When a stimulus is presented, there are a number of processes that takes effect before a response is initiated. Take a humble bumblebee buzzing around. According to all known laws of aviation, there is no way a bee should be able to fly. Its wings are too small to get its fat little body off the ground. However, for this example, we’ll assume that the bee is in fact, flying around and being a general annoyance to someone like Barry.
Firstly, Barry’s sensory neurons (in this case, a photoreceptor) sees the bee. Barry’s photoreceptor is a part of the Peripheral Nervous System. It sees the bee and sends nerve impulses (electrical signals) which reach the brain, through interneurons. These interneurons are part of Barry’s Central Nervous System, which consists of the brain and the spinal cord. This area coordinates all of Barry’s activities and what Barry does with this information. The brain has many different types of neurons, too much for Barry to possibly comprehend!
Secondly, the sensory neurons connect to the motor neurons via interneurons. They transfer information between the sensory neurons and the motor neurons. Finally, the nerve impulse, carried over many neurotransmitters, synapses, and neurons, is brought to the motor neuron, through the spinal cord (which has a lot of nerve cells).
The motor neuron controls our muscles and organs. For Barry, it’s the motor neurons that tell his arm to swat the bee away. There are upper motor neurons that travel from the brain to the spinal cord, and the lower motor neurons which travel from the spinal cord to the muscles.