Thalamocortical Physiology Flashcards
How are the neurons organized in the cortex
Well-organized
How are the neurons organized outside the cortex (the amygdala for ex)?
Not well organized
Where does the EEG measure the voltage?
surface
Does the EEG tell you about what’s happening deeply in brain?
Yes, even though only measures surface
Why wouldn’t the thalamus contribute directly to EEG signals?
EEG measures the summed activity of cells firing at the outer surface of the brain. The thalamus is too deep within the brain for its activity to be directly recorded by the EEG.
How does the thalamus actually contribute to an EEG signal?
The thalamus directs cortical activity via thalamocortical connections. For example, the during sleep the thalamocortical cells fire slow, 3 Hz delta waves in the cortical cells. This cortical activity is measurable by EEG.
How do thalamic relay neurons respond to hyperpolarization and what enables this response?
Thalamic relay neurons have T-type Ca2+ channels that possess activation and inactivation gates. The inactivation gates are closed at the normal depolarized resting potential (~-55mV) but are open when the cell is hyper polarized (~-85mV, by GABAergic thalamic reticular cells during sleep). The activated T-type Ca2+ channels generate Ca2+ spikes at a frequency of ~3 Hz.
Why are the EEG signatures found in absence seizures thought to arise from thalamocortical activity?
The EEG signatures found in absence seizures very closely resemble delta wave signals created by thalamocortical oscillatory activity. Also, such seizures cannot be replicated in mouse models that do not express T-type Ca2+ channels.
How might a mutation in the T-type Ca2+ channel lead to absence seizures?
Mutations in the T-type Ca2+ channels that alter the voltage dependence of the inactivation gate allow the firing of slow Ca2+ spikes at depolarized resting potentials (~-60mV). In humans, there is a predisposition for absence seizures in families with specific mutations in T-type Ca2+ channel genes.
What types of neurons confer ascending control of the thalamocortical circuits?
Thalamocortical circuit activity is regulated by axons ascending the brain stem from cholinergic, noradrenergic, and serotonergic neurons.
What are two epilepsy drugs that target the T-type Ca2+ channel?
Ethosuxamide and valproic acid
What happens when ascending cholinergic neurons fire in the thalamus?
Stimulation of cholinergic neurons in the reticular activating system causes release of acetylcholine in the thalamus and results in the awakening of the animal and interruption of thalamocortical slow waves in the EEG.
