Task 2 - Basal Ganglia Flashcards
Basal Ganglia
Set of nuclei deep within cerebral hemispheres:
- caudate
- putamen
- globus pallidus
- substantia nigra (base of midbrain)
- subthalamic nucleus (ventral thalamus)
Basal Ganglia functions
Subcortical loop linking cerebral cortex areas with upper motor neurons in primary and premotor cortex and in brainstem
- modulate activity in anticipation of and during movements
- important for voluntary motor movements
Striatum
Caudate + Putamen
- input zone of basal ganglia: mostly from association areas in frontal and parietal lobes
- > corticostriata pathways
Putamen input
Primary and secondary somatic sensory cortices in par. lobe
- secondary visual cortices in occipital and temporal lobe
- premotor & motor cortices in frontal lobe
- auditory association areas in temporal lobe
- > neural firing prior to limb and trunk movement
Caudate input
Multimodel association areas
Motor areas of frontal lobe controlling eye movements
-> neural firing prior to eye movement
Medium spiny neurons
Destination of neurons from cerebral cortex
- > allow integration from cortical, thalamic, and brainstem
- firing associated with occurrence of movement
- > axons arising converge in pallidum
- > connect striatum with other regions (brain stem, thalamus, globus pallidus)
Pallidum
- Globus pallidus
- Substantia nigra pars compacta
- > main sources of output from basal ganglia
Basal Ganglia Projections
Main output inhibitory:
- caudata & putamen: inhibitory to globus pallidus and substantia nigra pars reticulata
- > influences activity of upper motor neuronos located in motor cortex and brainstem
Basal Ganglia role
Gatekeeper in initiation of movements
-> facilitates initiation of motor programs that express movement and suppresses competing or non-synergistic motor programs that would interfere
Substantia nigra pars reticulata pathway
Caudate and Putamen inhibitory pathway towards substantia nigra pars reticulata
- > itself has inhibitory pathway to superior colliculus (eye movements)
- > if subst. nigra is inhibited enough superior colliculus is less inhibited leading to movement
Direct Pathway
From striatum (-) and globus pallidus internal segment (-) to VA/VL complex of thalamus (+) to frontal cortex -> functions to open physiological gates that initiate movement (release of thalamic neurons driving uppoer motor neurons from tonic inhibition)
Indirect Pathway
Reinforces suppression of inappropriate movements;
serves to increase tonic inhibition;
-striatum to external segment from which it projects to subthalamic nucleus (excites internal segment) and internal segment directly (-)
-> antagonizes activity of direct pathway
-> functions to close physiological gates that terminate movements
Gate Model of Basal Ganglia
Proposes that the output of the Thalamus as result of basal ganglia circuit only gives binary yes-or-no output back to cortex
Center-Surround Model of Basal Ganglia
Focused selection:
- Indirect pathway activation: surround region of globus pallidus (reinforces suppression)
- direct pathway activation: inhibition of center of neurons in internal segment
- > both active at same time: output also provides alternative action instead of just yes v no
Dopamine Modulation of Basal Ganglia Circuits
- Dopaminergic cells in substantia nigra pars compacta
- medium spiny neurons of striatum project to it and it projects back
- > dopamien modulation excites direct pathway via effect on D1 receptor
- > inhibites indirect pathway (effect on D2 receptor)
Result of dopamine modulation
Less inhibition of basal ganglia towards thalamus when goal of movement is associated with larger reward
Tonic Inhibition
Sustained, stron inhibition by default
-> no need of further input to sustain inhibition
Transient Inhibition
Inhibition that needs to be specifically activated
-> needs input to sustain inhibition
Movement Diseases
Parkinson’s
Huntington’s
Hemiballismus
Parkinson’s
Caused by loss of nigrostriatal dopaminergic neurons, which serve to increase responsiveness to direct pathway and decrease responsiveness to indirect athway
- > inhibitory outflow of basal ganglia abnormally high
- > hypokinetic movement disorder (failure of disinhibition)
- > difficulty initiating movements
Huntington’s
Degeneration of medium spiny neurons projecting to external segment of globus pallidus
- > absence of inhibition via indirect pathway
- > restraining influence of basal ganglia lost, upper motor neurons are activated by inappropriate signals
- > hyperkinetic movement disorder: undesired ballistic and choreiform movements due to failure of inhibition
Hemiballismus
Violent involuntary movements as result of damage to subthalamic nucleus (normally serves to increase inhibition of movements)
-> hyperkinetic movement disorder
Deep Brain Stimulation
Treatment for hyper-/hypokinetic movement disorders
- implantation of battery-powered generator units;
- > produce electrical discharges passed through wires to electrodes implanted bilaterally into brain
- common stimulation sites: internal segment of globus pallidus, subthalamic nucleus
- > provides stable and structured patterns of neural activity (can facilitate initiation and termination of movement)
- > can excite and inhibit nuclei
Motor Programs
result from innate programming of CNS circuits combined with epigenetic arrangements leading to development of appropriate and fine-tuned motor programs
