Senses: Balancing

Your Sense of Balance

The following is excerpted from Black Belt Magazine’s article, “The Real Sixth Sense.”

Balance is controlled by a combination of three senses; the vestibular [system], vision, and proprioception.

The sense of up and down is provided by the vestibular system located within the inner ear. It consists of three semicircular hollow tubes that are set at angles to each other. These tubes are filled with liquid that flows through the tubes depending on head tilt and movement. Lining the tubes are hair-like nerve endings that, when stimulated by the movement of the fluid, transforms this motion into a neural signal. This provides information on the position of the head, telling the brain when the head is tilted forward, backward, and side to side, similar to a carpenter’s level. While the vestibular system supplies information about head position, it does not communicate the overall positioning of the body itself. This information comes from two other sources: vision and proprioception.

Vision tends to dominate and override all other senses including our sense of balance. You can test how much vision influences your balance by first standing on one foot with your eyes open, and then with the eyes closed. Most people will begin to lose their balance with their eyes closed. However, vision is not essential to balance and in certain situations, its input is detrimental to performing complex physical movements. Gymnasts, acrobats, high platform divers, and martial artists must train their nervous system not to rely on the eyes for balance.

For example, dancers and figure skaters, when performing pirouettes, focus their eyes on a point in the horizon. As their body spins, the head and eyes remain focused on that spot until the neck will not twist any further, then the head turns around quickly ahead of the body and again focuses on that same spot. If you allowed the head to spin in tandem with the body, the overwhelming visual and vestibular sensations would cause immediate dizziness and disorientation. (Notice young children spinning and falling to the ground.) The same principle is needed to execute a spinning kick or hand technique. The eyes must focus on the target, while the body feels it way through the movement.

This feeling your way through a movement is done by the third sense involved in balance, Proprioception. (Also known as the kinaesthetic sense) This sense is the result of the combined information the brain receives from the Golgi tendon organs. These are sensors within every muscle that measures the amount of tension each muscle is exerting. This information enables us to sense physical movement and posture, as well as how heavy an object is, or how hard to throw a ball. It is also the sense of being  ‘in’ your body.

Information received from all three senses is processed, and directions are sent to the various muscles groups to maintain balance. Therefore, we know why the Samurai could not cross the gorge; he could not suppress his visual input over that of his proprioception that caused him to lose his balance on the log.”

(Originally published in Black Belt Magazine, The Real Sixth Sense)

Balance of course also references the more metaphorical sense of balance. Making time for activities that nurture the many facets of a balanced life – your balanced life – will actually make you feel like your sense of balance is stronger too.

Balance & the Chakras

The main function of the root chakra is to protect your physical body and provide a grounded connection; this aids you in making choices that reflect a sense of balance.

As the third eye or brow chakra connects us to our multi-sensory experiences, it too plays a role in the reliability of your balance.

Chakra balancing is about calibrating your entire system to work in alliance – imbalances in any chakra compromise your integrity and can be influential in this context.

Fun Facts

Most people rely on their sense of sight to manage balance. Strength of balance comes from severing that dependence.

Continued Learning

Super great content made for curious birds without a science background.

Balance – BBC Science & Nature: Human Body & Mind

The Real Sixth Sense – chinastrategies.com, originally published in Black Belt Magazine

Slow Down & Maintain a  Sense of Balance – Healthy Planet

Researchers Discover Gene that Contributes to Sense of Balance – wustl.edu

Balance Awareness Week – Vestibular Disorders Association

Senses: Hearing

Your Sense of Hearing

Ears are for hearing and balance. Both involve complex translations of vibrations into impulses, so the brain can understand them as sound and pressure changes.

The following is excerpted from A Primer on Hearing.

How sensitive is hearing?

Extraordinarily so. The ear can detect a sound wave so small it moves the eardrum just one angstrom, 100 times less than the diameter of a hydrogen molecule. Murray Sachs, director of biomedical engineering, likes to say that if there were nothing between you and the airport, 10 miles away, and if there were no other sounds, nothing for sound to reflect from–then theoretically, you could hear a piece of chalk drop at the airport.

