Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins Neuroscience for the Study of Communicative Disorders 4th Edition Subhash C. Bhatnagar, PhD
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins Chapter 14 Motor System 2: Cerebellum
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins CEREBELLAR ANATOMY Anatomy –Cerebellar cortex –Hemispheres (2) and Lobes (3) –Internal white substance –Four deep cerebellar nuclei –Three cerebellar peduncles
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
Cont’d Afferent Pathways –Inferior cerebellar peduncle Vestibular system Upright posture maintenance Momentary changes in rate & strength during ongoing movements
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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)
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins Cont’d –Intention (movement) tremor Impaired ability to dampen accessory movements Motion tremor- demand of cerebellum during movement
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins Cont’d –Hypotonia Decreased muscle tension, muscle becoming floppy Detection during passive manipulation of limbs Ipsilateral to the lesion side
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins Cont’d –Rebounding Impaired motor tone adjustments with loss of rapid & precise corrective response Loss in ability to predict, stop, or dampen movements
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins Cont’d –Disequilibrium Impaired integrated vestibular processing affecting the legs Unsteady gait Body wavering toward lesion site
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins Cont’d Cerebellar Pathologies –Cerebrovascular accident Vertebrobasilar artery- vascular supply to all cerebellar arteries
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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
Copyright © 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins 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