Wednesday, September 14, 2011

Why does your telephone have two holes? – Perspective on our Senses

We all know that we have five senses: seeing, hearing, tasting, touching, and smelling. Anyone disagree? Wanna put some money on it?

 


The five senses. Are these all there are? Notice how
four of the five are located on your face.

Our senses are the ways that we receive information about the world. Everything we know, feel, and interact with comes to us in  just a few select ways. And the ways we send out information are even more limited.


Would you consider all your senses of equal importance - which one would you hate most to lose? Neurophysiologically, humans are sight (visual sense) dominant. Thirty to forty percent of our cerebral cortex is devoted to vision, as compared to 8 percent for touch or just 3 percent for hearing (auditory sense).

Even though touch claims only 8% of our brain’s real estate, the tactile sense is really humans’ second dominant sense. Why do you think babies stick everything in their mouths? Our lips and tongue are the most sensitive areas for touch; the little ankle biters are just gathering information in the best way they know how at that point in their development.


Helen Keller was the first blind/deaf person in
America to graduate from college.
One of the most famous examples of how our senses affect our lives, but don’t have to control our lives, is Helen Keller. At the age of 19 months, Helen lost her sight and hearing as a result of an infection (probably rheumatic fever or meningitis).

Helen lost her most dominant sense (sight), but retained and made good use of her other dominant sense (touch). She even learned to speak by using touch – her fingers on her teacher's lips and throat help her to mimic the movements and vibrations. In general, Helen lost her ability to transduce (change energy from one form to another) waves of energy. Light waves could not be detected or changed to electrochemical nerve impulses, and neither could sound waves. 

Even without her abilities to sense waves, Ms. Keller retained her ability to sense chemicals and change those molecule/receptor interactions into nerve impulses that could be interpreted as taste (gustatory sense) or smell (olfactory sense). And she still had her important sense of touch.


There are many components included in our sense
of touch: pressure, pain, hot, cold.


Our sense of touch is actually a system of different inputs. Some scientists don’t lump them together, and state that humans actually have 10 senses. We have sensors that detect pressure (touch), hot, cold (yes, in terms of receiving information, hot and cold are different), nociception (pain), kinesthetic awareness (stretch receptors in our muscles tell us where our limbs are in space), and a vestibular sense or balance (in our ears we have the semicircular canals that tells us about where our head is in space).

Finally, some scientists consider the coordination of our inputs (proprioception) to be an 11th  sense. Any physical task that would require visual inputs of your position, balance inputs from your semicircular canals, and kinesthetic inputs from your muscles in order to make the proper responses would use proprioception – for example, most circus acts….. or motherhood. Since proprioception is a coordination of senses and not a direct intake of information about the world, I will let you decide if you think it belongs in the same category as the others.  Do you still think we have only five ways to bring in information?


Telephones are designed this way
because we talk and hear in two
different places.
Now let’s consider the opposite activity- sending out information. I offer no answer, but I like pondering the reasons why evolution developed some systems just for inputs and different systems for outputs. Take the telephone question in the title of this post. We take in and interpret sound waves through our ears, but we make sound waves with the muscles of our throat and diaphragm. Imagine what your smart phone might look like if your ears and mouth weren’t located so close to one another!

Not only are our input and output systems different, but the ways we transmit information are even more limited than the few ways we extract information. The principal way we send out information is by our muscular movements. Our muscles move us into and out of other people’s visual field, and our body’s posture, action, and expressions can also transmit information visually.

Muscles move our larynx to control the frequency of the sound waves that are generated by our diaphragm muscle pushing air out of our lungs. Our muscles also control our physical interaction with others; they can feel the pressure when me move to touch them. Sometimes our muscles generate enough pressure to cause pain.

Maybe we could provide more information if we were a cannibal’s meal (“She might not have had good taste, but she sure tasted good!” - Don Johnson in A Boy and His Dog). Bill Cosby always said he wouldn’t eat tongue because he didn’t want to taste anything that might taste him back.


The vomeronoasal organ, if adults have one,
is located forward of the olfactory bulb.
Finally, we might also communicate by the pheromones we produce. These are chemicals sensed by the vomeronasal organ (VNO), a part of the smell sense not associated with the olfactory bulb (see cartoon). Pheromones certainly affect social and physiologic behavior in lower animals; scent trails laid down by ants help the next ant find food or home. Hunters take advantage of pheromones to attract male deer or elk, as spiders use them to attract male moths to their web. But pheromone function in humans is more controversial. 


There are many companies that are more than willing to sell you pheromone concoctions aimed at increasing the physical attraction between a guy and a gal, but the latest research is equivocal at best.  Adult humans may retain a small VNO (up to 70% of adults show a VNO organ) and a gene for pheromone reception has been found to be expressed in the VNO.  Infants may sense and discriminate their mothers from other adults using pheromones.

Related to possible pheromone receptors in the VNO, recent research has shown that the human nose actually contains solitary chemosensory cells, with their receptors and signal pathways. This means that you have taste cells in your nose for umami, sour and bitter tastes! The purpsoe for these is not to taste the compounds that excite the receptors, but to signal that irritants are present. The signal pathways then trigger the trigeminal reflexes to get rid of noxious irritants - sneezing, watering eyes, runny nose, etc. Bitter receptors cells are most plentiful in the VNO, while the others are spread out evenly throughout the nasal mucosa.


Let’s review.
1. We have considerably more than five senses, but the actually number is a matter of some dispute. We settled on 10 senses for this post, but some scientists go all the way up to 17; and this doesn’t include seeing dead people or having common sense!
2. Our systems for inputs from the world don’t overlap with our outputs of information to the world. Our retinas and visual cortex don’t give off light, our smell receptors don’t produce odors, and our touch sensors don’t push on other people.

However, there is one of our senses that actually is a two-way street – you knew the exception was coming, didn’t you?  I won’t give it away, but we will HEAR about this exception in the next post.


Braun T, Mack B, Kramer MF (2011). Solitary chemosensory cells in the respiratory and vomeronasal epithelium of the human nose: a pilot study. Rhinology, 49 (5), 507-512



For more information, classroom activities or laboratories about the senses, proprioception, or pheromones:

Senses –

more than five senses –

proprioception –

pheromones –