In animals, where a greater percentage of fibers cross over, the left occipital cortex will input a greater proportion of the right visual field of the right eye and a smaller proportion of the right visual field of the left eye. In humans, where 50% of the axons cross over in the chiasm, the left occipital cortex inputs the right visual hemifield of both eyes, and the right occipital cortex inputs the left visual hemifield (orange and green pathways, respectively, in Figure 16-3). The right occipital cortex inputs from the left visual fields of both eyes (see green pathways in Figure 16-3). Consequently, the left occipital cortex receives the axons of the lateral retina of the left eye (inputting from the right visual field) as well as the axons of the medial retina of the right eye (inputting, again, from the right visual field) (see orange pathways in Figure 16-3). Saunders, Philadelphia.)Ĭrossover of optic nerve fibers occurs at the optic chiasm (Figure 16-3). (Modified from de Lahunta A : Veterinary Neuroanatomy and Clinical Neurology, 2nd ed. The wiggling line (∼) indicates axons crossing the midline of the brain. The cerebellum participates in modulating the menace response and integrating function of the motor cortex, using pathways that include: 10, longitudinal fibers of pons 11, pontine nucleus 12, transverse fibers of pons and middle cerebellar peduncle 13, cerebellar cortex 14, efferent cerebellar pathway 15, facial nuclei 16, facial muscles-orbicularis oculi. It is assumed that the visual cortex projects to the motor cortex, which in turn projects via the internal capsule (8) and crus cerebri (9) to the facial nuclei (15) in the medulla, and from there the facial nerve (cranial nerve VII) relays the efferent signal to the eyelid muscles (16). Afferent pathways that serve only the menace pathways (from the distal optic tract onward) are depicted in darker shades. Note that the afferent pathways common to the pupillary light reflex and menace response (up to the level of the proximal optic tract) are colored in lighter shades. As a result, menace testing in young patients may result in a false negative result, as the animal does not blink even though it can see.įigure 16-2 Anatomic pathway of the menace response: The afferent component of the response is relayed from the retina (1) by the optic nerve (2), through the optic chiasm (3), optic tract (4), lateral geniculate nucleus (5) and optic radiation (6) to the visual cortex (7) located in the occipital lobe. It is usually present by 5 to 7 days in foals and calves. Therefore this response may not become fully developed until 10 to 12 weeks of age in some small animals. The complexity of this pathway implies that the resulting blinking is not a reflex but a learned response. It is assumed that the visual cortex projects to the motor cortex, which in turn projects via the internal capsule and crus cerebri to the facial nuclei in the medulla, and from there the facial nerve (CN VII) relays the efferent signal to the eyelid muscles. The afferent component of the response is relayed by the optic nerve, through the optic chiasm, optic tract, lateral geniculate nucleus (LGN), and optic radiation to the visual cortex located in the occipital lobe. The anatomic pathways of the afferent and efferent components are depicted in Figure 16-2. The normal response to this threat is a rapid blink and closure of the palpebral fissure. To eliminate stimulation due to air movement or touching of hair, the menacing gesture may be made behind a transparent glass or plastic sheet. This eliminates the possibility of a blinking response generated by the visual, untested eye. Table 16-1 Summary of the Neuroophthalmologic ExaminationĬN II, optic chiasm, optic tract, lateral geniculate nucleus, optic radiation, visual and motor cortex, facial nucleus and nerve cerebellumĬN II, optic chiasm, proximal optic tract, CN III, sympathetic nerves, diencephalon-mesencephalon (pretectal and oculomotor nuclei)ĬNs III, IV, VI, vestibular system, brainstemĬN VIII-brainstem and vestibular system-CNs III, IV, VIįigure 16-1 The menace response of the right eye is tested while the left eye is being covered. This chapter reviews the examination, clinical signs, and diseases of the neuroophthalmologic patient. Therefore the workup of the neuroophthalmologic patient requires comprehensive neurologic and systemic examinations, in addition to a thorough neuroophthalmologic examination (Table 16-1). In addition, significant parts of the CNS are devoted to vision processing and ocular control. Facial nerve: CN VII-innervates the various muscles controlling the blink response.
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