
- they communicate with motor structures such as the supplementary motor area, premotor area, cerebellum and basal ganglia
What are the roles of the ventral posterior medial (VPM) and ventral posterior lateral (VPL) nuclei of the lateral nuclear group?
- 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
What is the role of the lateral posterior (LP) and pulvinar nuclei of the lateral nuclear group?
- 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
Where are the reticular nuclei of the thalamus found?
What is significant about the communications of these nuclei?
- 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
What type of neurones are found within the reticular nucleus?
What are the 2 main roles of this nucleus?
- it contains a thin sheet of inhibitory neurones that project deep into the rest of the thalamus
- the reticular nucleus has a role in:
- dampening down ascending sensory pathways during sleep
- modulating the state of alertness
How is the reticular nucleus able to dampen down ascending sensory pathways during sleep?
- 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
what are the 3 surgical approaches to the thalamus?
- interhemispheric transcallosal
- transcortical transventricular
- pterional transylvanian transinsular
What is involved in the transcortical transventricular approach to the thalamus?
Why is this approach less preferable?
- 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
What is involved in the pterional transylvanian-transinsular approach to the thalamus and when might this be used?
What is the associated risk?
- 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
What is involved in the interhemispheric transcallosal approach to the thalamus?
When can this be used and why is it a preferred approach?
- 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
What are the 3 most common complications of thalamic surgery and what injury causes them?
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
What are the following borders of the thalamus?
Where is the “critical surface” of the thalamus and why is it important?
What are the 3 “free” surfaces of the thalamus?
- 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
Where is the subthalamus located?
What does it lie medial to?
- it is inferior to the thalamus** and **lateral to the hypothalamus
- it is located medial to the internal capsule and cerebral peduncles
What is the role of the subthalamus?
Why is its functional grouping different to its anatomical grouping?
- 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
What is the main component of the subthalamus?
subthalamic nucleus
- this is associated with the motor functions of the basal ganglia and has roles in initiating smooth, controlled and coordinated movements
What are the following structures and which component of the diencephalon are they part of?
- 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
Where are the habenulas located?
What underlies this structure?
- 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
Where does the stria medullaris thalami run to and from?
What information is received by the habenula and what is its role?
- 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
What nuclei can the habenula connect to to induce an autonomic response?
- 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
Where is the pineal gland located?
What does it form the posterior aspect of?
- 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
What is the role of the pineal gland?
- it is an endocrine gland that releases melatonin** in response to **levels of light
- it has no direct connection to the CNS
Describe the indirect pathway from the retina involving the pineal gland
- 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
Why is the pathway from the retina to the pineal gland called the “indirect” pathway?
Why is melatonin production important?
- 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)
What are the boundaries of the hypothalamus?
What is the role of the hypothalamus?
- it has a role in regulating the internal environment and maintaining homeostasis
How is the hypothalamus involved in temperature regulation?
Which part of the hypothalamus coordinates this?
- 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
What is the result of a lesion to the anterior hypothalamus?
hyperthermia
- the person feels warm all the time as they are unable to initiate heat loss mechanisms
What are the 2 parasympathetic and 2 sympathetic nuclei of the hypothalamus?
Other than autonomic events, what are the other roles of the hypothalamus?
Parasympathetic nuclei:
- preoptic nucleus
- anterior nucleus
Sympathetic nuclei:
- posterior nucleus
- lateral nucleus
Other functions:
- also has roles in sexual and eating behaviour
How can the thalamus be divided into zones?
- the column of the fornix descends through the hypothalamus to reach the mamillary bodies
- this divides the hypothalamus into a medial and lateral zone
