"Nervous System"

       Your nervous system has two main parts the central nervous system ( CNS ) and theperipheral nervous system (PNS ). The central nervous system is made up of the spinal cord and the brain.
The peripheral nervous system is made of neurons and other things that put together the central nervous system with all different parts of the body.
       As the girl pictured below plays volleyball the PNS gets ands sends information to her spinal cord and her brain about what she feels and sees. Then the CNS processes the information that comes from the girls body parts and gives back information about what to do next. The PNS gets the information and makes the players body move right.
       The peripheral nervous system also puts together internal organs, like the stomach and the heart, to the central nervous system. You need to remember that the nervous system controls what goes on in the inside of your body, as well as how you react with the earth.

        You might be playing a trumpet. However, at the same time you might also be smelling popcorn, tapping your foot, and thinking about a soccer game. All of the actions are controlled by the body's " boss ", which of course is the brain. The brain is made of about 100 billion neurons and trillions of other supporting cells. Although the brain makes up only two percent of your body's weight, it uses around 20 percent of your body's energy.

        Think about all of the things you are sensing right now. While your eyes are seeing these words can you hear the sound of your own breathing or papers rustling? How do your clothes feel against your body? What tastes or smells are you aware of? All of these sensations are possible because of sensory receptorswithin your peripheral nervous system. Sensory receptors are cells that gather information from inside your body. Millions of sensory receptors in your eyes, muscles, tendons, skin, and internal organs are sensitive to different conditions, such as pressure changes, temperature, light, pain, vibrations, chemicals, and larger body movements.
        Information gathered by sensory receptors is picked up by sensory neurons - nerve cells of the peripheral nervous system that carry information from sensory receptors to the spinal cord and brain. The neurons do not actually move. They pass information by sending it from one sensory neuron to another one until it gets to the central nervous system.
        Once the brain is aware of what's out there, what happens next? The brain processes the signals from the sensory neurons. It figures out what needs to be done, and sends it back a message telling the body what action to do. These messages go through cells calledmotor neurons. Motor neurons carry messages from the spinal cord and brain to skeletal muscles and to organs, like the stomach and heart. Motor neurons tell the body to make many different kinds of movements. For example, they can cause a heart to pump blood faster or an eye to blink.
        Think of a girl eating a hamburger. What kind of information do you think are being carried by her sensory neurons? What messages could be carried by her motor neurons.

         When information like light, odor, touch, taste, or sound they form a sense organ. Messages from each one of these organs travel to a particular part of your brain that can process those messages.
        You've probably looked in a mirror a lot and seen your eyes. What you see is only a little part of these sense organs. As you can see in the picture the retina is a layer of light receptors at the back of the eye. The retina has four kinds of receptors. Three of them are cones. Every kind of cone is very sensitive to one of the three primary colors of light - green, red, or blue.
        Rods are the forth kind of light receptor. Rods are very sensitive to light and dark, movement , and shape. They can work in much lower levels of light than cones can. " Rod vision " occurs when light is dim, like in the evening, but with little color.
        recall that sound travels as waves. Sensory receptors in your ears are sensitive to these sound waves. Sound waves in the air go in the outer ear and make the eardrum vibrate. Three little bones in the middle ear pass the vibrations along to the inner ear, where they enter a snail - shaped structure. the structure has hair cells of different  lengths. As the hairs vibrate, the hair cells send a
message by way of the brain. The section of the brain receives the sound.
        Also helping a girl hear, her inner ear helps her to balance. It has fluid - filled tubes with hair cells that sense the position of your head. The cells send messages to your brain, which then controls your body movements to keep you balanced.

        Believe it or not, tiny hair cells in your nose are the sensory receptors for smell. Scientists think that when molecules from your favorite food drift through the atmosphere and into your nose, the hair cells respond to either the electric charge or the shape of the molecules. When the brain gets signals from the nose, it identifies the smell so that you are familiar with it.
        The sense of taste is closely linked with the sense of smell. TO try it yourself, try holding your nose while you taste an orange. Chances are that the orange will taste sour, but it will not have the flavor of the orange. There are only four main tastes that the taste buds on your tongue are sensitive to - sour, bitter, sweet, and salty. Find the places of each of these taste areas in the picture of the tongue below. Your brain blends the sour taste of the orange with its odor, and you can tell the flavor of the fruit.
       what is tough, stretchy, washable, waterproof, keeps your insides in, and covers your whole body? Your skin - the largest organ in your body. skin has different types of sensory receptors that are sensitive to pressure changes, temperature changes, vibration, and pain.
        Some skin receptors send their messages very slowly, like ones that react to something that always goes on - your clothes touching your body, for example. Other skin receptors send messages quickly, like when you put your finger on a hot surface. These messages let you to react immediately pulling your finger away before it is badly burned.

