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.
A lesion of the red nucleus – a coarse tremor – is a breakdown of this harmonic, over‐correcting each part of a movement.
In contrast to the anatomical complexity, signs of cerebellar disease are usually straightforward:
A lateral lobe lesion – a tumour or an infarct – causes rebound and past pointing of the upper limb and similar lower limb signs.
A vermis lesion – for example midline medulloblastoma – affects vestibular connection: truncal ataxia can be an early sign.
Nystagmus – coarse, fast phase towards the side of a lesion, sometimes dramatic – is an inconstant feature.
Sensory Pathways
Neurologists deal with the special senses – vision, hearing/balance, olfaction and taste – and the main five sensory modalities: touch, nociception, temperature, joint position, vibration and two‐point position. Neuroscientists use an alternative vocabulary: sensation is either conscious or non‐conscious and either afferent proprioceptive – from a limb, or enteroceptive – from gut, or heart.
Sensory Pathways in the Cord and Brain
Two major pathways deliver sensory information to the thalamus and thence to the cortex (Figure 2.8):
Spinothalamic pathways (nociceptive – pain, temperature);
Posterior columns → medial lemnisci (touch, position, movement, vibration).
