Let me demonstrate with an example – When a doctor taps on the knee tendon, the leg juts up in the air almost  instantaneously. 

The information from tapping of knee tendon (STIMULUS/SENSATION) is sent up the spinal cord by one NEURON – termed as SENSORY or AFFERENT NEURON. This information is transmitted electrochemically across the SYNAPSE located in the SPINAL CORD to the next NEURON in the CIRCUIT termed as the MOTOR or EFFERENT NEURON. The EFFERENT NEURON triggers the thigh muscle to contract resulting in the jutting up of leg – THE RESPONSE/ACTION