Menace reflex
Encyclopedia
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 neuron
es that innervate the head, neck, and body muscles affected by the reflex. The facial nerve is mediated through a corticotectopontocerebellar pathway.
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
Polioencephalomalacia
Polioencephalomalacia literally means softening of the cerebrocortical grey matter distributed in a laminar pattern. It is also called laminar cortical necrosis or cortical necrosis. PEM is a sporadic disease of unknown cause occurring in cattle, sheep and goats. PEM is most commonly seen in...
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
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
, oculomotor nucleus
Oculomotor nucleus
The fibers of the oculomotor nerve arise from a nucleus in the midbrain, which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle...
, and oculomotor nerve
Oculomotor nerve
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...
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)
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...
and facial (VII)
Facial nerve
The facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...
nerves. It is mediated by tectobulbar fibres in the rostral colliculi
Superior colliculus
The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus , but, even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a...
of the midbrain passing from the optic tract
Optic tract
The optic tract is a part of the visual system in the brain.It is a continuation of the optic nerve and runs from the optic chiasm to the lateral geniculate nucleus....
to accessory nuclei, and thence to the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
and lower motor neuron
Motor neuron
In vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
es that innervate the head, neck, and body muscles affected by the reflex. The facial nerve is mediated through a corticotectopontocerebellar pathway.