23. Cortex, Thalamus & Hypothalamus Flashcards
What are the three types of fibre that make up white matter?
Association Fibres – connect with areas in the same hemisphere Commissural Fibres – connect the two hemispheres Projection Fibres – connect the cortex with lower brain structures (e.g. thalamus)
How many layers of grey matter are there?
3-6 (they are usually numbered by roman numerals)
What is the neocortex?
A part of the cerebral cortex concerned with sight and hearing in mammals, regarded as the most recently evolved part of the cortex
Describe the different connections of the 6 layers of grey matter.
Layers 1-3 = mainly cortico-cortical connections Layer 4 = input from the thalamus Layer 5-6 = connections with subcortical, brainstem and spinal cord
What does layer 1 mainly consist of?
Neutropil – an area composed mostly of unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies
What type of neurone is found in layer 4?
Stellate neurones
What type of neurone is found in layer 5?
Pyramidal neurones
What are the two parts of the visual association cortex and what are they responsible for?
Dorsal Pathway – responsible for interpretation of spatial relationships and movements Ventral Pathway – responsible for form and colour
What is the role of the posterior parietal association cortex?
It creates a SPATIAL MAP of the body in its surroundings from multi-modality information
What could injury of this part of the association cortex lead to?
Disorientation Inability to read a map or understand spatial relationships Apraxia Hemispatial Neglect
Define apraxia.
Inability to make skilled movements with accuracy
What is the temporal association cortex responsible for?
Language Object Recognition Memory Emotions
What are the two main consequences of injury to the temporal lobe?
AGNOSIA – inability for the brain to interpret sensory information although the nerves carrying sensory information to the brain are fine E.g. visual agnosia – patients can see perfectly fine but they can’t interpret sympbols such as letters RECEPTIVE APHASIA –unable to understand language in the spoken or written forms
What are the consequences of visual association cortex lesions?
Prosopagnosia – inability to recognise faces
What is the role of the frontal lobe?
Executive functions e.g. planning, judgement, foresight, personality
What are the consequences of a prefrontal lobotomy?
Change in personality Inappropriate behaviour Lack of ability to remember and relate things over time Attention span and ability to concentrate are diminished
What two areas does the prefrontal cortex receive massive inputs from?
Sensory association cortex (somatosensory, visual and auditory) Dorsomedial Nucleus of the thalamus NOTE: lesion of the dorsomedial nucleus will have similar consequences to prefrontal lobotomy
If you give someone with a unilateral parietal lobe lesion something to draw, what will you expect him or her to do?
Hemispatial neglect – they will only draw half of it
What effect do temporal cortex lesions have on memory?
Impaired short-term memory They are effectively trapped in a 30 second window of memory
Describe hemispheric specialisation.
Right hemisphere = creative + artistic Left hemisphere = logical + scientific
What is a callosotomy?
A palliative procedure used for the treatment of seizures The corpus callosum is key for the interhemispheric spread of epileptic activity
State a type of imaging that uses the movement of water molecules in the brain to infer the underlying structure of white matter.
Diffusion Tensor Imaging – Tractography
State two types of brain stimulation testing and what it can be used for.
Transcranial Magnetic Stimulation (TMS) Magnetic field induces a current in the cortex This is method of focally stimulating different areas of the cortex and testing what each area is responsible for Transcranial Direct Current Stimulation (TDCS) This changes the excitability of neurones but does NOT directly induce neuronal firing Anode = increases neuronal excitability Cathode = decreases neuronal excitability TDCS could be used to reduce motion sickness by suppressing the area of the cortex associated with perceiving vestibular information
Describe and explain how PET scans work. What can it be used for?
A radioactive tracer is attached to a molecule to locate areas of the brain where that molecule is being absorbed The tracer emits positrons, which are then detected by the receptors It can be used in Parkinson’s disease to see the uptake of dopamine precursors by dopaminergic neurones
What is the difference between MEGs and EEGs?
MEGs = magnetoencephalography – measures magnetic fields EEGs = electroencephalography – measures electric fields
What is a major problem with MEGs and EEGs and how is this resolved?
It is quite noisy – there is a lot of background activity This is resolved by doing a trial of a large number of participants so that an average can be found Once the average has been found, it can be deducted from the captured signal to see the underlying activity
What is fMRI?
