The eye, rods and pupil responsesEdexcel A-Level Biology A: Revision notes
Section 1
Nervous coordination: stimulus to response
A stimulus is a change in the environment detected by a receptor. In the nervous system, impulses pass along a sensory neurone to the central nervous system (CNS), then along a motor neurone to an effector, which produces the response.
A reflex is a rapid, automatic, involuntary response to a stimulus. Because it follows a fixed pathway, it protects the body without the delay of conscious thought. The pupil reflex is an example: the receptors are in the retina, the effectors are muscles in the iris.
Section 2
How the pupil dilates and contracts
The iris controls the size of the pupil, the opening through which light enters the eye. It contains two antagonistic sets of muscle: circular muscles (ring-shaped) and radial muscles (running outwards like spokes).
- Bright light: the circular muscles contract and the radial muscles relax. The pupil constricts, so less light reaches the retina. This protects the retina from damage.
- Dim light: the radial muscles contract and the circular muscles relax. The pupil dilates, so more light enters and can be detected.
The reflex pathway is: retina (receptors), sensory neurones in the optic nerve, brain, motor neurones, iris muscles (effectors).
Do not say the muscles 'open' or 'close' the pupil. Say which muscle contracts and which relaxes.
Circular muscles contract = pupil gets smaller. Radial muscles contract = pupil gets larger.
Section 3
Rod cells and rhodopsin
Rod cells are photoreceptors in the retina of mammals. They are very sensitive to light, so they work in dim light.
Their light-sensitive pigment is rhodopsin, made of a protein, opsin, bound to a light-absorbing molecule, retinal (made from vitamin A). When rhodopsin absorbs light, retinal changes shape and rhodopsin splits into retinal and opsin. This is called bleaching. In the dark, rhodopsin is resynthesised from retinal and opsin, which takes time.
Section 4
A rod cell in the dark and in the light
In the dark: cation channels (sodium ion channels) in the rod membrane are open, so sodium ions flow in. The membrane is depolarised, and the rod continually releases neurotransmitter, which inhibits the neighbouring bipolar neurone. No action potentials pass to the brain.
In the light: rhodopsin splits into retinal and opsin. This leads to the closing of the cation channels, so sodium ions stop entering. The inside of the rod becomes more negative: it is hyperpolarised. A hyperpolarised rod releases less neurotransmitter.
Light does not depolarise a rod. It hyperpolarises it. This is the opposite of most receptors.
Section 5
From the rod to the optic neurone
Because the hyperpolarised rod releases less neurotransmitter, the bipolar neurone is no longer inhibited. It depolarises and stimulates the optic neurone. If the depolarisation reaches threshold, action potentials are generated in the optic neurone and travel along the optic nerve to the brain.
Summary: light, rhodopsin splits, cation channels close, rod hyperpolarised, less neurotransmitter, bipolar neurone no longer inhibited, action potentials in the optic neurone.
Put the steps in order and name each ion or cell. Most marks are for the sequence.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The eye, rods and pupil responses
- An ophthalmologist shines a bright light into one eye of each of two patients and watches the pupil. In the first patient the pupil constricts quickly. In the second patient, the motor neurones that supply the iris muscles have been damaged by a head injury, and the pupil does not constrict.Explain why the pupil of the second patient does not constrict.2 marks
- A driver leaves a brightly lit petrol station and pulls onto an unlit country road at night. For the first few minutes she cannot see the road edge clearly, but her vision gradually improves.Explain why the driver cannot see clearly at first on the dark road.2 marks
- Vitamin A is needed to make retinal. A man whose diet is deficient in vitamin A has difficulty seeing in dim light, a condition called night blindness. His rod cells contain less rhodopsin than normal.Explain why the man's lack of vitamin A makes it difficult to detect dim light.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).