Question

• What are the 5 components of a reflex arc? Identify (specifically) each component of the pupillary reflex arc?  

• when you shined the light into the left eye what happened to the left pupil? When you shined the light into the right eye what happened to the right pupil? Is this what you expected to happen, what should happen? Please explain your answers.  

• when you shined the light into the left eye what happened to the right pupil? When you shined the light into the right eye what happened to the left pupil? Is this what you expected to happen, what should happen? Please explain your answers. “Hint – both eyes constrict”

Answer 1

5 components of reflex arc :

• Receptor - where external stimulus is converted into impulse.
• Sensory neuron- carries information from receptor to interneurons in the spinal cord or brain
• Integrating centres - nuclei of spinal cord or brain serves as integration centers wich sends impulse from afferent sensory neuron to efferent motor neuron.
• Motor neuron- information from spinal cord to effector organ .
• Effector organ - from efferent motor neuron the effector organ receives impulses and produces appropriate response .

Pupillary reflex arc :

• Receptor - light falls on the retina , wich stimulates the impulse .
• Sensory neuron- this sensory stimulus is carried through optic nerve of ipsilateral eye .
• Integtation centre -  optic nerve relays information to pretectal nuclei in superior colliculus of midbrain of same side . From pretectal nuclei to edinger westphal nuclei of both right and left sides
• Motor neuron- edinger westphal nuclei of both right and left sides are stimulated wich sends impulse to preganglionic parasympathetic fibers through occulomotor nerve which synapse with ciliary ganglion and postganglionic parasympathetic fibers travel through short ciliary nerves.
• Effector organ - short ciliary nerves innervates sphincter pupillary muscles of the iris , which on stimulation causes constriction of pupil by releasing acetylcholine.  

When light is shined to left eye , left pupil constricts , when light travel through right eye right pupil constricts. This reflex is called direct pupilary light reflex. When light enters right pupil , stimulation of ipsilateral optic nerve wich will cause ipsilateral pretectal nucleus to be stimulated wich sends stimulus to edingerwestphal nucleus of both ipsilateral and contralateral sides , from where efferent motor neuron of both sides stimulated wich cause constriction in both the right and left pupil . So when light is shined through right eye , the left eye also constricts simultaneously this is called consensual pupillary light reflex , since pretectal nucleus stimulates not only the ipsilateral edingerwestphal nucleus but also contralateral edingerwestphal nucleus .