Ten Flashcards
What are the 3 major parts of the cerebellum? What are they comprised of? What are the 3 ways in which the cerebellum is connected to the brainstem? Where do they connect with it? What do they consist of?
The cerebellum or “little brain” is located in the posterior cranial fossa. It is comprised of three major parts: (1) A superficial mantle of gray matter, the cerebellar cortex, (2) Internal white matter formed by axons passing to or from the cortex, and (3) Three pairs of cerebellar nuclei buried deep within the white matter. Three pairs of peduncles,
formed by cerebellar afferent and efferent projections connect the cerebellum to the brainstem. The inferior cerebellar peduncles connect it with the medulla, the middle with the pons, and the superior with the midbrain.
How is the surface of the cerebellum organized? What are the different parts? What part of the body does each lobe primarily connect with? What is the largest part of the cerebellum?
The surface of the cerebellum is thrown into numerous parallel folds or folia, which are oriented in the transverse plane, that is, in an ear-to-ear direction. Two major transversely-directed fissures separate groups of folia into ten lobules and three lobes of the cerebellum. The small flocculonodular lobe is most inferior and lies behind the
posterolateral fissure. This is the most ancient part of the cerebellum and receives its major input from the vestibular apparatus (vestibulocerebellum). The anterior lobe is most superior and lies in front of the primary fissure. It appears somewhat later in evolution than the vestibulocerebellum and its main input is from the limbs via their spinal connections (spinocerebellum). Between the posterolateral and primary fissures is the largest part of the cerebellum, the posterior lobe. It is the newest part and has very strong connections with the cerebral cortex (cerebrocerebellum).
Sagitally, describe the sections of the cerebellum.
Sagittally the cerebellum consists of a median part, the vermis, and lateral expansions of the vermis, the hemispheres. Each hemisphere is divided into paravermal, or intermediate, and lateral hemispheric parts.
Describe the cytoarchitecture of the cerebellar cortex. What cells are in the different layers?
The cytoarchitecture of the cerebellar cortex is uniform throughout. Each folium is comprised of an inner part consisting of white matter, and an outer part consisting of the cortical gray matter. The cortex is formed by three layers of neurons and/or their processes:
(1) The molecular layer is characterized by its relative scarcity of neurons, (2) The Purkinje cell layer is formed by a monolayer of large flasked-shaped neurons, the Purkinje cells, and (3) The granule cell layer composed of densely packed, small granule cell neurons.
What does the molecular layer largely consist of? What else is there? What is found in the inner most cortical area? Where do all but one type of cerebellar afferent projections synapse? What signal do they send? What are their fibers like? Where do axons from the granule cells synapse? What are they called/where do they run/what are they like? What are climbing type afferents? Where do they originate? Where do they run? Where do they synapse? What is there signal? What is their function? What is the only excitatory neuron in the cerebellar cortex? What is the only efferent projection neuron? What signal does it send and to where?
The molecular layer contains chiefly the massive dendritic trees of the Purkinje cells interspersed with the axons of underlying granule cells.
Myriads of granule cells are found in the inner most cortical layer.
All the cerebellar afferent projections with the exception of one become excitatory mossy fibers and terminate on the granule cells. The granule cells give rise to axons that ascend past the Purkinje cell somata, enter the molecular layer and bifurcate forming the parallel plexus. These parallel fibers synapse on spines on the Purkinje dendrites.
Climbing fiber type afferents originate exclusively from the contralateral inferior olive and, after reaching the cerebellar cortex, they pass through the granule cell layer to climb upon the main Purkinje cell dendrites where they have numerous excitatory contacts. The climbingfibers are thought to teach the cerebellar cortex the programs for the coordination of movements.
Physiologically, the only excitatory neuron in the cerebellar cortex is the granule cell. Purkinje cells, the only efferent projection neuron in the cerebellar cortex, inhibit neurons in the cerebellar nuclei.
What are 4 things that damage to might cause ataxia?
- Sensory Axons
- Cerebellar Afferents
- Cerebellar Cortex or Nuclei
- Cerebellar Efferents
What are three types of inputs to the cerebellum? What is their function? Where do they receive input from?
• Input from the Spinal Cord - Spinocerebellum
Ongoing Movements
• Input from the Vestibular System - Vestibulocerebellum
Posture and Eye Movements
• Input from the Cerebral Cortex - Cerebrocerebellum
Planned – Highly Skilled – Rapid Movements
What are the three cerebellar nuclei? Where are they located? Where do they receive their signals from? What type of signals? How do they exert their influence?
