Thalamus and Hypothalamus

Question 1

Thalamus: identify and summarise the structure of the thalamus

Answer 1

Diencephalon: Thalamus + Hypothalamus + Subthalamic nucleus

• It is right in the middle of the brain, just under the posterior half of the corpus callosum
• If you look at it from the anterior aspect, you see that it has TWO HALVES 

• The halves are on either side of the 3rd ventricle 

• There is a little bridge that joins the two, this is a vestigeal structure - it doesn’t 
have a function in humans, most humans don’t have this connection at all 

• In the horizontal plane, the grey matter of the thalamus is seen on either side of the third ventricle 

• The thalamus is a collection of nuclei and each of the nuclei has a different function in the brain 


Organisation of the Thalamus

• Divided in TWO by the 3rd ventricle 

• It is a collection of individual nuclei with separate functions 

• Ipsilateral connections with forebrain - the part of the thalamus on the right 
will connect with the right hemisphere 

• The nuclei have separate functions but they are interconnected 
NOTE: Ipsilateral - belonging to or occurring in the same side of the body 


Function of the Thalamus 


• It is a relay centre between the cerebral cortex and the rest of the CNS 

• The connection is two way, it is reciprocal 

• There is also integration and modification of information as it goes through 
the thalamus 

• Involved in almost all sensory systems
• Enhances or restricts signals
• There is a nucleus within the thalamus for virtually every functional system 
within the nervous system 

• The only function that’s not represented within the thalamus is olfaction - this 
is evolutionarily the oldest sense within the brain 


Structure of the Thalamus 


• This is the left thalamus 

• The nuclei are named according to their position within the thalamus 

• You do NOT need to know the arrangement of nuclei within the thalamus 

• The nuclei can be classified into groups 


Question 2

Thalamic nuclei: explain the functional significance of the thalamic nuclei; explain the relationship between the intralaminar nuclei, reticular nucleus

Answer 2

​Thalamic Nuclei

It is classified based on the connection of the nuclei with the cortex: SARI 


• SPECIFIC 
Connected to primary cortical areas 

• ASSOCIATION 
Connected to association cortex 

• RETICULAR 
NOT connected to the cortex 

• INTRALAMINAR 
Connected to ALL cortical areas 
- Project to various medial temporal lobe structures (e.g. amygdala, hippocampus and basal ganglia)

Intralaminar Nuclei

• Project to various medial temporal lobe structures (e.g. amygdala, hippocampus and basal ganglia)
  
  • Amygdala = emotions, fear, anxiety
  • Hippocampus = memory
  • Basal ganglia = movement
• Mostly glutamatergic neurons (i.e. excitatory)
• Loss of neurons in this region associated with progressive supranuclear palsy* and Parkinson’s disease

Reticular nucleus

• Forms the outer covering of the thalamus
• Majority of neurons are GABAergic* (e.g. inhibitory)
• 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)

Specific Nuclei

• On the right is a list of the parts of the cortex that the nuclei on the left link to 

• Premotor and supplementary motor cortices lie alongside the primary motor 
cortex 

• There are TWO different nuclei that relay sensory information coming from 
different parts of the body: 
Below the neck - ventral posterolateral 
Head - ventral posteromedial 

• Lateral Geniculate - from the retina to the primary visual cortex 

• Medial Geniculate - from the inner ear to the primary auditory cortex 


Question 3

Functional Cortical Areas 


Answer 3

• All the white areas are association cortex 

• We can think of the association cortex as being divided into three areas in 
terms of thalamic function 

• Prefrontal Cortex - most of the frontal lobe except for the primary motor 
cortex and some other motor cortical areas 

• Parieto-tempero-occipital Cortex - consists of all of the cortex in the parietal, 
temporal and occipital lobes except for the primary cortical areas 

• Cingulate Cortex - it follows the line of the corpus callosum (can be viewed 
when looking at the medial surface of the cerebral hemisphere) 


Question 4

Association nuclei and reticular formation

Answer 4

Association Nuclei

• This is very simplified but gives an idea of how the nuclei are connected

Structure of the Thalamus continued

• There are two other types of nuclei, intralaminar and reticular 

• These nuclei are both part of the reticular activating system (RAS) 

• This is the system that maintains consciousness 


Thalamic Nuclei associated with the RAS 


• The reticular nucleus has no direct connections with the cortex 

• The reticular nucleus, however, does have intrathalamic connections with 
pretty much ALL other nuclei (it has a net like arrangement along the side of 
the thalamus) 

• Because of this, it is able to influence the flow of information through the other 
nuclei to the cortex 

• As part of the RAS, it gets activated by projections from the brainstem and it 
then increases the activity going through the other nuclei 

• This is an indirect way of influencing the activity of the cortex 


From the slide: 
Intralaminar Nuclei - diffuse cortical projections
Reticular Nucleus - intrathalamic projections

Both of these receive inputs from the reticular formation

This means that there is a direct and indirect way of affecting the activity of the cortex

