3.6.3Inquiry Process
Reflex Arc and Movement Control — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 143 of 160
Part 1 of 5Pathway of a Reflex Arc
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A receptor, for example a pain receptor in the skin, detects a harmful stimulus and generates impulses.
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A sensory neurone carries the impulses into the spinal cord through the dorsal root.
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A relay neurone in the spinal cord passes the signal to a motor neurone.
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The motor neurone leaves via the ventral root and carries impulses to an effector, a muscle.
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The muscle contracts and the limb is withdrawn, and impulses also travel up to the brain so the person feels the pain afterwards.
Part 2 of 5Reflex versus Voluntary Action
Reflex
- A rapid, automatic response that does not need conscious thought, which protects the body from damage.
Voluntary action
- A response decided consciously in the cerebral cortex, such as choosing to pick up an object.
Speed
- Reflexes are quicker because the pathway is short and the brain is not involved before the response occurs.
Survival value
- A fast withdrawal from heat or sharp objects reduces tissue damage, even if it is less flexible than a conscious choice.
Learned responses
- Some skilled actions,
- such as typing, begin consciously but later run with little thought,
- which shows the border is not always sharp.
| Feature | Reflex | Voluntary |
|---|---|---|
| Brain involvement | Not needed for action | Cortex decides |
| Speed | Fast | Slower |
| Learning | Inborn | Can be learned |
Part 3 of 5Control of Movement
Control of Movement
Motor cortex
- Sends signals down the spinal cord to motor neurones that cause voluntary muscles to contract.
Cerebellum
- Adjusts movements by comparing the planned movement with feedback from muscles and joints, giving smooth, balanced action.
Antagonistic muscles
- Muscles work in pairs, so when one contracts the other relaxes, for example biceps and triceps at the elbow.
Inhibition
- Relay neurones in the spinal cord can inhibit the antagonist, allowing the limb to move efficiently.
Part 4 of 5Worked Example
Question
- In a withdrawal reflex, impulses travel 0.60 m through myelinated neurones at 60 m/s, and two synapses add 0.5 ms each.
- Calculate the total time before the muscle starts responding.
- Conduction time = 0.60 ÷ 60 = 0.010 s = 10 ms.
- Synaptic delay = 2 × 0.5 = 1.0 ms.
- Total = 10 + 1.0 = 11 ms.
Answer About 11 ms, far faster than a decision made in the cortex.
Part 5 of 5Exam Checklist & Tips
3.6.3 – Key Points for the Exam
- Reflex arc The pathway from receptor to effector that produces a reflex.
- Relay neurone A neurone in the CNS that connects sensory and motor neurones.
- Antagonistic muscles Pair of muscles with opposite effects on a joint.
- Cerebellum Brain region that coordinates movement and balance.
Common Mistakes
- Saying the brain makes the decision in a reflex; the spinal cord does.
- Forgetting that impulses also reach the brain, which is why pain is felt.
- Believing both muscles of an antagonistic pair contract together.
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