When an environmental stimulus is received, the signal is usually transmitted through three types of nerve cells. in which order is the signal transmitted through these cells?

The correct answer is - sensory neurons → inter-neurons → motor neurons.  

Explanation:

The nerve system receives a signal from the external change or stimuli and transmitted it to the spinal cord and brain. The transmission of the signals or information is carried by three different types of nerve cells: sensory neurons, interneurons, and motor neurons.

Sensory neurons receive the signal from the external stimuli and converting it to the electrical impulses. Interneurons are the associative neurons that establish the connection between sensory neurons and motor neurons. Motor neurons are the efferent nerves that carry the signal from interneurons and brainstem or spinal cord and transmit to the muscles.

Thus, the correct answer is - sensory neurons → inter-neurons → motor neurons.