Lecture 16
W3005/4005
DB Kelley
The cerebellum, the vestbulo-ocular reflex and motor learning
A. Introduction
The cerebellum functions to modify the gain of motor output both during motor
planning (cerebrocerebellum) and during the excecution of movements
(spinocerebellum). The cerebellum also maintains the position of the head in
space and affects reflexes of the extraocular muscle to maintain the position of
gaze (vestibulocerebellum). The intrinsic circuitry of the cerebellum allows a
modification of its output via change in the frequency of firing of the Purkinje
cells. The climbing fiber input to the cerebellum is very powerful and produces
complex spikes which result in calcium influx; the mossy fiber input to the
cerebellum is less powerful and produces simple spikes (Fig. 41-5). The
preceding history of complex spike activity affects the probability of production of
simple spikes, a feature that suggests that the cerebellum may play a role in
motor learning. The Purkinje neuron provides the sole output from the cerebellar
cortex and that output (to the deep cerebellar nuclei) is inhibitory. Thus
modifications of cerebellar ouput are due to increases or decreases in inhibition;
inhibition is as, or perhaps even more, important than excitation for motor
circuits.
B. The anatomy and connectivity of the cerebellum.
A. What and where
• The cerebellum is a very large collection of neurons (half the neurons in the
CNS) that sits on top of the pons and medulla).
• Three large fiber bungles connect the cerebellum to the rest of the CNS: the
peduncles (superior, middle and inferior).
• The cerebellum consists of cortices (vestibulocerebellum, spinocerebellum and
cerebrocerebellum) and the deep cerebellar nuclei (dentate, fastigial, emboliform
and globose; the latter two make up the nucleus intepositus).
• The vesticbulocerebellum comprises the floccularnodular lobe; the
spinocerebellum is the more medial region of anterior cerebellar cortices and the
cerebrocerellum is the more lateral aspect.
• The basic repeating unit of the cerebellum is the folium; a leaflet of cells made
up of only 6 cell types (Pukinje, granule, stellate, basket and Golgi) arranged into
4 layers (molecular, Purkinje, granular and white).
C. Cerbellar macrocircuitry
• Different division of the cerebellum have different connection
• The vestibulocerebellum gets input directly from primary sensory neurons of the
vestibular syatem and well as the vestibular nuclei to which it projects to
influence spinal cord pathways (vestib. nuclei also affect oculomotor pathways,
see below).
• The spinocerebellum gets spinal and vestibular nuclei and its Purkinje cells
project to the fastigial and interpositus nuclei.
• The spinocerebellum has several complete maps of the body surface (Fig. 419); some are fractured
• The cerebrocerebellum gets input from pontine nuclei that themselves are
afferented by collaterals of neurons in the corticospinal tract. The output of the
cerebrocerebellum via the dentate nucleus to the thalamus and then to motor
cortex and premotor areas.
D. Cerbellar microcircuitry
• There are two major inputs to the cerebellum: climbing fibers (which come from
the inferior olive) and mossy fibers (which come from everywhere else).
• These form distinctive types of terminals on their target cells (mostly granule
cells for mossy fibers and Purkinje cells for climbing fibers).
• The granule cells give rise to parallel fibers which make synapses on the
dendritic trees (oriented by 90 degrees to the climbing fibers) of Purkinje cells;
each Purkinje cells receives ~200,000 parallel fiber inputs.
• The climbing fibers synapse directly on the Purkinje cell bodies; each Purkinje
cells receives input from a single climbing fiber and each climbing fiber
innervates, at most, 10 Purkinje cells.
• Inhibitory interneurons (eg. basket cells) sharpen the beam of parallel fiber
excitation
E. Functions of the cerebellum
1. Cerebrocerebellum
• Lesions of the dentate or of overlying cortex produce deficits in initiating
movements and in coordinating arm and leg movements (Fig. 41-130.
• Primate studies reveal that when the dentate is inactivated both the discharge
of neurons in primary motor cortex and the initiation of associated movements is
delayed.
• Rapid movements are characteristically overshot; terminal tremor results.
• Perceptual judgements of tone duration are also disrupted implying that
cerebellum functions also in cognition.
2. Spinocerebellum
• Information about ongoing movements are provided by information originating
in the dorso spinocerbellar tract while information in the ventral spinocerbellar
tract conveys information on CNS motor pattern generation and spinal circuit
activation.
• The spinocerebellum insures that movements are smooth by adjusting neural
output to compensate for changes in load.
• Lesions result in decreased activity of gamma motor neurons.
3. Vestibulocerebellum
• In addition to vestibular inputs, the vestibulocerebellum also receives auditory
and visual information.
•The vestibulocerebellum adjusts the gain of output to motor neurons that control
the position of the eyes in space so as to maintain gaze.
• The vestibuloocular reflex
• Learning and the vestibuloocular reflex; the Ito hypothesis.