Curious Folks Ask: 162 Real Answers on Amazing Inventions, Fascinating Products, and Medical Mysteries - Sherry Seethaler (2009)

Chapter 4. Bodily functions

Music of maturity

You can tell someone’s approximate age by listening to his or her voice. I also think women’s voices age more rapidly than men’s, because I can more readily tell it is an older lady than an older man. What happens to the vocal cords as a person ages?

Shakespeare wrote about the aging individual, “turning again toward childish treble, pipes and whistles in his sound” (As You Like It, Act 2, Scene 7). Tests with modern acoustic equipment validate these poetic observations. Older people’s voices can be distinguished by their characteristic decreases in loudness and clarity, changes in pitch, tremulousness, and breathiness.

The medical term for the normal age-related changes of the voice is presbylarynx. The prefix “presby” means elder. The larynx is the voice box. It lies in the middle of the neck and is composed of nine cartilages, held together by ligaments and controlled by muscles. Within the larynx are the vocal cords—paired ligaments covered by mucous membranes. Varying the length and tension of the ligaments produces sounds of different pitch.

All parts of the larynx have been observed to undergo age-related changes. Cartilage hardens, muscles atrophy, and nerves degenerate. The composition of the tissue in the vocal cords changes, which alters their mechanical properties. Dryness caused by diminished function of the mucus membranes in the larynx and decreased production of saliva affects the voice.

Respiratory health is also important, because air exhaled through the larynx creates the vibrations that produce sound. Therefore, the voice ages with decreases in the size and elasticity of the lungs, changes in the structure of the chest wall, and decreases in the force and rate of contraction of the muscles that control respiration.

Some physiological changes that age the voice differ by gender. In men, thinning of the outer layer of the vocal cords is common. As a result, the vocal cords may become bowed and fail to close completely, permitting air to escape through the gap and creating a wheezing sound. In women, the outer layer of the vocal cords tends to thicken, altering the vibration pattern and resulting in frequent breaks in pitch.

Changes in the thickness of the vocal cords are thought to be related to the testosterone/estrogen ratio, especially after menopause in women. Voice changes vary tremendously from person to person and appear to be more dependent on physiological age—overall health—than chronological age.

Prune people

What causes skin to wrinkle like a prune when a person is in a pool or bath?

The standard “stratum corneum” explanation is that we get wrinkly fingers and toes when water soaks into the outer layer of skin, the stratum corneum (Latin for “horny layer”). The stratum corneum is thickest on the palms and soles and consists of stacks of dead cells. When we dilly-dally in the tub, these dead cells absorb water and swell. The stratum corneum gets prune-like instead of puffy because it is firmly attached to the living skin beneath. Its surface area increases, but the surface area of the living skin stays the same. As a result, the stratum corneum buckles into a series of little ridges and valleys to accommodate its new surface area.

However, the observation that replanted fingers do not wrinkle after water immersion suggests that a different mechanism is responsible, or partly responsible, for wrinkling. A recent study found that blood flow to normal fingers decreased when people’s hands were immersed in warm water, but in fingers that had been successfully reattached after accidental amputation, blood flow did not change. Wrinkling occurred and blood flow declined in the normal fingers of the same hand, and even the normal portion of the injured finger up to the reattachment point. Nerve damage in the reattached fingers may explain the blood flow response difference.

Based on these observations, the researchers suggested that constriction of blood vessels plays a key role in wrinkling. The digits contain large numbers of glomus organs—clusters of large, convoluted arteries that are involved in temperature regulation. The glomus organs are attached to the upper and lower layers of skin, so if they shrink, they would cause the overlying skin to be pulled inward. Uneven skin folds would then form because of the varying levels of tautness between the upper and lower layers of skin, at and amid the attachment points that anchor together the two layers.

The constriction of blood vessels in warm water is considered to be paradoxical. Usually it is a cold environment that causes a decrease in blood flow in the extremities to conserve body heat. When hands are heated with warm air, rather than warm water, blood flow increases.

The blood vessel constriction mechanism explains the difference in wrinkling between normal and replanted fingers. But it does not explain why blood vessel constriction in response to a cold environment does not lead to wrinkling. It may be that stratum corneum swelling and blood vessel constriction must occur together to cause fingers and toes to get all crinkly.