What does hearing do for us?

It helps humans communicate by hearing and understanding speech, other species by hearing its less elaborate cousin, vocalization. “More generally,” says Eric Young, director of the Johns Hopkins Center for Hearing Sciences, “it’s our far sense. It notifies us of things we can’t see but that may be important, be it a prowler or the baby whimpering.” Hearing does that by being extraordinarily sensitive, and also by being able to compute where a sound is in space. .2.0.

What the nervous system gets is two streams of sound, one in the left ear and one in the right; it then calculates a sound’s time of arrival at each ear, the difference revealing roughly where the sound is in space (within about 1º of a circle). ( “Ha! The left ear got it sooner, so it’s off to the left, about there.”) Compared with vision, human hearing locates objects crudely. “But it’s good enough,” says Young, “that you can turn your eyes toward the object and try to find it.”

What good is earwax?

It does unpleasant things to insect intruders.

How does hearing work?

Mechanically, it’s like a Swiss watch. Any engineer would be proud to have invented a device of such precision.

You can think of the system as a relay race, except that the baton keeps transforming into something else: Energy enters the ear (see diagram) in the form of a sound wave, to be converted at the eardrum into mechanical vibrations of the middle-ear bones (the ossicles, the smallest bones in the body). These mechanical vibrations become pressure waves in the fluid of the inner ear (the cochlea), and the waves bend bundles of the cilia (Latin for hairs) of what are called hair cells. Each time cilia bend, hair cells start electrical signals firing toward the brain.

Moreover, at the same time it’s doing hey presto change-o, your ear mechanically boosts the signal by some 25 decibels in our best range of hearing.

How does the brain manage to get all the subtleties of sound and speech out of vibrations alone?

The auditory system does a lot of work before the cortex gets involved, more than other senses. Smell sensations go directly from receptor to olfactory bulb, while signals for sight and touch make three stops before they reach the cortex. But in hearing, there are five waystations, nerve cells that Young calls ‘calculational centers’, right in the brain stem.

The brainstem is the stemlike structure that connects the spinal column with the cerebral hemispheres, and its processing starts almost from scratch: the sound wave that enters your ear is inchoate. It might include bagpipe droning, trees rustling, air conditioner hissing, keyboard clicking, ambulance ululating, fax teeping, several people talking, and more. The nervous system must pick this jumble apart so you can tell one sound from another and pay attention to what matters–the person you’re talking to, let’s say.

Step one is pitch, which is handled by the hair cells in the cochlea. (Young people with normal hearing have about 15,000 in each ear.) The cells are arranged rather like a piano keyboard, on a long narrow membrane that spirals the length of the (spiral) cochlea, and each hair cell is sensitive to a particular frequency at a particular loudness. At one end of the membrane, hair cells react to high-pitched sounds, at the other to low ones, in between to in between.

Then nuclei in the brainstem take over, to locate the source of the sounds in space (as discussed), and to sort all those hundreds of tones into units by timbre, families of resonance. Between the two distinctions, we all know, with seeming lack of effort, that one set of sounds represents a bagpipe, another set footsteps.

Auditory signals also get sharper, because the clever brain stem deletes a clutter of echoes, so they never reach awareness. As your friend’s voice and piano playing bounce off the walls, fireplace, and ceiling, a processing center picks out the echoes as duplicates because they arrive a tad later. It deletes all but the original signal–a neat trick, given the complexity of the sound.

We do hear echoes like halloos at the Grand Canyon, of course. That’s because they come at longer intervals, so the brain stem construes them as separate sounds and sends them on to conscious awareness.

New and unfamiliar sounds do not get deleted, however. On the contrary, they tend to attract our attention, as you may have noticed the first time you heard an icemaker dropping ice cubes into the bin. In such a case, the brain stem may even trigger the motor cortex, making you jump and look around–a startle reaction, which is reflexive; the conscious mind is not involved.

