The traditional understanding that humans have five senses – sight, sound, smell, taste, and touch – is not entirely accurate. The sense of touch is actually a combination of several different senses working together.
The human body contains a network of sensory nerve cells with endings in the skin that detect various physical signals from the environment. Different sensations, such as a gentle touch, light pressure, or pain, are all detected by these sensory nerve cells.
In a recent study published in Science, researchers discovered a molecule called ELKIN1 in nerve cells that is specifically responsible for detecting gentle touch. This molecule converts the sensation of gentle touch into an electrical signal, which is the first step in the process of perceiving gentle touch.
When the skin is lightly brushed, tiny deformations occur, generating enough force to activate sensory molecules in specialized nerve endings. These molecular force sensors form a pore in the surface of the cell that opens when a force is applied, allowing an electrical current to flow. This current then travels along the sensory nerve to the spinal cord and up to the brain.
The researchers found that mice lacking the ELKIN1 molecule did not appear to sense gentle touch when a cotton bud was drawn across their paw. However, they retained their ability to sense other types of touch and environmental information.
This discovery highlights the existence of multiple specialized force-sensing proteins that enable us to distinguish different environmental signals and perceive different types of touch. ELKIN1 is the second touch-receptor molecule identified in sensory neurons, with the first being PIEZO2, discovered in 2010.
Studying these force-sensing molecules is challenging, as it requires isolating nerve cells and measuring electrical currents while applying controlled forces to the cells. While much of the research has been conducted on mouse neurons, efforts have been made to determine if ELKIN1 functions similarly in humans. Human stem cells were reprogrammed to produce specialized nerve cells that respond to touch stimuli, and it was found that ELKIN1 had similar functional properties in these human cells.
This research not only expands our understanding of how we perceive the world through touch but also raises the intriguing possibility that these molecular force sensors may give individual cells a nuanced sense of touch. Further research will continue to explore additional molecular force sensors and their role in helping cells and humans navigate their physical environment.