Basal Ganglia

Question 1

basal ganglia

Answer 1

refers to structures that yield distinctive types of movement disorders when damaged

Question 2

parts of BG

Answer 2

1. caudate: head, body, tail, c shaped, in wall of lat vent, connects mainly with frontal lobe
2. putamen: coincides with insula
3. globus pallidus: extensive BG outputs to thalamus, medially situated
4. substantia nigra: in BS
5. subthalamic nucleus: almost in BS

Question 3

what makes up the striatum?

Answer 3

caudate and nucleus accumbens

Question 4

what does the lat vent represent embryologically

Answer 4

telencephalon

Question 5

substantia nigra

Answer 5

in midbrain btwn cerebral peduncle and red nucleus, has 2 parts

Question 6

2 parts of substantia nigra

Answer 6

1. compact: densely packed, pigments neurons, provides diffuse modulatory, dopaminergic projections to striatum
2. reticular: closer to cerebral peduncle, non pigmented neurons, output nucleus

Question 7

BG related movement disorders

Answer 7

• involuntary movement, hyperkinetic disorders (chorea)
• difficulty initiating movement
• perturbed muscle tone (increase flexor/extensor tone, rigidity in PD, tone can also be decreased)

Question 8

huntington’s disease

Answer 8

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

Question 9

4 circuits of BG

Answer 9

• 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)

Question 10

motor loop

Answer 10

• 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

Question 11

cognitive loop

Answer 11

• 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

Question 12

limbic loop

Answer 12

• gives expression to emotions (smile, gesture, etc)
• rich in dopaminergic neurons
• loss may be why PD pts are expressionless

Question 13

occulomotor loop

Answer 13

• SNpr is tonically active when eyes are fixed on an object

- when saccade is about to start, loop is activated, disinhibiting the superior colliculus

Question 14

neuronal connectivity

Answer 14

• 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

Question 15

direct neural connectivity path

Answer 15

• 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

Question 16

indirect neuronal connectivity path

Answer 16

• 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

Question 17

how are GP and putamen organized

Answer 17

somatotopically,

• allows desired moves: direct
• disfacilitates undesired moves: indirect
• subthalamic nuc is imp in supressing unwanted moves by somatopic projections to GPi

Question 18

cardinal features of PD

Answer 18

1. bradykinesia
2. resting tremor
3. regidity

*onset 60 yo, SN compact part neurons contain Lewy bodies

Question 19

direct path and SN in PD

Answer 19

-striatal projection neurons express D1 receptors, are facilitatory

Question 20

indirect path and SN in PD

Answer 20

• striatal projection neurons in indirect path express D2 receptors->inhibitory
• bc SN is tonically active the direct path is favored and more active

Question 21

levadopa as PD tx

Answer 21

• idea is to replace dopamine lost due to degeneration of nigrostriatal fibers
• response can fluctuate over time and induce involuntary moves

Question 22

PD tx

Answer 22

• 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

Question 23

what makes up the lenticular nucleus

Answer 23

putamen and globus pallidus