The brain stem also handles the first steps of understanding speech: it ascertains that a particular series of sounds is speech. Then it deletes all but the sounds that matter to meaning in the hearer’s native language. Such sounds (for example oh and ah, puh and tuh) are called phonemes.

There are at least 60 phonemes, depending what sounds you count (English uses 40-some) and by the time a child is 6 months old, its brain is already specialized for its own language. The classic example is English vs. Japanese. Native English speakers have a notoriously hard time learning Japanese, because the meaning of individual English words does not depend on rising and falling inflections.

Many Japanese, conversely, cannot distinguish between L and R, because R does not exist in their language. “Of course they can hear R,” says Stewart Hulse, a psychologist in Arts & Sciences whose field is auditory processing. “If you test them: ‘Is this sound, ruh, the same as this sound, luh?’, they’ll say no. They can hear it. But they can’t hear R in spoken language, because their brain stem has thrown it out, before conscious awareness. It’s almost impossible to hear these things.”

By the time a sound arrives at the cortex, then, it has been analyzed for pitch, timbre, salience, and where it comes from, at a minimum.

What happens once the signals reach the cortex?

More processing. In general, the cortex is arranged in anatomical columns, literally stacks of cells that work together to store, decode, and process information (a discovery made in the somatosensory cortex by Hopkins’s great neuroscientist emeritus, Vernon Mountcastle).

At the point where sound signals reach the auditory cortex, columns initially correspond to frequencies reported by the hair cells. A single tone may activate a large area of cortex, though, in ways that are only murkily understood. Suffice it that as complex patterns of firing develop, the rest of the cortex gets involved, comparing the patterns with stored templates to tell you, ‘Oh! that’s just the refrigerator. Pay no attention.’

Music is thought to be processed in the right hemisphere, language in the left, both in structures that evolved from the auditory cortex itself. Note that the auditory cortex reports to the language center, not the other way around.

If you sit quietly and catalog the sounds around you, you may be surprised at how very many signals are out there. Yet what you consciously hear depends on which sounds you pay attention to, if any. If you’re reading, you may feel you hear nothing. If you’re deep in conversation, you hear the other person’s voice. But you won’t be aware of the icemaker’s clatter unless you’ve never heard it before; attention suppresses stimuli that are non- salient. Otherwise we would all go mad.

Context helps in the work of integrating signals, too. Next time you are listening to someone who mumbles or has a strong accent, notice how much it helps if you have some idea what the person is going to say, or at least what the topic is.

Is the auditory system especially fragile?

Actually, the ear protects itself well. The outer ear keeps the eardrum warm and out of harm’s way, while the middle ear can dampen most sounds that are loud enough to hurt the all-important hair cells. And when hair cells do get overexercised, they tend to quit for a time. That’s why the universe seems muted right after a loud concert.

Probably because of continued insults, however, hearing problems seem to be more widespread in the industrialized world than elsewhere. In the U.S., the major causes of hearing loss are thinning hair cells and sclerosis of the middle ear.

Loss of hair cells is permanent. ( “You have all the hair cells you’ll ever have at birth,” says Young.) It mainly affects soft sounds and high frequencies, the range where women and children tend to speak.

From A Primer on Hearing

Hypothesis: Opening muscles and nerve receptors in the ears increase sensitivity despite lost hair cells.

Hypothesis: Opening ears has a positive effect on nearsightedness.

Hearing & the Throat Chakra

Hearing and listening is associated with the throat (5th ) chakra, which governs communication and creativity. No wonder we get a lump in our throat when we feel challenged to express ourselves!

Fun Facts

Sound travels more quickly in higher temperatures than in cold ones.

More compiled fun facts about all the senses

Continued Learning

Super great content made for curious birds without a science background.

A Primer on Hearing – Johns Hopkins Magazine

Ear & Hearing Review – Neuroscience for Kids

Hearing: In Short – Johns Hopkins Magazine

Hearing – BBC Science

Balance – BBC Science

Chakras Chart: shows corresponding sense, area of consciousness, color vibration, musical vibration, gland, nerve and system of the body and element – The Brofman Foundation for the Advancement of Healing

Senses: Touching

Your Sense of Touch

Our sense of touch may be our most pervasive as it infiltrates every inch of the body, inside and out.

