REFLEX ACTION:
A reflex, or reflex action, is an involuntary and nearly movement in response to a stimulus. A reflex is made possible by nervous pathways called reflex arcs . which can act on an impulse before that impulse reaches the brain.
e.g When light acts as a stimulus, the pupil of the eye changes in size. Coughing or sneezing, because of irritants in the nasal passages. Knees jerk in response to a blow or someone stamping the leg.
REFLEX ARC:
The reflex arc is the nerve pathway involved in a reflex action, including at its simplest a sensory nerve and a motor nerve with a synapse between them.
The reflex arc contains
1) a Receptor or sensory organ,
2) a Sensory neuron,
3) a Reflex centre(brain or spinal cord),
4) a Motor neurone and
5) an Effector(muscle or gland).