Movement Disorders Flashcards
Pyramidal tracts
The term pyramidal tracts refers to upper motor neurons that originate in the cerebral cortex and terminate in the spinal cord (corticospinal) or brainstem. Nerves emerge in the cerebral cortex, pass down and may cross sides in the medulla oblongata, and travel as part of the spinal cord until they synapse with lower motor neurons (often via interneurons).
Extrapyramidal system
The extrapyramidal system is a part of the motor system network, carrying out involuntary motor actions. Extrapyramidal tracts are chiefly found in the reticular formation of the pons and medulla, and target lower motor neurons in the spinal cord that are involved in reflexes, locomotion, complex movements, and postural control. These tracts are in turn modulated by various parts of the central nervous system, including the nigrostriatal pathway, the basal ganglia, the cerebellum, the vestibular nuclei, and different sensory areas of the cerebral cortex.
Basal ganglia
Deep gray matter structures with strong thalamic connections, including the following structure:
• Striatum (Caudate + Putamen)
• Pallidum/Globus pallidus (internal and external segments)
• Subthalamic nucleus
• Substantia nigra (reticular, compacta)
The basal ganglia works via a direct and indirect pathway.
Direct pathway of basal ganglia
The direct pathway takes input from the cortex and produces excitation in the thalamus, via disinhibition of the internal part of globus pallidus (GPi) and the reticular part of substantia nigra (SNR).
There is a high inhibitory baseline signal from the GPI and the SNR to the thalamus, and when they are inhibited by signals from the striatum, it causes disinhibition in the thalamus.
Indirect pathway of basal ganglia
The indirect pathway takes input from the cortex and produces inhibition in the thalamus, via excitation of the internal part of globus pallidus (GPI) and the reticular part of substantia nigra (SNR). The indirect pathway travels through the globus pallidus externa (GPE) and the subthalamic nucleus (STN)
The role of substantia nigra pars compacta in basal ganglia
The dopaminergic projections of the pars compacta of the substantia nigra (SN ) modulate striatal activity by facilitating the direct pathway via the D1 receptors and
inhibiting the indirect pathway via the D2 receptors
The role of the basal ganglia
- Release and inhibition of movements
- Procedural implicit learning
- Reward-based learning and memory
- Automatised responses
“Processing in the cortical motor areas can be viewed as a competitive process in which candidate actions compete for control of the motor apparatus. The basal ganglia are positioned to help resolve the competition. The strong inhibitory baseline activity keeps the motor system in check, allowing cortical representations of possible movements to become activated without triggering movement. As a specific motor plan gains strength, the inhibitory signal is decreased for selected neurons. This movement representation breaches the gate, thus winning the competition.” (Gazzaniga, p. 357)
Effects of basal ganglia dysfuntion
- Parkinsonism (hypokinetic)
- Dystonia (hyperkinetic)
- Chorea (hyperkinetic; e.g., Huntington’s Disease)
- Tic disorders (hyperkinetic; e.g., Tourette’s Syndrome)
- Tremor (hyperkinetic)
- Myoclonus (hyperkinetic)
Neuropathology of parkinsonism
Loss of dopaminergic neurons in Substantia Nigra pars compacta
Motor symptoms of parkinsonism
- Tremors (at rest, slow, rhythmic, hand, leg, jaw or mouth)
- Rigidity (limb or torso stiffness)
- Bradykinesia (general slowness of movement, facial masking, micrographia)
- Postural instability
- Gait difficulties (propulsion, freezing)
Explain this representation of parkinsonism in the basal ganglia
Loss of the dopaminergic signaling from the SNc reduces the inhibitory activity along the direct pathway, resulting in increased inhibition from the GPi to the thalamus and thus a reduction in cortical activity and movement.
How was a clinical model of Parkinon’s disease discovered?
Seven young adults injected themselves with some bad heroin, which included a neurotoxin targeting neurons in the substantia nigra. They started developing parkinson’s symptoms, and scientists could use this discovery to induce parkinson’s in mice for research.
Most effective treatment for Parkinson’s symptoms?
Deep brain stimulation. using electrodes in a pace-maker-like manner, we’re able to alleviate most symptoms by stimulating GPi and STN
