Function Transmits impulse Connect sensory Transmit
from CNS to and motor impulse from
effector neurones to receptors to
form nerve CNS from
circuit sensory
Several axons held Typically found receptor cell
together make a in CNS
nerve
Location of End Middle Buds off middle
cell body
Dendrites Synapse with Synapse with Synapse with
effectors other neurones receptors
, A Level Notes GRM: Reflex Arc in Eyes Biology
The reflex arc; increases speed of responses (reduces damage)
• Controlling the intensity of light entering the eye; pupil reflex
• Too much light can damage cells called photoreceptors on the retina (more light; better sight; fight or flight)
An antagonistic pair of muscles work in one direction to create opposing forces to cause movement
Pupils reflex; (controlled by iris) contain antagonistic muscles called circular and radial muscles
• Circular – contraction causes constriction [parasympathetic]
• Radial – contraction causes dilation of pupil [sympathetic]
Process in conditions of bright light
1) Photoreceptors detect bright light striking the retina at the back of the eyes
2) Impulse sent from sensory neurone (in optic nerve) to CNS/coordinating cells in midbrain
3) These attach to relay neurones
4) Impulse sent along motor neurones in midbrain along parasympathetic (ANS) neurones
5) In iris, circular muscles contract and radial muscles relax
6) Pupils constrict / reduce in diameter, and less light enters the eyes
The giant axons of squid are the largest nerve cells in the animal kingdom; they also have a highly developed nervous system with
complex eyes and brain – hence are suitable samples for experimentation.
A person has damaged their sensory optic nerve in their left eye; what effect does this have on their pupil reflex?
→ No effect as the two sensory neurones combine to form one – this means the impulse from the other eye will still cause
the motor neurone of the damaged side to contract/relax muscles normally.
A person has severed the motor neurone controlling their left eye; what will happen when they walk into a dark room?
→ Damaged motor neurone means radial muscles won’t contract and the left pupil will stay the same diameter.
→ The right eye will still respond, as sensory and motor neurones are still intact, and the sympathetic nerves stimulate radial
muscles to cause dilation.
Part of the nervous system Structure in the pupil reflex
Receptor Rods and cones in the retina
Sensory nerve fibres Optic nerve
Coordinator Brain
Motor nerve fibres Oculomotor nerve
Effector Iris muscle
The purpose of the pupil reflex is to prevent damage to the retina from high-intensity light; in dim light conditions it ensures
maximum light reaches the retina
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