Basal Ganglia Flashcards
What are the components of the basal ganglia?
Subcortical region (nuclei within the brain)
- caudate and putamen make up the **striatum **
- globus pallidus (internal and external)
- subthalamis nuclei
**Brainstem **
- Substantia nigra pars compacta
- Substantia nigra pars reticulata
The caudate and putamen (striatum) receive input from multiple cortical, thalamic, and brainstem regions. Name them
Brainstem inputs
- dopaminergic axons from the substantia nigra pars compacta (Sn-C)
Cortical Inputs (= corticostriatal pathway)
- Greatest number of inputs is from frontal and parietal lobe association cortices
- Caudate inputs are primarily from 1) frontal lobe areas that control eye movements and 2) association cortices.
- Putamen receives input from 1ª and 2ª visual cortex, motor and premotor cortex and auditory association cortex
Logical organization of inputs to the caudate/putamen. That is related regions of the cortex will have their inputs to the caudate/putamen in close proximity to one another.
association cortex
a cortical region that receives and integrates input from multiple sensory modalities
medium spiny neurons projections (edit maybe)
Project out of striatum and to their targets at either GPi or GPe.
Axons that form the corticospinal path are actual collaterals from other cortical pathways (e.g. corticocortical, corticospinal).
Number of glutamatergic synpatic contacts between a single cortical axon and medium spiny neuron is small, but a medium spiny neurons each receive input from 1000s of different cells
therefore, the striatal medium spiny cells are integrating inputs from a large number of sources
MSNs receive input from..
Cortical input
local interneurons (onto soma)
DA neurons from Substantia Nigra pars compacta
Other striatal medium spiny neurons (not shown in lecture figure)
Type of potassium channels on medium spiny neurons , what’s the significance?
potassium inward rectifiers
Keep MSNs quiet and they require the simultaneous stimulation from several excitatory inputs (both glutamatergic cortical and DA substantia nigra inputs) to become activated.
When do MSNs fire? How does this correlate to the function of specific striatum areas?
- in anticipation of an impending movement
- precedes subsequent movement by intervals in the 100s of milliseconds range although activity can precede movement by several seconds
- indicates that striatum neurons play a role in selection/decision of appropriate movements and not other motor behavior elements (eg direction or amplitude)
Putamen: fires in anticipation of limb or trunk movement
Caudate: fires in anticipation of eye movements
Where do MSNs project to? What NT do they release?
GABAergic
project to both the internal and external globus pallidus (GP) and to the substanstia nigra pars reticulata (SN-R)
What is unique about the projections from the striatum to the GP and SN-R?
Substantial amount of convergence of input from the striatum (100 million neurons) to the GP and SN-R (700,000 neurons), but the pattern of innervation is unusual:
striatal MSNs weakly innervate many targets and strongly innervate one target.
Where does the GPi project?
thalamic targets which then project to the motor cortex
Where does the SN-R project to?
superior colliculus (eye movements)
What’s unique about GPi neurons?
GABAergic
tonically active
innervated by MSNs, target thalamus which they constantly inhibit
The thalamus has excitatory neurons that project to the motor cortex.
What happens when MSNs are activated?
they inhibit the inhibitory neurons in the GP-internal (disinhibition)
this removes the inhibition onto the thalamic neurons, allowing these excitatory neurons to stimulate their targets in the motor cortex
referred to as gating a movement.
Describe the steps in basal ganglia disinhibition and the generation of saccadic eye movements.
- Caudate is activatived, increases activity.
- This results in the SN-r inhibition
- The superior colliculus is active as a result of the disinhibition
- Motor behavior is generated! (eye movement)
Describe the basal ganglia disinhibition and generation of limb/body movements.
Putamen is activated
GP-i is therefore inhibited
This allows the thalamus to be active as a result of disinhibition
Motor behavior is generated!!
Direct pathway diagram
Cerebral cortex stimulates Cuadate/putamen
(substantia nigra pars compacta has positive D1 inputs to caudate/putamen to increase likelihood of appropriate motor action)
Caudate/putamen signal to GPi, which inhibits its activity
Inhibited GPi allows the thalamus to be disinhibited
thalamus stimulates frontal cortex for limb/trunk movement.
What is the indirect pathway?
involves the GP-e which has a tonic negative influence on the subthalamic nucleus
subthalamic nucleus stimulates GPi
overall, opposes the action of the direct pathway.
Note: Sn-C has D2 projections to the caudate/putamen areas associated with the indirect pathway and therefore make it harder for this pathway to fire.
Describe what is meant by center-surround and how the basal ganglia pathways may be viewed as a center-surround system.
- It is thought that the direct and indirect pathways are organized into center-surround structure to control the selection of the proper motor behavior.*
- That is, activate the desired behavior and suppress competing, but inappropriate behaviors.*
This is accomplished by the fact that the output of the direct pathway is focused to specific functional units,
- (physiologically, a result of striatum have strong synaptic input to a single target neuron in the GP-internal)
…whereas the output of the indirect pathway is more diffuse, inhibiting many functional units.
- (a result of subthalamic input activating multiple neurons in the GP-internal at more or less equivalent levels)
Exactly how the appropriate direct vs. indirect pathways are activated by cortical input is unclear.
