Basic Science
The size of the pupil is controlled by the activities of two muscles: the circumferential sphincter muscle found in the margin of the iris, innervated by the parasympathetic nervous system: and the iris dilator muscle, running radially from the iris root to the peripheral border of the sphincter. The iris dilator fibers contain α-adrenergic sympathetic receptors that respond to changes in sympathetic tonus and changes in the blood level of circulating catecholamines.
The pupillary light reflex arc begins in the retina (Figure 58.1). Considerable evidence exists that the visual cells of the retina, that is, the rods and cones, also serve as light receptors controlling pupillomotor activity. Fibers originating from the nasal neuroreceptor cells decussate in the optic chiasm to the opposite optic tract, whereas the temporal fibers continue in the homolateral optic tract. “Pupillary fibers” from both eyes within the optic tract pass via the superior quadrigeminal brachium and the superior colliculus to the mesencephalic pretectum and pretectal nuclei. Axons from each pretectal nucleus pass ipsilaterally and contralaterally to the ipsilateral and contralateral Edinger-Westphal (E-W) nucleus, a subnucleus of the oculomotor nuclear complex, The hemidecussation of the pupillary fibers at the optic chiasm and between the pretectal nuclei ensures that each E-W nucleus receives information about the level of incoming light from each eye. Hence, the pupils should be equal in diameter regardless of the level of vision of either eye. For example, in a patient with one blind eye, the pretectal nuclei would register and transmit to each E-W nucleus only one-half the normal level of illumination. The transmission of less pupilloconstrictor tone to each iris sphincter would result in slightly larger pupils but of equal diameter. Accordingly, anisocoria (unequal pupillary diameter) is not attributable to the angle at which light strikes the face, unilateral cataracts, or an asymmetric refractive error, unless there is local disease of the anterior segment.
Bạn đang xem: Bookshelf
Parasympathetic axons from the E-W nucleus join the outflow of the other oculomotor subnuclei to form the trunk of the oculomotor nerve. Pupillomotor fibers assume a superficial location in the nerve as it exits the mesencephalon in the interpeduncular space.
Xem thêm : DO YOU KNOW WHO YOU ARE? THE POWER OF BIBLICAL SELF-AWARENESS
In the orbit, the parasympathetic components synapse in the ciliary ganglion. Postganglionic fibers traveling in the short ciliary nerves innervate both the ciliary body, inducing lens accommodation, and the pupilloconstrictor muscles of the iris. The ratio of fibers innervating the ciliary body to those supplying the pupil is approximately 30:1. Acetylcholine serves as the neurotransmitter for both functions.
The pupillary near reflex consists of three separate, synergistic phenomena: accommodation, convergence, and pupillary constriction. The near reflex, in general, is a fundamental component of stereoscopic vision. Using the macaque monkey, Jampel (1967) showed that all three components of the near reflex can be elicited by electrical stimulation of the occipital association cortex. In fact, by slight variations in the position of the stimulating electrode or by changing the stimulus intensity, the various components could be obtained in partial combinations or, on occasion, alone. The exact anatomic pathways connecting cerebral cortex to midbrain are unresolved. There is evidence, however, derived mainly from clinical observations, that the fibers mediating pupillary constriction in the near reflex follow a more ventral course than those subserving the light reflex at the mesencephalic level.
Nguồn: https://buycookiesonline.eu
Danh mục: Info