Blinky

Why do some people blink more than others?

One reason is that some people have dry eyes. Tear film, consisting of a layer of mucus, a layer of salty water, and a layer of oil, protects the outer surface of the eye. When the tear film thins or breaks up, nerve endings in the eye are exposed to environmental pollutants, including smoke, smog, and vapors from paint and cleaning products. Blinking helps alleviate the irritation by sweeping debris from the surface of the eye and stimulating the meibomian glands in the eyelid to release oil into the tear film.

Certain medications, such as allergy medicines, may cause dry eyes. Contact lenses interfere with the maintenance of a uniform tear film. Women are much more likely to suffer from dry eyes than men, in part because eye cosmetics can cause the tear film to break up. In addition, tear production declines with age, especially in women. The decline is probably related to decreased levels of estrogen and testosterone, which scientists postulate may help maintain the health of the glands that produce the tear film.

Destabilization of the tear film is not the only factor that affects blink rate. Typical blink frequencies at rest are about 12 to 20 blinks per minute. Studies have found that blink rate increases during conversation and when someone is anxious, but it can be suppressed during visual tasks that require concentration, such as reading.

Blink frequency is also affected in diseases, such as Parkinson’s disease and Tourette’s syndrome, which involve alterations in dopamine—a chemical that nerve cells use to communicate with each other—in the brain. These diseases may affect a “blink generator” in the brain thought to control involuntary blinking.

Blepharospasm is a condition that results in the forceful closing of one or both eyes. It appears to be a blink reflex gone awry.

Twitchy

Why does a human face occasionally twitch or have muscle spasms?

Twitching is a sudden, involuntary contraction and release of a muscle. Minor eyelid or facial spasms occur frequently and can be induced by stress, fatigue, eyestrain, caffeine, and certain medications. The exact mechanism that leads to these twitches is unknown, but normally muscle fibers are stimulated to contract by the release of calcium from little storage compartments within a muscle cell.

Hemifacial spasm is a more serious condition that results when an artery presses on the nerve to the facial muscles. Involuntary movements of the face can also result from disorders involving an area of the brain called the basal ganglia.

Summertime blues

If the human body runs at 98.6 degrees, why do we consider it hot when it’s that warm outside?

To maintain a constant temperature, heat loss and heat production must be in balance. Our bodies produce heat as a byproduct of muscular activity and the chemical reactions of metabolizing food. Body heat radiates to the environment, but at a rate that decreases dramatically as the temperature of the environment increases.

When a part of the brain called the hypothalamus receives the message that the body is heating up, it sends out signals to make the blood vessels in the skin dilate. (This makes you feel more flushed but allows more heat to be released.) It also makes the sweat glands increase sweat output. Sweating cools you down because the evaporation of water uses heat.

If you have experienced the Midwest or East Coast during the three H’s (hot, hazy, humid), you will appreciate how much more effective sweating is in a dry climate like San Diego!

Staying cool

Why do some people sweat more than others?

Age is one factor. The ability to sweat increases with maturation. Compared to sweat glands in adults, those in children are less sensitive to increases in body temperature and produce sweat more slowly. Sweating capacity is also lower in older adults relative to younger and middle-aged adults.

Gender plays a role. Women have a greater sweat gland density—number of sweat glands per unit area. Men produce more sweat per gland. Overall, women have a slightly lower sweat rate than men.

Heat acclimation has a large effect on the production of sweat and its composition. A person who is not acclimated to the heat usually cannot produce more than a quart (or liter) of sweat per hour. After someone has been exposed to hot weather for a few weeks, the sweat rate can double or triple. At the same time, the concentration of sodium chloride in the sweat declines to conserve body salt.

Hormones control the changes in sweating that result from heat exposure. Sweat comes from the fluid between cells, which is supplied by the blood vessels. Therefore, sweat is filtered blood plasma—the liquid (cell-free) portion of the blood. The sweating-related decrease in the water content of the blood leads to the production of antidiuretic hormone by the pituitary gland and to the production of aldosterone by the adrenal glands.