Function MRI It detects changes in blood flow in the brain It relies on the fact that blood flow in the brain and neuronal activity are coupled – more active parts of the brain require increased blood flow
Which areas of the brain become more active when participants imagine positive events?
Amygdala Rostral anterior cingulate cortex
Where is the thalamus found within the brain?
It is right in the middle the brain just under the posterior half of the corpus callosum
What separates the two halves of the thalamus?
3rd ventricle Some people have a bridge connecting the two halves
Describe the connections of the thalamus with the forebrain.
Each half of the thalamus has ipsilateral connections with the forebrain
What is the main function of the thalamus?
It is a relay centre between the cerebral cortex and the rest of the CNS
What is the only function that is not represented within the thalamus?
Olfaction
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
Which nuclei connect with the motor cortex (primary, premotorand 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
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 do the anterior, lateral dorsal and dorsomedial nuclei connect with?
Prefrontal and Cingulate Cortex
What do the lateral posterior and pulvinar nucleus connect with?
Prefrontal and Parieto-tempero-occipital Cortex
What important system are the intralaminar and reticular nuclei a part of?
Reticular activating system – involved in maintaining consciousness
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 othernuclei to the cortex
What is the core of grey matter that runs through the brainstem and is involved in the reticular activating system?
Reticular formation
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
What is thalamic syndrome?
Syndrome that develops after thalamic stroke The symptoms depend on which part of the thalamus has been affected
What three 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 location and structure of the hypothalamus.
The hypothalamus is just below the thalamus and is divided by the 3rd ventricle It also has ipsilateral connections with the forebrain
How is the hypothalamus involved in maintaining homeostasis?
It coordinates these different mechanisms to maintain homeostasis: Autonomic nervous system Endocrine system Behaviour
State some forebrain structures that the hypothalamus has very close connections with
Olfactory system Limbic system
List some structures of the limbic system.
Hippocampus Amygdala Cingulate Cortex Septal Nuclei
What does the behavioural control exerted by the hypothalamus include?
Eating and drinking Expression of emotion Sexual behaviour Circadian rhythm Memory
Which nucleus is involved in the circadian rhythm?
Subrachiasmatic nucleus
How is our behaviour directed towards homeostatic goals?
There is a pleasure centre within the limbic system, which, whenever you’ve achieved homeostasis (e.g. eating food when you’re hungry), the activity of the pleasure centre increases
What are the presenting symptoms of hypothalamic tumour?
Polydipsia Polyuria Absent menses
What are some later symptoms of hypothalamic tumour?
Labile emotions, rage Inappropriate sexual behaviour Memory lapses Temperature fluctuation Thyroid, adrenal cortex and gonadal function decreases Hyperphagia
How common are strokes in US?
- 3rd leading cause of death in US - major cause of adult disability - about 800,000 people in US have a stroke each year - on avg, one American dies from a stroke q 4 minutes
Definition of a stroke? 2 main types?
- alteration of cerebral blood flow either from: brain ischemia: thrombosis, embolism, or systemic hypo perfusion or brain hemorrhage: intracerebral hemorrhage or SAH
source of anterior circulation of the brain?
- from internal carotid - majority of blood flow to the brain
Source of posterior circulation of the brain?
- verterbal-basilar
Cardiac sources of ischemic stroke?
- a fib - ASD/VSD - recent AMI - endocarditis - cardiac tumor - valvular disorder
What is the most common stroke type?
- ischemic stroke
Other etiologies of strokes?
- atherosclerotic plaques (emboli from rupture, lack of perfusion from stenosis of vessels) - vasculitis - prothrombotic state - cerebral hemorrhage (20% of strokes)
How is a-fib a source of a stroke? - how does anticoag help this? aspirin?
- embolization of intracardiac thrombi - most commonly from left atrial appendage - anticoagulation decreases the risk of stroke by up to 70% - aspirin decreases risk by 20-25%
Atrial and ventricular septal defects - source of stroke?
atrial: - if assoc with R to L shunt can cause stroke - patent foramen ovale: present in about 25% of general population, surgical or percutaneous closure ventricular: if assoc with R to L shunt can cause stroke
MI - as source of stroke?
- most common in pts after anterior wall infarction - left ventricular wall mural thrombi: large infarctions, LV dilation, CHF
Endocarditis source of stroke?
