Cerebellum, Balance and Coordination

Question 1

Describe the macroscopic structure of the cerebellum

Answer 1

• anterior and posterior lobe (primary fissure between)
• posterior and flocculonodular lobe (posterolateral fissure between)

Flattened orientation:

• lateral zone: dentate nuclei
• intermediate zone: interposed nuclei (emboliform and globose nuclei)
• vermis: fastigial nuclei
• medulla: vestibular nuclei (from flocculonodular node)

Question 2

What are the functional divisions of the cerebellum?

Answer 2

• vestibulocerebellum
• spinocerebellum
• cerebrocerebellum

Question 3

Describe the vestibulocerebellum

Answer 3

• flocculonodular lobes
• input = vestibular and visual areas
• output = vestibular nuclei
• controls equilibrium (balance) and eye movements

Question 4

Describe the spinocerebellum

Answer 4

• vermis and intermediate zones
• input = spinocerebellar, auditory, visual, vestibular systems and sensorimotor cortex
• output = vermis to fastigial nuclei to vestibular and reticular formation of pons and medulla
• controls anti-gravity muscles in posture and locomotion
• output = intermediate zone to interposed nuclei to red nucleus, thalamus, then cortex
• controls stretch reflexes and other somatosensory reflexes (error correction)

Question 5

Describe the cerebrocerebellum

Answer 5

• lateral zones
• input = cerebral motor cortex, premotor and somatosensory motor cortices
• output = dentate nucleus to thalamus to motor and pre-motor cortices
• creates feedback with cortical sensorimotor system for planning sequences of movement

Question 6

Describe the microscopic structure of the cerebellum

Answer 6

• molecular layer
• Purkinje cell layer
• granule cell layer
  (^ output to superior cerebellar peduncle)
• white matter layer = input from inferior and middle cerebellar peduncles
• climbing fibres
• mossy fibres
  (^ input from periphery)

Question 7

What are the main functions of the cerebellum?

Answer 7

• compares descending supraspinal signals with ascending afferent feedback (smooth and coordinated movement)
• pontocerebellum = sequence for motor activation
• vestibulocerebellum = maintains balance
• spinocerebellum = maintains posture
• Purkinje cells have modifiable synapses and can store motor info

Question 8

What are the effects of cerebellar damage?

Answer 8

• hypotonia
• ataxia (dysmetria, asynergy, dysdiadochokinesis)
• dysarthria
• nystagmus
• palatal tremor (rare)

Question 9

What are the connections of the vestibular system?

Answer 9

Membranous labyrinth (inner ear):

• filled with endolymph and encased in temporal bone
• vestibular part contains otolith organs (utricle and saccule) for sensing gravity and tilting of the head, and semicircular canals that detect head rotation

Question 10

Describe the mechanisms of otolithic organs

Answer 10

• detect changes in linear acceleration and head angle
• each hair cell synapses on an axon of the vestibular nerve (CN VIII)
• otoliths are dense particles connected to the gelatinous cap (where hair cells are immersed)
• they are moved by gravity which causes the cilia of the hair cells to deflect

Question 11

Describe mechanotransduction in hair cells

Answer 11

• hair cells containing cation channels project into otolithic containing gelatinous structure
• cilia and kinocilium provide directional info
• tilting of the head moves otolithic membrane cause kilocilium to bend
• tilting of the head that causes mechanical deformation towards the kilocilium opens K+ channels causing depolarisation
• opposite causes closing of K+ channels causing hyperpolarisation

Question 12

Describe how the semicircular canals respond to head rotation

Answer 12

• crista ampullaris = cluster of hair cells in the sensory epithelium that project into the gelatinous capula
• as head rotates this also causes rotational movement of the canal (endolymph does not move) causing hair cells to bend
• depending on direction they can excite or suppress NT release

Question 13

Describe the neural pathway of the vestibular system

Answer 13

• vestibular axons go directly to vestibular nucleus and cerebellum
• axons from otolith organs go to lateral vestibular nucleus, then through vestibulospinal tract to spinal motor neurons (posture)
• axons from semicircular canals go to medial vestibular nucleus, through the medial longitudinal fasciculus of thalamus to motor nerves of the trunk and neck muscles

Question 14

Describe the vestibuloocular reflex

Answer 14

• controlled by semicircular canals
• direct stimulation of ampullary nerves causes specific eye movements
• left afferent stimulation causes the eye to turn right (reflex)
• CN VI: lateral recti (abducens)
• CN III: medial recti (oculomotor)

Question 15

What are the causes of vertigo?

Answer 15

• Meniere’s Disease: excessive accumulation of endolymph in vestibular apparatus damaging the hair cells (also causes nausea, tinnitus and hearing loss)
• debris from otolithic membrane adhering to cupula in ampulla of posterior semi-circular canal
• lesions of vestibular aspect of CNVIII/central lesions affecting brainstem vestibular nuclei