4f. Central Balance Pathways

Question 1

Pathway

- Start

Answer 1

1. Cristae of semi-circular canals or macula of the sacculus or utriculus
2. Bipolar vestibular axons
3. Vestibular part of cranial nerve VIII (vestibulocochlear)
4. Vestibular nuclei (brainstem)

Question 2

Conscious Pathway

Answer 2

1. Vestibular nuclei
2. Partial decussation in the medial longitudinal fascicles
3. Medial geniculate nucleus of the thalamus (midbrain)
4. Temporal lobe of cerebral cortex

Question 3

Unconscious Pathway

Answer 3

1. Vestibular nuclei

Many different pathways:

• Spinal cord
• Descending reticular formation
• Ascending reticular formation
• Cerebellum
• Motor nuclei III, IV and VI

Question 4

Unconscious Pathway

- Spinal Cord

Answer 4

Neurones travel to the spinal cord via the vestibulospinal tract.

Undergo partial decussation along the way.

Support through motor processes

Question 5

Unconscious Pathway

- Descending Reticular Formation

Answer 5

Neurones travel to the descending reticular formation, especially the vomiting centres of the brain

Question 6

Unconscious Pathway

- Ascending Reticular Formation

Answer 6

Neurones travel to the ascending reticular formation and are involved in arousal and cardiovascular activity

Question 7

Unconscious Pathway

- Cerebellum

Answer 7

Neurones travel via the caudal peduncle to the fastigial nucleus of the ipsilateral flocculi-nodular lobe of the archicerebllum

Coordination and posture

Question 8

Unconscious Pathway

- Motor Nuclei III, IV and VI

Answer 8

Neurones travelling to motor nuclei III, IV and VI in the brainstem, which are involved in eye movements

Vestibular reflex nystagmus

Question 9

Vestibular Syndrome

- Clinical Signs

Answer 9

• Heat tilt towards lesion
• Fall towards lesion
• Circle towards lesion
• Travel sickness causing vomiting
• Nystagmus

Horses:
- Ventral strabismus

Question 10

Nystagmus

Answer 10

Vestibulo-ocular reflex

Eyeballs drift slowly towards the lesion then quickly snap back

Question 11

Vestibular Syndrome

- Clinical Signs Reason

Answer 11

Lesion on one side makes the animal think its falling in the opposite direction, so overcompensates and the animal falls and turns on the opposite direction

Question 12

Vestibulo-Ocular Reflex

- Sensory and Motor Arms

Answer 12

Sensory arm:
- Vestibular branch of cranial nerve VIII (vestibulocochlea)

Motor arm:

• Cranial nerve III (oculomotor) to medial rectus
• Cranial nerve VI (abducens) to lateral rectus

Question 13

Nystagmus

- 6 Types of Normal Nystagmus

Answer 13

• Rotational nystagmus
• Post rotation nystagmus
• Positional nystagmus
• Caloric nystagmus
• Optokinetic nystagmus
• Alcohol-induced nystagmus

Question 14

Rotational Nystagmus

Answer 14

Vestibulo-ocular reflex

Question 15

Post-Rotational Nystagmus

Answer 15

When the head stops rotating but the endolymph continues

Question 16

Positional Nystagmus

Answer 16

When the head is maintained in an unusual position for a period of time

Question 17

Caloric Nystagmus

Answer 17

Endolymph in one ear is chilled so created eddy currents which flow past crest and make you think you’re moving

Question 18

Optokinetic Nystagmus

Answer 18

Visual stimulus traveling along cranial nerve II (optic) where looking at a moving object induces the feeling of motion

Question 19

Alcohol-Induced Nystagmus

Answer 19

Ethanol partitions into the endolymph, reducing its density.

Cupula in the semi-circular canal become denser than the surrounding fluid, causing them to hang down under the effect of gravity, meaning that gravity is detected as a rotating force

Question 20

Nystagmus

- 4 Types of Abnormal Nystagmus

Answer 20

• Peripheral inner ear lesion
• Central vestibular nuclei lesion
• Searching nystagmus in blind animals
• Fixating nystagmus caused by cerebellar lesion