Cerebellar Contributions to Motor Control

1. Neuroscience for the Study of Communicative Disorders4th Edition Subhash C. Bhatnagar, PhD

2. Chapter 14Motor System 2: Cerebellum

3. CEREBELLAR ROLE IN MOVEMENT • Functional Details • Primary motor cortex- fine movement details • Premotor & supplementary motor cortices- movement planning • Cerebellum • Constant monitoring of cortical motor output • Correction of discrepant movements • Error- control device for rapid, alternating, and sequential movements • Functions • Modifications of cortically generated motor plan • Regulation of movement by monitoring all cortical motor output to muscles

4. Cont’d • Corrective efferents (decreasing or stopping movements) to the motor cortex & spinal cord • Regulation of • Muscle synergy and tone • Motion range and velocity • Movement strength and equilibrium • Motor learning regulation • No cerebellum participation in cognitive processing

5. Cont’d • Innervation Pattern • Ipsilateral cerebellar sensorimotor organization to input source & output targets • Effect of lesion on body ipsilateral to lesion site because of double crossing of cerebellar fibers

6. CEREBELLARANATOMY • Anatomy • Cerebellar cortex • Hemispheres (2) and Lobes (3) • Internal white substance • Four deep cerebellar nuclei • Three cerebellar peduncles

10. Cont’d • Cerebellar Cortex • Sensorimotor representation • Three Lobes • Paleocerebellum (anterior)- motor tone and walking position • Neocerebellum (posterior)- coordination of cortically directed skilled movements • Archicerebellum (floccular nodular)-equilibrium and eye movements

12. Cont’d • 3 Longitudinal Cerebellar Regions • Vermis- maintenance of body posture • Paravermal- ipsilateral movements • Lateral- skilled extremity movements • Deep Cerebellar Nuclei • Dentate- efferent nucleus for planning and limb coordination • Emboliform and Globose- regulation of ipsilateral movements • Fastigial- equilibrium

13. Cont’d • Cerebellar Peduncles • Inferior, middle, & superior cerebellar peduncles • Cerebellar connections to brainstem containing all afferent & efferent projections • Afferent to efferent projections ratio • 40:1

15. Cont’d • Afferent Pathways • Inferior cerebellar peduncle • Vestibular system • Upright posture maintenance • Momentary changes in rate & strength during ongoing movements

16. Cont’d • Middle cerebellar peduncle • Crossed afferents from motor cortex as mossy fibers; added– visual and auditory input for directional context • Efferent Pathways • Superior cerebellar peduncle • Crossed cerebellar (dentate, emboliform, & globose) projections to contralateral motor cortex

17. Cont’d • Additional efferents to • Dpinal cord • Modulation of muscle tone & reflexes • Brainstem and reticular formation • Cranial nerve nuclei for speech • Spinal motor neurons- ongoing control of muscle tone during movement • Vestibular nuclear complex- bidirectional connections for equilibrium

18. CEREBELLAR CORTEX • Structures • Cerebellar cortex- cellular layers • Molecular- parallel running fibers synapsing with Purkinje cells • Purkinje cell- axons penetrating granular cell layers & deep nuclei • Granular cells- synapsing onto mossy fiber as well as the spines of Purkinje cell dendrites

19. Cont’d • Cerebellar Circuitry Unit • Input axons • Excitatory to both- deep nuclei & cortex (outer cerebellar layers) • Output axons of Purkinje cell • Inhibitory to deep cerebellar nuclei • Integrated cerebellar circuitry feedback- essential for muscle synergy & tone

20. Cont’d • Cerebellar functional unit • Cerebellar cortex- deep nuclei & afferent & efferent fibers • Afferents- climbing or mossy fibers • Climbing fibers- specific to Purkinje cells & excitatory to deep nuclei cells • Highly developed in humans • Mossy fibers (all other input sources)- excitatory to deep nuclei & cerebellar cortex • Indirect innervation of Purkinje cells

22. Cont’d • Summated output from cerebellar parallel fibers and Purkinje cells with inhibitory (GABA) projections to deep nuclei • Cerebellar outputs- timing & strength of inputs to deep nuclei • Regulation of general excitability of the motor cortex

23. Cont’d • Cerebellar Output • To cortex, BG, RF & SC)- balanced by excitatory afferents & inhibitory Purkinje-mediated efferents • Timed efferents- essential for skilled movements • Altered sequence of excitatory & inhibitory events and timed impulses • Faulty outgoing cerebellar signals • Disrupted integrity of the neuronal circuitry of cortex, RF, and spinal cord • Altered synergy, tone, & equilibrium

25. CLINICAL CORRELATES • Cerebellar Plasticity • Smaller unilateral cerebellar lesions- easily compensable by retraining • Massive/bilateral lesions- long-term effects, unless occurring in young age • Vision- differentiation between disturbances of cerebellum and dorsal column–medial lemniscal system

26. CLINICAL CORRELATES • Romberg test- standing with extended arms in front, feet together, & eyes closed • Loss of proprioception- arms drift to downward and/or body tilt to side • Eyes opened- continued presence of arms drifting or unsteadiness suggestive of cerebellar abnormality or cerebellar malfunctioning

27. Cont’d • Tests of Cerebellar Dysfunction • Tandem gait -toes of the back foot touch heel of the front foot at each step • Finger-to-nose test • Alternating movements • Limb rebounding • Diadochokinesia

28. Cont’d • Common Cerebellar Impairments • Ataxia- lack of order & coordination in muscle activities • Segmented & clumsy movements • Bradykinesia-slow movements • Asthenia-mild muscular weakness • Asynergia-impaired direction & force of movement

29. Cont’d • Dysdiadochokinesia • Failure in sequential progression of motor activities • Clumsiness in rapid & alternating movements • To be tested on movements like tapping, articulation of sequenced phonemes, or rotating movements

30. Cont’d • Dysarthria • Impaired motor speech processes • Impaired ability for modifications & alterations in ongoing oral-facial movements • Implication of bilateral cerebellar lesions • Slow, slurred, & disjointed speech with each word spoken individually (scanning speech)

31. Cont’d • Dysmetria • Error in judgment of movement’s range or distance to target • Undershooting- falling short • Overshooting- extending past • Failure to incorporate range & distance of stationary & moving targets

32. Cont’d • Intention (movement) tremor • Impaired ability to dampen accessory movements • Motion tremor- demand of cerebellum during movement

33. Cont’d • Hypotonia • Decreased muscle tension, muscle becoming floppy • Detection during passive manipulation of limbs • Ipsilateral to the lesion side

34. Cont’d • Rebounding • Impaired motor tone adjustments with loss of rapid & precise corrective response • Loss in ability to predict, stop, or dampen movements

35. Cont’d • Disequilibrium • Impaired integrated vestibular processing affecting the legs • Unsteady gait • Body wavering toward lesion site

36. Cont’d • Cerebellar Pathologies • Cerebrovascular accident • Vertebrobasilar artery- vascular supply to all cerebellar arteries

37. Cont’d • Toxicity- chronic alcoholism • Progressive subacute cerebellar degeneration; gross cerebellar atrophy and the loss of cellular elements in the anterior lobe, most crucially the Purkinje cell • Symptoms- wavering gait, dysmetria, dyskinesia • Speech (monotonous and explosive) impairment- disappear with attenuation of blood alcohol level over time

38. Cont’d • Progressive cerebellar degeneration • Friedreich ataxia- autosomal recessive genetic degenerative condition • Symptoms- ataxia, dysarthria, tremor, weakness (lower to upper progression), loss of proprioception, nystagmus, dysmetria, & scanning speech • No medical treatment