- Menace reflex
The menace reflex is one of three forms of blink reflex. It is the reflex blinking that occurs in response to the rapid approach of an object. The reflex comprises blinking of the eyelids, in order to protect the eyes from potential damage, but may also including turning of the head, neck, or even the trunk away from the optical stimulus that triggers the reflex.
Stimulating the menace reflex is used as a diagnostic procedure in veterinary medicine, in order to determine whether an animal's visual system, in particular the cortical nerve, has suffered from nerve damage. Cortical damage, particularly cerebellar lesions, can cause loss of the menace reflex whilst leaving the other blink reflexes, such as the dazzle reflex, unaffected. The presence or absence of the menace reflex, in combination with other reflexes, indicates a locus of damage. For examples: An animal with polioencephalomalacia will lack the menace reflex, but will still have the pupillary light reflex. Polioencephalomacia damages the visual cortex, impairing the menace reflex, but leaves the optic nerve, oculomotor nucleus, and oculomotor nerve intact, leaving the pupillary light reflex unaffected. Contrastingly, an animal with ocular hypovitaminosis-A will suffer from degeneration of the optic nerve, affecting both reflexes, and such an animal presents with a lack of both reflexes.
Testing the menace reflex has to be done with care. Waving an object close to an animal's eyes or face does not necessarily demonstrate a functioning menace reflex, in part because the animal can sense such objects and react to them via other senses than sight. Clinical testing of the menace reflex usually involves such precautions as waving an object from behind a sheet of glass, so as to shield the animal from any drafts caused by the motion of the object through the air, which it might otherwise sense. Such reactions to non-visual stimuli are a widespread cause of false positives and false negatives when pet owners test their own animals for the presence of the menace reflex.
The neural pathway of the menace reflex comprises the optic (II) and facial (VII) nerves. It is mediated by tectobulbar fibres in the rostral colliculi of the midbrain passing from the optic tract to accessory nuclei, and thence to the spinal cord and lower motor neurones that innervate the head, neck, and body muscles affected by the reflex. The facial nerve is mediated through a corticotectopontocerebellar pathway.
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