The following is excerpted from A Primer on Touch.

What does touch do for us?

What does it not do might be the easier question, for the sense of touch pervades every inch of the body, inside and out. That’s why it is also called the somatosensory (body-sensing) system.

Without touch, we would be unable to coordinate even simple movements; to manipulate the environment (via the famous opposable thumb); or to know what is happening at the surface of the body. Without touch, we couldn’t walk or balance or heft a pot.

Conversely, it is thanks to touch that our hand jumps willy-nilly off a too-hot handle. That a surgeon can tease apart nerves so small they can only be seen at 30X through a microscope. That a pianist can render sounds ranging from thunder to mist droplets falling into stillness.

Neurologically, how does touch work?

In a variety of ways, each with its own type of nerve fiber and receptors.

To coordinate movement: Informed by the kinesthetic and proprioceptive nerve fibers (proprio- as in property), the somatosensory cortex can sense your every infinitesimal movement. It then clues the motor cortex to send orders accordingly. For instance, sight tells you where to reach for your coffee cup, but it takes constant feedback to let you slow your hand at just the right point in space, then close your fingers smoothly around the handle. At the same time, the body shifts subtly to maintain balance–again, guided by internal sensations.

No wonder it takes babies weeks to master the cup. Even the simplest-seeming acts would be impossible without our sensing the whereabouts of the body’s muscles, tendons, joints, and internal organs.

To manipulate the environment: How is it that you don’t hold a cup so tightly it breaks, or so loosely it drops from your hand? Here enter the ‘cutaneous mechanoreceptors,’ systems that respond to mechanical deformations of the skin. They help your brain gauge the right grip: just firm enough, as measured by pressure receptors. If the cup starts to slip, vibration receptors will report the movement and your grip will adjust, all with no conscious thought from you.

Kenneth Johnson, scientific director of Hopkins’s Krieger Mind/Brain Institute and a researcher in the neurology of touch, explains that pressure receptors work much as the eye does. “You probably know,” he says, “that when the cones and rods in your retina fire, they produce a recognizable picture of whatever you’re looking at? Well, this is like that. The finger sends off to the brain a picture that is exactly like the object itself. We call it isomorphic.” (Iso = same, morphic = form.)

For example, if you stroke a fingertip over a raised letter A (raised as in Braille), the skin is indented, ever so slightly. That causes several hundred neurons to fire, each sending its piece of A up its particular nerve fiber. Up the arm the signals go together, up the spine, up the brain stem, through the thalamus. As the journey ends, neuronal firing on the surface of the somatosensory cortex forms a recognizable A.

Receptors for vibration are deeper in the skin, and they work more like the auditory system. Indeed, Johnson says they fire “in such a way that it’s very hard for auditory neurophysiologists to tell which it was.” Tactile vibration is handled in a part of the brain not far from the auditory area.

Vibratory receptors “are extremely sensitive,” says Johnson. “Even at some distance, your hand can feel vibration of 100, not much more than the thickness of a cell membrane.” Such precision is highly useful for handwriting, machining tools, weaving fish traps, or anything else requiring fine motor control. For humans, its importance is right up there with the opposable thumb.

To protect our bodies, we also have ‘nociceptors’ (noci- as in noxious): two kinds for temperature (heat or cold), and two for pain (rapid pricking pain or slow burning pain). Cold receptors report only cold, and warmth receptors only heat. Both can register a sudden change as small as a hundredth of a degree, but quit at the point of tissue damage. Then pain receptors take over, as they do for other injuries. Presumably, we then take remedial action.

Also, a powerful somatic sensation ‘sensitizes’ the body, strengthening other sensations. Besides its entertainment value, this fact is also protective. Pain can leave the body responsive to even a whisper of sensation, like breeze on a sunburn. Youch! Sensitized, we stay in the shade, get off that ankle, etc. …

What happens after touch signals reach the brain?

