So what do the special senses all share in common? They all begin with cilia, the little hair-like projections that extend off the surface of cells. When it comes to the sense of smell and taste, the cilia bind molecules and an electrical change in the membrane is produced. When it comes to the sense of hearing and balance, the cilia are bent and an electrical change in the membrane is produced. When it comes to the sense of sight, photopigments arrayed within the cilia absorb light and an electrical change in the membrane is produced. Below is a figure of the three different cilia (shaded in green) tailored to detect changes in three different forms of energy:
Figure from Tomer Avidor-Reiss, Andreia M. Maer, Edmund Koundakjian, Andrey Polyanovsky, Thomas Keil, Shankar Subramaniam and Charles S. Zuker. 2004. Defining Specialized Genes Required for Compartmentalized Cilia Biogenesis Cell (04) 117:527-539.
We know these cilia are essential to our senses.
For example, there are a whole set of genetic diseases known as ciliopathies:
Ciliopathies are mainly of genetic origin and reflect a major cause for loss of smell and taste. Correction of defects in these signaling pathways will prove of value to correct the sensory defects present in these patients. Ciliopathies are a major cause of loss of taste and smell which has not been well defined. Cilia are the anatomical portion of both taste buds and olfactory receptor cells which provide the initial step (the “business end”) of both taste and smell perception.
Abnormalities in cilia function are mainly genetic in character and relate to abnormalities in each of four major sensory functions — vision, hearing, smell and taste.
In one sense, it is not surprising that cilia represent the business end of sensation. In essence, they are cytoskeletal extensions from the cell, as if life is reaching out to probe and sample its environment. The extensions are wrapped with membrane, and membrane serves as the very interface between the environment within the cell and the environment outside the cell. The actual detectors are membrane proteins that convert different forms of energy into electrical current. You can actually think of them as translators, translating different environmental states around you (vibration, light, concentration of various molecules, etc.) into the universal language of your nervous system – electrical impulses.
When you consider the manner in which our five special senses define our reality for us, it is humbling to realize it all begins with little antennae-like structures that emerge from our cells. In fact, since science itself is built on the foundation of observation, we can accurately say that science itself is built on the backs of cilia. Cilia facilitated their own detection.
And while all of this is fun to think about, can we take it further with help from the hypothesis of front-loading evolution? Need you ask?