The test is repeated for the other eye. With a cooperative patient the medial and lateral aspects of each eyeball retina can be tested. Care must be taken not to create too much air turbulence.
In the stoic patient it may be necessary to tap the eyelids being tested so that the animal is aware of the test. The entire peripheral and central visual pathway must be intact for a response to occur. This is a learned response, not a reflex, to a perceived threat. Normal function is demonstrated by a blink or retraction of the globe in response to the threat or even an aversive head movement.
Abnormal function is manifested by a reduced to absent blink or globe retraction. The afferent part of this response involves the same structures as the visual pathways. The information generated in the contralateral visual occipital cortex is forwarded to the motor cortex to initiate a motor response. This response requires intact ipsilateral facial nerve function as well as an intact ipsilateral cerebellum.
The neuronal pathways through the cerebellum are however not known. To localize or isolate a lesion responsible for dysfunction, other cranial nerve tests palpebral reflex; pupillary light reflex PLR would be required.
Anatomy Neuroanatomical Structures Evaluated - The afferent or sensory visual pathway optic nerve CN II , the optic chiasm , the optic tract, lateral geniculate nucleus, the optic radiations are evaluated by the menace response. Diagrammatic Representation of Anatomy Clinical Stimulus — As one eye is covered, the other eye is tested by directing a hand in its direction in a threatening manner. Clinical Response — In a normal animal the eyelids will close in response to the perceived threat.
Afferent Pathways — The hand movement or threat is initially detected by the retina and an induced nerve impulse travels down the optic nerve from the eye into the cranial cavity via the optic canal. It travels to the optic chiasm where the major portion of it will cross over to the other side.
Efferent Pathways — A normal response of closing the eyelids is mediated by impulses travelling via the motor portion of the facial nerve CN VII having left the brainstem and exiting the skull through the stylomastoid foramen. The facial nerve impulses cause contraction of the orbicularis oculi muscle closing the eyelids.
Pathway An image of the hand is projected through the eye to the retina. Electrochemical receptors in the retina are excited and the generated impulses are projected into the optic nerve. Objects in each temporal visual field are projected upon the nasal retina.
Objects in the nasal visual field are projected to the temporal retina. The nasal retinal field makes up the majority of fibers that cross and the temporal retinal field makes up the majority of fibers that remain ipsilateral. After crossing at the chiasm, the image is projected through the optic tract to reach the lateral geniculate nucleus in the thalamus. The pupillary light reflex allows the eye to adjust the amount of light reaching the retina and protects the photoreceptors from bright lights.
The iris contains two sets of smooth muscles that control the size of the pupil Figure 7. Both muscles act to control the amount of light entering the eye and the depth of field of the eye 1.
The pupillary light reflex neural circuit : The pathway controlling pupillary light reflex Figure 7. The lines ending with an arrow indicate axons terminating in the structure at the tip of the arrow. The lines beginning with a dot indicate axons originating in the structure containing the dot. Bilateral damage to pretectal area neurons e.
Recall that the optic tract carries visual information from both eyes and the pretectal area projects bilaterally to both Edinger-Westphal nuclei: Consequently, the normal pupillary response to light is consensual.
That is, a light directed in one eye results in constriction of the pupils of both eyes. Pupillary Dark Response. The pupils normally dilate increase in size when it is dark i. This response involves the relaxation of the iris sphincter and contraction of the iris dilator. The iris dilator is controlled by the sympathetic nervous system.
The pupillary dark reflex neural circuit : The pathway controlling pupil dilation involves the. Axons from the superior cervical ganglion also innervate the face vasculature, sweat and lachrymal glands and the eyelid tarsal muscles. When the superior cervical ganglion or its axons are damaged, a constellation of symptoms, known as Horner's syndrome , result.
This syndrome is characterized by miosis pupil constriction , anhidrosis loss of sweating , pseudoptosis mild eyelid droop , enopthalmosis sunken eye and flushing of the face. The accommodation response is elicited when the viewer directs his eyes from a distant greater than 30 ft. The accommodation near point response is consensual i.
The accommodation response involves three actions:. Pupil accommodation: The action of the iris sphincter was covered in the section on the pupillary light reflex.
During accommodation, pupil constriction utilizes the "pin-hole" effect and increases the depth of focus of the eye by blocking the light scattered by the periphery of the cornea Nolte, Figure , Pg.
The iris sphincter is innervated by the postganglionic parasympathetic axons short ciliary nerve fibers of the ciliary ganglion Figure 7. Lens accommodation: Lens accommodation increases the curvature of the lens, which increases its refractive focusing power. The ciliary muscles are responsible for the lens accommodation response. They control the tension on the zonules, which are attached to the elastic lens capsule at one end and anchored to the ciliary body at the other end Figure 7.
