Basal Ganglia Flashcards
Give a general description of the role of the basal ganglia in motor control.
There are four basal ganglia: Caudate/putamen (aka striatum), Globus pallidus, Sustantia nirgra, Subthalamic nucleus
Function of the basal ganglia:
•The basal ganglia play a role in modulation of movement (along with the cerebellum). The basal
ganglia serve as a feedback look to the ipsilateral motor and association cortices.
•The basal ganglia are responsible for much of the neurotransmitter synthesis in the brain.
•The basal ganglia are not involved in sensation or muscle strength.
•There is some evidence that the ganglia may contribute to cognitive and affective functions,
though this is not well understood
What is the major source of input to the basal ganglia?
Input to the basal ganglia comes from the cerebral cortex. Input comes into the caudate and putamen and then flows to the globus pallidus
Describe the character and probable cause of Parkinson’s disease.
- Resting tremor (3-6 Hz) lost during intended movement
- Increased muscle tone
- Difficulty with movement initiation
- Bradykinesis
- Shaking movements of head
- Tremulous speech
- Little change in facial expression
- Difficulty making “predictive” movements
There is a loss of dopamine neurons in the substantia nigra. Normally, dopamine is released diffusely into the striatum, resulting in excitation. Parkinson’s patients have reduced disinhibition due to low dopamine levels, making movement initiation more difficult. The cause of the resting tremor is not well understood.
Is the output of the basal ganglia inhibitory or excitatory?
The spontaneous output from the globus pallidus of the basal ganglia is inhibitory. Excitation of the caudate or putamen by the cerebral cortex can remove this inhibition.
What is the genetic cause of Huntington’s Disease and what areas of the basal ganglia are affected?
Huntington’s Disease is caused by an autosomal dominant mutation of the short arm of chr. 4 in the Huntington gene. The mutation involves an excess of CAG triplet-repeats (40+ versus the normal 17-34 repeats).
“Glutamate excitotoxicity” in Huntington’s patients leads to degradation of cholinergic and GABA-ergic striatal neurons. As a result, the effects of the striatum on the globus pallidus are lost. In addition, studies have shown that Huntington’s patients have an excess of dopamine in relation to acetylcholine. Patients choreic movements are more severe with either L-dopa or anti-cholinergic administration.
Describe the “direct path” from the cortex to basal ganglia and eventually back to cortex (which synapses are excitatory and which are inhibitory).
•Sensori-motor cortex→ putamen→ globus pallidus→ ventral anterior (VA) some ventral lateral (VL)
thalamus→ motor cortex (mainly supplementary motor area)
- Frontal association cortex→ caudate→ globus pallidus→ dorsomedial thalamus→ association cortex
- Paleo-cortex→ nucleus accumbens (a region between the caudate and putamen) for processing of emotional and drive-related behavior
Why does a stroke in the subthalamic nucleus cause hemiballismus?
The internal segment of the globus pallidus is normally excited by the subthalamic nucleus. Loss of excitation by the subthalamic nucleus results in reduced inhibitory outflow from the globus pallidus. Reduced inhibitory outflow leads to inappropriate excitation of the motor cortex through the thalamus and flailing movements result.
What type of stimulus (depolarizing or hyperpolarizing) would you predict would be used for the “deep stimulation” treatment of Parkinson patients?
Hyperpolarizing-
Deep brain stimulation of the subthalamic nucleus would need to be hyperpolarizing. Inhibition of the subthalamic nucleus leads to decreased excitation of the globus pallidus and decreased inhibition of the cortex.
Deep brain stimulation of the globus pallidus would need to be hyperpolarizing as well in order to inhibit the inhibition of the cortex.
