The Two Parts of the Nervous System
The nervous system is made up of the brain, the spinal cord and all the nerves that come run between your brain and spinal cord. Even though they all together, anatomists break down the nervous system down into two parts – the central nervous system and the peripheral nervous system.
The central nervous system (CNS) is the brain and the spinal cord – which runs down your back inside the bony vertebral column. The CNS receives the information, decides what to do with it and gives the orders.
The peripheral nervous system (PNS) is all the nerves outside the cerebral nervous system. That includes the nerves coming off the spinal cord and brain – the spinal nerves and cranial nerves.
The Peripheral Nervous System
The peripheral nervous system links the rest of your body to the spinal cord and brain. It does this with two jobs.
1) The first is bringing information in – the sensory input. The nerves deliver the sensory input to the central nervous system (the spinal cord and brain). When walk outside and see the sunlight that information is coming in through nerves to your brain. The same with the smell of flowers and the sound of birds singing – smell and sound are picked up by nerves and brought to your brain, which makes sense of them for you.
The peripheral nervous system brings in messages from all over your body – from your skin, muscles, joints and even your internal organs. If you eat too much food, you know it because you feel full. You know this because the peripheral nerves are giving that message to your brain and your brain says – no more French fries!
If you injure a peripheral nerve in your arm or leg, you may get some paralysis from that spot to the end of for fingers or toes for a while until the nerve grows back. Nerves grow very, very slowly though, so it may take a while. Scientists estimate that it can take as long as a month for every inch below the injury for nerve axons to grow back before you are all better.
2) The second job of the peripheral nervous system is to carry signals out – the motor output. If the brain wants to tell the muscles to move, it sends the signal out through the peripheral nerves to those muscles and you move it. If you have even had to chase the soccer ball down the field, it is the nerves in your peripheral nervous system that get the message from your brain and tell your legs to start running.
You don’t control all of the motor output of your peripheral nervous system. Some happens through the involuntary nervous system. Wave to a friend. You just controlled both voluntary muscles and voluntary nervous system motor signals. But if you have ever had a friend grab you from behind or seen an angry dog rush up to a fence to bark at you, you might gasp and jump without thinking about making that action happen. This is your involuntary nervous system at work. It controls all your everyday actions like heartbeat and digestion, but it also can give you a jolt of action in an emergency. This is called the fight or flight response and can save your life.
Sometimes messages coming into the nervous system need a very quick response. Think about how fast you move when you touch a hot stove. That message never even had time to reach your brain. It just arched around through the spinal cord to give a quicker response. This is called a spinal reflex. This is an important response to emergency moment when your body needs to respond quicker than the brain can act.