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A. Schematic diagram of the main neurotransmitter pathways and their effects in the cortical–basal ganglia–thalamic circuits. The blue lines indicate
neurons with excitatory effects; the black lines indicate inhibitory influences. The internal (medial) segment of the globus pallidus (MGP) and the zona
reticulata of the substantia nigra (SNpr) are believed to act as one entity that projects via GABA-containing neurons to the thalamus (ventrolateral and
ventroanterior nuclei) and to the pedunculopontine nuclei (not shown). Dopaminergic neurons (DA) arising in the pars compacta of the substantia nigra
(SNpc) have an excitatory influence on the direct striatopallidal fibers (via D1 receptors) and an inhibitory effect on the indirect striatopallidal fibers (via D2
receptors) that project to the external (lateral) pallidum (LGP) and subthalamic nucleus (STN). Dotted lines in the subsequent figures denote a reduction in
Source: Chapter 4. Abnormalities of Movement and Posture Caused by Disease of the Basal Ganglia, Adams and Victor's Principles of
activity of the pathway. (See text.) B. Corresponding physiologic state as conceptualized in Parkinson disease, in which hypokinesia is the main finding as
Neurology, 10e
a result of reduced dopamine input from the substantia nigra and pars compacta to the striatum via the direct pathway, which results in withdrawal of
Citation:
Ropper
Samuels
Kleinincreased
JP. Adams
and Victor's
of Neurology,
10e; 2014
Available
http://mhmedical.com/
inhibitory activity
of the
globusAH,
pallidus
and,MA,
in turn,
inhibitory
drive Principles
on the thalamic
nuclei, which
reduces
input at:
to the
cortical motor system. C.
Accessed:
May
07,
2017
Schematic diagram of the theorized mechanism in Huntington disease, a hyperkinetic movement disorder resulting from reduced inhibition by the striatum
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McGraw-Hill
All nucleus,
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within the indirect
pathway,
overdriving
of theEducation.
subthalamic
and causing excess activity in thalamocortical circuits. (See text.)