The ciliary muscles function as a sphincter and when contracted pull the ciliary body toward the lens to decrease tension on the zonules see Figure 7. The decreased tension allows the lens to increase its curvature and refractive focusing power. When the ciliary muscle is relaxed, the ciliary body is not pulled toward the lens, and the tension on the zonules is higher.
High tension on the zonules pulls radially on the lens capsule and flattens the lens for distance vision. The ciliary muscles are innervated by the postganglionic parasympathetic axons short ciliary nerve fibers of the ciliary ganglion. Convergence in accommodation: When shifting one's view from a distant object to a nearby object, the eyes converge are directed nasally to keep the object's image focused on the foveae of the two eyes.
This action involves the contraction of the medial rectus muscles of the two eyes and relaxation of the lateral rectus muscles. The medial rectus attaches to the medial aspect of the eye and its contraction directs the eye nasally adducts the eye.
The medial rectus is innervated by motor neurons in the oculomotor nucleus and nerve. The accommodation neural circuit : The circuitry of the accommodation response is more complex than that of the pupillary light reflex Figure 7. Ocular motor control neurons are interposed between the afferent and efferent limbs of this circuit and include the. During accommodation three motor responses occur: convergence medial rectus contracts to direct the eye nasally , pupil constriction iris sphincter contracts to decrease the iris aperture and lens accommodation ciliary muscles contract to decrease tension on the zonules.
An excellent way to test your knowledge of the material presented thus far is by examining the effects of damage to structures within the ocular motor pathways.
The observed motor loss s provide clues to the pathway s affected; and the muscle s and eye affected provide clues to the level of the damage. Cranial nerve damage : Damage to cranial nerves may result in sensory and motor symptoms.
The sensory losses would involve those sensations the cranial nerve normally conveys e. The motor losses may be severe i. The cranial nerves involved in the eye blink response and pupillary response are the optic, oculomotor, trigeminal and facial nerves.
The patient, who appears with a bloodshot left eye, complains of an inability to close his left eye. When asked to rise his eyebrows, he can only elevate the right eyebrow. When asked to close both eyes, the right eyelid closes but the left eyelid is only partially closed. Touching the right or left cornea with a wisp of cotton elicits the eye blink reflex in the right eye, but not the left eye Figure 7.
However, the patient reports he can feel the cotton when it touches either eye. He can smile, whistle and show his teeth, which indicates his lower facial muscles are functioning normally.
Physical examination determines that touch, vibration, position and pain sensations are normal over the entire the body and face. There are no other motor symptoms. Pathway s affected : You conclude that structures in the following motor pathway have been affected. When lower motor neurons are damaged , there is a flaccid paralysis of the muscle normally innervated. The action of the muscle will be weakened or lost depending on the extent of the damage. There will be a weakened or no reflex response and the muscle will be flaccid and may atrophy with time.
The Facial Nerve. Section of the facial nerve on one side will result in paralysis of the muscles of facial expression on the ipsilesional side of the face. There will be an inability to close the denervated eyelid voluntarily and reflexively. The eyelids may have some mobility if the oculomotor innervation to the levator is unaffected. The patient complains of a badly infected left eye. When he is asked to close both eyes, both eyelids close.
Touching the right cornea with a wisp of cotton elicits the eye blink reflex in the both eyes Figure 7. However, touching the left cornea with a wisp of cotton does not elicit the eye blink reflex in the either eye Figure 7. The patient cannot detect pinpricks to his left forehead. However, he reports that pinpricks to rest of his face are painful.
He can blink, wrinkle his brows, smile, and whistle and show his teeth, which indicates his facial muscles are functioning normally. Physical examination determines that touch, vibration, position and pain sensations are normal over the entire the body and over the lower left and right side of his face.
Pathway s affected : You conclude that structures in the following reflex pathway have been affected. The Trigeminal Nerve. Section of the trigeminal nerve will eliminate somatosensory sensation from the face and the eye blink reflex e.
However, light touch of the right cornea will elicit a bilateral eye blink. The effect of sectioning the trigeminal nerve is to remove the afferent input for the eye blink reflex.
The patient complains of pain in her left eye. Her left pupil appears dilated and is not reactive to light directed at either the left or right eye Figure 7. The right pupil appears normal in size and reacts to light when it is directed in the right or left eye.
Both eyelids can be elevated and lowered and both eyes exhibit normal movement. Touch, vibration, position and pain sensations are normal over the entire the body and face.
Parasympathetic Innervation of the Eye. Section of the parasympathetic preganglionic oculomotor nerve or postganglionic short ciliary nerve innervation to one eye will result in a loss motor of both the direct and consensual pupillary light responses of the denervated eye.
Section of the left short ciliary nerve or a benign lesion in the left ciliary ganglion will result in no direct response to light in the left eye and no consensual response in the left eye when light is directed on the right eye a. When the damage is limited to the ciliary ganglion or the short ciliary nerve, eyelid and ocular mobility are unaffected.
0コメント