Diencephalon Flashcards
What does the diencephalon consist of
Thalamus and hypothalamus
What is important to remember about the thalamus
Core of the brain- relay station- nearly all pathways will have a synapse in thalamus
What are the 3 structures found in the diencephalon
Thalamus
Subthalamus- important for neurosurgeons- where electrodes are inserted for brain stimulation treatment in Parkinson’s (to stop tremors- part of basal ganglia)
Hypothalamus
Which structure is in close connection with the hypothalamus
The pituitary gland
Where is the thalamus found within the brain
Sits ventral to the lateral ventricles
Thalamus divided in two by 3rd ventricle (Some people have a bridge connecting the two halves).
Just below it hypothalamus
Two corticospinal tracts encapsulate thalamus
Can see two subthalmic nuclei- and beneath this the substantia nigra- thalamus well connected with the midbrain below it
Organised into discrete nuclei (thalamus is large area of grey mater)* ( *In neuroanatomy, nuclei are clusters of neurons with a similar functions and connection)=- with the exception of the basal ganglia.
)
Look at midline structures anteriorly it will be the basal ganglia- but the more posterior you go in coronal slices- will see thalamus in midline
Summarise the functions of the thalamus
Relay site for numerous inputs/outputs
Key relay centre to cortical sensory areas
Involved in almost all sensory systems (except olfactory- which is limbic system- no emotional connections to thalamus)
Enhances or restricts signals
What is the main function of the thalamus
It is a relay centre between the cerebral cortex and the rest of the CNS
Describe how thalamic nuclei are named.
They are named based on their location within the thalamus
What is the classification of thalamic nuclei based on and what are the four different classes?
The classification is based on the connections of the thalamic nuclei with the cortex
SPECIFIC – connected to primary cortical areas
ASSOCIATION – connected to association cortex
INTRALAMINAR – connected to ALL cortical areas
RETICULAR – not connected to the cortex (some nuclei are part of the reticular activating system)
Which nuclei connect with the motor cortex (primary, premotor and supplementary)?
Ventral lateral
Ventral anterior
Which nuclei relay sensory information from different parts of the body?
Head – Ventral posteromedial
Below the neck – Ventral posterolateral
Which nucleus is connected to the primary visual cortex?
Lateral geniculate nucleus
Which nucleus is connected to the primary auditory cortex?
Medial Geniculate Nucleus.
Where is the sensory cortex located
In the post-central gyrus (parietal lobe)
Describe the projections of the intra-laminar nuclei
Project to various medial temporal lobe structures (e.g. amygdala, hippocampus and basal ganglia)
Amygdala = emotions, fear, anxiety- commonly affected area of the brain in pathology- will present with an inherent fear and anxiety of everything (found in anterior temporal lobe)
Hippocampus = memory (just behind amygdala at floor of fourth ventricle)
Basal ganglia = movement (control of the initiation of movement)
Describe the neurones of the intra-laminar nuclei
Mostly glutamatergic neurons (i.e. excitatory)
Loss of neurons in this region associated with progressive supranuclear palsy* and Parkinson’s disease
Mostly motor neurones affected
Describe progressive supranuclear palsy
*PSP = rare brain disorder causing problems with walking and balance
How do the intralaminar nuclei modulate the activity of the cortex?
The reticular formation projects up to the thalamus to the intralaminar nuclei and the intralaminar nuclei, because of their diffuse cortical projections, can modulate the activity of the cortex
Describe the structure of the intra-laminar nuclei
The thalamus is organized around a Y-shaped collection of white matter called the ‘internal medullary lamina’. This splits the thalamus into three sections – the anterior, lateral and medial sections. Each section is composed of several cell groups with particular inputs and functions, and there are also groups of cells inside the medullary lamina.
Describe the neurones of the reticular nuclei
Forms the outer covering of the thalamus
Majority of neurons are GABAergic* (e.g. inhibitory)
Describe the connections of the reticular nuclei
Unlike other thalamic nuclei, they don’t connect with distal regions, but with other thalamic nuclei
Receive inputs from collaterals of their axons from thalamic nuclei
Therefore, reticular nucleus acts to modulate thalamic activity (negative feedback)
Involved only in the local circuitry of the thalamus as to modulate the internal activity of the thalamus
Describe how the reticular nuclei affect cortical activity
The reticular nuclei don’t have any direct connections with the cortex but they do have widespread intrathalamic connections with all other thalamic nuclei so it can influence the flow of information from the other nuclei to the cortex
What is meant by the reticular formation and where is it located
core of grey matter that runs through the brainstem and is involved in the reticular activating system
Set of interconnected pathways in the brainstem
Send ascending projections to forebrain nuclei
Ascending reticular activating system (ARAS)
Describe the role of the reticular activating system
Involved in consciousness and arousal
Degrees of wakefulness depend on ARAS activity (increased activity = increased wakefulness)
Both intralaminar and reticular nucleus receive inputs from ARAS
§ The reticular formation projects through the brainstem and is controlling of level of consciousness of the brain – and with the cortex connections of the intra-laminar nuclei, all parts of the cortex can be affected by changes of consciousness.
Describe the organisation of the hypothalamus
§ Located in the diencephalon below the thalamus.
§ Divided into the left and right hypothalamus by the third ventricle.
§ Each side is largely a collection of individual nuclei with separate actions and ipsilateral connections with forebrain structures.
§ Mammillary bodies lie directly below the hypothalamus.
Where is the hypothalamus found
Divided into two by 3rd ventricle (hypothalamic nuclei found at the floor and walls of the 3rd ventricle)
Collection of individual nuclei with distinct functions
Largely ipsilateral* connections with other nuclei
Essentially, what is the hypothalamus involved in
The four Fs:
Fighting
Fleeing
Feeding
Mating
How is the hypothalamus involved in maintaining homeostasis?