At that point signals are pre-sorted, by the nature of the receptors, for pain, cold, and the like, and for intensity by the speed at which the nerves fire. Now begins integration.

Underneath each cell group of the homunculus are a set of columnar brain structures like those discussed for hearing, but devoted to touch. Here, as a signal moves deeper in its column, it becomes more precise–the brain recognizes edges and motion, for example, as the visual system does–but also more abstract and difficult to study. (A neurological code for edge-ness wouldn’t necessarily look like an edge, for instance.) Coded impulses fan out to other parts of the brain, becoming more and more enigmatic.

Let’s go back to the A: At the second level in, the A is still perceptible, but blurred. “By the fourth level,” Johnson says, “neurologists have no idea what’s happening. Deep neural readouts, when they can be obtained, look chaotic.”

As we touch that A, “how do we know whether it’s an A or a B?” asks Johnson. “The image of A must be stored in memory as a pattern, a template.” The brain compares the A from the finger with its template and says ‘aha! it’s an A!–that’s pattern- recognition. People do it magnificently. Machines do not.’

Johnson says the template must be in an abstract, all-purpose kind of form, because the brain does not care whether the A comes from sight or touch. It is not fooled by small print, Olde English lettering, or anything else. “We recognize A-ness,” says Johnson, “and B-ness, and the -ness of our mother’s handwriting, and of whatever else we happen to have learned, much of it non- verbal. We recognize things in an instant, even in forms we’ve never seen before.”

From A Primer on Touch

Touch & the Heart Chakra

Touch is associated with the heart (4th) chakra, which governs self-acceptance, social identity, love and peace. Our abilities to integrate duality (like mind and body, yin and yang, male and female, ego and unity) also come from the fourth chakra. No wonder even simple touch is intimate for many.

Fun Facts

Each fingertip has more than 3,000 touch receptors (most for pressure), each reporting events in overlapping fields about one-tenth of an inch across.

More compiled fun facts about all the senses

Continued Learning

Super great content made for curious birds without a science background.

A Primer on Touch – Johns Hopkins Magazine

Skin, or, “Hey! Your epidermis is showing!” – Neuroscience for Kids

Sensory map: how our cerebral cortex processes touch via the receptors – Neuroscience for Kids

Touch – BBC Science, Human Body and Mind interactive section

Acupressure points: for self-healing – BBC Health

Somatosensation Experiments: fun with touch for the whole family – Neuroscience for Kids

Learn about Dermatones – Human Anatomy Online

Chakras Chart: shows corresponding sense, area of consciousness, color vibration, musical vibration, gland, nerve and system of the body – The Brofman Foundation for the Advancement of Healing

Senses: Tasting

Your Sense of Taste

The following is excerpted from A Primer on Taste.

Tastebuds alone can detect only sweet, sour, salty, and bitter. “If you lick a pink ice cream cone,” says Donald Leopold, an otolaryngologist at Hopkins’s Bayview Medical Center, “your tongue tells you it’s cold and sweet and smooth, but your sense of smell tells you it’s strawberry. Probably 80 percent of what you eat, you appreciate through your sense of smell.” That’s why if you have a cold, you could mistake a bite of onion for apple.

If smell is so crucial to taste, why do we have tastebuds?

Authorities agree that tastebuds have high survival value. They screen our food for important molecules that do not vaporize– salt, for instance–and therefore cannot be smelled.

Hungry Forefather: ‘Hmm. Is this good to eat?’ He nibbles. ‘Hey, it’s sweet!’ Kneeling, he crams berries into his mouth. Sweetness marks glucose, a good source of energy.

Ten feet away, Foremother makes a face and spits. The root she just sampled tastes bitter, a characteristic of many poisons. She moves over to the berries.

Humans and other animals are overwhelmingly attracted to sweetness, and it is the taste that degrades least in old age. We also crave salt, a universal bodily need; wars have been fought over salt. As for sour, the taste of high acidity, Kenneth Johnson, scientific director of Hopkins’s Krieger Mind/Brain Institute, suggests it’s a marker for hydrogen ions.