- emboli from vegetations
Cardiac tumor as a source of stroke?
- obstruction of blood flow - can lead to arrhythmias (like a-fib) - embolization of tumor fragments
Valvular disorders as a source of a stroke?
- native valves: rheumatic mitral stenosis is most commonly assoc with stroke - MVP: may have fibrinious deposits on valve - prosthetic heart valves: mechanical valves require lifelong anticoag - repaired cardiac valves: require only anticoag short term
How common is hemorrhagic stroke?
20% of al strokes - spontaneous intracerebral hemorrhage (10%) - SAH - other 10%: intracranial aneurysm, and arteriovenous malformations
What are the causes of spontaneous intracerebral hemorrhage?
- assoc with poorly controlled HTN: commonly located in basal ganglia and less commonly in pons, thalamus, cerebellum or cerebral white matter - lacunar infarcts are assoc with HTN or DM - bleeding disorders - amyloid angiopathy: amyloid deposits lead to weakening of cerebral blood vessels resulting in a stroke
Causes of SAH?
- trauma - spontaneous SAH is usually related to a ruptured AVM or aneurysm - abnorm vascular composition (amyloid angiopathy or dissection) - illict drug use such as cocain or amphetamines - intracranial arterial dissections - 20% may have no ID cause
Most common site for intracranial aneurysm? What determines the risk of rupture?
- most commonly located in circle of willis - aneurysm is usually asx until rupture - size and location determine risk of rupture: in general size over 1 cm carries a high risk of rupture
What is an arteriovenous malformation (AVM)?
- abnormal arterial to venous connection - arteries and veins are tangled up - veins are under high pressure which leads to rupture - also assoc with risk of seizure - occurs in 0.01% of population, 1-2% of all strokes and 9% of SAH - may be assoc with heredirary hemorrhagic telangiectasia (HHT; osler-weber-rendu syndrome)
Subtypes of strokes?
- hemorrhagic: intracerebral hemorrhage SAH - ischemic: anterior circulation posterior circulation lacunar
What is an intracerebral hemorrhage? Major causes?
- arterial bleeding directly into the brain parenchyma - major causes: HTN, trauma, bleeding disorder, amyloid angiopathy, illicit drug use, AVMs - accum of blood over minutes to hours forming a localized hematoma - *** neuro sxs increase gradually as hematoma grows - brain tissue is destroyed as hematoma enlarges, pressure created by blood and surrounding brain edema is life-threatening - large hematomas have a high mortality and morbidity - goal of tx is to contain and limit the bleeding
2 main causes of SAH? What is happening?
- ruptured aneurysm (most common) or AVM - bleeding into CSF and space surrounding brain - aneurysm bleeds into CSF under arterial pressure and increased the ICP (this causes sxs) - bleeding lasts a few seconds but rebleeding is common
Main tx goal of SAH?
- Identification of source of bleeding and tx before rebleeding occurs - other goal of tx is to prevent brain damage due to delayed ischemia related to vasoconstriction of intracranial arteries: blood within the CSF induces vasoconstriction which can be intense and severe (can cause another stroke!)
2/3 of all ischemic strokes affect what circulation in the brain? 2 main arteries of this circulation?
- anterior circulation - MCA and ACA
What is the most commonly affected vessel in ischemic strokes?
- MCA: 96% of all anterior circulation strokes (due to direct flow from internal carotid artery and its large size) - 3% in ACA - 1% in entire ICA distribution
What arteries would be involved in posterior circulation stroke? What areas of the brain does this supply?
- verterbral artery - basilar artery - posterior cerebral artery - blood supply to posterior portion of brain, including occipital lobes, cerebellum, and brainstem
Outcomes of posterior circulation strokes?
- terrible! - 20% of all strokes - 20-60% have unfavorable outcomes - basilar artery occlusion: 90% mortality, 8-14% of all posterior circulation strokes
What are lacunar strokes?
- small lesions (less than 5 mm) that occur in penetrating arterioles in basal ganglia, pons, cerebellum, internal capsule, thalamus, and deep cerebral white matter
Outlook of lacunar strokes? How do these appear on CT?
- less morbidity and mortality than other strokes - on CT sometimes seen as “punched out hypodense areas” but sometimes no abnormalities can be seen
Anterior strokes occur from occlusion off of what artery?
- internal carotid artery or its branches