Reticular formation

• Set of interconnected pathways in the brainstem
  
  • Send ascending projections to forebrain nuclei
    
    • Ascending reticular activating system (ARAS)
• 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

Reticular Activating System

• With the RAS you have a core of grey matter through the brainstem called the reticular formation 

• The reticular formation projects up to the thalamus (to the intralaminar nuclei) 

• Then, the intralaminar nuclei, because of their connections to all areas of the 
cortex, can modulate the activity of the cortex 

• Generally speaking, the greater the level of activity coming through the 
reticular formation into the thalamus, the more activated and stimulated the 
cortex becomes 

• This translates behaviourally to your level of consciousness/awareness 

• If you have some damage that interrupts this system, you will become 
unconscious and go into a coma 

• The intralaminar nuclei are a very important part of this because they have 
connections with all parts of the cortex and so can modulate the activity of the cortex 


Question 5

Thalamic Syndrome 


Answer 5

• There aren’t many diseases or syndromes that affect the thalamus except for thalamic syndrome - this develops after a thalamic stroke 

• The exact pattern of symptoms depends on which part of the thalamus has been affected - demonstrating the multifunctional nature of the thalamus 

• The most likely thing to happen is a change in sensation - due to disturbance of the nuclei that relay somatosensory information from the body to the primary somatosensory cortex 

• Changes that occur: 


Sensation

1. Reduced - feeling of numbness 

2. Exaggerated 

3. Altered - going from one modality to another e.g. a light touch may 
be perceived as a burning feeling


Pain

1. Central, non-localised pain 

2. This is not easily treated because the normal analgesics have no 
effect 


You may be able to use opioids against it

More likely you’ll get anti-depressants and anti-convulsants

Emotional Disturbance

• The nuclei that transmit information to and from the association cortex are often associated with the limbic system 

• Limbic System - circuit of structures within the forebrain that’s involved with setting your mood and controlling your emotions 


Question 6

Hypothalamus: recall the functions of the hypothalamus and interactions with the endocrine and autonomic systems and the brain controlling mood and behavior

Answer 6

Hypothalamus 


• Lies below the thalamus 

• It is divided through the middle by the 3rd ventricle 

• Collection of individual nuclei with distinct functions
• The optic chiasm and pituitary stalk are seen just at the front of the hypothalamus 


Coronal View of the Hypothalamus 


• The 3rd ventricle is the little black line in the midline 

• The big grey area on either side of the 3rd ventricle is the thalamus 

• Just below the thalamus you get the grey matter of the hypothalamus 

• There are little white lines going through the hypothalamus 


Organisation of the Hypothalamus

In terms of structure it is basically the same as the thalamus

• Divided in two by the 3rd ventricle 

• Collection of individual nuclei with separate functions 

• Largely ipsilateral connections with the forebrain 

• Involved in 4 F’s

1. Fighting
2. Fleeing
3. Feeding
4. Mating

Function of the Hypothalamus 
The hypothalamus is involved in homeostasis and it coordinates several different mechanisms that provide homeostasis: 


• Autonomic Nervous System 

• Endocrine System 

• Behaviour 

• Neurons in hypothalamic paraventricular nucleus with direct projections to pre-autonomic neurons in spinal cord
• Connections with endocrine systems
• Control of behaviour E.g. feeding behaviour

Paraventricular nucleus

• Sends projections to autonomic nervous system and posterior pituitary gland

Parvocellular: small cells – spinal cord – heart and vasculature

Magnocellular: larger cells - more secretory connecting to posterior pituitary reducing vasopressin and oxytoxin.

Associated Forebrain Structures 


• The hypothalamus has very close connections with some structures in the forebrain: 
Olfactory System 
Limbic System 

• Structures within the limbic system include: hippocampus, amygdala, cingulate 
cortex, septal nuclei 


Behavioural Control 


• Behavioural control includes: 
Eating and drinking Expression of emotion Sexual behaviour Circadian rhythm Memory 

• If you’re hungry then the nucleus involved in regulating appetite gets more active and this makes you go and eat 

• Once you’ve eaten, the appetite nucleus goes down in activity and the satiety nucleus goes up 

• The limbic system is important in setting your mood and anything that makes you happy will reinforce this homeostatic behaviour 

• Expression of emotion also has a close connection with the ANS (e.g. you get pale when you’re scared) 

• Sexual behaviour is important in making sure that the species survives 

• The suprachiasmatic nucleus is involved in controlling the circadian rhythm - it 
synchronises your body functions with the time of day 

• Memory - allows you to learn so that you don’t make the same mistakes again 
e.g. waiting a long time between meals and getting hungry 

• Structures within the hypothalamus are part of the circuit that consolidates 
memory 

• How is our behaviour directed towards homeostatic goals? 
It is because the hypothalamus and the limbic system have a motivational process 


There is a pleasure centre within the limbic system that whenever you’ve achieved homeostasis like eating food when you’re hungry the activity of the pleasure centre increases

• This motivational system is very powerful and it’s great for reinforcing beneficial behaviour 

• However, the motivational system can go wrong in the case of addiction