nerve anatomy

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Anatomy of a Nerve

Your entire nervous system is made up of billions of nerve cells called neurons. Neurons come in all shapes and sizes; some are minute, some are as long as three feet. Neurons form systems of "one-way streets." Messages can travel only one direction along these one-way streets. They can't back up or turn around. When a neuron is activated, we say it is firing. Its receiving antennae (dendrites) receive an electrical signal at one end, send the signal along the length of the neuron (axon), and, with a tiny release of chemicals, transmit the signal (via the terminal buttons) across a gap called a synapse to the next neuron. The process is repeated until the message reaches its destination.

Neurons are specialized by function. There are three kinds of neurons. Sensory neurons gather information, from inside and outside the body. (Inside the body, they monitor such things as warmth, pain, sugar or water levels; outside the body they receive information from the five senses—sight, hearing, smell, taste, and touch.) Interneurons pass the signals along, routing them toward their appropriate destinations. They are not connected to muscles or organs, but instead communicate only with other neurons. They serve as connectors between the sensory neurons and the third kind of neuron—the motor neuron. Motor neurons carry return messages from the brain and spinal cord (the central nervous system) to the muscles, glands, and organs. Some sensory messages do not require the brain's attention. When you are walking, for instance, you do not have to remind yourself to move your legs. This motor activity is handled at a lower level in the central nervous system, by the spinal cord. If you should stumble, however, the new message would be switched to the brain for instructions.

There are times when it is neither necessary nor convenient for information to be sent to the brain for decisions. In such cases the spinal cord "short circuits" the incoming messages and handles the problem itself. Reflex actions happen this way. You experience a reflex action when the doctor taps your knee with a rubber mallet. The knee "jerks" before you have had time to think about it. If, for some reason your head was missing, the knee would still jerk.

Other motor actions are also carried out by reflex action. In an emergency incoming information might reach the brain too late to protect you. Then the interneurons in your spinal cord switch the signals from the sensory to the motor nerves for immediate action. This is what happens if you put your hand on a hot stove. Your hand jerks away and the action registers in your brain an instant later.

So, where are we in the fishhook problem? The message traveled to the brain via sensory neurons. Following instructions from the brain, interneurons passed the "ouch" message from the sensory neurons to the motor neurons. The motor neurons rushed an order to the muscles in your arm and hand to disengage your finger from the fishhook. How long did that take? Nerve signals or impulses have been estimated to travel at approximately 250 miles an hour. That means your brain can receive information from the tip of your toe, process it, and send a return signal to your toe in a few hundred milliseconds.