Cortex: Movement Force, Direction and Synergy
Movements are produced by neuronal groups in the motor cortex. These groups act synergistically to control force, direction and timing – and they communicate with sensation – to produce fine, skilled movements.
Pyramidal System Anatomy
Figure 2.2 outlines the principal motor pathway from cortex to anterior horn cells.
Note:
Pyramid: within rostral medulla
Decussation of the pyramids: within caudal medulla
Cortico‐spinal axons synapse on cord anterior horn cells.
Extrapyramidal System and Basal Ganglia Region
The word extrapyramidal is used in various ways. In neurology, extrapyramidal describes disorders such as Parkinson’s disease – the slowing, stiffness and/or tremor. Extrapyramidal is also sometimes used to include dyskinesias, such as chorea, hemiballismus or dystonia. In neuroanatomy, as a more general term, extrapyramidal relates to the basal ganglia region (Figure 2.3), that is:
Figure 2.2 Descending corticospinal pathways.
Source: Champney (2016).
Figure 2.3 Oblique coronal section: putamen, caudate, globus pallidus, subthalamic nucleus, substantia nigra.
Source: Champney (2016).
The striatum (caudate nucleus, putamen of lentiform nucleus, nucleus accumbens);
Globus pallidus (GP) – lateral and medial parts. The GP extends into the pars reticularis of the substantia nigra;
Subthalamic nucleus
Pars compacta of the substantia nigra.
Figure 2.4 A striatal motor loop. SMA ‐ supplementary motor area, VLN ‐ ventral lateral nucleus of thalamus, STN ‐ subthalamic nucleus, GPL ‐ globus pallidus (lateral), GPM ‐ globus pallidus (medial), SNpc ‐ substantia nigra pars compacta, CST‐ corticospinal tract.
Source: Fitzgerald (2010).
Basal Ganglia Circuits
Neuronal servo‐loops commence and end in the motor cortex. All pass through the striatum (putamen + caudate nucleus) and return via the thalamus, and within each loop there are two pathways: direct and indirect.
Transmission through each loop is controlled via the pars compacta of the substantia nigra to the lateral globus pallidus, where axons make two principal types of synapse, on excitatory D1 (dopaminergic, direct pathway) and inhibitory D2 (indirect pathway) receptors. Further receptors are now recognised in the D receptor series.
In normal subjects, the nigro‐striatal tract is active, selecting preferentially the excitatory, direct pathway and thus leading, via the loop back to the cortex to activation of the supplementary motor area before a movement, and thence to a movement itself (Figure 2.4). This early activation of the cortex underlies the electrical readiness potential (Bereitschaftspotential).
Such servo‐loops modulate, for example:
Cognition/motor intention, contraction strength, suppression, speed control, storage of programmes
Limbic (memory) loop: cortex→nucleus accumbens→ventral pallidum→thalamus→cortex.
Cerebellar System
Zones of the cerebellum are illustrated in Figure 2.5.
The cerebellar peduncles and deep cerebellar nuclei are shown in Figure 2.6.
The essential cellular anatomy of the cerebellar cortex is shown in Figure 2.7.
Figure 2.5 Zones of the cerebellum.
Source: Fitzgerald (2010).
Figure 2.6 Cerebellar peduncles & nuclei: posterior view.
Source: Champney (2016).
Figure 2.7 Cerebellum: cortical micro‐anatomy.
Source: Fitzgerald (2010).
Afferent and Efferent Cerebellar Pathways
Afferent pathways include:
Spino‐cerebellar: posterior and anterior spino‐cerebellar tracts – proprioceptive data from spinal cord.
Ponto‐cerebellar: originates in the cerebral cortex, and enters via middle cerebellar peduncle.
Vestibulo‐cerebellar: vestibular nuclei, enters via inferior peduncle.
Efferent pathways project to the vestibular system, to the cord, thalamus, motor cortex and to the red nucleus,.
The cerebellum and red nucleus in the midbrain tegmentum have a role in learned movement. The system modulates new motor activity:
The red nucleus is a relay between cerebral cortex and the olive – the red nucleus is inhibitory, to the ipsilateral olive.
When there is imbalance between movement intended (cerebral cortex) and movement already learned (cerebellum), the red nucleus is thought to modulate, to achieve harmony.
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