Some of the stimulation reaching the contralateral (right) pretectal nucleus also comes from the decussating (crossing over) optic nerve impulses, although this is not depicted here.ĢD direct PLR pathway - The direct PLR pathway can be seen here to be ultimately an ipsilateral pathway causing constriction of the pupil being stimulated.ĢD consensual PLR pathway to parasympathetic nucleus of CN III - The consensual or indirect PLR pathway can be seen here to cause papillary constriction of the right pupil when the left one is stimulated. Stimulation of the contralateral CN III parasympathetic nucleus results in this diagram from optic nerve transmission which does not cross over at the optic chiasm (25% in the dog) but does so at the pretectal nucleus. The Consensual PLR Pathway - Direct stimulus of the left eye results in an indirect constriction of the right pupil which is termed the consensual response. The oculomotor nerve travels to the ciliary ganglion where it synapses and from which the short ciliary nerves leave and innervate the pupil constrictor muscles.ĭorsolateral view of the direct pathway - This dorsolateral view details the complete direct PLR pathway. From here the pathway crosses back over to synapse in the nucleus of cranial nerve III.ĭirect pathway - This lateral view of the direct pathway details the efferent component which starts in the parasympathetic nucleus of CN III and exits as the oculomotor nerve from its origin in the midbrain. It is important to note that this response can also result from the light impulses which cross over in the chiasm and also stimulate the right CN III, although this is not depicted in this movie.ĭorsal view of direct pathway - This dorsal view of the afferent pathway shows how the impulse starts in the retina and crosses over in the optic chiasm to synapse in the pretectal nucleus of the thalamus. From here it synapses in the left pretectal nucleus and then can cross over to stimulate the paramsympathetic nucleus of the right oculomotor nerve (CN III). In this movie you will see that stimulation of the left eye can result in the left optic nerve transmitting impulses to the left optic tract and not crossing over. The consensual response results in pupillary constriction of the eye not directly stimulated by light, although the response is slightly reduced compared to the eye being directly tested. From here, the short ciliary nerves innervate the pupillary constrictor muscles to cause constriction of the pupil. The oculomotor nerve leaves the skull via the orbital fissure and synapses behind the eye in the ciliary ganglion. This explains why the direct response is greater than the consensual response.Ĭlinical Response - The direct response is seen in the eye which is stimulated by light and pupil constriction is stronger in this eye than in the opposite eye (consensual response).Īfferent Pathway - Stimulation of the retinal receptors by light initiate an impulse which is transmitted from the eye via the optic nerve, with the majority then crossing over at the optic chiasm to the contralateral optic tract.Įfferent Pathway - The efferent pathway begins in the parasympathetic nucleus of cranial nerve III (oculomotor nerve) located in the midbrain (mesencephalon) on the stimulated side. The proportion of axons that decussate is higher than the one that do not decussate. There are also neurons that do not decussate and which project to the oculomotor nucleus on the contralateral side of the stimulated eye. Most of the axons arising from this nucleus decussate again and synapse in the parasympathetic component of the oculomotor nucleus (ipsilateral to the stimulated eye) in the mesencephalon. While axons involved in vision reach the conscious level after synapse with the lateral geniculate nucleus, the axons involved in the PLR synapse with a third neuron in the pretectal nucleus. The afferent part of the PLR shares some common pathways (up to the level of the optic tract) with the visual pathways. Pupillary Light Reflex Clinical Exam Anatomy Pathway Sections Further Reading
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |