The Hydra is a simple, yet elegantly designed organism. It has no brain, but the neurons that form its nerve net allow it to peacefully propel itself through the water by expanding and contracting its gastrovascular cavity. Thanks to its nerve net, the Hydra can also respond to stimuli; however, this response is limited to the same, reflexive motor response regardless of the direction of the stimulus.
Unlike the Hydra, humans have more sophisticated, varying responses to stimuli. This is possible because humans have a central nervous system, which consists of the brain and the spinal cord, and a peripheral nervous system. The brain can be thought of as the “master control” of the human body. It receives sensory input from our environment, processes this information, and broadcasts instructions to the rest of our body accordingly .
For example, when the brain senses that there is a threat (such as the deafening roar of a lion), it activates the sympathetic nervous system and produces what is referred to as the “fight-or-flight response.” This response alters our physiology in ways such as increasing our heart rate, slowing our digestion, etc. However, after our initial reaction, our reevaluation of the event can alter our initial behavioral response.
For example, if you end up figuring out the “lion” was actually one of your friends playing a prank on you, then your heart rate will slow and your digestion will gradually resume to its normal rate. However, in the scenario that there is actually a lion, your body will still exhibit symptoms of the “fight-or-flight” response because your brain still perceives that there is a threat.
Without our brain, we would not be able to perceive and appropriately respond to the complex, ever-changing cues in our environment.