cerebellum

2. cerebellum Functional Anatomy

4. cerebellum • ‘little brain’ • 10% of brain mass • As many neurons as the rest of the CNS • Every kind of sensory input reaches the cerebellar cortex, which projects via deep cerebellar nuclei to various sites in brain stem and thalamus • Has few ways to influence motor neurons directly

5. Location of cerebellum: • Largest part of hindbrain • Occupies most of posterior cranial fossa • Lies behind pons & medulla • forming roof of 4th ventricle • Separated from posterior part of cerebrum • by tentoriumcerebelli

6. Important facts • The hemispheres, vermis, flocculus, nodule, and tonsil are major landmarks of the cerebellar cortex. • Afferent fibers end in the three-layered cerebellar cortex. • The Purkinje cells have axons that end in the cerebellar nuclei.

7. Motor part of the brain, serving to maintain equilibrium and coordinate muscle contractions. • Makes a special contribution to synergy of muscle action (i.e., to the synchronized contractions and relaxations of different muscles that make up a useful movement).

8. Ensures that contraction of the proper muscles occurs at the appropriate time, each with the correct force. • Participates in learning patterns of neuronal activity needed for carrying out movements and in the execution of the encoded instructions.

9. Imagined movements are accompanied by an increase in cerebellar blood flow that is larger than the increase detected in the motor areas of the cerebral cortex. • Evidence also suggests that the cerebellum has sensory and cognitive functions.

10. The cerebellum consists of a cortex, or surface layer, of gray matter contained in transverse folds or folia plus a central body of white matter. • Four pairs of central nuclei are embedded in the cerebellar white matter. • Three pairs of cerebellar peduncles, composed of myelinated axons, connect the cerebellum with the brain stem.

13. Cerebral peduncles: • Joined to the brain stem via: • Superior cerebellar peduncle –> Midbrain • Middle cerebellar peduncle –> Pons • Inferior cerebellar peduncle –> Medulla

14. Anatomical divisions • On the basis of transverse fissures into lobes • Dorsolateral fissure separates flocculonodular lobe from the rest of the cerebellum • So deep that flocculonodular lobe appears pinched off from rest of cerebellum • Primary fissure divides body of cerebellum into anterior and posterior lobes

20. Functional divisions • The Vestibulocerebellum is the flocculonodular lobe and receives input from the vestibular nerve and nuclei. • The spinocerebellum consists of the vermis of the anterior lobe together with the adjacent medial or paravermal zones of the hemispheres; the spinocerebellar tracts and cuneocerebellar fibers, which convey proprioceptive and other sensory information, terminate here.

21. The Pontocerebellum/cerebrocerebellum comprises the large lateral parts of the hemispheres and the superior vermis in the posterior lobe; afferents are from the contralateralpontine nuclei.

24. Contents of Cerebellar Peduncles • The superiorcerebellar peduncle contains cerebellar efferent fibers, the ventral spinocerebellar tract, and tectocerebellar fibers. • The middlecerebellar peduncle consists of fibers from the contralateralpontine nuclei

25. Contents of Cerebellar Peduncles • Inferiorcerebellar peduncle contains olivocerebellar and dorsal spinocerebellar fibers and the vestibulocerebellar and fastigiobulbar connections.

27. Central Nuclei-deep cerebellar nuclei • Four pairs of nuclei are embedded deep in the cerebellar white matter; in a medial to lateral direction, they are the fastigial, globose, emboliform, and dentate nuclei • The fastigial nucleus is close to the midline, almost in contact with the roof of the fourth ventricle.

28. The interposed nucleus (comprising two cell clusters, the globose and the emboliform nuclei) is situated between the fastigial and dentate nuclei. • The prominent dentate nucleus has the irregular shape of a crumpled purse, similar to that of the inferior olivary nucleus, with the hilus facing medially

29. Its efferent fibers occupy the interior of the nucleus and leave through the hilus.

30. Input to the cerebellar nuclei • sources outside the cerebellum • the Purkinje cells of the cortex

31. extrinsic input • Pontocerebellarfibres • Spinocerebellarfibres • Olivocerebellar fibers • Fibers from the precerebellar reticular nuclei • Most of these afferents are collateral branches of fibers proceeding to the cerebellar cortex

32. A few rubrocerebellar fibers end in the interposed nucleus • fastigial nucleus receives afferents from the vestibular nerve and nuclei. • fastigial nucleus projects to the brain stem through the inferior cerebellar peduncle • efferents from the other nuclei leave the cerebellum through the superior peduncle and end in the brain stem and thalamus.

35. Whereas the input to the central nuclei from outside the cerebellum is excitatory, the input from Purkinje cells, which use GABA as their transmitter, is inhibitory. • Crudely processed information in the central nuclei is refined by the inhibitory signals received from the cortex.

37. combination of the two inputs maintains a tonic discharge from the central nuclei to the brain stem and thalamus. • discharge changes constantly according to the afferent input to the cerebellum at any given time.

38. Functional division • Longitudinal • According to function • Most medial-vermis • Either side- cerebellar hemisphere

39. Hemisphere divided into a medial part= intermediate/paravermal zone • Larger lateral hemisphere • Vermis more prominent on inferior surface • Division based on patterns of connections and functional differences

40. There is some overlapping of the divisions • for example, both spinocerebellar and pontocerebellar fibers terminate in the cortex of the paravermal zones.

41. functional divisions • The vestibulocerebellum is the flocculonodular lobe and receives input from the vestibular nerve and nuclei. • The spinocerebellum consists of the vermis of the anterior lobe together with the adjacent medial or paravermal zones of the hemispheres • the spinocerebellar tracts and cuneocerebellar fibers, which convey proprioceptive and other sensory information, terminate here.

42. The pontocerebellum comprises the large lateral parts of the hemispheres and the superior vermis in the posterior lobe • afferents are from the contralateralpontine nuclei. There is some overlapping of the divisions; for example, both spinocerebellar and pontocerebellar fibers terminate in the cortex of the paravermal zones.

44. Vestibulocerebellum-afferents • Vestibular ganglion and nuclei [ipsilateral]- some terminate in fastigial nucleus, which also receives collaterals from axons going to cerebellar cortex • Contralateral accessory olivary nuclei –have collateral branches to fastigial nucleus

45. Vestibulocerebellum- efferents • Some Purkinje cell axons from the vestibulocerebellar cortex proceed to the brain stem (an exception to the general rule that such fibers end in central nuclei) • Most terminate in the fastigial nucleus and end as climbing fibers in the cortex of the flocculonodular lobe

46. Fibers from the cortex and the fastigial nucleus traverse the inferior cerebellar peduncle to their termination in the vestibular nuclear complex and in the central group of reticular nuclei

47. Connections of vestibulocerebellum and vestibular nuclei. Afferents-blue; efferents-red;neurons-black

48. Summary of function • influences motor neurons through the vestibulospinal tract, the medial longitudinal fasciculus, and reticulospinal fibers. • concerned with adjustment of muscle tone in response to vestibular stimuli. • coordinates the actions of muscles that maintain equilibrium and participates in other motor responses, including those of the eyes, to vestibular stimulation

49. The posterior vermis also contributes to the cerebellar control of eye movements.

50. Spinocerebellum-afferents • SOMATIC SENSORY SYSTEMS • DORSAL AND VENTRAL SPINOCEREBELLAR TRACTS- data from proprioceptive endings, touch and pressure receptors[Dorsal tract from trunk and leg; ventral tract, mainly from leg] • CUNEOCEREBELLAR FIBRES- equivalent for arm and neck to dorsal spinocerebellar tract • ALL TRIGEMINAL NUCLEI