Antidiuretic hormone stimulates the kidneys to reabsorb water. Aldosterone stimulates the kidneys to reabsorb sodium. Repeated days of exercise in the heat can increase the volume of the blood plasma and the fluid between cells by 20 percent. Retention of water and salt prepares the body for subsequent sweat losses.

Aldosterone also stimulates the reabsorption of sodium and chloride by the cells that comprise the long, coiled tube of the sweat gland. However, potassium, calcium, magnesium, and other electrolytes found in sweat are not conserved, because the sweat gland does not have a mechanism to reabsorb them.

Sweating is initiated more quickly in physically fit people. More copious amounts of sweat are produced compared to less-fit people exercising at the same relative intensity (not engaging in the same task, but exerting themselves equally hard with respect to their own limitations).

Body size and composition can also play a role in sweating by limiting the body’s ability to radiate heat to the environment so that more heat must be lost via evaporation. Other influences include hormonal imbalances and medications that stimulate the part of the nervous system that controls sweating.

Low thermostat

I have always sweated profusely. My normal body temperature is 96.8, and this is not a transposed figure. Is it possible that with my very low body temperature I suffer more in temperatures that others find chilly?

Average normal body temperature is 98.6 degrees Fahrenheit (37 degrees C), but temperatures as low as 95.9 degrees Fahrenheit (35.5 degrees C) and as high as 101.2 degrees Fahrenheit (38.4 degrees C) have been recorded in healthy people.

Maintenance of body temperature occurs through the balance of thermal energy generation from metabolizing food and the loss of thermal energy to the environment by conduction to other objects, convection due to air currents, radiation of infrared energy, and evaporation of sweat.

At rest, conduction, convection, and especially radiation account for most of the thermal energy transferred to the environment. The hotter a body is in relation to the environment, the more effective are these ways of getting rid of excess thermal energy. So someone with a naturally low body temperature must rely more on sweating to cool down.

Temperature is tightly regulated in humans, and relatively small increases in body temperature trigger sweating. On the other hand, camels can allow their body temperature to increase more than 10 degrees Fahrenheit (5.6 degrees C), which reduces the need for evaporative cooling through sweating and conserves water.

Sweaty gourmet

My daughter sweats when she eats, regardless of the temperature of the food or weather. I have never seen anyone else react the same. The sweat pours down her face.

Gustatory sweating—sweating in response to food—has various causes. Spicy food can stimulate the nerves that control the sweat glands. Also, thermal energy is generated as a byproduct of the digestion, absorption, and storage of food.

The amount of thermal energy generated in response to consuming an identical meal varies considerably among individuals. Gustatory sweating can also occur as a rare complication of diabetes.

Frey’s syndrome is a special case of gustatory sweating that occurs when the nerve that controls the salivary gland is damaged by an accident or infection. The nerve’s regrowth may be misdirected so that it connects with the nerve fibers that control the sweat glands. If this happens, any of the stimuli that would normally cause salivation—eating, the smell of food, or even the thought of food—can cause sweating on one or both sides of the face.

Impulsive impulses

When I observe my finger touching my toe, the touch feeling in finger and toe and the visual observation all occur simultaneously. How can the three nerve impulses (6 feet, 3 feet, and 4 inches) arrive at the brain simultaneously? I understand that nerve impulse speed is about 6 feet per second. This seems awfully slow, since I seem to feel the touch instantaneously.

If all nerve impulses traveled that slowly, you would be in trouble if you were a giraffe! Some nerve impulses do travel as slowly as 3 feet per second (about 1 meter per second), but others travel at speeds of over 200 feet per second (70 meters per second). Impulses travel more slowly along axons—long processes of the nerve cell—with smaller diameters.

The speed also depends on whether an axon is surrounded by myelin. Myelin consists of layers of membrane produced by special cells that envelop the nerve cells. Myelin acts as an electrical insulator and dramatically increases the speed at which the nerve impulse can travel. In diseases such as multiple sclerosis, in which myelin is destructively removed from the nerve, nerve impulses are slowed.

