The *Six* Senses (and maybe more...)

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In the previous we discussed the "Five" senses:  Sight, sound, touch, smell, taste - and then added some more sensory functions to the list.  I argue that a *sixth* sense: "proprioception" should be added to the list, since it provides the essential knowledge about body condition and position to the brain - much more than just touch alone.

Then too, there are multiple subdivisions within the visual and auditory systems.  We could easily argue that both auditory and visual information is further divided into simple (pitch or color), complex (harmony or shapes) and movement-related sensation.  The supporting argument for such a division is in the different pathways each type of information takes into and through the brain.  The counter argument is that auditory and visual information arise from just the one respective type of sensory neuron (cochlear hair cell or retinal photoreceptor) and the information gets combined in the association cortex for each respective "sense".

Then again we list taste and smell as two separate senses, when in some considerations they are a single sense.  Both sensory neurons are chemoreceptors, and both require that the chemical being sensed is dissolved in liquid.  Like an extreme dry mouth blunts the sense of taste, nasal dryness also blunts the sense of smell.  The interior of the nose warms and humidifies the air we breathe not only to protect the sensitive airway, but also because it is the only way we can detect scents.  Try this some time - sniff your favorite spice dry from the container, then place a very small amount onto paper, sniff again; then put that same small amount into boiling water and sniff the steam!

Still, taste and smell are justified as separate senses by their brain pathways - smell is conducted from the vomeronasal organ in the nasal passages via the Olfactory Nerve to the Olfactory Bulb and then to the Pyriform Cortex and Limbic structures of the diencephalon.  Taste is conducted from the sensory neurons in the taste buds directly to the Thalamus by the Facial Nerve and Glossopharyngeal Nerve.  From the Thalamus, Taste is represented in the Somatosensory Cortex in the Parietal Lobe.  However, as discussed in the prior blog, with visual and auditory systems, divergent neural pathways have historically been no impediment to definition of a single sense.

Another curious fact about the senses of taste and smell comes from studies of persons who have lost their sense of smell - either through chemical means, disease or trauma.  Most folks know that sinus congestion impairs the sense of smell and taste.  However, in the absence of a sense of smell, other neurons can take over.  The Trigeminal Nerve which connects to sensory neurons in the skin of the face can sense strong chemical odor.  If you ever feel your cheeks burning from strong spice (Wasabi!) or peppers, you will know that your Trigeminal sensation is working well.

However, these chemical receptors cause a further problem for subdividing the sense of touch.  The tactile sense is commonly considered to be the primary sense of majority of the body.  But what of vibration, hot, cold or pain?  The sensory neurons in joints and muscles also respond to vibration, thus vibration is common classed with proprioception rather than touch.  The skin also contains many chemical sensitive neurons that react when the surrounding cells are damaged and release internal contents into the spaces between cells.  In  addition to specialized receptors for heat and cold, the chemoreceptors are responsible for the continuing "burning" sensation that follows *after* too much heat or cold has damaged the skin.  The majority of "pain" sensation is in reality the activation of chemoreceptors following damage to the cells of the body - and in some cases, the neurons themselves are damaged and continue to send signals along pain pathways even though the source of the damage is long gone.

This leads to two last issues with tactile or body sensations: 

The first is the mapping of sensation to regions of the skin.  The technical term is "dermatomes" and these are the areas of skin served by a common nerve, or region of the spinal cord.  Neurologists map dermatomes to localized nerve damage.  Loss of sensation on a particular part of the body can be traced to a specific spinal nerve, region of the spinal cord, and ultimately to regions of Thalamus and Somatosensory cortex.  Regions that require fine touch - fingertips, tongue, lips - have many very small dermatomes.  Regions of the body that do not require a very precise localization of touch - chest, abdomen, back - have very few, large dermatomes.  The "sensory homunculus" (right) emphasizes this different by the *size* of the represented area - indicating that more cortex (and hence more, smaller dermatomes) are occupied by certain body regions. 

One particular dermatome that I have problems with stretches almost from the hip to the knee along the outside of my right leg.  This large area is served by one nerve - the Lateral Subcutaneous Femoral Nerve - and because I damaged it many years ago, this entire region of my leg is plagued by numbness and occasional burning pain.

Which brings up the final point:  Phantom pain.  Just because a limb or dermatome is missing or has damage to the nerve does not mean that the brain just manages to ignore the connections where sensory information for that region should appear (see the "sensory homunculus", above).  Damage to nerves is often felt as a severe burning pain.  Even when the nerve is dead and gone, the neurons of the spinal cord, Thalamus and Somatosensory Cortex can still receive occasional random signals. When this occurs, the brain interprets the signal as activation of the respective sensory inputs - and in particular as pain - from the missing nerve or limb. 

Even the internal organs, which don't have dermatomes and specific topographic mapping to the somatosensory cortex can be represented as pain on the surface of the body.  Most people realize that both heart and stomach pain can be felt in the chest, kidney pain in the small of the back, etc.  This is due to the fact that the nerves handling pain sensation from those organs enter the spinal cord at the same point as the respective dermatomes - leading to "referred pain" than is confusing to the patient, but well known to the neurologists and neuroscientists who study pain and sensation. 

So the "five" senses aren't really five.  There may be six if we count proprioception - or four if we combine taste and smell.  However, if we count *all* of the subsets, I come up with 12!  The brain is a complex creation, and we haven't even gotten to the "mythical sixth sense" of which so many writers are fond.

But we will!  Rest assured, there is much more to come in The Lab Rats Guide to the Brain!