Basal Ganglia Flashcards
What are the basal ganglia?
Collection of large subcortical forebrain nuclei
What are the main nuclei of the basal ganglia?
- Caudate
- Putamen
- Globus pallidus (internal and external)
What are these nuclei also referred as?
Corpus striatum
Grey matter crossed by white fibres from motor cortex giving striped appearance
What are the midbrain nuclei that are functionally connected to the basal ganglia?
- Substantia nigra
2. Subthalamic nucleus
Where do the major inputs to the basal ganglia come from?
Cerebral cortex (all lobes)
What are the outputs of the basal ganglia?
Inhibitory
Directed to parts of the thalamus that supply almost all parts of the frontal lobes
What connects the nuclei of the basal ganglia?
Sequential GABA inhibitory connections
What are the symptoms of basal ganglia dysfunction?
Problems with either excess (hyperkinesia) or paucity (hypokinesia) of movement
What is hyperkinesia?
Excessive movement
- Chorea
- Athetosis
- Ballismus
- Dyskinesia
What is chorea?
Unexpected ‘dancing’ movement
What is athetosis?
Writhing movement of hands or face
What is ballismus?
Flailing ballistic movements
What is dyskinesia?
Unpredictable movements
What is hypokinesia?
Reduction in movement
- Bradykinesia
- Rigidity
What is bradykinesia?
Slowness
Parkinsonism
What is the extrapyramidal motor system?
Basal ganglia disorder
Misleading term as still involves pyramidal motor system (corticospinal tract)
Abnormal pattern of activity through normal pyramidal tract
What is the direct pathway in the basal ganglia?
- Corticostriate fibres activate neurons in caudate/putamen
- These neurons inhibit neurons in globus pallidus internus
- These neurons tonically inhibit neurons in the thalamus
Thus activation of this pathway leads to disinhibition of thalamus, permitting movement
What is the indirect pathway in the basal ganglia?
- Corticostriate fibres activate different neurons in caudate/putamen
- These neurons inhibit neurons in globus pallidus externus
- These neurons inhibit neurons in the subthalamic nucleus
- These neurons excite neurons in the globus pallidus internus
Thus activation of this pathway increases inhibition, preventing movement
What causes Parkinson’s disease?
Degeneration of substantia nigra leads to loss of dopamine causing imbalance in basal ganglia pathways
Causes reduced activity in direct pathway and increased activity in indirect pathway
What do D1 receptors do?
Dopamine receptor
Facilitates pathway to globus pallidus internus
What do D2 receptors do?
Dopamine receptor
Suppresses pathway to globus pallidus externus
How is Parkinson’s disease treated surgically?
- Controlled lesions of globus pallidus (pallidotomy) to reduce inhibition of thalamus
- Deep brain stimulation of subthalamic nucleus to reduce activity in subthalamic nucleus
What is the role of the dorsal striatum?
Motor function
- Motor circuit with input from sensorimotor cortex and output to primary motor cortex and SMA
- Oculomotor circuit with input from visual association cortex and output to frontal eye fields
What is the role of the ventral striatum?
Higher level function such as deciding between options and motivation for action
- Prefrontal cortex association circuit with input from association cortex and output to prefrontal cortex (executive function)
- Limbic circuit with anterior cingulate gyrus and orbitofrontal cortex
