Compare accommodation chains and predict a supplied tension constraint
Use all four links without inventing a medical diagnosis.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall receptors, electrical impulses, CNS organisation and the distinction between a stimulus and an effector response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which variable is constrained in the stated model?
What you will learn
- Compare complete near and distant states
- Predict within a stipulated mechanical model
Compare accommodation chains and predict a supplied tension constraint
| Feature | Meaning |
|---|---|
| Near | Contract → slack → rounder → stronger |
| Distant | Relax → taut → flatter → weaker |
| Constraint | Taut ligaments despite allowed contraction |
| Inference | Normal near shape transition not established in this model |
Compare complete near and distant states
Comparing accommodation requires a complete chain for each distance. Near: ciliary muscles contract, suspensory ligament tension decreases, the lens becomes rounder and thicker, and refraction becomes stronger. Distant: ciliary muscles relax, ligaments become taut, the lens becomes thinner and flatter, and refraction is weaker than in the near state.
The shape difference follows from tension on an elastic lens. A diagram showing only a thick lens does not establish whether the ciliary muscles or ligaments have been identified correctly. Include their states to explain why the lens can change, and avoid saying that the lens contracts itself or that the ligaments are the actively contracting muscle.
Predict within a stipulated mechanical model
In a stipulated paper model, suppose the ligaments are held taut even after a near-object signal and ciliary contraction. The usual reduced-tension step is prevented, so the normal slack-ligament rounder-lens transition is not established by that model. Keep the stated muscle contraction rather than deleting it: the specified constraint is on the tension step.
This prediction concerns a supplied mechanical constraint, not a learner’s vision. The scenario gives no individual examination, measurements or treatment evidence. It also does not guarantee every alternative focusing mechanism is absent. Explain the first constrained link and its consequence for the taught lens chain, rather than extrapolating to a diagnosis or claiming that pupil dilation solves the missing lens adjustment.
Sources for this lesson
Pause and explain
Which conclusion stays within this paper model?
Worked example
Predict the near-chain consequence of ligaments stipulated to remain taut.
Show the worked solution
- Ciliary contraction is allowed, but tension cannot decrease.
- The normal slack-ligament rounder-lens transition is not established.
Answer The stipulated taut constraint prevents the normal reduced-tension and rounder-lens near transition.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write both complete accommodation chains.
Give me a hint
Keep all four links in each.
Compare my reasoning
- Near: contract, slack, rounder, stronger.
- Distant: relax, taut, flatter, weaker.
Near: ciliary contract, ligaments slack, lens rounder/thicker, stronger refraction. Distant: ciliary relax, ligaments taut, lens flatter/thinner, weaker refraction.
Look for these in your work
- Include every near link
- Include every distant link
Ciliary contraction occurs but ligaments are stipulated to stay taut. Identify the first constrained link and the normal transition affected.
Give me a hint
Keep the given contraction.
Compare my reasoning
- Ligament tension cannot decrease as usual.
- The normal rounder-lens near transition is not established.
The reduced ligament-tension step is prevented despite the given ciliary contraction; the usual slack-ligament rounder-lens near transition is not established in this model.
Look for these in your work
- Keep earlier contraction allowed
- Locate the constrained tension step
Does the taut-ligament paper scenario diagnose a student or prove pupil widening will restore near accommodation?
Give me a hint
Match the conclusion to the supplied evidence.
Compare my reasoning
- There is no personal examination evidence.
- Pupil diameter control is distinct from the lens tension mechanism.
No. The scenario supports only the stipulated mechanical prediction; it neither diagnoses a student nor establishes that pupil widening restores the normal lens-shape transition.
Look for these in your work
- Limit the conclusion
- Separate pupil and lens mechanisms
Common mistakes
- Erasing the given muscle contraction.
- Using pupil widening as the missing lens-shape adjustment.
What can you explain now?
Which conclusion stays within this paper model?
Compare with the explanation
The normal reduced-tension rounder-lens near transition is not established
It follows the first constrained step.
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Make it stick
Compare every focusing link and predict the first affected step under a supplied constraint.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.