The spindle fibers and other receptors involved in proprioception, the scientific term for sensory information that contributes to one's sense of positioning and movement, are among the slowest to adapt. Scientists often use the term "tonic receptors" to refer to receptors that adapt slowly.
Adaptation of central nervous system receptors occurs when a receptor's sensitivity level to external stimuli changes. An example is the phenomenon of dark adaptation, in which a person's visual system adapts to lower levels of light in the environment. For this to occur, both the color receptors, called the cones, and the night-vision receptors, called the rods, experience an increase in their sensitivity to light. This occurs because the amount of light-sensitive chemicals increases in dark conditions; the result is that the person's eyes are less able to detect light. Cones are able to adapt to dark conditions within five to 10 minutes, while it takes rods 20 to 30 minutes to adapt, demonstrating that different types of receptors adapt at different speeds.
Adaptation also occurs with the senses of smell and touch. For example, people possess the ability to perceive a very small amount of perfume, yet they often stop noticing the smell after a while despite the perfume still being present. Similarly, the water in a pool may initially feel very cold to a swimmer, but he begins to feel more comfortable as his touch receptors adapt to the temperature.