Cerebellum structure and function

Question 1

Differences in lesions btwn U/LMNs, basal ganglia, and cerebellum

Answer 1

• UMN and LMN lesions result in paralysis/weakness and either hyperreflexia or hyporeflexia
• Basal ganglia lesions result in hypo/hyperkinetic disorders (not weakness/paralysis)
• Cerebellar lesions result in poor coordination (cerebellar ataxia), not weakness/paralysis

Question 2

Function of cerebellum

Answer 2

• Coordinates and predicts motor activity by adjusting the timing and speed of muscle contraction/relaxation
• Works through the cortical UMNs and sum cortical UMNs
• Cerebellar control of extremities is ipsilateral and control of trunk is bilateral

Question 3

Anatomy of the cerebellum

Answer 3

• Divided into 3 lobes: anterior, posterior and flocculondular (at the inferior end)
• The fine grooves of the cerebellum are the folia
• There are 3 cerebellar peduncles (inferior, middle, superior)
• Superior cerebellar peduncle (SCP): connects midbrain w/ cerebellum (mostly efferent)
• Middle cerebellar peduncle (MCP): connects the pons w/ the cerebellum (afferent)
• Inferior cerebellar peduncle (ICP): connects the medulla w/ the cerebellum (afferent + efferent)
• Nuclei of the cerebellum: dentate, emboliform, globose, and fastigial (Don’t Eat Greasy Foods)

Question 4

Microanatomy of the cerebellum

Answer 4

• Has white matter (inside) and grey matter (outside) and 3 layers
• Molecular layer (most outer) contains dendrites of purkinje cells and axons of granule cells (and other stuff)
• Purkinje layer contains cell bodies of purkinje cells (reside in the border btwn the granular layer and molecular layer), dendrites project to molecular layer, axons project to deep cerebellar nuclei
• Granular layer contains cell bodies of granule cells who’s axons project to the molecular layer (for purkinje cells)

Question 5

Fibers (afferents) of the cerebellum

Answer 5

• All afferents of the cerebellum ultimately activate purkinje cells (3 fiber types)
• Mossy fibers come from the SC, pontine nucleus, and vestibular nucleus and synapse on granule cells
• Climbing fibers come from the inferior olivary nucleus and synapse on purkinje neurons
• Parallel fibers come from granule cells and synapse on purkinje neurons

Question 6

Functional subdivisions of the cerebellum

Answer 6

• From medial-> lateral/anterior: fastigial, globose, emboliform, dentate (emboliform+dentate together called interposed nucleus)
• There is a homunculus of the cerebellum: trunk is controlled by the midline (vermis) w/ limbs being controlled by the hemispheres: proximal limbs medial and distal limbs lateral (both anterior and posterior sections have their own homunculi)
• 3 functional subdivisions: vestibulocerebellum, spinocerebellum, cortico/pontocerebellum

Question 7

Vestibulocerebellum subdivision

Answer 7

• The 2 flocculi on either side connected by a part of the inferior vermis (nodule) together form the flocculonodular lobe
• This subdivision projects to the deep nucleus fastigial nucleus
• Receives info from the vestibular nucleus regarding balance
• Pathway: vestibular nucleus->fastigial nucleus-> vestibular nucleus

Question 8

Spinocerebellum subdivision

Answer 8

• Superior and inferior vermis except nodule, the medial and lateral zones of the hemispheres
• Projects to the interposed nucleus
• Receives information about proprioception form the SC
• Pathway: SC-> interposed nucleus-> SC and cortex

Question 9

Cortico/pontocerebellum subdivision

Answer 9

• Receives info from cortex but on the way to cerebellum it synapses in pons
• Comprises most of the lateral cerebellar hemispheres
• Projects to the dentate nucleus
• Receives info from cortex about the planning of movements and sends info back to cortex fine tuning those movements
• Pathway: cortex-> dentate-> cortex

Question 10

Vestibulocerebellum pathway 1

Answer 10

• Responds to changes in movements detected by inner ear
• Coordinates contractions of muscles of the trunk and proximal parts of the limbs to maintain balance, also controls eye muscles
• Afferents: arise from VIII (vestibular) nerve, cell bodies in vestibular ganglion, and synapse on the vestibular (VIII) nuclei in the upper medulla
• From the vestibular nucleus, the afferents enter the inferior cerebellar peduncles as mossy fibers, to terminate on the granule cells (flocculonodular lobe). These cells then synapse on the purkinje cells

Question 11

Vestibulocerebellum pathway 2

Answer 11

• Efferents: the purkinje cells project to the fastigial nucleus. From here axons leave the cerebellum via the ICP and synapse back on the vestibular nucleus and also on the reticular formation (also upper medulla)
• These motor centers influence the LMNs controlling trunk and proximal limb muscles via descending tracts arising from the 2 nuclei (vestibulospinal and reticulospinal tracts)

Question 12

Spinocerebellum pathway

Answer 12

• Monitors all ongoing movements in real time in all parts of the body (particularly the distal limbs) to ensure coordination
• Vermal and medial part of hemispheres control trunk/proximal limbs and lateral parts control the distal limbs
• 2 tracts: posterior which conveys info about joint position/muscle activity and anterior which relays info about the influence of CST on muscle activity
• Afferents: convey unconscious proprioceptive info from neuromuscular spindles and neurotendinous organs (golgi)
• Tracts are either truly ipsilateral (posterior) or cross twice to thus be ipsilateral as well (anterior)

