Tuesday, December 22, 2009

Pupillary Pathway

The Pupillary Light Reflex Pathway

The pupillary light reflex pathway consists of two parts:  Afferent pupillary light reflex and Efferent pupillary light reflex. 

Afferent Pupillary Light Pathway

The afferent pupillary light pathway originates in the retinal receptor cells and passes through the optic nerve, optic chiasm, and optic tract. (Figure 1)  Pupillary fibers follow the optic tract (posterior third of the optic tract) and separate from the optic tract just anterior to the lateral geniculate body.  They then enter the midbrain, where they synapse to pretectal nucleus.  The pupillary fibers leave the pretectal nucleus and distributes approximately equally to both Edinger-Westphal nuclei.  This tract is called the tectotegmental tract.  Thus, the optic tract carries pupillary fibers from both eyes, and the tectotegmental tract carries pupillary fibers from both pretectal nuclei. (6)  From these papillary fiber arrangements, pupils constrict according to direct pupilary reflex or consensual light reflex.     
Efferent Pupillary Light Pathway

Parasympathetic Pupillary Pathway

The efferent pupillary light pathway begins at the Edinger-Westphal (E-W) nuclei.  This is located on the dorsal aspect of the third cranial nerve nucleus in the anterior dorsal mesencephalon at the level of the superior colliculus.  Efferent pupillary fibers from the E-W nuclei are carried in the superficial layer of the third cranial nerve to the cavernous sinus.  The efferent pupillary fibers eventually end in its inferior division, where they pass through the superior orbital fissure and synapse in the ciliary ganglion.  The anatomical location of the efferent pupillary fibers superficially on the third cranial nerve becomes critical, when evaluating patients with third cranial nerve palsy.  It is clinically important to note that the pupillary fibers are located superficially between the brain stem and the cavernous sinus.  Finally, postganglionic parasympathetic pupillary fibers synapse and pass through the short ciliary nerves to the iris sphincter and ciliary muscles.  93% to 97% of these parasympathetic fibers supply the ciliary muscles and 3 % to 7 % of the remaining supplies the iris sphincter muscles. (7)  The short ciliary nerves not only carry parasympathetic pupillary fibers, they also carry sensory and sympathetic pupillary fibers.

Sympathetic Pupillary Pathway (Oculosympathetic)

The pupillodilator system is controlled by the sympathetic nervous system. The sympathetic nervous system is divided into central (first-order) neuron, preganglionic (second-order) neuron, and postganglionic (third-order) neuron. (Figure 2) The sympathetic fibers arise in the posterolateral area of the hypothalamus and descend, uncrossed, in the lateral portion of the midbrain, pons, medulla, and cervical spinal cord to the ciliospinal center of Budge at C8 to T2. This section of the sympathetic pathway is the central (first-order) neuron and is located in the brainstem and cervical cord. The preganglionic fibers travel upward in the sympathetic chain over the apex of the lungs and through the stellate ganglion, the inferior cervical ganglion, around the subclavian artery and through the middle cervical ganglion to the superior cervical ganglion at the carotid bifurcation. (1) The preganglionic (second-order) neuron is located in the chest and in the neck. The postganglionic fibers travel to the iris via the carotid plexus, the cavernous sinus and the long ciliary nerves. The postganglionic fibers run upward around the internal carotid artery into the cavernous sinus where they join with the ophthalmic division of the trigeminal nerve. They emerge from the cavernous sinus and pass into the orbit through the nasociliary branch of the ophthalmic division. Finally entering the eye through the long ciliary nerves and terminating at the iris dilator muscle. The postganglionic (third-order) neuron starts from the base of the skull and passes through the cavernous sinus to the orbit. The postganglionic fibers also distribute to orbital vasomotors, lacrimal gland and the smooth muscles of the upper and lower lids (Mueller) through the ophthalmic artery branches.

Near Pupillary Reflex Pathway
The contraction of the pupil at near is not true reflex but believed to be an associated movement.  It is independent of any change in illumination.  When gaze is directed from a distance to a near object, a triad of responses occurs:  convergence, accommodation and pupillary constriction.  However, the contraction of the pupil at near does not depend on either the accommodation or the convergence and vise versa.  The pupillary near response depends on a supranuclear connection between the neurons serving the pupillary sphincters, the ciliary body muscles and the medial recti.
It is believed that the afferent pathway of the pupillary near response follows the visual pathway to the striate cortex (higher cortical centers).  From the striate cortex, information is relayed to the front eye fields, then to the oculomotor nucleus and the Edinger-Westphal nucleus, bypassing the pretectal nuclei in the dorsal midbrain.  It is believed that these arrangements of the light and near pupillary pathways cause light-near dissociation, when the dorsal midbrain and pretectal nuclei are damaged.  Finally, the medial rectus muscles are innervated via the oculomotor nerve.  The iris sphincter and ciliary body muscles are innervated by the parasympathetic pathway.

1 comment:

  1. where can i download the video clip? 'cause i need it as an example to my homework. Thanks. :D