Light reflex

The rd rd cl mouse has provided a powerful model to characterize the ocular non-rod, non-cone photoreceptors of mammals using action spectrum techniques. The first completed action spectrum was for the pupillary light reflex PLR (Lucas et al 2001). The results demonstrated that the PLR in rd rdcl mice is driven by a single opsin/vitamin A-based photopigment with a 479 nm. The... 

Intraocular pressure is reduced by 40%. The pupil first dilates and then constricts, although the light reflex is maintained until surgical anaesthesia is achieved. The corneal, conjunctival, eyelash and eyelid reflexes are abolished. 

Physiological Basis of Pupil Size and the Light Reflex.128... 

PHYSIOLOGICAL BASIS OF PUPIL SIZE AND THE LIGHT REFLEX 7.2.1 Pupil Size... 

Sequential photographs can be used to monitor pupil size over an extended time. If the pupil is illuminated with infrared fight and infrared-sensitive film is used, recordings can be made in total darkness. Although this method was used in seminal studies of the pupillary light reflex and other dynamic applications,6 7 it is seldom used today because of the high cost of film, processing time, and limited temporal resolution. 

As shown in Figure 7.1, there are several components of the light reflex that may be evaluated with dynamic pupillometers. From studies in cats, monkeys, and rabbits, Lowenstein and... 

Lowenfeld12 identified the components of the fight reflex that were controlled by parasympathetic and sympathetic innervation of the smooth muscles controlling pupil diameter. They concluded that the parasympathetic nervous system must be intact to observe the light reflex the sympathetic nervous system influences the shape of the reflex. For example, in the absence of sympathetic innervation, the constriction velocity is increased and the dilation velocity is decreased. Conversely, in situations of increased sympathetic tone, the constriction is sluggish and incomplete, and the pupil slowly returns to its baseline size. The effects of abused drugs on these and other components of the light reflex were studied in the experiment described below. 

The constriction amplitude of the light reflex differed significantly among the treatment conditions (Figure 7.2). A two-way ANOVA indicated significant differences among drug conditions... 

As summarized in Table 7.1, only the high dose of marijuana significantly changed (reduced) the velocity of dilation of the pupil during the recovery phase of the light reflex. 

In the laboratory experiment described above, pentobarbital (450 mg) caused a small but significant decrease in pupil size and a reduction in the constriction velocity of the light reflex. The maximal effect was measured 300 min after oral drug administration. Nystagmus (rhythmical oscillation of the eyeballs) and ptosis (drooping of the upper eyelid) are the eye signs that are most often attributed to ingestion of barbiturates, benzodiazepines, ethanol, and other CNS depressants.26 30 31... 

Both indoleamine (e.g., lysergic acid diethylamide, LSD psilocybin) and phenethylamine hallucinogens (e.g., mescaline) increased pupil diameter.26 There have been no systematic studies of the effects of these drugs on dynamic measures of the light reflex. Phencyclidine (PCP) does not cause marked changes in pupil size or light reflex. However, subjects intoxicated with PCP often show horizontal and vertical nystagmus.26... 

Fosnaugh, J.S., Bunker, E.B., and Pickworth, W.B., Daily variation and effects of ambient light and circadian factors on the human light reflex, Meth. Find. Clin. Exp. Pharmacol., 14, 545, 1992. 

Radzius, A. et al., A portable pupilometer system for measuring pupillary size and light reflex, Behav. Res. Method Instrum. Comput., 21, 611, 1989. 

Pickworth, W.B. et al., Intravenous buprenorphine reduces pupil size and the light reflex in humans, Life Sci., 49, 129, 1991. 

Lanting, P. et al., The cause of increased pupillary light reflex latencies in diabetic patients the relationship between pupillary light reflex and visual evoked potential latencies, EEG Clin. Neurophysiol., 78, 111, 1991. 

When cocaine is applied to the conjunctiva, it produces local anesthesia (which may not extend to the iris), local anemia (which extends to the iris but not to the retina), and submaximal dilatation of the pupil by peripheral sympathetic stimulation. Mydriasis occurs also on systemic administration. The accommodation is impaired, but the light reflex is preserved there is some exophthalmos. The intraocular pressure is usually lowered, but may be increased. The mydriasis and its associated phenomena are not produced by most of the cocaine substitutes. 

The pupillary light reflex may help differentiating metabolic cause from structural brainstem lesion in comatose patients (Tokuda et al. 2003). The light reflex is very resistant to metabolic dysfunction. An abnormal light reflex, especially when unilateral, points to a midbrain lesion. Bilateral diencephalic lesions or metabolic coma may cause bilateral small pupils well reacting to light ( diencephalic pupils ). 

Larson M. D., Sessler D. I., McGuire J., andHynson J. M. (1991) Isoflurane, but not mild hypothermia, depresses the human pupillary light reflex. Anesthesiology 75, 62-67. 

During an acute imilateral attack, pupil testing reveals RAPD, because demyelinating disease can disrupt the impulses traveling within the pupillary fibers of the light reflex pathway. Color vision is reduced in most cases. Contrast sensitivity is reduced in cases of MS and may remain reduced after visual recovery occms. The ONTT reported that diffuse visual field loss occurred in 48.2% of eyes and that altitudinal field defects or other nerve fiber bimdle-type defects were present in 20.1% of eyes. Significantly, there was asymptomatic visual field involvement in the fellow eye in 68.8% of patients. 

Scopolamine was considerably more active chan atropine in this series of experiments, a dose of 0.03 mg/kg producing ataxia in six of six dogs and Increasing heart rate by 24%. A dose of scopolamine at 0.05 mg/kg rendered four of four dogs ataxic and caused them to engage In obstinate progression. This dose also resulted In abolition of the light reflex of the iris, paralysis of salivation between 45 and 135 min after the dose, and a 59% increase in heart rate. 

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