4f. Central Balance Pathways Flashcards
Pathway
- Start
- Cristae of semi-circular canals or macula of the sacculus or utriculus
- Bipolar vestibular axons
- Vestibular part of cranial nerve VIII (vestibulocochlear)
- Vestibular nuclei (brainstem)
Conscious Pathway
- Vestibular nuclei
- Partial decussation in the medial longitudinal fascicles
- Medial geniculate nucleus of the thalamus (midbrain)
- Temporal lobe of cerebral cortex
Unconscious Pathway
- Vestibular nuclei
Many different pathways:
- Spinal cord
- Descending reticular formation
- Ascending reticular formation
- Cerebellum
- Motor nuclei III, IV and VI
Unconscious Pathway
- Spinal Cord
Neurones travel to the spinal cord via the vestibulospinal tract.
Undergo partial decussation along the way.
Support through motor processes
Unconscious Pathway
- Descending Reticular Formation
Neurones travel to the descending reticular formation, especially the vomiting centres of the brain
Unconscious Pathway
- Ascending Reticular Formation
Neurones travel to the ascending reticular formation and are involved in arousal and cardiovascular activity
Unconscious Pathway
- Cerebellum
Neurones travel via the caudal peduncle to the fastigial nucleus of the ipsilateral flocculi-nodular lobe of the archicerebllum
Coordination and posture
Unconscious Pathway
- Motor Nuclei III, IV and VI
Neurones travelling to motor nuclei III, IV and VI in the brainstem, which are involved in eye movements
Vestibular reflex nystagmus
Vestibular Syndrome
- Clinical Signs
- Heat tilt towards lesion
- Fall towards lesion
- Circle towards lesion
- Travel sickness causing vomiting
- Nystagmus
Horses:
- Ventral strabismus
Nystagmus
Vestibulo-ocular reflex
Eyeballs drift slowly towards the lesion then quickly snap back
Vestibular Syndrome
- Clinical Signs Reason
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
Vestibulo-Ocular Reflex
- Sensory and Motor Arms
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
Nystagmus
- 6 Types of Normal Nystagmus
- Rotational nystagmus
- Post rotation nystagmus
- Positional nystagmus
- Caloric nystagmus
- Optokinetic nystagmus
- Alcohol-induced nystagmus
Rotational Nystagmus
Vestibulo-ocular reflex
Post-Rotational Nystagmus
When the head stops rotating but the endolymph continues
Positional Nystagmus
When the head is maintained in an unusual position for a period of time
Caloric Nystagmus
Endolymph in one ear is chilled so created eddy currents which flow past crest and make you think you’re moving
Optokinetic Nystagmus
Visual stimulus traveling along cranial nerve II (optic) where looking at a moving object induces the feeling of motion
Alcohol-Induced Nystagmus
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
Nystagmus
- 4 Types of Abnormal Nystagmus
- Peripheral inner ear lesion
- Central vestibular nuclei lesion
- Searching nystagmus in blind animals
- Fixating nystagmus caused by cerebellar lesion