Question 13

Posterior spinocerebellar tract

Answer 13

• First neuron is in the DRG of spinal nerves T1 and below (only for lower limbs- don’t need to know upper limbs)
• Fibers conveying proprioception (from spindles and golgi) enter the dorsal horn and synapse in the nucleus of clarke (T1-L2) which resides in the intermediate gray horn (just lateral to white commissure)
• The axons from here extend to the lateral funiculus (just posterior to the anterior spinocerebellar tract and lateral to the CST, on the lateral wall of the SC), which ascends all the way to the medulla
• In the upper medulla the tract enters the ICP to reach the spinocerebellar cortex (purkinje cells) in the anterior lobe on the ipsilateral side of info origin
• Purkinje axons projected to interposed nucleus

Question 14

Anterior spinocerebellar tract

Answer 14

• Arises form scattered spinal border cells of the anterior horn (around the anterior/lateral border of the ant horn)
• These cells receive info from muscles (sensory- where am I?) and from the CST (where I should be) to monitor what movements should happen vs what movements are happening
• These axons cross (also only for lower limbs) the anterior commissure to form the tract (just posterior to the STT, lateral to anterior horn) which travels up to the pons
• In the upper pons the tract enters the SCP to reach the cerebellar white matter where it crosses back to the side of origin and ends on the spinocerebellar cortex on the ipsilateral side
• The purkinje cells receiving this info project to the interposed nucleus

Question 15

Efferents of the spinocerebellar tract

Answer 15

• Axons of the interposed nucleus cross at the decussation of the SCP in the lower midbrain to reach 2 contralateral targets: red nucleus and ventral lateral (VL) nucleus of the thalamus
• The red nucleus axons will cross back over to the ipsilateral side (of info origin) before ending on LMNs that regulate muscles of trunk and proximal extremities
• The VL axons project to the motor cortex on the same side as the VL, which is contralateral to info origin. Since the motor cortex operates the muscles on the contralateral side, the VL projects to the motor cortex that will control muscles on the ipsilateral side of the info origin
• Thus all spinocerebellar information receives info from and generates corrections for muscles on the ipsilateral side of the interposed nucleus

Question 16

Corticocerebellum pathway

Answer 16

• Responds to info provided by all parts of the cortex, primarily the pre-motor area (PMA) and the supplemental motor area (SMA), is involved in motor learning
• Afferents: axons arise from the primary motor cortex, PMA and SMA (these fire 100 msec before the onset of a movement message) pass thru corona radiata, anterior limb of IC, and descend thru lateral (legs) and medial (upper/face) 1/3rds of crus cerebri to enter pons (corticopontine fibers)
• These end ipsilaterally in the gray matter of the pons by synapsing on pontine nuclei
• Axons form pontine nuclei (pontocerebellar fibers) cross midline and enter the MCP to end on granule cells of the cortex (ends contralateral to info origin)-> purkinje cells-> dentate nucleus

Question 17

Efferents of the corticocerebellar pathway 1

Answer 17

• Efferents: Purkinje cells send fibers to the ipsilateral dentate nucleus, which sends fibers to exit the cerebellum thru the SCP and cross at midbrain
• Some terminate in the contralateral (from origin) red nucleus (ipsilateral to info origin)
• Others continue w/o synapsing and terminate in the VL nucleus of the thalamus on the contralateral side (ipsilateral to info origin)

Question 18

Efferents of the corticocerebellar pathway 2

Answer 18

• Cortical projections of VL go to the SMA on the ipsilateral side (via IC)
• Thus the corticocerebellar tract manages the motor cortex that generated the information in the first place
• Each side of the corticocerebellum is connected w/ the opposite motor cortex
• Thus the corticocerebellum regulates muscle coordination on the ipsilateral side as it
• These muscles include distal limb, muscles of speech and eye

Question 19

Overall functions of cerebellum

Answer 19

• Does not influence sensation or strength, fairly inactive at rest
• Coordinates skeletal muscle contractions and relaxation (timing, speed, duration)
• Coordinates posture, gait, and voluntary movements
• Vestibulocerebellar responds to changes in head position to adjust contractions in neck, trunk, and proximal limbs for gait and posture
• Spinocerebellar response to sensory receptors (spindles and golgi mostly) and interneurons (info on the state of activity of CST) to compute adjustments and corrections for muscle length and tension during contractions
• Corticocerebellar participates in planning voluntary movements involving the distal parts of the extremities by receiving motor programs from SMA, PMA, and PMC. Fine tunes the plans before movement occurs be feeding back to these areas and altering the programs

Question 20

Midline lesions of the cerebellum

Answer 20

• Lesions affecting the vermis will affect the spinocerebellum that regulates the trunk
• Common cause is medulloblastoma
• Sx: loss of equilibrium (swaying side-side when standing or walking): truncal ataxia
• Wide gait
• Limb movements unaffected
• Eye problems

Question 21

Hemispheric lesions

Answer 21

• Mostly affect the corticocerebellum that controls the limbs (can also affect the medial spinocerebellum)
• Common causes: astrocytoma, abcess, infarct, others
• Predominantly affect limbs and decrease muscle tone, coordination of voluntary movements (limb ataxia)
• Sx: dysmetria (overshooting/undershooting motions due to errors in range), dysdiadochokinesia (can’t repeat movements/no regular rhythm)
• Intention tremor: absent at rest and appearing mostly @ end of movement
• Dysarthria (Slurred) speed due to poor coordination of palatal, laryngeal and tongue muscles

Question 22

Anterior lobe syndrome

Answer 22

• Degeneration of the anterior part of the superior vermis and adjacent medial areas (results form chronic alcoholism)
• Affects lower limbs, causes a “drunk-like walk” (ataxia), imbalance while standing or walking
• Suggests the somatotopy isn’t continuous and may be represented in a fragmented manner (fractured somatotopy)