3.5.3Additional lessons
Reflex Arc and Movement Control — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 67 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Pathway of a Reflex Arc
The reflex arc is the shortest route from stimulus to response.
1
A stimulus, such as heat on a fingertip, is detected by a receptor in the skin.
2
The receptor generates an impulse in a sensory neurone, which travels through the dorsal root into the spinal cord.
3
In the grey matter of the spinal cord the sensory neurone synapses with a relay neurone.
4
The relay neurone passes the signal to a motor neurone, whose axon leaves the cord through the ventral root.
5
The motor neurone stimulates an effector, such as a muscle in the arm, which contracts to pull the hand away.
6
The brain is informed afterwards, so you feel pain only after the withdrawal has already begun.
Part 2 of 4Features of Reflexes
Rapid
- Rapid Few synapses are involved and the brain is bypassed, so the response is quicker than a conscious decision.
Involuntary
- Involuntary The response occurs without conscious thought and is automatic in the same way every time.
Protective
- Protective Reflexes such as withdrawal and blinking reduce damage from harmful stimuli.
Override
- Override The brain can sometimes override a reflex, for example by holding a hot dish rather than dropping it.
Part 3 of 4Control of Voluntary Movement
Control of Voluntary Movement
Motor cortex
- Motor cortex This part of the cerebral hemispheres sends impulses down the spinal cord to start voluntary movements.
Cerebellum
- Cerebellum It compares planned and actual movement and fine-tunes timing and balance, so actions are smooth.
Skeletal muscles
- Skeletal muscles Motor neurones stimulate muscle fibres to contract, and muscles can only pull, not push.
Antagonistic pairs
- Antagonistic pairs One muscle contracts while the other relaxes, for example the biceps and triceps bend and straighten the elbow.
| Feature | Reflex action | Voluntary action |
|---|---|---|
| Starts in | Receptor | Brain (motor cortex) |
| Conscious thought | No | Yes |
| Speed | Very fast | Slower |
| Example | Withdrawal from heat | Picking up a pen |
Part 4 of 4Exam Checklist & Tips
3.5.3 – Key Points for the Exam
- Reflex arc The neural pathway from receptor to effector that produces an automatic response.
- Relay neurone A neurone inside the CNS linking sensory and motor neurones.
- Antagonistic muscles A pair of muscles that act in opposite directions across a joint.
- Motor cortex The part of the brain that initiates voluntary movement.
Common Mistakes
- Saying reflexes go through the brain before the response; the response is decided in the spinal cord.
- Forgetting the relay neurone and drawing the sensory neurone connected straight to the motor neurone.
- Stating that both antagonistic muscles contract together to produce movement.
- Writing that the cerebellum starts voluntary movement; it coordinates and refines it.
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