Page 339 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

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tracts from the entire cortex and use glutamate Function of the Cerebellum
as their transmitter (! D). Once activated, The cerebellum contains as many neurons as
neurons of the striatum release an inhibitory the rest of the brain combined. It is an impor-
transmitter (GABA) and a co-transmitter— tant control center for motor function that has
either substance P (SP) or enkephalin (Enk., afferent and efferent connections to the cortex
! D; ! p. 55). The principal output of the basal and periphery (! F, top panel). The cerebel-
ganglia runs through the pars reticularis of the lum is involved in the planning, execution and
control of movement and is responsible for
substantia nigra (SNr) and the pars interna of
Central Nervous System and Senses atum (! D). ates with higher centers to control attention,
the globus pallidus (GPi), both of which are in-
motor
adaptation
new
movement
to
hibited by SP/GABAergic neurons of the stri-
sequences (motor learning). It is also cooper-
etc.
Both SNr and GPi inhibit (by GABA) the ventrolateral
thalamus with a high level of spontaneous activity.
Anatomy (! F, top). The archeocerebellum (floc-
Activation of the striatum therefore leads to disinhi-
culonodular lobe) and paleocerebellum (pyramids,
bition of the thalamus by this direct pathway. If,
uvula, paraflocculus and parts of the anterior lobe)
however, enkephalin/GABA-releasing neurons of the
are the phylogenetically older parts of the cerebel-
striatum are activated, then they inhibit the pars ex-
lum. These structures and the pars intermedia form
terna of the globus pallidus (GPe) which, in turn, in-
the median cerebellum. The neocerebellum (poste-
thalamic nucleus induces glutamatergic activation of
genetically younger part of the cerebellum and
SNr and GPi. The ultimate effect of this indirect
forms the lateral cerebellum. Based on the origin of
pathway is increased thalamic inhibition. Since
their principal efferents, the archicerebellum and ver-
the thalamus projects to motor and prefrontal cor-
mis are sometimes referred to as the vestibulocerebel-
12 hibits (by GABA) the subthalamic nucleus. The sub- rior lobe of the body of the cerebellum) is the phylo-
tex, a corticothalamocortical loop that influences lum, the paleocerebellum as the spinocerebellum, and
skeletal muscle movement (skeletomotor loop) via the neocerebellum as the pontocerebellum. The cere-
the putamen runs through the basal ganglia. An bellar cortex is the folded (fissured) superficial gray
oculomotor loop projects through the caudate nu- matter of the cerebellum consisting of an outer
cleus, pars reticularis and superior colliculus and is in- molecular layer of Purkinje cell dendrites and their af-
volved in the control of eye movement (! pp. 342, ferents, a middle layer of Purkinje cells (Purkinje so-
360). Descending tracts from the SNr project to the mata), and an inner layer of granular cells. The outer
tectum and nucleus pedunculus pontinus. surface of the cerebellum exhibits small, parallel con-
The fact that the pars compacta of the substantia volutions called folia.
nigra (SNc) showers the entire striatum with
dopamine (dopaminergic neurons) is of pathophy- The median cerebellum and pars intermedia of
siological importance (! D). On the one hand, the cerebellum mainly control postural and
dopamine binds to D1 receptors (rising cAMP levels), supportive motor function (! F1,2) and oculo-
thereby activating SP/GABAergic neurons of the stri- motor function (! pp. 342 and 360). Input:
atum; this is the direct route (see above). On the The median cerebellum receives afference cop-
other hand, dopamine also reacts with D2 receptors ies of spinal, vestibular and ocular origin and
(decreasing cAMP levels), thereby inhibiting efference copies of descending motor signals to
enkephalin/GABAergic neurons; this is the indirect
route. These effects of dopamine are essential for the skeletal muscles. Output from the median
normal striatum function. Degeneration of more cerebellum flows through the intracerebellar
than 70% of the dopaminergic neurons of the pars fastigial, globose, and emboliform nuclei to
compacta results in excessive inhibition of the motor motor centers of the spinal cord and brain
areas of the thalamus, thereby impairing voluntary stem and to extracerebellar vestibular nuclei
motor function. This occurs in Parkinson’s disease (mainly Deiter’s nucleus). These centers con-
and can be due genetic predisposition, trauma (e.g., trol oculomotor function and influence loco-
boxing), cerebral infection and other causes. The motor and postural/supportive motor function
characteristic symptoms of disease include poverty
of movement (akinesia), slowness of movement via the vestibulospinal tract.
(bradykinesia), a festinating gait, small handwriting The lateral cerebellum (hemispheres)
(micrography), masklike facial expression, muscular mainly takes part in programmed movement
hypertonia (rigor), bent posture, and a tremor of rest- (! F3), but its plasticity also permits motor
326 ing muscles (“money-counting” movement of adaptation and the learning of motor
thumb and fingers). sequences. The hemispheres have two-way
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Despopoulos, Color Atlas of Physiology © 2003 Thieme
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