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Learn4BI1Paper 1B · Paper 1 (core content)Structure and functions in living organismsNervous system and the eye
Structure and functions in living organisms · Subtopic 8 of 21

Nervous System and the Eye

The central nervous system (brain and spinal cord) coordinates responses. Impulses travel: stimulus → receptor → sensory neurone → relay neurone (CNS) → motor neurone → effector → response. Reflexes are fast, automatic and protective. Neurones meet at synapses, where neurotransmitters diffuse across the gap. In the eye, the cornea and lens focus light on the retina (rods for dim light, cones for colour). The pupil reflex: in bright light, circular muscles of the iris contract, making the pupil smaller. Accommodation: for near objects, ciliary muscles contract, suspensory ligaments slacken, and the lens becomes fatter.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Describe the parts of the nervous system and types of neurone.
  • Describe a reflex arc and the role of synapses.
  • Describe the structure of the eye, the pupil reflex and accommodation.
Pause and think

You pull your hand away from a hot pan before you even feel the pain. How?

Section 2 of 14

Core idea

Start hereFaster than thought

Touch something hot and your hand moves away in a fraction of a second, before your brain even knows what happened. That's a reflex.

The big idea

Receptors detect stimuli; neurones carry impulses through the CNS to effectors. Reflexes are automatic shortcuts; the eye adjusts automatically to light and distance.

1. Reflex arc

The reflex arc

Through the spinal cord, not the conscious brain.

2. The eye

Worked exampleWalking into a dark room

Describe the pupil reflex on entering a dark room.

Goal
Muscles and effect.
Strategy
Dim light → dilate.
  1. radial muscles contract\text{radial muscles contract}
    Circular muscles relax.
  2. pupil dilates\text{pupil dilates}
    More light reaches the retina.
AnswerPupil widens to let more light in.
✓ CheckOpposite of the bright-light response ✓.
Key takeawayAccommodation Near: ciliary muscles contract, suspensory ligaments slacken, lens fatter. Far: ciliary muscles relax, ligaments tighten, lens thinner.
Pause and think

Why do synapses make impulses travel in one direction only?

Summary

  • Receptor → sensory → relay → motor → effector.
  • Synapses use neurotransmitters.
  • Pupil reflex and accommodation.
Key words
Reflex
A fast, automatic response to a stimulus.
Synapse
Junction between two neurones.
Accommodation
Changing lens shape to focus at different distances.
HistoryRamón y Cajal. Santiago Ramón y Cajal stained brain tissue in the 1880s and showed that the nervous system is made of separate cells with gaps between them, now called synapses.
Fun factFast messages. Impulses in some myelinated neurones travel at about 120 m/s, over 400 km/h.
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