basal ganglia Flashcards
(22 cards)
What are the basal ganglia?
Series of interconnected nuclei forming series of segregated parallel loops with cerebral cortex.
What are the basal ganglia loops associated with?
Each loop associated with a different cortical area. Means that signals are processed separately for skeletomotor, oculomotor, associative and limbic systems etc
What do basal ganglia loops to for each system
Loops are presumed to complete similar computations for each separate system
What is the body movement loop?
(Primary motor, premotor, supplementary motor cortex)
Motor, premotor, somatosensory cortex ->putamen-> lateral globus pallidus, internal segment-> ventral lateral and ventral anterior nuclei -> cortical targets
What is the oculomotor loop?
For frontal eye field, supplementary eye field:
Posterir parietal, prefrontal cortex -> caudate (body) -> globus pallidus, internal segment; substantia nigra pars reticulata -> mediodorsal and ventral anterior nuclei -> cortical targets
What is the prefrontal loop?
Dorsolateral prefrontal cortex
Dorsolateral prefrontal cortex -> anterior caudate -> globus pallidus internal segment; substantia nigra pars reticulata -> mediodorsal and ventral anterior nuclei -> cortical targets
What is the limbic loop?
Anterior cingulate, orbital frontal cortex
Amygdala, hippocampus orbitofrontal, anterior cingulate, temporal cortex -> ventral striatum -> ventral pallidum -> mediodorsal nucles -> cortical targets
Where does the basal ganglia receive information from?
From the whole cortex except the primary visual and auditory cortices
What are the main basal ganglia circuits and what are their effects?
Basal ganglia has a neuronal circuit.
2 pathways within each loop
Direct pathway runs directly through the basal ganglia whereas the indirect pathways goes via the subthalamic nucleus and then the Globus Palidus.
What is the direct pathway?
Originates from D1 medium spiny neurons (MSN) in th striatum (putamen). Shortest pathway. Activation of this pathway increases cortical activity by releasing the thalamus from inhibition thus creating “dis-inhibition”.
This is a pro-movement / pro-activity pathway
What is the indirect pathway?
Originates from the D2 MSN in the striatum (putamen)
Net result of activation of this pathway decrease cortical activity due to increased inhibition on the thalamus
Anti-movement
What is dis-inhibition?
Dis-inhibition occurs when tonic inhibition is “released” by an inhibitory input
This occurs in the direct pathway when the internal globus pallidus (Gpi) is inhibited by input from the striatum.
What is the difference between the direct and indirect pathway?
Direct CTX-> striatum -> internal global pallidus -> thalamus -> cortex. D1 dopamine receptors involved. Net effect is the production of movement
CTX -> striatum -> external globus pallidus -> striatum -> internal globus pallidus -> thalamus -> cortex. Involves D2 dopamine receptors. Net effect is anti-movement
What is the influence created by dopamine in terms of spiny neurons?
D1 medium spiny neurons (MSN) are excited by dopamine so D1 is a direct pathway. Dopamine acts to increase movement via a direct pathway,
D2 medium spiny neurons (MSN) : dopamine inhibits the inhibitory action of the indirect pathway so presence of dopamine allows movement
What produces symptoms of basal ganglia disorders?
A loss of cells which produce dopamine reducing movement. Result from imbalance in circuits of basal ganglia.
The balance between the indirect and direct pathways is what allows for movement.
In hypokinetic disorders there is insufficient direct pathway output and excess indirect pathway output.
In hyperkinetic disorder there is an excess direct pathway output and insufficient indirect pathway output
What can unbalance the basal ganglia circuits?
In Parkinson’s loss of cells in the substantial nigra (which produce dopamine) occurs
- Most PD has no known cause, 5-15% has a genetic cause, can also be vascular PD where stroke leads to damage to the dopaminergic pathways following stroke and other neurodegenerative diseases and neuroleptic drugs
In Huntington’s damage occurs in the striatum which can be seen as an enlargement of the ventricles. This spreads late into other areas of the brain.
Subthalamic nucleus stroke
- damages the subthalamic nucleus blocking the indirect pathway
- continuous movement resulting
What are parkinson’s symptoms?
Tremor (pill rolling)
Rigidity
Bradykinesia
Postural instability and gait disorder
Dysarthria
Mask face
Micrographia
PD treatments can also lead to dopamine induced dyskinesia
Involuntary erratic withing movements of the face, arms, legs or trunk
What occurs in Parkinson’s disease?
A loss of dopamine producing cells means that the indirect pathway occurs as dopamine cannot disinhibite the thalamus leading to anti-movement results. Substantia nigra is overactive in PD
Results in akinesia - lack of movement and bradykinesia, slowness of movement
What occurs in huntingtons disease (HD)
Early:
- DA levels are increased wheras expression of DA receptors is reduced
Increased DA ‘tone’ produced characteristic hyperkinetic abnormal dance-like movements (Chorea)
Late
- DA levels are significantly decreased
Reduced excitatory thalamic transmission to the cortex
Hypokinetic / akinetic symptoms that resemble those of a parkinsonian state
what are symptoms of huntington’s disease?
Jerky, random and uncontrollable, dance-like movements called chorea
Personality changes of disinhibition, compulsive behaviours and worsening addictions
What can treatment of PD lead to and cause symptoms of?
Surgery for PD and drug treatments can overstimulate the direct pathway leading to:
Impluse control disorders
Similar issues as occur in Huntington’s
Hypersexuality
Impulsive and compulsive shopping
Pathological gambling
Compulsive hobbyism
Too much indirect can have opposite effects including apathy which is a common cognitive symptom of PD and basal ganglia disorder
What are the main functions of the basal ganglia?
Movement regulation
Skill learning
Habit formation
Reward systems
Selection of appropriate behaviours
Self-initiation
