Balance – an introduction

57 Balance – an introduction


Balance is maintained by equal input from the vestibular receptors found bilaterally in the inner ears (see Fig. 57.1). The receptors detect a change in head position and the impulses generated are transmitted into the brainstem via the vestibular branch of the vestibulocochlear nerve (CN VIII). Damage to the receptors, or the vestibular nerve before its entry into the medulla oblongata, results in a clinical syndrome known as peripheral vestibular disease.


image

Figure 57.1 Brain T2WI. Semicircular canals visible on T2 as ‘rubber duck’ shaped outlines (arrows). Air-filled tympanic bullae (stars).


The vestibular nerve enters the rostral medulla and terminates in the ipsilateral vestibular nuclei, with a few fibres directly entering the cerebellum via the caudal cerebellar peduncle. From the vestibular nuclei, the information is relayed to the:


1. Cerebellum, via the caudal cerebellar peduncle.

Axons from the vestibular nuclei project to the flocculonodular lobe and the fastigial nucleus of the cerebellum. Lesions in this location give rise to the syndrome known as paradoxical vestibular disease.


2. Spinal cord, via the vestibulospinal tract.

The vestibulospinal tract originates in the lateral vestibular nucleus and projects caudally to interneurons which connect with ventral horn cells throughout the entire spinal cord. The vestibulospinal tract facilitates ipsilateral extensors, and inhibits ipsilateral flexors and contralateral extensors with the net effect of providing ipsilateral extensor tone. A lesion in this tract or the lateral vestibular nucleus, or the ipsilateral vestibular receptor creates the opposite effect: the contralateral extensor tone increases, causing the animal to lean and fall to the side of the lesion.


3. Cranial nerve nuclei III, IV, VI, via the medial longitudinal fasciculus (MLF).

The medial vestibular nucleus gives rise to the MLF, which projects rostrally in the brainstem to the cranial nerve nuclei III, IV, and VI. This coordinates eye movement with the position of the head and gives rise to the vestibulo-ocular reflex (VOR). Uneven input occurring in unilateral vestibular disease gives rise to spontaneous abnormal nystagmus. Bilateral destruction of the vestibular receptors from a congenital defect or a drug-induced degeneration removes the afferent stimulus and no nystagmus, physiological or pathological, can be generated. Destruction of the MLF within the brainstem is accompanied by coma. Absence of the VOR in this case signals a grave prognosis for recovery.


The MLF also projects caudally to the cervical and cranial thoracic spinal cord and has the same function as the vestibulospinal tract.


4. Reticular formation within the brainstem.

The vestibular nuclei project to the emetic centre within the brainstem which explains vomiting occurring with an acute loss of balance.

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Balance – an introduction

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