Myelin is rare in invertebrate organisms but is ubiquitous among vertebrates. Not all vertebrate axons are myelinated, but sensory nerves and nerves involved in movement are myelinated. Therefore, it takes only a fraction of a second for a nerve impulse to travel from the toe to the brain. As a result, the difference in impulse arrival times from the toe, finger, and eyes is too small for us to consciously distinguish.

American Lilliputians

How do our bodies know when to stop growing so that we do not become giants?

We would seem like giants to some populations of the past. A higher standard of living (better nutrition, less infectious disease) in many developed nations has led to significant increases in height with each generation. For example, in the past century, average height has increased about 4 inches in Japan and many European countries.

Intriguingly, Americans, who were the tallest in the world from colonial times to after World War II, have been surpassed by the Dutch, Swedes, Norwegians, Danes, British, and Germans, according to a study in Economics and Human Biology led by economist John Komlos. Komlos argues that universal access to health care and greater social equality in Northern Europe, relative to the United States, have led to healthier and taller populations.

Whatever the explanation, an immigration-related change in demographics does not seem to be it. When Komlos compared only non-Hispanic, non-Asian people who were born in the United States, Americans were still shorter than their Northern European counterparts.

Height is controlled by genetic programs that lead to the production of growth hormone and a cocktail of other hormones in our bodies. Exactly how environmental factors influence growth is not well understood, but scientists have a pretty good idea of the mechanisms through which hormones exert their influence on height.

Growth hormone is produced by the pituitary gland—a tiny organ near the base of the brain. In about 1 in 20,000 people, the pituitary gland produces too much growth hormone. If this happens in children before puberty, it can cause gigantism—excessive growth of the long bones in the limbs, as well as muscle and organ overgrowth.

Elongation of the bones in the arms and legs occurs at growth plates—regions near the ends of the bone consisting of cartilage. Stimulated by growth hormone, the cartilage cells reproduce, and the cartilage is later converted to compact bone. A variety of other hormones play a role in the proliferation and maturation of cartilage and the process by which it is removed and replaced with bone. Exercise also stimulates bone growth.

At puberty, the sex hormones (estrogen, testosterone) initially boost the release of growth hormone and lead to a growth spurt. Later, higher levels of sex hormones close the growth plates by causing the cartilage-producing cells to die and be replaced with bone.

Therefore, after puberty, an excess of growth hormone does not lead to gigantism. Instead, it can cause acromegaly—growth of soft features, resulting in enlarged feet, hands, and facial features.

Hair-raising

Why is it when you get scared the hairs on your arms and legs stand up?

Hair standing on end goes by many names: the pilomotor reflex, horripilation, cutis anserina, or, simply, goose bumps. It is part of the fight-or-flight reaction and is not unique to humans. You have probably seen a frightened feline assume a Halloween cat pose with fur puffed out, or a dog develop a bristling ruff when confronted by a rival.

Of course, humans are not particularly fuzzy mammals (with a few exceptions making an appearance on beaches and at poolside), and our pilomotor reflex does little to convince our enemies that we are bigger and should not be messed with. It may, in contrast, help us become consciously aware of our own fear response and make us more attentive to potential dangers in the environment.

Cold also causes our hair to stand on end. Again, this response works better for fuzzier mammals or birds. Lifting the hair or plumping feathers traps a layer of air close to the skin, which provides extra insulation.

Some people get goose bumps when listening to beautiful music or in other pleasurable situations. Stress and strong emotions (good or bad) activate the sympathetic nervous system, which prepares the body to respond to the stress. The sympathetic nervous system causes the contraction of a tiny muscle—the arrector pili (also called the erector pili)—that is attached to each hair follicle, the elongated pit that contains the hair. When the muscle contracts, it elevates the hair follicle to form a goose bump.

Puny puckers

How come we get goose bumps on our arms and legs, but not on our face?

Goose bumps can occur on the face. Facial hair follicles have arrector pili muscles that can elevate the follicle. But goose bumps do seem to be less obvious on the face.

The explanation is not the size of the hair or follicle, because a study found that average hair diameter and follicle diameter were similar on the face and on the body.

