What is the first event in a reflex action?

The first event in a reflex action is the detection of a stimulus by a sensory receptor. A stimulus is any change in the environment that can cause a response. Examples of stimuli include touch, light, sound, heat, or pain.

When a sensory receptor detects a stimulus, it generates an electrical impulse that travels through a sensory neuron towards the spinal cord. The sensory neurons are specialized cells that can convert different types of stimuli into electrical impulses, which can then be transmitted to the nervous system.

For example, if you accidentally touch a hot stove, the sensory receptors in your skin detect the heat and generate an electrical impulse that travels through a sensory neuron towards the spinal cord. The sensory neuron is responsible for transmitting this impulse to the spinal cord, where the information is processed and an appropriate response is generated.