Basal Ganglia Flashcards
basal ganglia
refers to structures that yield distinctive types of movement disorders when damaged
parts of BG
- caudate: head, body, tail, c shaped, in wall of lat vent, connects mainly with frontal lobe
- putamen: coincides with insula
- globus pallidus: extensive BG outputs to thalamus, medially situated
- substantia nigra: in BS
- subthalamic nucleus: almost in BS
what makes up the striatum?
caudate and nucleus accumbens
what does the lat vent represent embryologically
telencephalon
substantia nigra
in midbrain btwn cerebral peduncle and red nucleus, has 2 parts
2 parts of substantia nigra
- compact: densely packed, pigments neurons, provides diffuse modulatory, dopaminergic projections to striatum
- reticular: closer to cerebral peduncle, non pigmented neurons, output nucleus
BG related movement disorders
- involuntary movement, hyperkinetic disorders (chorea)
- difficulty initiating movement
- perturbed muscle tone (increase flexor/extensor tone, rigidity in PD, tone can also be decreased)
huntington’s disease
chorea
- degeneration of striatum (caudate, nucleus accumbens)
- chorea, rigidity, cognitive problems
- autosomal dominant, trinucleotide repeat, onset around 50 yo
- nearly continuous, rapid movements of face/limbs, rhythmic
4 circuits of BG
- circuits include multiple parallel loops that modulate cortical output
- cortical start and end points determine loop function
1. motor loop (learned movements)
2. cognitive loop (motor intentions)
3. limbic loop (emotional aspect of movement)
4. occulomotor loop (voluntary saccades)
motor loop
- BG cannot initiate movements but is active during all movements
- role in motor control is to regulate descending motor path, by modulating corti in motor areas
- 2 recognized paths
1. direct: 5 sets neurons
2. indirect: 7 sets of neurons
cognitive loop
- role in motor learning, planning movements ahead, esp when intended move is complex
- when new motor task is practiced to the point it can be “automatically” executed, motor loop takes over
limbic loop
- gives expression to emotions (smile, gesture, etc)
- rich in dopaminergic neurons
- loss may be why PD pts are expressionless
occulomotor loop
- SNpr is tonically active when eyes are fixed on an object
- when saccade is about to start, loop is activated, disinhibiting the superior colliculus
neuronal connectivity
- presence of neurons w/ inhibitory and excitatory NTs in same neuronal path is key to fine tuning path output
- disinhibition (inhibiting a silent neuron) is a major feature of neuronal activity in BG
-can have direct or indirect pathway
direct neural connectivity path
- Go pathway
- excitatory connections: glutamate (corticostriate, thalamocortical)
- inhibitory connections: GABA (GPi and SNr)
- cerebral cortex->int and ext capsule (inputs: cortical afferents to striatum)->striatum->GPi (outputs GPi and SNr)->thalamus->int capsule->cerebral cortex
*selectively facilitates neurons that mediate desired movements
indirect neuronal connectivity path
- cortical input and striatal output->inhibit GPe output->increase thalamic inhibition->decrease cortical output
- loss of subthalamic fibers disinhibits thalamus, results in failure to suppress cortical outputs and involuntary movement
- selectively disfacilitates neurons to suppress unwanted movements
how are GP and putamen organized
somatotopically,
- allows desired moves: direct
- disfacilitates undesired moves: indirect
- subthalamic nuc is imp in supressing unwanted moves by somatopic projections to GPi
cardinal features of PD
- bradykinesia
- resting tremor
- regidity
*onset 60 yo, SN compact part neurons contain Lewy bodies
direct path and SN in PD
-striatal projection neurons express D1 receptors, are facilitatory
indirect path and SN in PD
- striatal projection neurons in indirect path express D2 receptors->inhibitory
- bc SN is tonically active the direct path is favored and more active
levadopa as PD tx
- idea is to replace dopamine lost due to degeneration of nigrostriatal fibers
- response can fluctuate over time and induce involuntary moves
PD tx
- VA/VL and GPi are deep in brain and near int capsule, risky to remove
- DBS: implant electrodes in subthalamus to provide long term stim and overcome abnormal activity in these structures
- stem cell therapy
what makes up the lenticular nucleus
putamen and globus pallidus