    Most neurons, or nerve cells, are skinnier than the period at the end of this sentence, yet they are the longest cells in the body. Some neurons are more than a meter long! Neurons are like other cells, but they have parts that let them to get and send messages.
        A cell body of a neuron has the nucleus. Molecules that the neuron needs to function and live are made in the cell body. Dendrites, like twigs and branches on a tree, extend out from other neurons. How does the structure of the dendrites help them gather information?
        As the dendrites help them gather information, they form messages called nerveimpulses. The nerve impulses go from the dendrites through the cell body, and along the axon.

        How does your brain instruct your heart to beat or your legs to move? How does information about something you hear, feel, or see travel through your body? All of  these things happen because of the movement of nerve impulses through the nervous system. Each dendrite of a neuron gets messages from another neuron. When a lot of information is received, a small electrical charge which is a nerve impulse, which goes through the neuron. It carries messages from the dendrite, to the cell body ,and then through the axon to the other end of the neuron.
        Between a dendrite of one neuron and the axon of another neuron, there is a gap called a synapse. When a nerve impulse gets of the end of an axon, chemical messengers  let out from the axon into the synapse. The chemicals cross the synapse and attach themselves to a dendrite on the next neuron. When enough of the chemicals become attached, the dendrite forms a nerve impulse. The nerve impulse then goes through the cell body and along the axon to the end of the neuron. The process continues as the nerve impulse passes from neuron to neuron.
      When nerve impulses pass through sensory receptors, the impulses go to the the brain and spinal cord where they're processed and interpreted. When nerve impulses go through motor neurons, they finally reach organs and muscles, causing them to function and move properly.
        Some diseases can hurt the nervous system. For example, Lou Gehrig's disease - named for a baseball player who had the disease - causes motor neurons to die and shrink. As a result, the muscles in the body get weaker through not enough use.
        multiple sclerosis is another disease that causes muscles to get weak by damaging neurons. Multiple sclerosis kills a protective cover that is around the axons of neurons.
        Parkinson's disease damages brain cells that produce a chemical needed to control movement. People that have this disease have trouble controlling their movements or tremble.
        Researchers are searching to find cures for these diseases, as well as finding out more on how the nervous system works. For example, in recent years, scientists have found out that the more you use your brain, the more dendrites its neurons develop. More dendrites mean that the neurons can gather more information which can help the brain take action and make decisions.

        Suppose you're a baseball player and you're up at bat when a pitched ball comes to close to your head. Before you even had a chance to think about it you jump to the side to keep yourself safe. By the time your brain figures out what you see, the rest of your nervous system reacts. Any unplanned, quick response that happens without your brain thinking about it is a reflex. Reflexes are simple responses that can help you adjust to your surroundings and protect your body from danger. Some common reflexes are blinking, sneezing, and coughing. When you hear an unexpected, loud sound, you probably jerk your body and look toward the sound. What ways could this reflex help you.
        To understand how reflexes work think of what happens when you prick your finger on a cactus. First, sensory receptors in your finger sense pain. Nerve impulses from the receptors go through sensory neurons to the spinal cord. One or more neurons in the spinal cord transfer the nerve impulses right to he motor neurons, which carry the impulses to your arm. The arm muscles move, pulling your finger away from the cactus very quickly - without you having to think about it.
        Meanwhile, the spinal cord transfers nerve impulses from sensory neurons to your brain. When the impulses arrive there, the brain interprets what just happened and gives you the message: " OUCH! Pain. " Luckily for you, your finger was already out of danger.
        Compare how nerve impulses act in responses that involve a reflex - for example, when you scratch an itch - nerve impulses go along sensory neurons, through the spinal cord, and then to the brain where the information is processed before any action is taken. In a reflex response, nerve impulses set the action in motion right away, without waiting for the brain to make a decision of what to do.