Your Mechanoreceptors

By probing your hand with a toothpick, try to locate the most sensitive regions. You will probably find that different parts of your hand are sensitive to larger or smaller signals or have different adaptation rates. These experiments work best if you close your eyes. If you trust your neighbor, even better results can be obtained through cooperation.

After a few minutes of experimentation, I'll ask the class to discuss about a few observations - such as the location for the most sensitive parts of the hand, the fastest and slowest adapting receptors, and any other interesting features.

What is your observation?

The different adaptation rates are also distributed throughout the hand in various ways. Where would you expect the slow and fast adapting receptors to be?

Based on your experiments, where are they?

Another interesting feature of these mechanoreceptors is the spatial resolution. Break your toothpick in half, and simultaneously touch two places on your hand. With your eyes closed, you should be able to tell that there is a minimum resolvable separation between the contact points. See if you can see how this minimum resolvable separation is different in different places. Interesting places to test include the tip of your tongue, the lips, the forehead, and the back of your neck.

Do you notice any variation in the resolution for these locations?

Figure 1. Cat's sensory system carries the information in the form
of a constant magnitude pulse train. The magnitude of the pulse is
independent of the size of the stimulus. Pulse density varies with
the stimulus magnitude.

The output signal of mechanoreceptors is generally in the form of a stream of voltage pulses. The amplitude of the signal being transmitted is represented as a pulse density. In this regard, the nervous system is essentially transmitting digital signals, rather than analog signals. The pulses are analogous to TTL pulses from a voltage-controlled oscillator in an electrical circuit. There are many advantages to transmission of digital data rather than analog data. Many of these same advantages are present in telecommunications, and modern systems are beginning to use digital telecommunications for most applications.

One interesting thing to keep in mind are the limitations of human mechanoperception. Given the finite density of these cells, and the location of some of them far below the skin surface, it is interesting that we can derive so much information. If designing a replacement system (virtual reality, anyone?) it is important to remember that only detectable sensations are worth simulating. This probably means that Virtual Reality gloves do not need to accurately reproduce the entire stimulus - only those which your hand can detect. At the same time, it is interesting to sort out how you are able to derive so much information from such a limited sensing capability.