Co-ordinates homeostatic mechanisms by:
§ Regulating ANS via connections with the spinal cord.
§ Acting as an endocrine organ via the pituitary gland.
§ Controlling behaviour via connections with forebrain structures.
Which nucleus in the hypothalamus has connections with the ANS
Direct connections with autonomic nervous system
Neurons in hypothalamic paraventricular nucleus with direct projections to pre-autonomic neurons in spinal cord
Wing shaped nucleus at top of 3rd Ventricle
Describe the potential applications of optogenetics in obesity therapeutics
Optogenetic stimulation of GABAergic LH neuron terminals in PVN results in feeding.
If we could genetically modify these cells to respond to light and thus stimulate the inhibitory signal with light- it reduce appetite- thus having a potential role in the treatment of obesity.
Describe the projections of the paraventricular nucleus
Sends projections to autonomic nervous system and posterior pituitary gland- made up of parvocellular (small) and large magnocellular neurones (large)
Parvocellular neurones project to intermediolateral cell column to project onto autonomic neurones- to influence the heart, vasculature and kidneys.
Magnocellular neurones project to the posterior lobe of the pituitary gland- to control the release of vasopressin and oxytocin.
Describe the role of the PVN in feeding behaviour
PVN also involved in feeding behaviour
PVN lesions cause hyperphagia and weight gain
PVN receives input from hypothalamic nuclei involved in feeding (such as arcuate nucleus).
Optogenetic stimulation of GABAergic LH neuron terminals in PVN results in feeding.
Where is the supra-chiasmatic nucleus found
present rostrally (anteriorly) above optic chiasm for circadian rhythm (supraoptic above optic nerves)
Describe the role of the supra-chiasmatic nucleus in the circadian rhythm
Light sitmulates melanopsin expressing retinal ganglion cells which travel up the retinohypothalamic tract to the suprachiasmatic nucleus.
Neurones then synapse onto the PVN
Neurones from PVN synapse onto intermediolateral cell column in spinal cord
These neurones synapse in superior cervical ganglion and then synapse on pineal gland to control release of melatonin
This controls hours of wake with light-dark cycle.
What can lesions in the supra-chiasmatic nucleus lead to
Disrupted cycle
May be an early sign in Parkinson’s= as pathology travels up the brainstem and so can affect the RAS- leading to REM behaviour sleeping disorders- 80% of such patients progress to Parkinson’s.
Does the thalamus have ipsilateral or contralateral projections
Each is a collection of individual nuclei with separate functions and connections with ipsilateral forebrain structures.
Describe the association nuclei of the thalamus
Association nuclei have more diffuse reciprocal connections with association cortex.
The anterior, lateral dorsal and dorsomedial nuclei connect with parts of the limbic system (cingulate and prefrontal cortex).
The lateral posterior and pulvinar nuclei connect with the association cortex at the junction of the parietal, temporal and occipital lobes and the prefrontal cortex.
The association cortex can be divided into three areas based on thalamic function. What are these three areas?
Prefrontal Cortex
Parieto-tempero-occipital Cortex
Cingulate Cortex
What is thalamic syndrome
Syndrome that develops after thalamic stroke
The symptoms depend on which part of the thalamus has been affected
What 3 main changes occur in thalamic syndrome
CHANGE IN SENSATION – reduced, exaggerated, altered
PAIN – central, non-localised (not easily treated because normal analgesics have no effect – may need to use opioids or anti-convulsants/anti-depressants)
EMOTIONAL DISTURBANCE – the nuclei that transmit information to and from the association cortex are associated with the limbic system
Describe the consequences of traumatic brain injury
§ Most damage is from ‘shearing forces’ which damages the axons in the brain (white matter tracts).
§ Neuroinflammation (microglia activation) in the thalamus from TBI occurs for a VERY long time after an accident.
§ Damage in the white matter tracts, travels down the axons into the thalamus which causes thalamus inflammation.
o Damage, however, does not track back out into the cortex.
Describe the associated structures of the hypothalamus
§ Olfactory system.
§ Limbic system – hippocampus, amygdala, cingulate cortex and septal nuclei.
§ Behaviour directed towards well-being triggers the reward system in the hypothalamic-limbic circuitry, leading to reinforcement of that behaviour.
List the behavioural control influences of the hypothalamus
§ Eating and drinking. § Expression of emotion. § Sexual behaviour. § Circadian rhythm. § Memory.
Describe the consequences of hypothalamic damage
§ Structural damage includes:
o Craniopharyngioma and other tumours – i.e. glioma, meningioma, dermoid, chordoma, hamardoma.
o Inflammatory diseases – Sarcoidosis, Langerhans cell histiocytosis.
§ Patients with damage to the hypothalamus can have:
o Hormone deficiencies.
o Diabetes Insipidus and possible adipsia (doesn’t have any thirst at all).
o Cannot regulate food intake – infundibular (arcuate nucleus) and PVN.
o Forgets things – mammillary bodies.
o Temperature fluctuations – preoptic area.
o Aggression – pre-frontal cortex.
Describe the influences of the hypothalamus
§ The hypothalamus can influence: o Sexual activity. o Cardiac arrhythmias. o Alzheimer’s disease. o Narcolepsy & Cataplexy – orexin neurones are missing in patients with this. Basically the hypothalamus is involved in everything.
Where does the hypothalamus sit
Hypothalamus: sits below thalamus, with the optic chiasm and pituitary stalk just rostral to itself
