The Cerebellum, Balance and Coordination

Question 1

What connects the cerebellum to the medulla? And what is the function of this pathway?

Answer 1

• Inferior peduncles connect cerebellum to medulla – sensory information from muscle proprioceptors

Question 2

What connects the cerebellum to the pons? And what is the action of this pathway?

Answer 2

• Middle peduncles connect cerebellum to pons – voluntary motor activities by motor cortex

Question 3

What connects the cerebellum to the midbrain? And what is the action of this pathway?

Answer 3

• Superior peduncles connect cerebellum to midbrain – neurons in deep cerebellar nuclei and communication with motor cortex via thalamus

Question 4

What is the cerebellar output?

Answer 4

occurs via deep nuclei - tonically activated by climbing and mossy fibres

From lateral to medial:

• Dentate nucleus
• Emboliform nucleus
• Globose nucleus
• Fastigial nucleus

**Emboliform + globose nucleus = interposed

Question 5

What is the action of climbing and mossy fibres in the cerebellum?

Answer 5

integrate sensory information (excitatory)

Question 6

What is the action of the golgi and purkinje cells of the cerebellum?

Answer 6

inhibitory output

Question 7

Describe the function of the cerebellum

Answer 7

Stimulation of the cerebellum causes neither sensation nor significant movement. However, loss of cerebellum leads to severe abnormalities of motor function

Question 8

Describe the function of the cerebellum as a comparator

Answer 8

• Cerebellum compares descending supraspinal motor signals with ascending afferent feedback information
• Movement smoothly and accurately coordinated

Question 9

Describe the function of the cerebellum as a timing device

Answer 9

• Pontocerebellum – creates a sequence for motor activation
• Vestibulocerebellum – maintains balance
• Spinocerebellum – maintains posture

Question 10

Describe the action of the cerebellum in initiating and storing movements

Answer 10

• Modifiable synapse (Purkinje cell)
• So can store motor information and update it

Question 11

Give an overview of the motor regulatory function of the cerebellum

Answer 11

• Modulates motor output based on integration of:

1. Activity in the pre-motor & motor areas as well as spinal motor circuits (information about potential motor output)
2. Sensory feedback from vestibular system, visual system, and ascending proprioceptive information (about real motor input)

• Modulation by the cerebellum is affected at the motor cortex and brainstem

Question 12

What is thought to be the primary role of the cerebellum?

Answer 12

• Primary role of cerebellum thought to be a supplement and correlate activities of other motor areas e.g. correction of rapid muscular movement initiated by cortex (typing, musical instrument)

Question 13

What is the action of the cerebellum due to inputs from muscle spindles and golgi tendon organs

Answer 13

• Secondary role in control of postural mechanisms
  
  • Exerts control on a moment-by-moment basis
• Third role in maintenance of tone of muscles
• Cerebellum informed of movement by cortex before it occurs, then cerebellum processes sensory information to generate an error signal which is fed back to cortex and movement Is adjusted

Question 14

What are the main clinical features of cerebellar damage?

Answer 14

• Hypotonia/Reduced muscle tone
• Incoordination/Ataxia
• Dysarthria (scanning speech)
• Nystagmus
• Palatal Tremor/Myoclonus

Question 15

Why does hypotonia/reduced muscle tone occur in cerebellar damage?

Answer 15

• Due to reduced input from Deep Cerebellar Nuclei neurone (DCNN) via descending motor pathways to muscle spindle

Question 16

Asynergy

Answer 16

• inability to coordinate contraction of agonist and antagonist muscles

Question 17

Dysmetria

Answer 17

inability to terminate movements (intention tremor/past pointing)

Question 18

Dysdiadochokinesis

Answer 18

• inability to perform rapidly alternating movements

Question 19

Dysarthria

Answer 19

• Inability to articulate words (slurred and delivered slowly) due to inco-ordinated oropharyngeal musculature

Question 20

Nystagmus

Answer 20

• Rapid jerky eye movements – disruption between vestibular nucleus and oculomotor nuclei

Question 21

Palatal tremor/Myoclonus

Answer 21

• Rare condition
• Hypertrophy of inferior olive which causes damage to dentate nucleus of cerebellum and red nucleus in midbrain

Question 22

Describe the inputs and outputs that control balance and co-ordination

Answer 22

• Inputs
  
  • eye
  • joint position, proprioception, superifical sensation
  • Labyrinthine activity
  • cortex cerebellum, reticular formation, extra-pyramidal system
• Outputs
  
  • cortical awareness of head/body/motion
  • Eye movement/control of oculomotor activity
  • control of posture
  • control of motor skills

Question 23

What is the function of the auditory system and the vestibular apparatus?