It may be because hair follicles are much more numerous on the face and head than on the rest of the body. Since the skin puckers at the site of the goose bump, the skin surrounding the goose bump must be pulled tighter. If the hair follicles are close together, as they are on the face, as the arrector pili contracts to lift the hair, the tightening of the skin between the hair follicles would oppose the lifting and result in flatter, less noticeable goose bumps. Facial skin is also thicker, and therefore more resistant to puckering, than skin on the forearms and calves, where goose bumps are very noticeable.

Earplugs

What are the source and purpose of earwax?

Earwax, or cerumen, is produced in the outer third of the ear, in the auditory canal. It is a mixture of secretions from the sebaceous or oil-producing glands and from modified apocrine or sweat glands. Cerumen lubricates the ear and prevents it from getting dry and itchy. It has antimicrobial properties and traps dust and debris.

Earwax also helps clean the ear because the skin in the auditory canal migrates out of the ear very slowly (about 1 millimeter every couple of weeks), carrying the wax that adheres to the skin, along with the dirt trapped in the wax.

Itchy and scratchy

When you have an itch on your back, and you or someone else scratches it, why does the itchy spot seem to move from one spot to another? Sometimes scratching makes your entire back itchy. Why?

Detection and alleviation of itchiness involve nerve pathways for itch, tickle, and pain. The pathways are distinct, but each involves receptors in the skin to detect the sensation, nerves to relay the information to the brain, and nerves to relay information from the brain back to the skin.

Scratching reduces itchiness by removing whatever is causing the itch, such as a hair or an insect. If the cause of the itch cannot be removed—for example, because the skin has launched an allergic reaction to the saliva in a mosquito bite—we may find ourselves scratching until it hurts. The pain signal occupies the central nervous system so that it “forgets” about the itch signal, at least temporarily. The sting of rubbing alcohol also helps soothe the itch of an insect bite for this reason.

When someone else scratches you, the receptors for tickle can be activated. When we touch our own bodies, inhibitory signals from the brain suppress the tickle response. Inhibitory signals from the brain also kick in to shut down the itch response when a large area of the skin is scratched or rubbed, so it may feel like you need to scratch your entire back to make the itch go away.

Scratching can sometimes make the situation worse, because scratching may cause mast cells in the skin to release histamine, which causes inflammation and itchiness. Scratching is more likely to stimulate the release of histamine if someone is already experiencing an allergic reaction or has very dry skin.

Heart-stopping

Does your heart stop when you sneeze?

No. The heart’s rhythm is controlled by a natural pacemaker—the sinoatrial node, a group of cells located in the right atrium of the heart. These cells create an electrical impulse by pumping charged particles out of the cell and then allowing them to flow back in. Conducting cells transmit the electrical impulse to all parts of the heart to initiate muscle contraction. Sneezing does not stop this electrical activity.

On the other hand, the nervous system and circulating hormones, such as adrenalin, alter the rate of the electrical activity in the sinoatrial node to increase or decrease heart rate. Just as exercise increases the heart rate, sneezing works many muscles; therefore, it is possible for a “sneeze attack” to increase the heart rate.

Sneeze grimace

Why can’t you sneeze with your eyes open?

A close relationship exists between the protective reflexes of the nose and eyes. When something like pollen irritates the mucous membranes in the nose, the trigeminal nerve is stimulated, and it relays the message to a sneeze integration center in the medulla at the base of the brain.

The sneeze center is mission control for the sneeze reflex, and it coordinates three simultaneous actions. It commands the respiratory muscles to produce an explosive inspiration and then expiration. It causes the glands in the nose to produce mucous, and it triggers facial muscles to close the eyes and grimace.

Photon allergy

Why do I always sneeze when I step into bright sunlight?

You have a photic sneeze reflex, also known as ACHOO Syndrome. If you want to impress your friends, ACHOO stands for autosomal dominant compelling helio-ophthalmic outburst. (I’m not sure why it’s not ADCHOO, but at least sneeze scientists have a sense of humor.)

About one-quarter of the population has this reflex, and it is thought to be genetic. The reflex varies in strength, with some people affected only in bright sunlight and others affected by camera flashes or other light sources.

The number of sneezes initiated in bright light varies among individuals. And some people even have a sneeze reflex when they rub the corner of their eye, pluck their eyebrows, or comb their hair.

