Chapter 6: Cerebellum and Its Connections
What does the cerebellum consist of?
Two cerebellar hemispheres joined by a narrow median vermis.
What are the peduncles?
Three paired bundles of nerve fibers (superior, middle, and inferior) that connect the cerebellum to the brainstem.
What are the three lobes of the cerebellum?
- middle (or posterior)
What is the primary fissure?
A fissure between the anterior and middle lobes of the cerebellum.
What is the uvulonodular fissure?
A fissure between the middle and flocculonodular lobes of the cerebellum.
What is the horizontal fissure?
A fissure along the margin of the cerebellum separates the superior from the inferior surfaces.
What is the cerebellar cortex?
A large, folded sheet of gray matter, forming folia, each containing a core of white matter.
What is the arbor vitae?
A branched appearance of folia seen in medial sections of the cerebellum.
What are the three layers of the cerebellar cortex?
- external molecular layer
- middle Purkinje cell layer
- internal granular layer
What two types of cells comprise the molecular layer of the cerebellum?
- outer stellate cells
- inner basket cells
What types of cells comprise the Purkinje cell layer of the cerebellum?
Large, flask-shaped Golgi type I neurons with many branches covered by dendritic spines.
What types of cells comprise the granular layer of the cerebellum?
Small cells with densely staining nuclei and scanty cytoplasm, with parallel fibers, running at right angles to the dendritic processes of the Purkinje cells.
What are the four intracerebellar nuclei?
- dentate nucleus
- emboliform nucleus
- globose nucleus
- fastigial nucleus
What are the intracerebellar nuclei composed of?
Large, multipolar neurons with simple branching dendrites and axons that form the cerebellar outflow.
What are the three groups of fibers that make up white matter in the cerebellum?
What are intrinsic fibers?
Fibers that do not leave the cerebellum but connect different regions of the organ.
What are afferent fibers?
Fibers that form the greater part of the white matter of the cerebellum, proceeding to the cortex.
What are efferent fibers?
Fibers that constitute the output of the cerebellum and commence as the axons of the Purkinje cells.
What are the two major inputs to the cerebellar cortex?
- mossy fibers
- climbing fibers
What is the effect of climbing and mossy fibers on the cerebellar cortex?
They are excitatory to the Purkinje cells.
Where do climbing and mossy fibers originate?
Climbing fibers are terminal fibers of olivocerebellar tracts; mossy fibers are terminal fibers of all other cerebellar afferent tracts.
How do climbing fibers contact Purkinje neurons?
Each Purkinje neuron makes synaptic contact with only one climbing fiber, but each climbing fiber makes contact with 1 to 10 Purkinje neurons.
How do mossy fibers contact Purkinje neurons?
A single mossy fiber may stimulate thousands of Purkinje cells through the granule cells.
If mossy and climbing fibers are the only source of cerebellar input, then what is the function of the stellate, basket, and Golgi cells?
They are inhibitory interneurons that modulate excitation from climbing and mossy fiber input.
What is the role of glutamate in the cerebral cortex?
It is used as an excitatory neurotransmitter between climbing and mossy afferent fibers and Purkinje cells.
What is the role of norepinephrine and serotonin in the cerebral cortex?
They are used by afferent fibers to modify the action of the glutamate on the Purkinje cells.
What are the superior cerebellar peduncles connect?
Bundles of nerve fibers connecting the cerebellum to the midbrain.
What do the middle cerebellar peduncles connect?
Bundles of nerve fibers connecting the cerebellum to the pons.
What do the inferior cerebellar peduncles connect?
Bundles of nerve fibers connecting the cerebellum to the medulla oblongata.
What are the three pathways from the cerebral cortex to the cerebellum?
- corticopontocerebellar pathway
- cerebro-olivocerebellar pathway
- cerebroreticulocerebellar pathway
Describe the corticopontocerebellar pathway.
Corticopontine fibers arise in the cerebral cortex, and terminate on the pontine nuclei, which give rise to transverse fibers of the pons which terminate as mossy fibers in the cerebellar cortex.
Describe the cerebro-olivocerebellar pathway.
Cortico-olivary fibers arise in the cerebral cortex, and terminate on the inferior olivary nuclei, which give rise to fibers that terminate as climbing fibers in the cerebellar cortex.
Describe the cerebroreticulocerebellar pathway.