Some neuroscientists would add a fifth basic taste: delicious, a translation of the Japanese word ‘umami.’ Delicious is defined by the taste of MSG, a flavor-enhancer that was first isolated from a seaweed the Japanese had been adding to food for centuries. And what is MSG? monosodium glutamate, an amino acid. As a rule, the taste ‘delicious’ signals amino acids, building blocks of protein, therefore good for you.

And there you have it: As with sex, crafty Nature has put the pleasure principle to work. In general, what tastes good will keep the species going. What does not, may not….

So tell me about tastebuds. Where are they, and how do they work?

Go to a brightly lit mirror and take a good look at your tongue, something you may not have done since fourth grade. You’ll see bumps scattered on the surface, small round ones at the front and sides, larger ones in the back. These are technically called papillae, from the Latin word for bumps.

Each bump has from one to several hundred tastebuds (visible only with a microscope), and each tastebud has 50 to 150 taste receptor cells (taste cells). In all, most people have 2,000 to 5,000 tastebuds.

So, in comes a forkful of food. Gnash, gnash, chop, chop, work the tongue, pause to savor. Specialized receptors all over the mouth take note of texture, temperature, and nippy spices, while each taste cell reacts to the nearest molecule of food. It reports its verdict–sweet, sour, bitter, or salty–by firing an action potential, launching an electrical signal that eventually reaches the brain.

Receptors near the tip of the tongue are especially sensitive to sweetness. Salty and sour tend to be sensed on the sides of the tongue, and bitterness at the rear. But of course sweet can be tasted somewhat at the back of the mouth, bitter at the front, and so on.

Nor are the taste cells themselves rigidly specialized. Experiments show that while each has a taste it consistently prefers, 9/10ths report on two or more of the basic tastes. Taste cells also vary in their threshold; a cell might prove impervious to sucrose, but respond to the much sweeter saccharine.

These variations help the nervous system sort flavors out with some precision. Tea, for instance, causes rapid firing from receptors for both acidity and bitterness, plus sweetness signals depending how much sugar you put in.

The tongue’s message to the brain, then, is more like a symphony than a single tune-up A. The action potential from each taste cell–let’s be conservative and say some particular mouth has 300,000 of them–zings up the nerve axons into the brain stem, the thalamus, and finally the cortex. The cortex also gets reports about temperature and texture and delivers the conscious experience of the bite: Strawberry ice cream, but let’s not buy this brand again. It’s too sweet.

From A Primer on Taste

Taste & the Sacral Chakra

Taste is associated with the sacral (2nd) chakra, which governs self-gratification, nurturing and identification with the emotional body. Depth of feeling, sexual fulfillment and the ability to accept change flow as a result of the second chakra too. No wonder psychologists equate foods we select with characteristics of our personalities.

Fun Facts

Probably 80% of what you perceive as taste actually comes from smelling functions.

Chili peppers do not stimulate a heat receptor but actually chemically irritate or burn pain receptors in the mouth. Perhaps people who love spicy food have a bit of a culinary S&M fetish!

Compiled fun facts about all the senses

Continued Learning

Super great content made for curious birds without a science background.

A Primer on Taste – Johns Hopkins Magazine

That’s Tasty! – Neuroscience for Kids

In Short: Taste & Smell – Johns Hopkins Magazine

Are you a supertaster? – BBC Science & Nature: Human Mind & Body

Flavour and personality – BBC Science & Nature: Human Mind & Body

Taste, BBC Science & Nature: Human Mind & Body

Chakras Chart: shows corresponding sense, area of consciousness, color vibration, musical vibration, gland, nerve and system of the body and element – The Brofman Foundation for the Advancement of Healing

Senses: Smelling

Your Sense of Smell

The following is excerpted from A Primer on Smell.

