Lesson 7 of 10 / Homeostasis and response
Nervous coordination and reflexes
How can a hand withdraw before a conscious decision?
In this lesson: Trace a reflex response through receptors, sensory, relay and motor neurones to an effector.
About 6 min
The key ideaA 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.
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.
Is this true? "Every reflex bypasses the brain entirely."
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.