basal ganglia

Question 1

list the four major subcortical nuclei of the basal ganglia

Answer 1

1. striatum
   • STR
   • caudate nucleus + putamen
2. globus pallidus
   • external (lateral) = GPe
   • internal (medial) = GPi
3. subthalamic nucleus
   • STN
4. substantia nigra
   • pars compacta = SNc
   • pars reticulata = SNr)

Question 2

the striatum consists of …?

Answer 2

caudate nucleus + putamen

Question 3

lenticular nucleus consists of …?

Answer 3

putamen + pallidum

Question 4

what are the major connection of the basal ganglia?

Answer 4

• major input = striatum
• major output = Gpi and pars reticulata of substantia nigra

• basal ganglia work closely with supplementary cortex (not so much with premotor cortex)
—–> indirectly modulate pyramidal pathways via supplementary cortex

• input into extrapyramidal pathways
• no direct connections to spinal cord

Question 5

cortico-basal ganglia-cortical loop: direct pathway

Answer 5

diagram

1) cortex sends excitatory input to striatum
2) excited neurons inhibit major output (Gpi + SNr)
3) Gpi and DNr inhibited, cannot inhibit thalamus = DISINHIBITION
4) therefore thalamus excites cerebral cortex

→ activation of the direct pathway disinhibits the thalamocortical tract
→ increases thalamocortical tone tone
→ initiate and promote voluntary movements in accordance to cortical motor command

Question 6

cortico-basal ganglia-cortical loop: indirect pathway

Answer 6

1) cortex send excitatory input to striatum
2) excited neurons strongly inhibit external pallidum (GPe)
3) Gpe inhibited, cannot inhibit STN = DISINHIBITION
4) strong excitation of GPi and SNr by STN
5) strong inhibition of thalamus
6) thalamus cannot excite cerebral cortex

→ reduces thalamocorticol tone
→ suppressing involuntary movement

Question 7

what is unique about the projections from the striatum?

Answer 7

projections from the STN is the only excitatory (glutamatergic) intrinsic connection of the basal ganglia.

Question 8

how does the substantia nigra pars compacta influence the basal ganglia loops?

Answer 8

functions as a modulator of the cortical input into the striatum

dopamine acts on excitatory D1 receptors to stimulate direct pathway

dopamine acts on inhibitory D2 receptors to inhibit indirect pathway

overall, SNc disinhibits thalamocortical neurons and facilitate voluntary movements

Question 9

how do the basal ganglia connect to the spinal cord?

Answer 9

basal ganglia do not have a direct connection to spinal cord

GPi + SNr → pedunculopontine nucleus (PPN)* → reticulospinal tract → spinal cord

• the pedunculopontine nucleus (PPN) is located in the pontomesencephalic tegmentum

Question 10

where are the dopamine inputs into the striatum?

Answer 10

dopamine has input to both ventral and dorsal striatum

VTA (ventral tegmental area) has dopamine input into the ventral striatum (Ncl. accumbens) for reward feeling (e.g. food, drinks, drugs, sex)

SNc has dopamine input into the dorsal striatum to initiate movements thxat are predicted to having rewarding effects (reinforcement learning)

Question 11

cortico-basal ganglia-cortical loop: hyperdirect pathway

Answer 11

1) primary motor cortex sends excitatory signal to subthalamic nuclei
2) subthalamic nuclei send excitatory signal to GPi + SNr
3) GPi + SNr strongly inhibit thalamus (VA, VL)
4) thalamus cannot excite primary motor cortex

results in fast inhibition of all motor programs

Question 12

list and describe two hyperkinetic disorders of the basal ganglia

Answer 12

HEMIBALLSIM
• higher excitatory control compared to normal
• degeneration of STN = under-activity in indirect pathway
• uncontrollable, rapid ballistic movement of limbs

CHOREA HUNTINGTON
• higher excitatory control to thalamus
• degeneration of putamen = under-activity in indirect pathway
• involuntary and rapid limb and trunk movement

Question 13

list and describe one hypokinetic disorder of the basal ganglia

Answer 13

PARKINSON’S DISEASE
• degeneration of SNc = reduction in dopamine input = over-activity of indirect pathway + under-activity of direct pathway
• thalamus receives strong inhibitory tone = strong reduction in excitatory input from thalamus to cortex
• reduced voluntary movement = bradykinesia

Question 14

what are two possible treatments for parkinson’s disease?

Answer 14

1) lesion of subthalamic nucleus
2) lesion of globus pallidus internus

effectively reduce parkinsonian signs and dyskinesias by respectively normalizating or eliminating abnormal and excessive output from the internal pallidal segment