Main->Readings->5th Grade Readings->Control Systems ->Part 2
Vocabulary |
The brain, locked away from the outside world like a prisoner inside the skull, can't work all by itself. It needs to send and receive signals from the rest of the body. The spinal cord is the main path for many of these signals, but the spinal cord too is hidden away from the outside world, protected by the vertebral column. To get and send signals, the brain has to depend on the many messenger nerves which reach through every part of the body, from the tip of your finger to the end of your toe.
There are two main types of messenger nerve cell. One type is for gathering information ( sensory nerve cells), and the other is for sending signals to the muscles ( motor nerve cells). When the brain gets signals from the eyes, ears, nose, skin, tongue, and so on, those signals are coming in from the sensory nerve cells. When the brain sends a signal to do something, for instance snap your fingers or lift your foot, it is sending those signals to the motor nerve cells. Though these actions happen very quickly, it does take time for signals to reach the brain and for the brain to send signals back, and this time is called the reaction time. Reaction time is very fast, but it is possible to measure it.
Some animals (the jellyfish for instance) get by without a real brain. Even complicated animals like insects have a very small brain which is difficult to see. They can use their senses, move, and eat without having to think about it.
Even humans can do some things without having to think at all. If we had to think about breathing, or making our hearts beat, or moving food through the digestive system, we'd be too busy to do anything else. We call these actions involuntary. Many involuntary actions are controlled by the brain, even though they do not involve conscious thought. However, there are some things humans do which do not involve the brain at all.
These actions are called reflexes. A reflex is a fast, automatic action which does not involve the brain. Reflexes act to protect the body from harm, because they are very fast, even faster than conscious actions.
The knee-jerk test in the doctor's office (known as the "patellar" reflex) is an example of a reflex. When you are tapped on the tendon just below the kneecap, your lower leg jerks out. It happens quickly and without conscious thought, so it may surprise you when it happens, even though you're doing it yourself. It feels as though it just "happens" to you.
The tapping signal triggered sensory nerve cells in your muscle, and the sensory nerves sent a message to the spinal cord. But the message didn't go up the spinal cord to the brain. Instead, the spinal cord sent a message to the motor neurons in the muscle, and just like that the muscle contracted and kicked your leg for you.
There are many other reflexes in the human body, and we do not always know why they are there. For instance, we know why a human baby has a sucking reflex. It allows it to feed itself and stay alive. But why does a human baby have a reflex that causes it to grab your finger if you place it in the center of the palm? How does that help keep a baby alive?
Go on to Lesson 3: The Endocrine System
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This page last updated August 15, 2002