Nervous System
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What happens in your nervous system when you react to stimuli? Did you know that the brain is sometimes uninvolved with reflexes? Find out the answer to these questions, and more, in this lesson on the human nervous system.
Central Nervous System
Not everything we do is decided by our brains. There are some automatic reactions that don't involve your brain. I know that seems weird, but think about how your leg involuntarily kicks when the doctor taps your knee with a rubber mallet. Your spinal cord is responsible for this knee-jerk reaction, not your brain.
Your spinal cord is made up nerves, which are in turn made up of bundles of neurons. You can think of all of these neurons as baseball players passing balls, which are chemical signals.
Sometimes sensory neurons carry messages to the brain. Other times, like when you accidentally touch a hot burner on the stove, they only get as far as the spinal cord before producing a natural reflex. Your spinal cord sends signals to your motor neurons without any input from your brain so you can pull your hand back before you get burned. No time is wasted 'thinking' about what to do. When every moment counts, the nerves in your spinal cord take control!
Don't get me wrong, your brain gets involved most of the time when you touch something. For example, when you feel a mosquito land on your arm, your sensory neurons send signals to interneurons in your brain. Then, your brain sends signals to your motor neurons to swat the mosquito.
Together, your brain and your spinal cord make up your central nervous system. At the core of your body, they are the center of the control system.
Peripheral Nervous System
Your sensory neurons and your motor neurons make up your peripheral nervous system. Your central nervous system is like the coach and your peripheral nervous system is like the team. The coach calls the plays, but the team is on the front line of action, and sometimes players have to react on their own.
Your peripheral nervous system is made up of sensory neurons (which take in information about what you see, hear, taste, touch and smell) and motor neurons (which send information to your muscles and glands so you can react). The sensory neurons are the ones that tell your brain there's a mosquito biting you, and then your brain tells your motor neurons to swat it with your hand.
Just as your central nervous system has both conscious and unconscious reflexes (remember the knee jerk?), your peripheral nervous system has some functions that you're aware of while they're happening, and others (like digesting food) that you do without thinking. The things you're conscious of involve your somatic nervous system. When you swat a mosquito, it's your somatic nervous system kicking in. It controls your external muscles and skin. But, other internal activities happen more or less automatically, like your mouth watering or sweating. While it is possible to regulate how fast your heart is beating or how fast you're breathing, these are primarily the realm of the autonomic nervous system; they happen whether you're paying attention to them or not. Autonomic responses are automatic.
Okay, so we can break down these automatic responses further, into sympathetic and parasympathetic responses. Your sympathetic nervous system gets you ready to fight or flee when you're faced with a crisis, and your parasympathetic nervous system calms you down and allows you to relax. So, when you hear the fire alarm go off in your building, your sympathetic nervous system gets your heart racing to ready you to act. But, when you realize it's a false alarm, your parasympathetic nervous system slows your heart back down and allows you to relax.
Overview
When it all works together, your nervous system allows you to react to any situation. Your central nervous system is the command center, and your peripheral nervous system is the front lines. The front lines have both automatic, or autonomic, internal reflexes like sweat and fear, and conscious reactions that involve your somatic nervous system in your muscles