Neurodegenerative Diseases Of The Basal Ganglia Cause Movement Disorders

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The basal ganglia are subcortical brain structures that function in sensory-motor integration and in the planning and initiation of skeletal muscle movements. The striatum (caudate nucleus and putamen), globus pallidus (external and internal divisions), substantia nigra (pars compacta and reticulata), and subthalamic nucleus are the major components of the basal ganglia (Fig. 8). The basal ganglia do not control movement through direct connections with lower motor neurons, but rather the functions of the basal ganglia are executed by the frontal cortex through corticobulbar and corticospinal projections to brain stem and spinal motor neurons, respectively. The operation of the basal ganglia involves forebrain-diencephalon-mid-brain circuitry loops. The different circuits within the basal ganglia utilize different neurotransmitters (Table V). The primary input to the basal ganglia originates from the neocortex and is directed to the striatum (Fig. 8). This corticostriatal projection uses the excitatory neurotransmitter glutamate. The primary output center of the basal ganglia is the globus pallidus, which conveys signals to back to the neocortex through the thalamus (Fig. 8). The pallidothalamic projection is inhibitory; in contrast, the thalamocortical projection is excitatory (Table V). Thus, the globus pallidus functions to inhibit the excitatory thalamic drive of neocortex. Somatic movements occur when thalamic neurons are released from tonic inhibition. This release occurs when corticostriatal projections excite striatal neurons that can phasically inhibit the neurons in the

Figure 8 Basal ganglia circuits control movements. The basal ganglia comprise (left panel) the caudate nucleus (CN), putamen (P), globus pallidus external (GPe) and internal (GPi) divisions, the subthalamic nucleus (STN), and the substantia nigra compact (SNc) and reticular (SNr) divisions. The cerebral cortex and thalamus (T), although not part of the basal ganglia, participate in the connectivity loops (right panel). See Table V for connections. The major excitatory input to the striatum (the caudate nucleus and putamen) is from the cerebral cortex (right panel). The striatum in turn projects to the globus pallidus and the substantia nigra reticular division. Striatal activity is modulated by extensive dopaminergic input from the substantia nigra compacta. The major output of the basal ganglia is directed toward the thalamus, originating from GPi and SNr (not shown). The thalamic projection to the cerebral cortex (premotor and supplementary motor areas) drives the activity of the motor cortex, which executes somatic movements.

Figure 8 Basal ganglia circuits control movements. The basal ganglia comprise (left panel) the caudate nucleus (CN), putamen (P), globus pallidus external (GPe) and internal (GPi) divisions, the subthalamic nucleus (STN), and the substantia nigra compact (SNc) and reticular (SNr) divisions. The cerebral cortex and thalamus (T), although not part of the basal ganglia, participate in the connectivity loops (right panel). See Table V for connections. The major excitatory input to the striatum (the caudate nucleus and putamen) is from the cerebral cortex (right panel). The striatum in turn projects to the globus pallidus and the substantia nigra reticular division. Striatal activity is modulated by extensive dopaminergic input from the substantia nigra compacta. The major output of the basal ganglia is directed toward the thalamus, originating from GPi and SNr (not shown). The thalamic projection to the cerebral cortex (premotor and supplementary motor areas) drives the activity of the motor cortex, which executes somatic movements.

Table V

Primary Connections of the Basal Ganglia and Their Major Neurotransmitters0

Table V

Primary Connections of the Basal Ganglia and Their Major Neurotransmitters0

Projection

Neurotransmitter

Neocortex to striatum

glutamate

(corticostriatal)

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