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The Nervous System
Introduction to the Nervous System

The Nervous System In Detail

Runninig, biking, swimming, playing sports, dreaming, flying, jumping, singing, moving, surfing, jogging, reading, eating, drinking, fighting, all these things wouldn't be possible with out yor nervous system. Your nervous system is made up of the brain, spinal cord, nerves, and sense organs such as your eyes and ears. Your nervous system carries information from your body to the brain and back.

Any thing that makes an organism react is called a stimulus. When an organism reacts to a stimulus it is called a response. There are two kinds of responses; a voluntary response and an involuntary response. Voluntary responces are when you control the response such as swatting a fly. Your heart beat however is involuntary.

The Neuron

In your nervous system there are many cells to help it do what it is designed to do. The cells that carry informtion through your nervous system are nuerons. The messages that a neuron carries are called nerve impulses. Neurons are shaped so that they can carry these nerve impulses. Nuerons have a large body containing the nucleus, thread-like extensions called dendrites, and an axon.

The dendrites carry impulses towards the neuron's cell body while the axon carries impulses away. Nerve impulses begin in the dendrite and go from there to the cell body, and on to the axon. A nueron has only one axon but can have many dendrites. There can, however, be more than one tip on the axon. Another name for the axon and dendrites are nerve fibers. Nerve fibers are aranged in parallel bundles and are covered with connective tissue. A bundle of nerve fibers is called a nerve.

There are three kinds of neurons in your body. these neurons are sensory neurons, interneurons, and motor neutrons. Sensory neutrons pick up stimuli from the environment and converts it into a nerve impulse. The nerve impulse travels along the sensory neuron until it reaches an interneuron. Interneurons carry nerve impulses from one neuron to another neutron. Sometimes interneutrons pass impulses from sensory neurons to motor neutrons. A motor neutron sends an impulse to a muscle, and then the muscle reacts.

How a Nerve Impulse Travels

Each nerve impulse begins in the dendrites of a neuron. It moves toward the cell body, and on to the axon tip. the nerve impulse travels along with the neuron in the form of electrical and chemical signals. When the impulse reaches the end of the neuron, it can pass to another neuron or it can travel to a muscle or organ. This point is called a synapse. When a nerve impulse passes at a synapse the dendrite and axon tip do not touch. The axon tips release chemicals that carry the impulse across.

Divisions of the Nervous System

Your nervous system is made up of two divisions, the central nervous system and the peripheral nervous system. the central nervous system is made up of the brain and spinal cord, while the peripheral nervous system is made up of everything else.

The Central Nervous System

The main parts of the central nervous system is the brain and spinal cord. The brain, which is located in your head, controls most of the functions of the body. The spinal cord is a very thick column of nervous tissue that links your brain to a lot of the nerves in the peripheral nervous system.

Your brain contains about 100 billion interneurons. these neurons can recieve messages and give messages to about 1,000 more. There are three main regions in the brain that process information; the cerebrum, cerebellum, and the brain stem. the cerebrum is the largest part of the brain. Different parts of the cerebrum control different movements and thoughts. The cerebrum is divided into two halves. The right half is associated with creativity and artistic ability. the left half is associated with logical thinking, such as mathematics. The cerebellum is responsible for for coordinating your muscles and helps keep your body. The impulses that tell your feet to walk start in the cerebellum. The brain stem, which lies between the cerebellum and the spinal column, controls your involuntary actions such as breathing and heart rate. The spinal cord is is merely the link between your brain and peripheral nervous system.

The Peripheral Nervous System

The peripheral nervous system contains the network of nerves that branch out from the spinal cord, and connect it to the rest of the body. There are 43 pairs of nerves in the peripheral nervous system. Twelve of the nerves pairs are in the brain and the other 31 ore in the spinal column. The nerves in the spinal column are called spinal nerves. One nerve in each pair goes to the right side, while the other nerve goes to the left side.

