Glossary
reflex arc
A reflex arc is the pathway a reflex travels — receptor, sensory neuron, a synapse in the CNS, motor neuron and effector — without waiting for the brain.
A reflex arc is the circuit a reflex follows, and the point of it is speed. It begins at a receptor that detects a change, such as stretch in a muscle or pain in the skin. A sensory (afferent) neuron carries that signal to the central nervous system, where it synapses directly or through an interneuron onto a motor (efferent) neuron, and the motor neuron carries the command out to an effector — a muscle or a gland — which responds. The whole loop can be completed in the spinal cord without the brain being involved, which is why you pull your hand from a hot surface before you feel the pain. The brain is still informed, through other branches of the same pathway, and it can influence a reflex, but it is not required for it. In the stretch reflex the sensory neuron synapses straight onto the motor neuron, a two-neuron arc that keeps posture steady without conscious control.
Taught in
Related terms
- neuron — A neuron is a nerve cell that receives, carries and passes on information as an electrical impulse, and its working parts are the dendrites, the cell body and the axon.
- synapse — A synapse is the junction where a neuron passes its signal to another cell, across a narrow gap, by releasing a neurotransmitter.
- myelin sheath — The myelin sheath is fatty insulation wrapped around some axons that speeds the impulse by letting it jump between the nodes of Ranvier.
- effector — An effector is what carries out a feedback loop's response — a muscle that contracts or a gland that secretes.
- somatic nervous system — The somatic nervous system is the motor division that carries commands to skeletal muscle, the muscle a person moves on purpose.
Source: The anatomy and physiology guide in this course, whose organ and tissue descriptions follow the US National Cancer Institute's SEER Training Modules and MedlinePlus consumer health pages. Both are US federal government material, not subject to copyright in the United States. The definition above is written for this course in our own words. (SEER Training Modules accessed 2026-10-26; definition written 2026-10-27)