Scientists are not exactly sure what causes ACHOO Syndrome. It is known that the sneezing integration center at the base of the brain receives neural inputs from other parts of the brain as well as the nose.

In photic sneezers, bright light may directly or indirectly stimulate nerves that usually respond when something irritates your nose. This information gets sent to the sneezing integration center, which in turn sends signals to coordinate the diverse muscle groups needed for the sneeze.

Sleeping beauty

What goes on in your body while you are sleeping?

Until the late 1950s, the dominant view was that sleep was simply an idling state. However, electroencephalograms (EEGs), which record fluctuations of electrical activity in groups of nerve cells in the brain, have shown that the sleeping brain is active and that sleep is composed of identifiable stages that occur in cycles throughout the night.

About 30 to 45 minutes after falling asleep, a person enters slow-wave sleep, which is characterized by slow-frequency brain waves. As a sleeper progresses through stages 1 to 4 of slow-wave sleep, the EEG records brain waves that are progressively slower frequency and higher voltage.

The muscles are relaxed during slow-wave sleep, but the sleeper shifts posture regularly. Heart rate and blood pressure decrease. Stage-4 sleep is the deepest and most difficult to interrupt. Someone awakened from stage-4 sleep feels groggy and confused.

By about 90 minutes after the initiation of sleep, the sleeper has progressed back through stages 4 to 1 of slow-wave sleep, and the EEG pattern changes abruptly. The EEG records low-voltage, high-frequency brain waves, similar to those observed in the waking state. This is rapid eye movement or REM sleep, and if awakened, most sleepers will recall dreaming. Sleepers awakened from slow-wave sleep may recall an image or emotion, but rarely a story-like dream.

The pons—an area at the base of the brain—keeps the body in a state of paralysis throughout REM sleep, although the muscles controlling eye movements and respiration are not inhibited. During REM sleep, the body even ceases to regulate its temperature.

Cats with damage to the pons appear to act out their dreams, such as stalking and pouncing as if they were chasing mice. People can also have “REM behavior disorder.” One sleeper, who had been dreaming he was a football player charging an opponent, woke up with a gash on his head from tackling his dresser.

Depriving people of sleep right after they are trained to do a task interferes with learning, even when people are tested a week later, after recouping their sleep. Brain-imaging studies with animals reveal that the pattern of brain activity that occurs during the learning of a task, such as navigating a maze, is replayed during sleep. Greater replay during sleep translates into greater learning.

The exact mechanisms through which sleep facilitates learning and memory are not understood. However, certain genes known to play a role in changing connections between nerve cells are switched on in the brain during post-training sleep.

Yawning maw

I have often wondered what triggers a yawn.

According to folk belief, we yawn because we are not breathing in enough oxygen. The deep inhalation that is a major feature of the yawn makes this idea appealing, but compelling evidence exists that this explanation is not entirely correct.

When people were made to breathe air with higher-than-normal levels of carbon dioxide, their respiration rate increased, but they did not yawn more than people breathing normal air. The number of yawns also did not change when people breathed pure oxygen. Therefore, respiration rate, rather than yawning, seems to regulate oxygen intake.

So why do we yawn? One possibility is that yawning stimulates us to stay awake. In support of this hypothesis, studies have shown that people yawn frequently in the hour before they go to bed but rarely yawn when they are trying to fall asleep. People also yawn frequently while driving. Zoo and laboratory animals yawn before their normal feeding time. Yawning seems to occur when it is important to stay awake.

How would yawning help us stay awake? Some scientists think yawning may dilate the arteries that bring blood to the brain, thereby increasing cerebral blood flow.

The exact trigger for the yawn remains elusive. Certain research suggests that an oxygen sensor, located in a part of the brain known as the hypothalamus, initiates the yawn in response to low levels of oxygen in the brain. Since blood carries oxygen, this research is consistent with the idea that yawning causes a jump in blood flow to the brain, but it does not explain why breathing air containing less oxygen does not induce yawning.

Many different brain chemicals can induce or inhibit yawning, but because the effects of these chemicals are often studied by injecting them into the brains of anesthetized animals, it is unclear which play a role under normal conditions.

