Basal Ganglia

Question 1

Role of basal ganglia in motor control

Answer 1

• modulation of motor performance via interconnections with the motor cortices.
• major feedback loop to motor and association cortex
• in general, serve a role in motor program selection.

Question 2

Major source of input to basal ganglia

Answer 2

• Cerebral cortex
• primarily: frontal lobe areas and sensorimotor areas that have a greater role in the planning and initiation of movement.

Question 3

Characteristics of Parkinson’s disease

Answer 3

• Resting tremor, difficulty initiating movements, slowness
• etiology: loss of dopamine neurons in the substantia nigra.

Question 4

Probable mechanism of Parkinson’s

Answer 4

• Substantia nigra sends dopaminergic axons back to the striatum.
• The action of dopamine in striatum is probably largely excitatory, since Parkinson’s disease, which includes degeneration of substantia nigra neurons and loss of dopaminergic input, is marked by a paucity of movement.

Question 5

Output of basal ganglia: inhibitory vs. excitatory

Answer 5

• Inhibitory.
• GPI → (GABA -) @ Thalamus → (Glu +) Cortex.
• output of the basal ganglia is directed to the thalamus via the pallidothalamic fibers from the GPI which are GABA-nergic and from there back to the cortex.
• Basal Ganglia provide resting inhibition of the thalamus until a signal comes from the cortex causing the disinhibition of the the thalamus via the direct pathway.

Question 6

Genetic cause of Huntington’s Disease

Answer 6

• autosomal dominant
• site of mutation @ short arm of the 4th chromosome.
• Normal humans have 17-34 CAG repeats, HD have > 40.

Question 7

Characteristics/presentation of Huntington’s Disease

Answer 7

• Striatal damage produces athetosis or chorea.
  
  • Chorea = continuous rapid movements of the face, tongue or limbs (usually distal portions).
  • Athetosis indicates slow, writhing, ceaseless movements of hand, sometimes lips, tongue, neck and foot.

Question 8

HD impact on basal ganglia

Answer 8

• caudate = most affected region of basal ganglia

Question 9

“Direct path” from cortex to basal ganglia and back to cortex

Answer 9

• Cortex → excites through striatum Caudate/Putamen with glutamate
• glutamate → which inhibits GPI with GABA →
• stops inhibiting Thalamus with GABA → which can then excite the cortex with glutamate

Question 10

Stroke @ subthalamic nucleus impact

Answer 10

• subthalamic nucleus = integral piece of the indirect pathway.
• If stroke that destroyed the STN then a great degree of thalamic inhibition would be lost ==> hemiballismus
  
  • e.g. stroke @ small ganglionic branch of posterior cerebral artery
• Disinhibition would rule, hence the flailing of arms and legs on one side.

Question 11

Type of stimulus used in deep brain stimulation for Parkinson’s patients

Answer 11

• Hyperpolarizing.
• Main problem in Parkinson’s = STN and GPI cause inhibition of the thalamus.
• DBS would work to remove such inhibition, otherwise known as encourage disinhibition.
• Hyperpolarizing the STN or GPI would do just that without the need for medications.