Smell “gives us information about place, about where we are,” says Randall Reed, a well-known Hopkins neuroscientist whose specialty is the sense of smell. And smell tells us about people. “Whether we realize it or not, we collect a lot of information about who is around us, based on smell,” says Reed. And about food.

Odors can also warn of trouble–spoiled food and leaking gas (today), cave bears (once), or fire (any eon)–even at a distance. “It’s a great alerter,” offers Donald Leopold, a Hopkins otolaryngologist. “If someone lights a cigar three offices down, you know it right away.”

Smell can also evoke remarkably intense emotion, just with a simple scent. Music stimulates emotion, too, but it typically takes an entire symphony or song to make anyone burst into tears. … Say the smell is purple petunias, which have a rich spiciness no other petunia has, and say your mother died when you were 3. You wouldn’t need to identify the smell, or to have conscious memories of your mother or her garden, to feel sad when that sweet tang drifts up to the porch.

Smell in that broad sense is thought to be ancient, because its fundamental structures are similar in species all the way from fish to moths to primates. In all species, chemical receptors do their work in a fluid medium (mucus for human smell), as would be expected if this sense formed in primeval seas. All smell receptors are self-renewing (practical, since they’re exposed to the world at large, including toxins). And all smell receptors report directly to the brain (in primates to the old part of the cortex), without intervening synapses in the thalamus.

Compared with other mammals, how well do people detect smells?

That depends what you mean by ‘how well.’ We are low on receptors: Current estimates say that humans have roughly five million olfactory receptor cells, about as many as a mouse. A rat has some 10 million, a rabbit 20 million, and a bloodhound 100 million.

“Across species, there is a relatively good correlation between the number of receptor cells and olfactory acuity,” says Reed. “You can hardly find the olfactory bulb in a human brain–it’s a pea-sized object. In a mouse, it’s a little bigger. It’s bean- sized in a rat, about the size of your little finger in a rabbit, and the size of your thumb in a bloodhound.”

While we may not have the olfactory range of other creatures, the receptors we do have are as sensitive as those of any animal. Several recent papers indicate that humans are capable, at least in experimental conditions, of smelling a single molecule. If so, in that sense not even a bloodhound could hope to do better.

We can also think, making conscious (and successful) efforts to sort smells out. A trained ‘Nose,’ a professional in the perfumery business, can name and distinguish some 10,000 odors. Reed says that a master perfumer can sniff a modern scent that has a hundred different odorants in it, go into the lab, and list the ingredients. “In a modest amount of time, he comes back with what to you or me would smell like a perfect imitation of that perfume. It’s amazing.” Similarly, using smell alone, trained wine tasters can tell you a wine’s alcohol content, year of production, grape variety, and even the district in which the grapes were grown.

While a few people do have a dramatically better sense of smell, most of us probably just don’t pay attention. ‘Noses’ say that their abilities are a matter of training, in which the important thing is to practice, to make the distinctions conscious, and to attach words to each one. That makes sense, given the acute sense of smell found in the few remaining aboriginal peoples of the world, for whom smell remains a matter of survival.

Physically, how does the sense of smell work?

Smell receptors in the innermost parts of the nose bond to gas molecules from the air. The receptors then send electrical signals to the olfactory bulb, which signals the orbitofrontal cortex, where the firing pattern reveals to the rest of the brain what smells so nice (or bad) out there. Most receptors (though not all, according to preliminary research in Don Leopold’s lab) are arrayed in two dime-sized patches, one per nostril.

The genetics and biochemistry of the system are a hot field of study, and much of what’s known is new. The first gene for a smell receptor was discovered as recently as 1991, by a team from Columbia. That triumph broke the subject wide open.

Several hundred receptors sounds like a lot, except that the world has thousands and thousands of different odors. How can we distinguish so many?

Because we don’t need a special receptor for each individual odor. Current thinking holds that each receptor type bonds to one or several specific molecules, according to shape. Researchers often describe the system in terms of locks and keys, because only the right odorant will unlock the receptor. The fit must be precise even to the molecule’s handedness.

