Trace the sequence of events through a reflex arc which occurs when a bright light is focused on eye.

Reflex arch is a neural pathway that does not directly pass into the brain, but synapse in the spinal cord. When bright light is focused on our eyes, this signal do no directly passes into the brain but synapses in the spinal cord which allows us to respond at a much faster rate and this results in immediate closing of our eyes in response to bright light.

When a bright light is focussed on eye, receptor cell receives the message and passes on to sensory neuron, then it goes to brain, brain reverts back the message by motor neuron which contracts the pupil.

Receptor → Sensory neuron → Brain → Motor neuron → Eye → Eye muscle contracts.

• The iris controls the size of the pupil and therefore only lets a certain amount of light to enter the eye

• The iris contains two sets of smooth muscles- circular muscles and radial muscles.

• The circular muscles are arranged in concentric rings around the pupil, while the radial muscles run radials.

• These muscles are antagonistic

• The pupil reflex is the reflex contraction and relaxation of the antagonistic muscles of the iris in response to changes in light intensity. It causes a change in the pupil size.

• The pupil dilates at low light intensities, and constricts at high light intensities

• This allows sufficient light to enter the eye for vision in dim light, while screening off excess light at high light intensities to prevent damage to the retina

• The pupil in the human eye remains circular when contracted or dilated