The nerves of the peripheral nervous system can be divided into two groups, the somatic and autonomatic. the somatic nerves control voluntary actions, and the autonomatic nerves control involuntary actions.

Sometimes your body moves with out you thinking about it. When this happens it is called a reflex. A reflex is an automatic response that occurs without conscious control. Usually your movements are are controlled by the brain. With reflexes your body feels that you are in danger and acts with out your brain. Sensory neurons travel to your spinal cord, and transfer the the impulse to interneurons. These interneurons take the impulse back to the point of origin and signal that body part to move. Without reflexes we could get seriously hurt.

A concussion is when your brain gets knocked into your skull. This produces a bruislike injury to your brain. Spinal cord injuries can occur when your spinal cord is cut or crushed. When this happens the axons are broken and impulses cannot pass through them. This usually results in paralysis.

The Senses

THe sense organs pick up information about the world around you, change the information into a nerve impulse, and send the impulse to your brain.


Your eyes enable you to see what is around you. Your eyes respond to the stimulus of light. They then convert that light into nerve impulses, which the brain can read in order for you to see. When light hits your eye it first strikes your cornea, which is a light layer of tissue. It then passes to your pupil. The pupil is the opening where light enters your eye. The iris is a circular structure that regulates how much light gets into your pupil. Light that passes through the pupil it hits the lens. the lens focuses light. Once the light goes through the lens it strikes the retina. The retina is a layer of cells that lines the back of the eye. It contains about 130 million receptor cells that respond to the light. These receptor cells are separated into two types: rods and cones. Rods allow you to see black white and gray. Cones enable you to see colors. When the light strikes the rods and cones nerve impulses travel to teh cerebrum.


Your ears are the sense organs that respond to sound. Sound is produced by vibrations. These vibrations can be produced by anything. Your outer ear is designed to catch sound vibrations in the ear. The vibrations travel down a canal called the ear canal. At the end of the ear canal is the eardrum. When sound vibrations hit the eardrum it begins to vibrate. These vibrations pass to three small bones called the hammer, anvil, and stirrup. The hammer passes the vibrations to the anvil, and the anvil to the stirrup. The stirrup vibrates against a membrane. The membranes channel the vibrations to the cochlea. The cochlea is a snail-shaped tube that is lined with cells that respond to sound. Finally these cells are stimulated and send the impulses to the cerebrum.

Smell and Taste

The sense of smell and taste aer closely related. Both depend on chemicals in food or in the air. The chemicals trigger receptors in the mout and nose. Then nerve impulses travel to the brain. The human nose can distinguish about 50 different smells, but can only taste five sensations. These are sweet, sour, salty, bitter, and a meatlike taste called umami. You experience a very wide variety of tases when you eat because you are influenced by taste and smell.


Unlike your other senses the sense of touch is found all over your skin. Your skin has many different touch receptors, some that respond to hard pressure and some to light. Others respond to temperature change and some to pain. The receptors the respond to light touch are in the top part of your skin. Receptors deeper in the skin pick up the feeling of pressure.


A drug is any chemical taken into the body that causes changes in a persons body or behavior. Deliberately misusing drugs is called drug abuse. Any drug can be misused. When drugs like alchohol, marijuana, or cocaine are misuused the effects can be devastating. Drugs like these are dangerous because they have imediate effects on your brain and other parts of your nervous systam. There are many diferent kinds of abused drugs. Depressants are drugs that slow down the activity of the centralnervous system. Depressants may cause relaxed muscles and sleepyness. Stimulants are like the opposite of depressants. They make the heart beat faster and the breathing rate increase. Inhalants produce mood altering effects. Some althletes take steroids to improve their perfomance. Steroids are chemicals that are similar to hormones. Alchohol is a drug that is found in many drinks. Alchohol is a very powerful depressant. The more alchohol you ingest the worse off you'll be. Alchohol absorbs into the organs and can cause som very bad effects.

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