Corticoreticular fibers arise in the cerebral cortex, and terminate in the reticular formation, which gives rise to to the reticulocerebellar fibers that enter the cerebellar hemisphere.
What is the function of pathways from the cerebral cortex to the cerebellum?
They convey control from the cerebral cortex.
What are the three pathways from the spinal cord to the cerebellum?
- anterior spinocerebellar tract
- posterior spinocerebellar tract
- cuneocerebellar tract
What is the nucleus dorsalis (Clarke column)?
A nucleus at the base of the posterior gray column where axons entering the spinal cord from the posterior root ganglion terminate.
Describe the anterior spinocerebellar tract.
Axons entering the spinal cord terminate on the nucleus dorsalis, which gives rise to fibers that ascend in the contralateral lateral white column, crossing back in the cerebellum to terminate as mossy fibers in the cerebellar cortex.
Describe the posterior spinocerebellar tract.
Axons entering the spinal cord terminate on the nucleus dorsalis, which gives rise to fibers that ascend in the posterolateral part of the lateral white column to terminate as mossy fibers in the cerebellar cortex.
Describe the cuneocerebellar tract.
Fibers originate from the nucleus cuneatus of the medulla oblongata, terminating as mossy fibers in the cerebellar cortex.
What is the function of pathways from the spinal cord to the cerebellum?
They convey information about muscle spindles, tendon organs, and joint receptors.
What are the four efferent cerebellar pathways?
- globose-emboliform-rubral pathway
- dentatothalamic pathway
- fastigeal vestibular pathway
- fastigeal reticular pathway
Describe the globose-emboliform-rubral pathway.
Axons from the globose and emboliform nuclei synapse with the contralateral red nucleus, which gives rise the rubrospinal tract, which crosses over to ipsilateral motor neurons.
Describe the dentatothalamic pathway.
Axons from the dentate nucleus synapse in the contralateral ventrolateral nucleus of the thalamus, which gives rise to fibers that synapse in the motor cortex, which gives rise to the cortocospinal tract, which crosses over to ipsilateral motor neurons.
Describe the fastigeal vestibular pathway.
Axons from the fastigial nucleus synapse in the lateral vestibular nucleus, which gives rise to the vestibulospinal tract to ipsilateral motor neurons.
Describe the fastigeal reticular pathway.
Axons from the fastigial nucleus synapse in the reticular formation, which gives rise to the reticulospinal tract to ipsilateral motor neurons.
What is the significance of cerebellar output being directed through descending pathways?
The cerebellum has no direct neuronal connections with the lower motor neurons but exerts its influence indirectly through the cerebral cortex and brainstem.
What is the clinical significance of cerebellar pathways being principally ipsilateral?
A lesion in one cerebellar hemisphere gives rise to signs and symptoms that are limited to the same side of the body.
What is the fucntion of the cerebellum?
To coordinate, by synergistic action, all reflex and voluntary muscular activity.
What are the effects of acute cerebellar lesions?
They produce sudden, severe symptoms and signs, but patients usually show complete recovery, possibly due to compensation from other areas of the CNS.
What are the effects of chronic cerebellar lesions?
They produce less severe symptoms and signs, possibly because other areas of the CNS have time to compensate for loss of cerebellar function.
What is dysdiadochokinesia ?
The inability to perform alternating movements regularly and rapidly (e.g. pronatation and supination the forearms).
What is a pendular knee jerk?
A series of flexion and extension at the knee joint following tapping of the patellar tendon, which occurs in cerebellar disease, because of loss of influence on the stretch reflexes.
What is nystagmus?
Rhythmical oscillation of the eyes.
What is pendular nystagmus?
Nystagmus in the same rate in both directions.
What is jerk nystagmus?
Nystagmus quicker in one direction than in the other.
What is dysarthria?
Difficultly speaking, which occurs in cerebellar disease because of ataxia of the muscles of the larynx.
What is vermis syndrome?
A condition involving muscle incoordination of the head and trunk but not the limbs, most commonly caused by medulloblastom in children.
What is cerebellar hemisphere syndrome?
A condition that may be caused by tumors of one cerebellar hemisphere, which may produce unilateral muscle incoordination on the ipsilateral side.
What is acute alcohol poisoning?
One of the most common diseases affecting cerebellar function, which occurs as the result of alcohol acting on GABA receptors on the cerebellar neurons.