D’YOUVILLE COLLEGE
BIOLOGY 659 - INTERMEDIATE PHYSIOLOGY I
MOTOR SYSTEMS III
Lecture 19: Cerebellum, Basal Ganglia
1.
Cerebellum:
• organization: (figs 56 – 1, 56 – 2, 56 – 3 & ppts. 1 - 3)
- two hemispheres separated by central vermis
- each hemisphere features anterior, posterior & flocculonodular lobes
- each hemisphere also features intermediate and lateral zones
- topographical map (cruder than motor cortex) spans vermis & adjacent
intermediate zones; trunk associated with vermis, limbs with intermediate zones
• cortex and deep nuclei: (fig. 56 – 7 & ppt. 4)
- cortex – three layers (from deep to superficial): granular layer (contains
granular cells), Purkinje layer & molecular layer (contains large dendritic trees of Purkinje
cells)
- deep (central) gray matter includes paired fastigial nuclei, interposed
nuclei & dentate nuclei
• cortical functional unit: (ppt. 5)
- afferent fibers (via glutamate) excite deep nuclear cells to increase output
signal and also cause inhibitory output from Purkinje cells, which causes deep
nuclear cells to decrease output signal
- mossy fibers (afferents) synapse with deep nuclear cells and with granule cells;
granule cells synapse with Purkinje cells; mossy fiber input (via granule cells) causes
relatively weak inhibitory signal (of short duration) from Purkinje cells
- climbing fibers (afferents) synapse with deep nuclear cells and with Purkinje
cells; climbing fibers elicit strong, prolonged inhibitory signal from Purkinje cells
Bio 659 - lec 19
- p. 2 -
- interneurons: most are inhibitory, except for granule cells; Purkinje cells are
excited by afferents (& granule cells), but inhibited by other interneurons
- Purkinje cells make inhibitory synapses (via GABA) with deep nuclear
cells; Purkinje axons are the only efferents from the cortex
- circuits generate turn-on/turn-off, followed by turn-off/turn-on output to
modulate commands to agonist/antagonist muscle groups (produces enhanced
excitation followed by damping); mossy fiber pathway initiates this output; climbing fiber
pathway modulates it according to comparison of motor cortex intention vs.
proprioceptive image of action
• neuronal pathways:
- afferent pathways (fig. 56 – 4 & ppt. 6): corticopontocerebellar tract sends
fibers from motor cortex through contralateral pons to lateral zones via middle cerebellar
peduncle
- olivocerebellar, vestibulocerebellar & reticulocerebellar tracts (from motor
cortex, basal ganglia, brainstem and from vestibular apparatus) share inferior cerebellar
peduncles with dorsal spinocerebellar tract (mostly from muscle & tendon proprioceptors)
- ventral spinocerebellar tract follows superior cerebellar peduncle (carries
fibers with information about motor image that has arrived at dorsal horn) – efference copy
from cord motor circuits & lateral motor system)
- efferent pathways (fig. 56 – 6 & ppt. 7): signals from vermis via fastigial
nuclei communicate with vestibular nuclei (equilibrium adjustments) and with pontile
& medullary reticular nuclei (posture muscle adjustments)
- vestibulocerebellum (ppt. 8) occupies flocculonodular lobes & adjacent
vermis; output from fastigial nuclei; responsible for coordinating muscular adjustments
to maintain equilibrium; appears to have anticipatory function (making adjustments
immediately prior to a change in equilibrium)
Bio 659 - lec 19
- p. 3 -
- signals from intermediate zone via interposed nuclei communicate with
thalamus, motor cortex, red nucleus, reticular formation & basal ganglia (adjustments
for coordination of muscular function of distal limb parts) . (fig. 56 – 8 & ppt. 9)
- spinocerebellum (ppt. 10) occupies vermis and intermediate zones of
anterior & posterior lobes; output from interposed nuclei is responsible for damping
muscular movements (prevents excessive oscillations & perfects correspondence between
motor image & actual muscle performance)
- signals from lateral zone via dentate nuclei pass to premotor & to primary
and association somatosensory cortices (via thalamus) & red nucleus (fig. 56 – 6 &
ppt. 7)
- cerebrocerebellum (ppt. 11) occupies large lateral zones of cerebellar
hemispheres; output from dentate nuclei is responsible for planning sequences of actions
and timing of sequences of actions to facilitate smooth transitions between patterns of
muscular movements- 'sequencing'
2.
Basal Ganglia: (fig. 56 - 9 & ppt. 12)
• caudate nucleus – medial comma-shaped region of gray matter, arching over
thalamus; lies deep within frontal, parietal, occipital and temporal lobes
• putamen and globus pallidus – lateral to caudate nucleus and thalamus,
separated from medial structures by fiber tracts (internal capsule) to/from cortex
• subthalamic nucleus & substantia nigra (midbrain)
• neuron circuits: (figs 56 – 10 & ppt. 13): extensive connections with several areas
of cortex (prefrontal, premotor & supplementary motor, somatosensory association
areas, visual & auditory); two main functional circuits (poorly understood):
Bio 659 - lec 19
- p. 4 -
- putamen circuit (fig. 56 – 11 & ppt. 14) – coordinates complex actions (piano
playing, using scissors, writing, vocalization, catching/throwing a ball, eye
movements, etc.)
– dysfunction results in loss of muscle control (flailing – lesion in
subthalamus, flicking – lesion in putamen, writhing actions – lesion in globus
pallidus, or rigidity, tremors or loss of motor control (=Parkinson’s disease) – lesion in
substantia nigra)
- afferents from cortex (premotor & supplementary motor areas,
somatosensory areas) to putamen
- feedback pathways from putamen to globus pallidus mainly to thalamus &
primary motor cortex; accessory connections involve subthalamus and substantia nigra
(neurotransmitter = dopamine)
- caudate circuit (fig. 56 – 12 & ppt. 15) – cognitive control of sequential actions
(e.g. at sight of danger, turn and flee); also, timing of movements and scaling intensity
(e.g. writing large or small versions of letters)
- afferents from extensive areas of cortex (motor, auditory, visual,
somatosensory association area)
- feedback pathways to frontal cortex (prefrontal & premotor areas)
• neurotransmitters (fig. 56 – 14 & ppt. 16) : dopamine (inhibitory) from
neurons of substantia nigra – deficiency associated with Parkinson’s disease
- GABA (inhibitory) & Ach (excitatory) from neurons of putamen and caudate
nucleus – deficiency associated with Huntingdon’s chorea
- additionally, neurotransmitters from several other areas of the nervous
system interact with the basal ganglia system; these include glutamate, serotonin,
norepinephrine, enkephalin