In this lesson, we will
The nervous system is a complex network of nervous tissue that carries electrical messages. It includes the brain, spinal cord, and many nerves that run throughout the body.
What happens when you touch something hot?
If you touch something that is very hot, your hand moves away quickly. Have you ever wondered why that happens?
If you touch a hot surface, the nerves in your skin shoot a message of pain to your brain. The brain then sends a message back telling the muscles in your hand to pull away.
Try this experiment.
Dim the lights in a room. After a few minutes, look at the eyes of another person and note the size of the pupil (the black center spot in the middle of the eye). Turn the room lights back on. Check the size of the pupils again. The pupils should now be smaller. This is the pupillary response: it "automatically" keeps out excessive light that may damage the eye.
Think about this:
You touch a hot dish and jerk back your hand.
When dust gets into your eyes, you tear and your eyelids flutter automatically.
You smell French fries and your mouth waters.
The doctor taps your knee and your foot kicks out.
These kinds of responses are called reflexes. Reflexes are important since they protect us and help us to stay alive. Our body organs are controlled mostly by reflexes.
The survival of organisms depends on their ability to sense and respond to stimuli in their environment. Sense organs of the body take in information from an organism’s surroundings and send them to the brain.
The brain controls what you think and feel, how you learn and remember, and the way you move and talk. But it also controls things you're less aware of — like the beating of your heart and the digestion of your food.
Think of the brain as a central computer that controls all the body's functions. The rest of the nervous system is like a network that relays messages back and forth from the brain to different parts of the body. It does this via the spinal cord, which runs from the brain down through the back. It contains threadlike nerves that branch out to every organ and body part.
The network of cells that make up the nervous system are called nerve cells or neurons. There are several hundred billion nerve cells in the human body. The brain itself contains over 100 billion nerve cells. There are actually other cells that surround neurons in the brain called glial cells. They greatly outnumber neurons and are thought to support neurons.
When a message comes into the brain from anywhere in the body, the brain tells the body how to react. For example, if you touch a hot stove, the nerves in your skin shoot a message of pain to your brain. The brain then sends a message back telling the muscles in your hand to pull away. Luckily, this neurological relay race happens in an instant.
When you are riding a bicycle and about o fall, your nervous system senses that you are losing your balance. It responds by sending messages to your muscles. Some muscles contract while others relax. As a result, you gain your balance again. How did your nervous system accomplish all of this in just a split second? You need to know how the nervous system transmits messages to answer that question.
Nerve cells transmit messages via electrochemical signals. Ions like sodium, potassium, and chloride are important in the changes that occur in the electrical potential of the cell membrane as the impulse moves along the neuron. The difference in concentration of charged ions inside and outside of the nerve cell creates a voltage across the cell membrane. This is kind of like the voltage in a chemical battery! Once the impulse has moved from the dendrites to axon to axon terminals, it will pass to another nerve cell, muscle, or gland. There is always a tiny space, however, between a neuron and the neuron, muscle, or gland that it ‘communicates’ with. This space is called the synapse. Information is sent across the synapse by chemicals called neurotransmitters. These chemicals reach the neuron, muscle or gland to send the message.
The major divisions of the nervous system are the central nervous system and the peripheral nervous system.
The brain is composed of four major parts: the cerebrum, the diencephalon, the cerebellum, and the brain stem.
The spinal cord relays information between the brain and the rest of the body. It is protected by bony vertebrae. It also controls reflexes below the head, like pulling your hand away when you touch a hot stove.
The peripheral nervous system is divided into the somatic component and the autonomic component.
The neurons that make up the somatic component are under voluntary control. For example, the neurons associated with muscles in your arm that move when you think about raising your hand would be associated with the somatic component of the peripheral nervous system.
The neurons that make up the autonomic component are those associated that work automatically without you having to think about them working, like breathing, digestion, sweating, and shivering. The autonomic nervous system has two parts: the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system prepares the body for sudden stress, like if you witness a robbery. When something frightening happens, the sympathetic nervous system makes the heart beat faster so that it sends blood quickly to the different body parts that might need it. It also causes the adrenal glands at the top of the kidneys to release adrenaline, a hormone that helps give extra power to the muscles for a quick getaway. This process is known as the body's "fight or flight" response.
The parasympathetic nervous system does the opposite: It prepares the body for rest. It also helps the digestive tract move along so our bodies can efficiently take in nutrients from the food we eat.
