The cerebellar cortex is composed of five types of neurons arranged into three layers (Fig. 5.18). The molecular layer is the outermost and consists mostly of axons and dendrites plus two types of interneurons, stellate cells and basket cells. The next layer contains the dramatic Purkinje cells, whose dendrites reach upward into the molecular layer in a fan-like array. The Purkinje cells are the only efferent neurons of the cerebellar cortex. Their action is inhibitory via GABA as the neurotransmitter. Deep to the Purkinje cells is the granular layer, containing Golgi cells, and small local circuit neurons, the granule cells. The granule cells are numerous,- there are more granule cells in the cerebellum than neurons in the entire cerebral cortex!
Afferent axons to the cerebellar cortex are of two types: mossy fibers and climbing fibers. Mossy fibers arise from the spinal cord and brainstem neurons, including those of the pons that receive input from the cerebral lution. The lateral cerebellar hemispheres increase in size along with expansion of the cerebral cortex. The three divisions have similar intrinsic circuitry, thus, the function of each depends on the nature of the output nucleus to which it projects.
The vestibulocerebellum is composed of the flocculo-nodular lobe. It receives input from the vestibular system and visual areas. Output goes to the vestibular nuclei, which can, in a sense, be considered as an additional pair of intrinsic cerebellar nuclei. The vestibulocerebellum functions to control equilibrium and eye movements.
The medially placed spinocerebellum consists of the midline vermis plus the medial portion of the lateral hemispheres, called the intermediate zones. Spinocerebellar pathways carrying somatosensory information terminate in the vermis and intermediate zones in somatotopic arrangements. The auditory, visual, and vestibular systems and sen-sorimotor cortex also project to this portion of the cerebellum. Output from the vermis is directed to the fastigial nuclei, which project through the inferior cerebellar peduncle to the vestibular nuclei and reticular formation of the pons and medulla. Output from the intermediate zones goes to the interposed nuclei and from there to the red nucleus and, ultimately, to the motor cortex via the ventrolateral nucleus of the thalamus. It is believed that both the
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Mossy fiber i Cerebellar circuitry. The cell types and ac' tion potential pathways are shown. Mossy fibers bring afferent input from the spinal cord and the cerebral cortex. Climbing fibers bring afferent input from the inferior olive nucleus in the medulla and synapse directly on the Purkinje cells. The Purkinje cells are the efferent pathways of the cerebellum.
cortex. Mossy fibers make complex multicontact synapses on granule cells. The granule cell axons then ascend to the molecular layer and bifurcate, forming the parallel fibers. These travel perpendicular to and synapse with the dendrites of Purkinje cells, providing excitatory input via glutamate. Mossy fibers discharge at high tonic rates, 50 to 100 Hz, which increases further during voluntary movement. When mossy fiber input is of sufficient strength to bring a Purkinje cell to threshold, a single action potential results.
Climbing fibers arise from the inferior olive, a nucleus in the medulla. Each climbing fiber synapses directly on the dendrites of a Purkinje cell and exerts a strong excitatory influence. One action potential in a climbing fiber produces a burst of action potentials in the Purkinje cell called a complex spike. Climbing fibers also synapse with basket, Golgi, and stellate interneruons, which then make inhibitory contact with adjacent Purkinje cells. This circuitry allows a climbing fiber to produce excitation in a single Purkinje cell and inhibition in the surrounding ones.
Mossy and climbing fibers also give off excitatory collateral axons to the deep cerebellar nuclei before reaching the cerebellar cortex. The cerebellar cortical output (Purkinje cell efferents) is inhibitory to the cerebellar and vestibular nuclei, but the ultimate output of the cerebellar nuclei is mostly excitatory. A smaller population of neurons of the deep cerebellar nuclei produces inhibitory outflow directed mainly back to the inferior olive.
Lesions Reveal the Function of the Cerebellum Lesions of the cerebellum produce impairment in the coordinated action of agonists, antagonists, and synergists. This impairment is clinically known as ataxia. The control of limb, axial, and cranial muscles may be impaired depending on the site of the cerebellar lesion. Limb ataxia might manifest as the coarse jerking motions of an arm and hand during reaching for an object instead of the expected, smooth actions. This jerking type of motion is also referred to as action tremor. The swaying walk of an intoxicated individual is a vivid example of truncal ataxia.
Cerebellar lesions can also produce a reduction in muscle tone, hypotonia. This condition is manifest as a notable decrease in the low level of resistance to passive joint movement detectable in normally relaxed individuals. My-otatic reflexes produced by tapping a tendon with a reflex hammer reverberate for several cycles (pendular reflexes) because of impaired damping from the reduced muscle tone. The hypotonia is likely a result of impaired processing of cerebellar afferent action potentials from the muscle spindles and Golgi tendon organs.
While these lesions establish a picture of the absence of cerebellar function, we are left without a firm idea of what the cerebellum does in the normal state. Cerebellar function is sometimes described as comparing the intended with the actual movement and adjusting motor system output in ongoing movements. Other putative functions include a role in learning new motor and even cognitive skills.
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