Lesson 7 of 10 / Homeostasis and response
Nervous coordination and reflexes
How can a hand withdraw before a conscious decision?
I can trace a reflex response through receptors, sensory, relay and motor neurones to an effector.
I can describe brain, spinal cord and nerves in coordination.Syllabus
Syllabus K325, Topic 8(j). Describe brain, spinal cord and nerves in coordination.
I can trace receptors, neurones and effectors in reflexes.Syllabus
Syllabus K325, Topic 8(k). Trace receptors, neurones and effectors in reflexes.
About 6 min
Make a guess. You are not marked.
You touch a hot pan and pull your hand away before you feel pain. Where was the response coordinated?
Show the answer
In the spinal cord, through a relay neurone.
A reflex arc goes receptor, sensory neurone, relay neurone, motor neurone, muscle. The brain is told afterwards.
The key idea
A reflex is a rapid, automatic response. A reflex arc links a receptor to an effector through sensory, relay and motor neurones.
Choose a stage to trace the signal. This is a diagram exercise.
A skin receptor detects the stimulus. It is the detector, not the effector.
Green: selected stage. Purple background: relay in the spinal cord. Arrows show information flow across neurones and synapses; the brain also receives information.
Explanation
The brain and spinal cord form the central nervous system. Nerves connect them with receptors and effectors around the body. A neurone is a nerve cell; a nerve contains bundles of nerve fibres. This system coordinates and regulates body functions using nerve impulses.
In a withdrawal reflex, a receptor detects a potentially damaging stimulus and an impulse travels along a sensory neurone towards the spinal cord. At synapses, information passes to a relay neurone and then to a motor neurone. The motor neurone carries impulses to a skeletal muscle.
The muscle is the effector. Its contraction moves the hand away from the stimulus. The spinal cord coordinates this withdrawal without waiting for a conscious decision. Information also travels to the brain, allowing awareness of the event.
Not every reflex is coordinated in the spinal cord. A pupil reflex uses pathways through the brain. In each case, trace the appropriate route: a receptor detects the stimulus, neurones carry information, and an effector produces the response. Effectors can be muscles or glands.
Exam diagrams usually show the reflex arc on a cross-section of the spinal cord, with the back (dorsal side) at the top. The butterfly-shaped grey matter in the middle contains relay neurones and the cell bodies of motor neurones. The white matter around it contains nerve fibres that carry impulses up to the brain and down from it. The sensory neurone enters through the dorsal root; its cell bodies sit in a swelling on that root called the dorsal root ganglion. The relay neurone lies wholly inside the grey matter. The motor neurone leaves through the ventral root. Draw arrows on the neurones to show the impulse travelling one way: receptor to sensory neurone to relay neurone to motor neurone to effector.
Step by step
- 1
Start with a receptor
Name what detects the stimulus; do not make the muscle detect it.
- 2
Trace the neurones in order
Sensory, relay in the relevant CNS region, then motor.
- 3
Finish with the response
Name the effector and its action, such as a muscle contracting to withdraw the hand.
Worked example
A pathway interruption
In a diagram, the motor neurone to a muscle is interrupted. Sensory input still reaches the spinal cord. Why does that muscle not produce the withdrawal?
One way to explain it
Impulses cannot pass along the interrupted motor neurone to the muscle. The effector therefore does not receive the signal needed for this contraction, even though sensory input reaches the CNS.
Why this answer works
- Locate the interruption after the relay.
- Do not claim the receptor has stopped detecting the stimulus.
- Link missing motor output to the missing response.
Watch out for this
A student says: "Every reflex bypasses the brain entirely." What is wrong with this?
Show the answer
Withdrawal can be coordinated by the spinal cord while the brain also receives information. Other reflexes, including the pupil reflex, are coordinated through the brain.
Check your understanding
Which route correctly describes a simple withdrawal reflex?
Show the answer
Receptor -> sensory neurone -> relay neurone in spinal cord -> motor neurone -> muscle
The muscle is the effector, and its contraction produces the withdrawal response.
Quick recall
How can a hand withdraw before a conscious decision?