For instance, a left-handed carvone molecule smells like spearmint, while its right-handed twin smells like caraway seeds. Clearly, right- and left-handed versions fit different receptors, therefore send along different signals (as is also true of smell in other animals and bacteria).

Receptors mostly overlap, many of them responding to the same odorants. Researchers infer that from their work with specific anosmias, conditions in which people cannot smell some particular substance because they lack the receptors–which is most likely if the receptor is coded by only one gene. No gene, no receptor, no smell.

But there are only about 30 such anosmias, which means there must be many thousands of smells that are handled either by many receptors, or by combinations of receptors. These are the smells any person with normal olfaction can detect. Reed says, “Take phenyl ethyl alcohol, which smells like rose. If there are 10 different receptors that all detect some level of phenyl ethyl alcohol, you will not find people who cannot smell rose. I’m certainly not aware of any.”

Two people may not perceive a scent in the same way, though. Because we all have different genes, including genes for smell receptors, we each have our own combination of receptors, including those for phenyl ethyl alcohol. Each combination makes a different firing pattern in the brain–but we all call it ‘rose.’ (Well, sort of. This chemical is the ‘rose’ in drugstore perfume.)

Complex aromas, like a genuine rose or the rose-scented perfume Joy, consist of many different odorant molecules. Therefore they trigger a variety of receptors, giving each of us a unique firing pattern that we can recognize, and name, with practice.

Do we pick up odors equally from both nostrils?

Surprise! Maybe not, according to preliminary, unpublished research by Leopold. He and his team took CAT scans of 90 patients who said they had lost their sense of smell in either the right or left nostril. The scans showed, however, that regardless of which nostril was supposedly affected, a significant number of patients had anatomical obstructions in the left nostril and the left nostril only.

“What we took this to mean,” says Leopold, “was that the majority of these 90 patients were using only the left side of their noses, [at least while their smell was being tested]. They were ignoring their right nostril.”

Do most people have one dominant nostril, then, just as most are right-handed? “That’s the logical leap we’d like to make,” admits Leopold. But the data aren’t there yet.

Why is it that even an overwhelming odor–except garlic– seems to go away after a few minutes?

The system adapts, primarily through a biochemical mechanism discovered by King-Wai Yau, a neuroscientist at the medical school: Calcium cations enter the olfactory receptors and prevent them from sending signals. “Essentially, they turn down the volume,” says Reed.

All senses have variations on the theme, ways to damp stimuli, and a good thing, too. We’ve all had times, such as cleaning up after a child vomits, when that damper is essential. Or imagine walking from a shadowed doorway into sunlight, which can be more than a millionfold brighter–yet we’re only dazzled for a few seconds, because our eyes adjust so quickly.

From A Primer on Smell

Research also shows that like our hands, we may have a dominant nostril!

Smell & the Root Chakra

Smell is associated with the root (1st) chakra, which governs self-preservation, vitality, heredity and identification with the physical body. Covering such innate concepts, it is no wonder our sense of smell is our most primitive sense and the first to awaken upon birth.

Fun Facts

If your sniffer is at peak performance, you can distinguish 4,000-10,000 different smells! Even so, bloodhounds can smell a range of odors 1,000 times better than humans.

Sometimes people with head injuries experience ‘phantom smells’ that no one else experiences. This is often due to injured nerve fibers, which lucky can grow back with a little guidance and luck.

Compiled fun facts about all the senses

Continued Learning

Super great content made for curious birds without a science background. Seriously, read up!

A Primer on Smell – Johns Hopkins Magazine

In Short: Taste & Smell – Johns Hopkins Magazine

Smells & Nostalgia – BBC Science & Nature: Human Mind & Body

Sniffing the Decades – BBC Science & Nature: Human Mind & Body

The Nose Knows – Neuroscience for Kids

Smell – BBC Science & Nature: Human Mind & Body

Chakras Chart: shows corresponding sense, area of consciousness, color vibration, musical vibration, gland, nerve and system of the body and element – The Brofman Foundation for the Advancement of Healing