lesson
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Touch a scorching hot frying pan, and your hand jerks away before you even realize you burned yourself.
This lightning-fast, automatic reaction is an involuntary reflex, carried out by a dedicated pathway called a reflex arc.
How does your nervous system trigger this physical movement without waiting for your conscious brain to decide?
The Seven-Stage Pathway
A stimulus is any detectable change in your environment, such as intense heat or a sharp prick, detected by specialized cells called receptors.
The impulse travels from a sensory neurone to a relay neurone in the spinal cord, then through a motor neurone to an effector (like a muscle), triggering a response.
The sensory neurone carries the impulse to the spinal cord, the relay neurone transfers it across the central nervous system, and the motor neurone signals the effector to carry out the physical response.
The sequence of a spinal reflex arc follows a precise seven-stage pathway: Stimulus→Receptor→Sensory Neurone→Relay Neurone (Spinal Cord)→Motor Neurone→Effector→Response.
📊Interactive diagram
So what actually happens at the exact point where these different nerve cells connect inside your spine?
Inside the Spinal Cord
The sensory neuron carries the electrical impulse into the spinal cord, where it passes the signal across a microscopic gap called a synapse to a short relay neuron.
In 1906, neurophysiologist Charles Sherrington first showed that these chemical junctions allow the central nervous system to coordinate fast, unidirectional reflex circuits.