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
different pathways of dopamine release
Mesolimbic pathway – VTA to ventral striatum (NAcc)
Mesocortical pathway – VTA to frontal cortex
Nigrostriatal pathway – SNc to striatum
what is dopamine seen as?
as a reward signal, which potentiates the glutamate transmission in the cortex and leads to strengthening the connections between the cortex and striatum
what does repeated application of dopamine rewards cause?
leads to LTP of their synapses, to reinforce behaviour and lead to habit forming
dorsolateral prefrontal cortex
undergoes prolonged periods of maturation and is heavily connected to the BG and dopamine.
is active when considering cost-benefit of decision-making
what does the wisconsin card sort test do?
observes the executive control of behaviour – DLPFC lesions experience:
perseverance errors (unsuccessful strategy) and capricious errors (abandon successful rule)
two models of action selection
- the central executive
- affordance competition
what is the central executive view?
we respond to our environment by using sensory input to perceive the world around us, then make a central executive decision and implement this through the motor system
separate modules in the central executive view
Perception, cognition, and action (sense, think, act) are three separate modules within the serial model of action selection
what is the motor system in the central executive view?
the motor system is simply the output stage
what is the affordance competition model?
parallel systems are used as actions compete within sensorimotor maps, which are influenced by various factors such as rewards
how are decisions made in the affordance competition model?
through a ‘disturbed consensus,’ meaning motor control is integral to decision making.
affordance competition and executive control
executive control biases the competitive processing of action selection in particular directions.
Multiple actions are considered, and competition narrows down choices as information accumulates
BG and the affordance competition model
decision-making involves consensus among various brain regions, with the BG influencing behavioural biasing until a specific action is determined
