Please take a moment and notice how your fingertips sense the tabletops. Notice the different mechanisms for sensing location, roughness, table edge, table temperature. And, try to pay attention to how you use your fingers to detect certain signals, such as roughness or temperature.
In class, we usually pass around a selection of different sandpapers with coarseness ranging from very fine to very coarse. The students are asked to handle the sandpaper as they normally would, and then to see how well they can distinguish the different pieces with their eyes closed. For example, if the sandpaper is squeezed between the tips of thumb and finger and held still, is it possible to tell rough from smooth? If so, what characteristics of the sandpaper are really being detected - roughness or stiffness or thickness. Also, pay attention to the kinds of finger motions that are used to distinguish these samples, and think about the characteristics of the sensors in the fingers that are being taken advantage of.
One important characteristic of the sensors in your skin is referred to as the Rate of Adaptation. Most human mechanoreceptor cells respond to a change in the external stimulus (pressure, temperature, etc) by producing voltage pulses across neurons. Immediately after the change in external stimulus, these pulses begin to appear. Over some time, the pulse rate declines and eventually returns to the original passive level. The rate of adaptation is the rate at which the mechanoreceptor pulse rate returns to normal after a change in stimulus. Simply put, sensors with adaptation do not provide information about static signals - only about changing signals. To use such a sensor to sense a static quantity, like roughness, it is necessary to make the roughness produce a time-varying contact force on the tactile sensors in the fingers.
The mechanoreceptors in your skin may be separated into distinct categories:
Pacinian Corpuscles are rapidly adapting mechanoreceptors in your skin and are often the most sensitive cells to very small changes in the stimulus, such as the tactile force. These rapidly adapting cells return to a normal rate of pulses in less than 0.1 second. These delicate mechanoreceptors are generally found in the subcutaneous layer of the skin, where they are protected from the abuses which may occur at the surface. These receptors are used in human perception to detect surface roughness as the fingertips are dragged across a surface, or very small vibrations in machines. Because of their location far below the surface and the role of the skin in transmission of signals, it is not necessary or useful to have a high areal density of these receptors. The skin acts to distribute the applied forces over relatively large areas (maybe 10x the thickness of the skin), so spacing closer than tenths of a millimeter would not add any additional sensitivity.
Meissner's Corpuscles and hair follicle receptors are good examples of mechanoreceptors with moderate adaptation rates. These receptors can be located near the surface of the skin, and adapt to changes on time periods of order 1 second. Some experiments with the hair on your arm should confirm these adaptation rates. Think about the sorts of things such sensors (located around hair follicles) would be useful for in an outdoor setting. If you've been camping recently, you might recall that these sensors are the ones you use most effectively to detect insects on your skin (mosquitoes, ticks, flies, etc). Since these insects are a threat to human survival at some level, evolving a capability to detect and remove such insects would be of obvious value.
Ruffini Endings, Merkel's Cells, and Tactile Disks are examples of slow adapting mechanoreceptors. These receptors are generally located near the surface of the skin, and are responsible for much of the static perceptive capabilities. For example, the sensitivity to temperature at the skin is generally of a slow-adapting type, as are many tactile sensors useful for maintaining grip on an object. The adaptation time scale for these cells can be from 10 to more than 100 seconds. Experiment with grasping of an object in the air, like a pencil or a cup of coffee. Close your eyes and think about how it is that you overcome the adaptation in these sensors to avoid dropping objects.
In fact, it is interesting to give some thought to the whole process of grasping objects in the air. You have all developed a set of skills for holding drinks in your hand with a minimum of effort. Think about how often you mistakenly crush the coffee cup in your hand, or about how often the cup slips completely through your fingers. Aside from falling asleep, these events are extremely rare. However, the task of holding a cup of liquid is an extremely complicated one. Think about all the forces that must be balanced and maintained, and remember that the sensors used in this task have very odd temporal response, and that ALL of them eventually stop sending information about the forces on the fingertips if those forces are constant. Nevertheless, all of you are able to accomplish this task without much direct feedback control being applied - in fact it might be completely unconscious!