Basal Ganglia Flashcards
what is the BG, and what does it assert influence on?
old system predating the neo-cortex, that exerts influence on motor and prefrontal areas via the Thalamus and also on limbic circuits
what does BG play a major role in?
in action selection, reinforcement learning, and cognition and decision-making – filters actions ready for execution
what does the BG consist of?
of a collection of nuclei that sit above the brainstem, but below the cerebral cortex:
structure of the BG
- striatum = input structure that the cortex projects information through to enter the BG.
o caudate, putamen, and nucleus accumbens each receive input from a different area of the cortex. - globus pallidus
o interna (GPi)
o externa (GPe) - subthalamic nucleus (STN)
- substantia nigra and the pars compacta (SNc)
what does the cortex project to?
the striatum via an excitatory connection, with two following pathways - direct and indirect
direct pathway of cortex-striatum projection
striatum inhibits the GPi, which inhibits the STN.
this double inhibition releases the Thalamus from inhibition, resulting in an increase in Thalamic activity via disinhibition
(disinhibits the thalamus)
indirect pathway of cortex-striatum projection
striatum inhibits the GPe, which inhibits the STN, before exciting the GPi.
as this becomes more active, it increases inhibiton of the Thalamus (gateway back to the cortex)
(increases inhibition of the thalamus)
how is the balance between BG pathways regulated?
regulated by the substantia nigra, which innervates the striatum with dopamine.
this has one excitatory and one inhibitory connection
dopaminergic connections
D1 receptors are excited by dopamine and innervate the direct pathway.
D2 receptors are inhibited by dopamine and directly innervate the indirect pathway.
what are the consequences for neuronal loss in the SNc?
on movement, cognitive control, and decision-making
where can consequences for neuronal loss in the SNc be seen?
this can be seen in PET scans where (PD) patients have an asymmetric loss of uptake on the tracer vs. normal patients when BG binds to D receptors
weak direct pathways -> akinesia
the loss of dopaminergic innervation to the striatum causes severe imbalances in components of the BG circuit – no longer drives the direct pathway nor inhibits the indirect pathway
how does akinesia affect action plans?
unable to be selected efficiently, and those selected are slow and unrefined due to excessive inhibition from the indirect pathway – precisely what happens in PD
cardinal symptoms of PD
- Absence/slowness of movement (akinesia/bradykinesia)
- Stiffness or rigidity
- Tremor at “rest” (much reduced with active movement)
explanations of PD
- degeneration of dopaminergic neurons in the substantia nigra
- disordered motor signals are sent to the SMA
- imbalanced limbic system causes motivation and emotional disturbances
what does L-DOPA do?
attempts to reinstate the balance between the pathways, by flooding the striatum with dopamine
why has neurosurgery been suggested to treat PD?
suggested to reduce excess inhibition in the indirect pathway
strong direct and weak indirect pathways -> hyperkinesia
too much dopamine can push the balance of BG circuits too far towards the direct pathway, as this excitation can cause premature selection of action or uncontrolled selection of inappropriate actions
examples of other cortical-based ganglia loops
- motor
- oculomotor
- executive/associative
- emotion/motivation
loops
what does BG act as during eye movements?
a “filter” to add contextual information onto reflexive control of eye movements, through inhibiting the superior colliculus except for the target of interest
input and output of BG
receives diverse cortical information from different systems
output modulates activity in the Thalamus to shape selection and self-initiation of appropriate behaviours
different learning systems
cortex = learning relationships between stimulus conditions and responses
BG = reinforcement learning
cerebellum = learning through error correction
what does reinforcement learning rely on?
reward and punishment, rather than error.
reinforcement signal does not directly provide information about how to improve performance – must use reward-based learning to maximise future rewards and yield intelligence behaviour
what does intracortical electrical stimulation (ICS) or reward cause?
causes a release of dopamine from SNc to the striatum. Stimulating the substantia nigra causes a release of dopamine into the striatum