Answer 23

• Auditory system provides sense of hearing
• Vestibular generates out sense of balance

Question 24

What is the function of different aspects of the ear?

Answer 24

• Pinna collects and focuses sound waves
• External auditory canal, also skin lined ends up at tympanic membrane (eardrum)
• Middle ear is air filled chamber, bounded by tympanic membrane on one side and oval window on the other
• Eustachian tube connects middle ear to nasopharynx, allows pressure equalisation

Question 25

What is the membranius labyrnith filled with?

Answer 25

* Labyrinth filled with endolymph and encased in temporal bone

Question 26

What is the auditory part of the membranous labyrinth?

Answer 26

cochlea

Question 27

What is the vestibular part of the membranous labyrinth?

Answer 27

* Vestibular part contains 2 structures: * Otolith organs (detect gravity and head tilt) * Semi-circular canals (detect head rotation)

Question 28

Describe the actions of the hair cells of the saccule?

Answer 28

* Mechanoreceptors that respond to minute movement changes * Consists of one large kinocilium and 50-150 stereocilia * Hair cells of cochlea lose there kinocilium with age; vestibular hair cells do not

Question 29

What is the function and action of the otolithic organs?

Answer 29

(Saccule and Utricle) * Detect changes in linear acceleration (i.e. that experienced as a car starts or stops) and angle * Macula is orientated vertically in saccule and horizontally in utricle when head upright * Each hair cell synapses on an axon of vestibular nerve (part of CN VIII) * Otoliths are particles of calcium carbonate; they are denser than the endolymph * Otoliths are pulled by gravity and move the gelatinous cap in the same direction; cilia of hair cells are deflated

Question 30

Describe the mechanotransuduction in hair cells

Answer 30

* Hair cells, project into an otolithic (ear stone) containing gelatinous structure * Cilia + kinocilium – provide directional information * Movement of otolithic membrane (from tilting or translational movement) bends the cilia/kinocilium in a very direction specific way * A 0.5-micron movement of the kinocilium opens/closes the hair cell cation channels

Question 31

How do the semi-circular canals function?

Answer 31

* Sensitive to angular acceleration (head rotation movements) * Hair cells clustered in sensory epithelium crista ampullaris * Hair cells project into gelatinous cupula, all hair cells orientated and they either excite or suppress transmitter release depending on direction of movement

Question 32

What is the vestibular nervous pathway?

Answer 32

* Vestibular axons from CN VIII make direct connections to vestibular nucleus, which project via vestibulospinal tract to spinal motor neurons – posture * Axons from semi-circular canals project to medial vestibular nucleus, which project via medial longitudinal fasciculus to motor nerves of trunk and neck muscles – keeps head straight as body moves

Question 33

Describe the vestibulo-ocular reflex

Answer 33

semi-circular canals control eye movements * Direct stimulation of ampullary nerves elicits specific eye movements * Stimulation of afferents from left horizontal canal causes eye to turn right (vestibulo-ocular reflexes) * Specific adaption to allow gaze to remain steady during head movements * Lateral recti – cranial nerve VI (abducens nucleus) * Medial recti – cranial nerve III (oculomotor nucleus)

Question 34

What nerve is assocaited with the lateral recti of the eye?

Answer 34

cranial nerve VI (abducens nucleus)

Question 35

What nerve is associated with the medial recti of the eye?

Answer 35

* cranial nerve III (oculomotor nucleus)

Question 36

What is Ménière's Disease?

Answer 36

* Clinical symptoms – vertigo, nausea, tinnitus and hearing loss * The vestibular apparatus is bathed in endolymph * Normally drains to venous sinus * Ménière’s cause is unknown, but there is an excessive accumulation of endolymph and damage to hair cells * Accumulation of endolymph is probably due to poor drainage

Question 37

What is vertigo?

Answer 37

* Sensation of turning or rotation in space in absence of actual rotation

Question 38

What are the clinical symptoms of vertigo?

Answer 38

* nausea, vomiting and gair ataxia

Question 39

What is vertigo caused by?

Answer 39

* Caused by debris from otolithic membrane adhering to cupula in ampulla of posterior semi-circular canal * Can be due to lesions of vestibular aspect of CNVIII or central lesions affecting brainstem vestibular nuclei

Question 40

Summary of the cerebellum

Answer 40

* Cerebellum important in coordination and learning of movements

Question 41

Summary of the vestibular system

Answer 41

* Vestibular system important in balance, postural reflexes and eye movements

Question 42

What is the action of the cerebellum and the vestibular system together?

Answer 42

* Together give us information about our body’s position and movement in our environment

Question 43

Draw a diagram of the pathways in the brain

Answer 43