What are the types of cortical stimulation (awakening)?
2 forms:
Direct neuronal stimulation
Neurohormonal systems
What happens if there is no cortical excitation?
Coma; cerebrum is useless without constant activation by the lower centers
What drives wakefulness?
Reticular formation which sits in the brain stem and its activity radiates out to the cerebral cortex
What are the 2 parts of the reticular formation?
Excitatory area
Inhibitory area (sleepiness results from activation)
What kind of action does the inhibitory area perform?
Its only known function is to inhibit the excitatory area from activating the rest of the brain. No direct effect on the cortex observed.
What CN is located directly inferiorly to the excitatory area? Why is this important?
CN5 (trigeminal nerve)
CN5 recieves sensory signals which can activate the excitatory area.
Where in the brainstem does the reticular excitatory area sit?
In reticular formation of pons and midbrain.
Where does the reticular excitatory area send signals?
Upwards towards the cortex and downwards towards antigravity muscles and controls levels of spinal reflexes
What do antigravity muscles do in response to excitatory area stimulation?
They become stronger and produce higher amounts of tension
What are the types of actions that the Reticular Excitatory Area has?
1) Rapid transmission to excite the cerebrum. (gigantocellular neurones: excitatory and produce acetylcholine) [rapid action rapid destruction]
2) Progressive build up for seconds to minutes. Large number of smaller neurons, slower signal: Also often excitatory and controls long term excitation of the brain. (Dopamine, Noradrenaline, Serotonin)
What is the locus coeruleus?
The origin of the noradrenaline system.
What does the noradrenaline system do?
It acts mostly in an excitatory way in fibers spread throughout the cortex.
It is active during wakefulness and non-REM sleep and inactive during REM sleep.
What is the action of the dopamine system?
Can be both excitatory and inhibitory.
Fibers innervate specific regions of the brain
Where is the dopamine system located?
Starts off at substantia niagra, arcuate vasciculus and VTA
What does the serotonin system do?
Usually inhibitory, fibers to diencephalon and specific cortex regions.
Induces sleep, controls pain, and mood control
Where is the serotonin system located?
Raphe nuclei
What does acetylcholine system do?
Usually excitatory and causes feeling of being acutely awake and excited nervous system.
It also drives REM sleep
Where is the acetylcholine system located?
Gigantocellular neurons in REA
Why are Glutamate and GABA the most important neurotransmitters to understand?
They are the most abundant excitatory neurotransmitters in CNS.
What does glutamate do?
Binds NMDA (Na and Ca channel) and AMPA (Na channel) and produce exctation. It is excitatory.
What does GABA do?
It is the most common inhibitory NT in CNS and it binds GABAa (Cl channels) or GABAb (GPCR -> potassium channels)
What enzyme converts glutamate to GABA?
Glutamate decarboxylase
What excites excitatory area?
Peripheral sensory signals. (Pain signals strongly excite the brain)
Feedback:
Cortex activation (motor/thought) -> REA -> Cortex activation (thalamic feedback does the same)
+ve feedback; any cerebral activity.
What are some functional brain imagine techniques used?
Electroencephalography (EEG)
Positron Emission Tomography (PET)
Functional Magnetic Resonance Imaging (fMRI)
What are the types of resolution that can be achieved from imagine of the brain?
Spatial resolution which gives accuracy of location in space
Temporal resolution which gives accuracy in determining the timing of activity.
What is the best method of scanning for spatial resolution? What is the worst?
Worst: EEG
Best: PET
What is the best and worst scanning method for temporal resolution?
Excellent: EEG
Pretty bad: fMRI
Terrible: PET
What is electroencephalography?
An old technique in which the electrical current moving in the brain is measured via many small electrodes.
What are the limitations of EEG?
The current has to flow through both the cranium and the scalp and so changes in electric potential are only detected if they are major.
What is EEG good for?
Diagnosing pathologies of electrical activity, sleep abnormalities, coma, confirmation of brain death, psychiatric disorders-gamma rhythm
What kind of signals can EEG detect?
Volume currents which become distorted as they follow lines of least resistance through the skull tissue.
What do corticothalamal loops tell us? How do they show up on the EEG?
The brain is keeping itself awake which means no external signals are likely activating them (i.e occipital lobe synchronous waves show eyes are closed). they show up as synchronous alpha waves.(8 - 13 Hz)
What happens to EEG pattern during deep sleep?
Waves are much larger due to corticothalamal loops. During dreams (REM sleep) the waves are smaller again.
What are the types of rhythms of EEG?
Beta rhythm (small amplitude high frequency resulting in alert cortical state)
Alpha rhythm (Awake medium amplitude medium frequency waves resulting in awake and relaxed state)
Delta rhythm (Large amplitude low frequency which is seen in deep sleep)
Gamma rhythm (has very high frequency and is consistent with elevated states of consciousness)
What happens when GABA is knocked out?
If you suppress inhibitory signal of GABA you create excitation which is seen as a lower amplitude higher frequency signals.
What is a coma?
Widespread cortical damage resulting in loss of consciousness.
Damage to ascending reticular activating systems causes permanent sleep
What causes seizures?
Brief episodes of abnormal excessive or synchronous neuronal activity in the brain.
Above what point of the pons is damage going to induce a coma?
Above the entry of CN5
What are the types of seizure effect?
Momentary loss of awareness (absence seizure)
Uncontrolled jerking movement (tonic-clonic seizure)
How are PET scans conducted?
a “tracer” containing positron-emitting isotopes of organic molecules/compounds is injected intravenously (Fluoridated glucose 18F-FTG) and this flows to area of highest metabolic needs (most active) and so brain activity will use the fluoridated glucose resulting in it showing up on the scan.
What are the spatial and temporal resolution of PET scans?
Spatial resolution = 5 - 10mm
Temporal resolution = minutes
What conditions are PET scans good for diagnosing?
Cancers
What does fMRI measure?
Haemoglobin dissociation (Hb:O vs Hb + O)
When group of neurons fire more signals they increase their metabolic rate and therefore that creates more oxygenated blood flow to the area and in turn more oxygen dissociation from haemoglobin
