Activity 2: Pupillary Light Reflex
Background
The pupillary light reflex is an autonomic reflex. It is mediated through two cranial nerves (optic and oculomotor). The optic nerve conveys sensory impulses from the retina (ganglion cells functions receptors) to the thalamus; the oculomotor nerve conveys motor impulses from the midbrain to the circular smooth muscles (sphincter pupillae) of the iris. Some of the sensory impulses bypass the thalamus and terminate in the midbrain (pretectal nucleus). From here internuncial fibers relay information to parasympathetic nuclei (Edinger-Westphal) located on both sides (right and left) of the midbrain. Notice that fibers that crossover to the contralateral (opposite) side are responsible for causing responses on the opposite side (non-stimulated side). Concerning the pupillary light reflex, if we stimulated the right eye with a beam of light, we would see a pupillary constriction (response) in the right eye. In addition, there is papillary constriction in the left eye. The response you see on the left eye is called a consensual response. Consensual responses are characteristic of autonomic reflexes and are absent in somatic reflexes. A consensual response, therefore, is defined as a response observed on the non-stimulated side of the body.
In the midbrain, the parasympathetic fibers join somatic motor fibers from the oculomotor nuclei, and both emerge (from the brainstem) as components of each oculomotor nerve. The parasympathetic fibers of the oculomotor nerve then innervate the sphincter pupillae while the somatic motor fibers innervate the extrinsic muscles that move the eyeball.
The pupils of a dying person often dilate markedly and do not contract to bright light. In persons who have taken narcotics (such as heroin or morphine) the pupils constrict greatly (called “pinpoint pupils”) and do not dilate. This fact is often used to determine if a person is intoxicated with narcotics. The pupillary reflexes are also important indicators in general anesthesia; the anesthetist checks the degree of dilation or constriction of the eyes (as well as other signs) to determine the depth of unconsciousness.
The purpose of this experiment is to learn how to perform an autonomic reflex, to see the normal responses of autonomic reflexes, and to emphasize the clinical importance of this type of reflex. This activity also gives the student an opportunity to see the similarities and differences (anatomically and physiologically) of reflexes.
Materials needed
willing subject (lighter colored irises are easier to see the pupils)
keylight , penlight, or cell phone light
3″ x 5″ index card
metric ruler
Procedure
Draw five half-circles of different diameters (2mm, 3mm, 4mm, 5mm, and 6mm) at 1mm intervals at the edge of the index card. The half-circles will be used to measure the diameter of the subject’s pupils.
Measure the diameters of the subject’s pupils in a dimly lit room. Record the results.
Ask the subject to place his or her hand (vertically) between the eyes, in front of the nose. Formulate your hypothesis about what to expect when light is introduced to the pupils. Direct a beam of light to the subject’s left eye for a few seconds. NOTE: the hand between the eyes should prevent stimulation to the right eye. Now, observe what happens to both pupils as you stimulate the left eye. Repeat two or three times.
Stimulate the left eye again and measure the diameters of the constricted pupils (right and left eyes). Record them.
Results
Generate a FULL lab report for this lab using the scientific method format. This format should include purpose, hypothesis, materials and methods, results, and conclusion sections for EACH of the two activities you are performing. Use the Lab Report template Download Lab Report templateas your guide.
