International A Level Biology / The nervous system and neurones
The spinal reflex arc
Edexcel International A Level Biology (YBI11), chapter 23: The nervous system and neurones, specification point 8.3. Free lesson.
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A spinal reflex arc is a short, automatic pathway that produces a rapid response to a stimulus. Its speed helps protect the body, and the spinal cord coordinates the response before the brain is informed.
How the spinal cord is arranged
In a cross-section, the grey matter is inside and has a butterfly-like shape. It contains cell bodies, dendrites and synapses. The white matter surrounds it and contains myelinated axons carrying impulses to and from the brain. The central canal is a narrow, fluid-filled channel. The dorsal root is at the back of the cord. Sensory neurones enter through it. The dorsal root ganglion is a swelling on this root. A ganglion is a cluster of cell bodies, and this one contains sensory neurone cell bodies. The ventral root is at the front. Motor neurones leave through it.
How an impulse travels through the reflex arc
When your hand touches a hot pan, a receptor in the skin detects the heat and starts impulses in a sensory neurone. The pathway then carries the impulse through the spinal cord to a muscle, producing a response.
Why a spinal reflex is fast
A spinal reflex is coordinated by a relay neurone in the spinal cord. Compared with a voluntary action, it needs fewer synapses, a shorter pathway and no conscious decision. Each synapse causes a short delay, so having fewer helps make the response quicker. A spinal reflex has only two synapses in the cord. A voluntary action has many more. The reflex pathway goes into the spinal cord and straight out. A voluntary pathway travels to the brain and back. A reflex is automatic and innate, meaning you are born with it. Voluntary actions need a conscious decision and are often learned.
How the reflex protects you
The rapid response moves your hand away before much damage is done. The spinal cord coordinates the withdrawal; the brain does not decide to move the hand. Separate neurones in the white matter carry impulses up to the brain, so you feel the pain after the movement. For a three-mark explanation of speed, state that there are fewer synapses, the pathway is shorter because the spinal cord coordinates the response, and no conscious decision is needed.