The cerebellum influences motor activities almost exclusively through the cerebellar nuclei. These paired neuronal masses, embedded in the medullary white matter near the roof of the fourth ventricle, are, from medial to lateral, the fastigial, interposed, and dentate. Each of these receives excitatory impulses from collateral branches of the cerebellar afferent projections and inhibitory impulses from Purkinje cells in certain parts of the cerebellar cortex, and, in turn, each exerts its influence via certain parts of the brainstem.
What does the flocculonodular lobe do? Where do direct and indirect impulses come from? Where do they pass to get into the cerebellum? Where do they end up in the cerebellum?
The Flocculonodular Lobe Coordinates Axial and External Ocular Muscles for Equilibrium and Eye Movements.
Direct and indirect impulses from the vestibular apparatus in the inner ear carry information about the position of and movements of the head. The direct vestibulocerebellar impulses reach the cerebellum via the vestibular nerve without synapsing. The indirect vestibulocerebellar impulses come from the vestibular nuclei. Both groups enter the cerebellum in the medial part of the inferior cerebellar peduncle, the juxtarestiform body, and pass chiefly to the flocculonodular lobe and the adjacent parts of the vermis.
What does the anterior lobe do? What is an example of a pathway that shows the manner in which this happens? How does it receive information from the corticospinal tracts?
The Anterior Lobe Fine Tunes Movements While They Are Occurring
The vermal and paravermal parts of the anterior lobe are chiefly concerned with the coordination of limb movements while the movements are being executed and, hence, it has strong connections with the spinal cord. Clinical findings suggest that the lower limb is represented more anteriorly, the upper limb more posteriorly, and the trunk medially.
Discrete information chiefly from muscle spindles and tendon organs of individual lower limb muscles reaches the cerebellum through the dorsal spinocerebellar tract. This tract arises from the dorsal nucleus of Clarke (nucleus thoracicus), which forms a column of neurons in the medial part of lamina VII from about C8 to L2. The dorsal nucleus receives input directly from Ia and Ib collaterals ascending from the lumbosacral parts of the graciletract. The axons of the dorsal nucleus of Clarke ascend ipsilaterally as the dorsal spinocerebellar tract and enter the cerebellum via the restiform body, the large lateral part of the inferior cerebellar peduncle.
Information pertaining to activity in the corticospinal tracts reaches the anterior lobe via the pontine nuclei. This information comes from collaterals of the corticospinal fibers. From the pontine nuclei, pontocerebellar fibers cross and enter the cerebellum through the contralateral
middle cerebellar peduncle from which they are projected to the lateral parts of the anterior lobe. Through these connections the anterior lobe receives information about the impending influence of the corticospinal tract on an ongoing movement.
What does the posterior lobe do? What is the pathway for its afferents?
The Posterior Lobe Programs Coordination of Skilled Movements
By far, the largest group of cerebellar afferents is the cortico-ponto-cerebellar projections. Most of the corticopontine fibers arise from the sensorimotor, premotor, and posterior parietal parts of the cerebral cortex, although the association areas of all the lobes contribute heavily. The corticopontine fibers reach the ipsilateral pontine nuclei by coursing through the internal capsule and cerebral crus. The pontine nuclei give rise to the transverse pontine fibers which, after crossing and proceeding through the contralateral basilar pons, form the massive middle cerebellar peduncles which project to the posterior lobe.
What is the efferent pathway for the Flocculonodular lobe?
Axons from Purkinje neurons in the flocculonodular lobe influence the vestibular nuclei and the adjacent reticular formation indirectly through the fastigial nuclei and directly from the Purkinje cells. The fastigiobulbar projections as well as the direct flocculonodular projections reach the vestibular nuclei through the juxtarestiform body (inferior cerebellar peduncle). Vestibulospinal and vestibulo-ocular projections then descend and ascend in the medial longitudinal fasciculus to reach the motor neurons innervating the axial muscles and the external ocular muscles.
What is the efferent pathway for the anterior lobe?
Axons from Purkinje neurons in the anterior lobe, especially its vermal and paravermal parts, influence the fastigial and interposed nuclei. Through the fastigial nucleus and its connections with the vestibular nuclei, mainly the lateral, vermal parts of the anterior lobe
have a strong influence on the ipsilateral extensor muscles, especially those in the proximal parts of the lower limbs. Through the interposed nuclei and its decussation, the paravermal parts of the anterior lobe influence the ipsilateral flexor muscles, especially those in the more distal parts of the upper limbs.
What is the efferent pathway for the posterior lobe?
Axons from Purkinje neurons in the lateral parts of the posterior lobe project to the dentate nucleus, the major source of fibers of the superior cerebellar peduncle. These dentatofugal fibers pass to the contralateral ventral lateral nucleus (VL-thalamus). From here thalamocortical fibers project to the primary motor cortex, the origin of the pyramidal system.
