Of all the organs in our bodies, the brain is the most complex and deeply tied to what makes us human. Billions of nerve cells (neurons) in the nervous system make trillions of connections that are the basis of our thoughts, feelings and actions. Our genes make sure our nervous system is generally organized in the same way as everyone else’s, and our experience helps us decide how to respond to the information our nervous system gives us.

The brain receives, processes and responds to information about various conditions inside and outside the body. Neurons convey information through electrical signals. Other specialized cells called glia protect and enhance the function of neurons.

Other neurons and specialized receptor cells in the spinal cord, peripheral nervous system and autonomic nervous system collect different kinds of sensory information (the senses of smell, sight, sound, touch and taste) and send it to the brain.

The peripheral nervous system is made up of more than a billion neurons that run throughout the body that connect the nerves in our brain and spinal cord to our organs, limbs and skin.

The autonomic nervous system manages the sensory information that keeps our heart beating, blood flowing and lungs filling. We’re not very aware of the autonomic nervous system most of the time, but it controls other essential life processes that are critically important for our survival. The brain also receives information about the position of our body and limbs in space, but we are not very aware of these messages—until our spatial awareness and gravity are challenged when we take a ride on a roller coaster!

Once the brain has processed the incoming information, it tells the nervous system how to respond. Sometimes the response is immediate—for example, when our doctor taps our knee with a rubber hammer during a physical. This is called a two-neuron reflex, also known as the knee jerk or patellar tendon reflex. A tap in just the right place stretches a sensory neuron in our muscle, which causes a motor neuron—a nerve cell that controls muscle movement—to activate and contract the muscle.

Other times, the brain may not act until after comparing the information it gets to the internal condition of our body and our memories of past experiences. Experience shapes the way our brains perceive, process and react, even to the same events. For example, if we’ve just finished a meal and see a favorite dessert, we might decide not to eat it because we are full. Or, we might eat it because we are still a little hungry or maybe we’ll eat it despite being full because we remember how delicious it tastes.

So, the next time you’re faced with a piece of cake or pie, see where your nervous system takes you.