Ask students the following questions: What does your dentist use to numb your mouth when you have a cavity filled? What is the benefit of these preparations? What are some potential problems with them? Why do teeth need so many nerves and blood vessels? How does anesthesia work? Why do we need to feel pain anyway? 
 

Pain is an important safety feature of the human body because without it, no one would have any warning of injury. Nerves transmit pain messages by a combination of chemistry and electricity. When a nerve receives a pain stimulus over a certain intensity, it "fires" by changing the arrangement of positive and negative charges across its cell membrane. When the message reaches the end of the nerve cell, chemicals known as neurotransmitters spill out into a space (synapse) and stimulate the receiving areas (receptors) on the next nerve cell. The signal passes from nerve to nerve very quickly until it reaches the brain, where the message registers as pain (and you say "Ouch!").

Because nerves send messages by a combination of chemistry and electricity, interference in either area can relieve pain. Traditionally, dentists have used a shot of lidocaine (a substitute for Novocainô) to numb the tooth so the patient can't feel the drill. This is a little alarming in itself when the cavity is in a lower jaw tooth; the only available nerve to numb is way in the back of the mouth, so the needle is several inches long. The numbing effects of lidocaine last a while, too, which can be embarrassing. Ever try to talk when your mouth is numb?

One nonchemical procedure, which dates back to the 18th century, avoids needles altogether by stimulating the tooth's nerve with electricity to numb it. Although it can't be used for everything, this electronic anesthesia has proven useful for some simple dental procedures. There are generally two electrodes, placed inside or outside the mouth (sometimes one in and one out). The patient controls the degree of stimulation by turning a knob on a small switch box. When the stimulation is turned off, the numbness goes away immediately.

Anesthesia works in a number of ways. Some anesthetic drugs block certain receptors. Others inhibit biochemicals that increase the nerve's likelihood of firing. Scientists disagree on how electrical stimulation works, although some think it somehow results in the release of natural painkilling substances in the brain called "endorphins." 
 

1. Are you a little nervous about going to the dentist? Do you think nervousness makes pain worse? How can you calm yourself?

 2. Some anesthetics have side effects. How should these drugs be regulated?
 
 
 
 

 Because pain and touch receptors on nerves are closely related, an area of skin more sensitive to touch will often be more sensitive to pain. You can investigate some phenomena that cause changes to the sense of touch; whatever numbs the sense of touch may also be a potential anesthetic. 

A classic experiment on the sense of touch is called the "two-point discrimination test," which means noticing whether one or two objects are pressed against your skin. More sensitive areas of skin will be able to tell one from two, even when the two are very close together. Less sensitive areas will require that the objects be widely spaced. A numbed area should become less sensitive.

Materials

 
• several toothpicks
• blindfolds (one for each person ? be careful not to share blindfolds)
• rulers
• tape
• ice or ice water
• a source of fairly strong vibration (personal massager works well) 
• pen and paper for keeping records

1. Tape two toothpicks to a ruler (in the same direction as the markings). Measure the distance between them.

 2. Touch either a single toothpick or the two side-by-side toothpicks to a blindfolded subject's inside forearm. Record whether the subject can distinguish between the two.

 3. Touch with toothpicks that are different distances apart until you can conclude the minimum distance for distinguishing one toothpick from two. 

4. Try to alter the sense of touch by applying ice or vibration to the same skin area. What do you notice? What is the shortest distance between toothpicks that can be distinguished? Does the ice make the skin more or less sensitive? Does the vibration make the skin more or less sensitive?

 5. Design a similar test for the first finger. Is the first finger more or less sensitive than the forearm? Does the ice or vibration have more or less of an effect on the finger than on the forearm? Repeat the test with loud music or static sounds. Does this alter the sense of touch in any way?
 
 

Questions

1. Why do you think the ice or vibration works to numb your sense of touch? 

2. Why are some areas of your body more sensitive to pain than others?