P. 550

An Urban Experience
M. Berendtm1
1Department of Veterinary Clinical Sciences, Dyrlaegevej 16, 1870 Frb. C, Denmark
Maintaining balance is a complex process which involves vision, a sense of gravity, the ability to direct movement and adjust the speed of movement, and the ability to maintain body posture. Sensory input from vision, the vestibular apparatus (special proprioception) and general proprioception is used to regulate the muscles of the eyes, head and body in order to secure posture and coordinated interaction of the position of the eyes and different body parts during all kind of activities.
Vestibular anatomy
The Vestibulocochlear nerve (CN VII) contains a cochlear division and a vestibular division which is concerned with hearing and balance respectively. This text will focus
on the vestibular component which is concerned with the sense of balance and spatial orientation (special proprioception).
Peripheral and central vestibular system
The vestibular system can be divided into a peripheral vestibular component including receptors for special proprioception located in the membranous labyrinth (continuous with the cochlea). The membranous labyrinth consists of two otolith organs (the utricle and the saccule with its macula) which observe orientation in a vertical orientation and linear movement (gravity), and the three endolymph  lled semicircular canals located at right angles to each other and their connected ampullae (the site for the cristae ampullaris), which detect movements of the head in any plane including rotational. The dendritic parts of the bipolar sensory neurons forming the vestibular nerve  bers are connected with the peripheral receptors (hair cells of crista ampullaris, macula utriculi and macula sacculi). From here axons travels through the internal acustic meatus close to the cochlear component of CN VIII to the petrosal bone where the vestibular
 bers form the vestibular ganglion. The vestibular nerve continues to enter the rostral part of the brain stem at the cerebellomedullary angle. Most axons terminate in the four vestibular nuclei (representing the central vestibular component of the vestibular system), whereas a minor portion of axons continue directly to the cerebellar peduncle to enter the cerebellum in the fastigal nucleus and  occulonodular lobe (the direct vestibulocerebellar tract). The vestibular nuclei recieves information from formatio reticularis, mesencephalon & cerebellum and send information to spinal cord motor neurons, nuclei
for extraocular muscles & cerebellum. Together these integrated structures are orchestrating that the position and activity of the head and body, and movements of
the eyes, are at all times coordinated. Communication between the vestibular system and the cranial nerve nuclei of the cranial nerves associated with eye movements, CN III: Oculomotor nerve, CN IV: Trochlear nerve and CN VI: Abducent nerve, is conducted through the Fasciculus Longitudinalis Medialis situated in the core of the brainstem. Physiological nystagmus will only be present if these structures are intact and functional.
Structures that are anatomically closely related to the peripheral vestibular system are the Facial nerve which travels very close to the vestibular nerve in the petrosal bone and postganglionic sympathetic nerve  bers which travels from the internal carotid sympathetic plexus close to the tympanic bulla.
Structures anatomically closely related to the central vestibular system in the brain stem are the Ascending Reticular Activating System (ARAS) which is responsible for maintaining arousal, neighboring cranial nerve nuclei and roots (CN V:Trigeminal nerve, CN VI: Abducens nerve, CN VII: Facial nerve and CN IX: Glossopharyngeal nerve), ascending proprioceptive pathways and descending UMN pathways.
Clinical signs of vestibular system dysfunction
Dysfunction of the vestibular system is characterised
by problems with keeping balance and associated ataxia. The animal will have a wide-based stance. The most signi cant and consistent clinical sign of vestibular dysfunction is head tilt. Other clinical signs such as circling, leaning, falling, rolling, pathological nystagmus (involuntary repetitive eye movements expressing a slow and a fast phase) and vestibular strabismus (ipsi-lateral ventral deviation of the eye ball) may also be present. In the majority of cases, vestibular dysfunction is caused by unilateral lesions, and in those cases head tilt, circling, leaning, falling and rolling will with few exceptions
be directed towards the lesion. With nystagmus, the slow eye movement phase is towards the lesion ( the pathological phase), whereas the fast eye movement phase is away from the lesion (the compensatory phase). Nystagmus may be horizontal, vertical, rotatory or of changing direction.
In rare cases of bilateral vestibular disease, the animal will not display asymmetrical signs but is characterized by abnormal head movements where the head is swinging from one side to the other.
Please note that middle-aged to older individuals may experience transient vestibular signs with no indication of central involvement (so-called idiopathic vestibular

   548   549   550   551   552