The nervous system is a complex structure within our bodies that allows us to sense and perceive the world around us. At its most basic level, the nervous system consists of a large array of neurons spread throughout the body which work to send messages to our ultimate control center, the brain. Neurons (known commonly as nerves, nerve cells) relay signals from a part of the body experiencing an internal or external sensation or stimuli to the brain for processing and response. So, imagine you do not know if a stove is hot or not, so you place your palm on it to test the temperature. Pain signals from the nerves in your hands travel toward the brain to register this pain and you reflexively pull your hand away. This is the work of the nervous system.
There are two major subdivisions of the nervous system: the central nervous system (CNS) and the peripheral nervous system (PNS). The peripheral nervous system consists of neurons known as sensory neurons and motor neurons that connect the body to the central nervous system. Motor neurons send signals from the brain to the muscles and typically help with general movement. Sensory neurons pick up stimuli from sensory receptors throughout the body to the central nervous system. The central nervous system consists of the brain and the spinal cord, two of the vital components of signal transfer in the nervous system. This is where our reflexes are controlled. The spinal cord receives a signal from a sensory neuron and the signal splits, one signal being sent back to the original sensed area and the other being sent to the brain. I can demonstrate this with a personal example. During my first semester of college, I had a chemistry lab where we had to use dry ice (which is incredibly cold). I wore gloves to pick up the clump of dry ice, but I felt it burning into my palm, so I instinctively dropped it. So, the sensory neurons in my palm felt the burning stimuli, which sent a signal to my spinal cord. My spinal cord split that signal, sending one message back to my palm, prompting me to drop the dry ice, and another message to my brain, telling me that what I just touched was way too cold to lay flat in my palm.
From the peripheral nervous system, there are two more divisions. One is the autonomic nervous system and the other is the somatic nervous system. The somatic nervous system is the division that controls voluntary movements of skeletal muscles. This is what allows us to move when we sense that we must move, like if we are in danger. The autonomic nervous system is responsible for the movement of the body’s internal organs, such as the heart. Some examples of the autonomic nervous system in action is heart rate and the dilation of our pupils when exposed to bright light. Further dividing the autonomic nervous system is the sympathetic and parasympathetic nervous systems. The sympathetic nervous system has the task of arousing the body, inciting what we know as our “fight-or-flight” response. When we are threatened by something, our heart rate rises, and this is from the sympathetic nervous system. The parasympathetic nervous system does the exact opposite and calms our body. This will lower our heart rate or blood pressure after an incident in which the sympathetic nervous system has risen these attributes.
To illustrate these concepts, I will use another personal example. When I studied abroad in the Netherlands, I was walking with a friend when I could feel we were being followed. As I sensed him approaching us, my heart began to beat faster and I started sweating. This was the work of my peripheral nervous system; my autonomic and sympathetic nervous system activating my fight-or-flight response. Because of this, my legs started to move faster, due to a signal from my brain to my motor neurons to pick up the pace. This was my somatic system kicking in. Once my friend and I made it to our destination safely, I was able to calm down; I stopped sweating and my heart rate slowed. This was my parasympathetic nervous system at work, reversing the actions of the sympathetic nervous system. The nervous system controls just about every aspect of our body’s movements and actions in some way or another.