An enigmatic feature of yawning is that it appears to be contagious. Perhaps you have yawned while reading this answer. Seeing someone yawn, and reading about and thinking about yawning, can cause humans to yawn.

Although nearly all vertebrates (even fish, frogs, and birds) yawn, until recently humans were the only species known to yawn contagiously. However, recent research has shown that chimps yawned more when watching a video of other chimps yawning. Images of grinning chimps did not have the same effect. Not all the chimps were susceptible to contagious yawning, but neither are all humans.

The fact that yawning is contagious has led researchers to suggest that it may have evolved as a way of synchronizing the social behavior of groups. College lecture halls are a good place to observe vestiges of this evolutionary mechanism.

Smells good

Everything in our world has a scent. Has anyone ever been able to identify how many scents and odors exist in our world?

If we humans were to count all the scents in the world, we would come up with a different number than the other members of the animal kingdom. Dogs, for instance, can detect odors at concentrations almost 100 million times lower than humans can.

There is also much variation among humans in how well we can smell: Some people are unable to perceive certain smells, women generally have a more sensitive sense of smell than men, and as we age we lose our ability to discriminate between smells.

You get a whiff of something because small, volatile molecules from that thing have become airborne, and you have breathed them in. New paint smells because molecules in the paint are evaporating and dissolving in the mucus that lines your nasal passages. When all the volatile molecules have evaporated, paint loses its smell.

At the top of your nasal passages are two postage stamp-sized patches of cells that contain olfactory, or scent, receptors. Estimates of the number of olfactory receptor cells vary widely. Humans likely have somewhere in the range of 10 million of these cells, while scent-tracking bloodhounds have about a billion.

Your brain finds out about a smell when molecules bind to the olfactory receptors in your nose. Scent molecules activate different receptors, with each type of receptor thought to respond to no more than a handful of different smells. The pattern of activation of olfactory receptors seems to work something like a bar code from which the brain determines the smell’s identity.

Some controversy exists among scientists about just how olfactory receptors become activated, but currently the most compelling explanation is that smell molecules activate receptors into which they fit, like a key in a lock.

Of the five senses, smell remains the most difficult for scientists to explain. Coffee, bacon, and cigarette smoke all have hundreds of volatile molecules, yet we do not detect the individual components. But we can detect the distinct fragrances of coffee, bacon, and cigarette smoke when all three are mixed together.

Previously scientists estimated that we should be able to distinguish 10,000 different smells. However, from our current understanding of smell discrimination, in theory we should be able to distinguish an almost infinite number of smells.

One of the most enigmatic features of smell is how the mere hint of an aroma can conjure up powerful memories. For example, the smell of apple pie might take you back to your grandmother’s kitchen. This is because information about smell is sent to the hippocampus, a part of the brain concerned with emotion, motivation, and certain kinds of memory.

Tip of the tongue

Please explain this apparently common phenomenon. You are trying to remember the name of someone you knew a long time ago. Despite earnest and repeated attempts to recall the name, it eludes you. When you are no longer trying to think of it, the name suddenly pops into your brain.

The very mechanisms that help us concentrate can cause thoughts to shy away from us like skittish horses, only to return when we have stopped pursuing them.

To make knowledge more accessible, our brains suppress conceptual distractions through a process known as retrieval-induced forgetting. Researchers have most commonly studied this active process of forgetting using word retrieval tests.

For instance, people learn lists of category–exemplar pairs (fruits–apple, fruits–plum, fruits–banana) for several categories (fruits, sports, cars, dog breeds). They then practice retrieving some of the exemplars when cued with the category and the first two letters of the exemplar (fruits–pl__). Later, they are given the categories and asked to recall all the exemplars from each category.

As expected, retrieval practice improves recall of the reviewed material. Surprisingly, recall of the category–exemplar pairs that were not practiced is worse than it is when people do not practice retrieving any of the category–exemplar pairs at all. In other words, the recall of one memory causes the suppression of related memories.

Brain imaging has shown that retrieval-induced forgetting is adaptive because it reduces the demands on the cognitive control mechanisms needed to recall one of the competing memories.

Unfortunately, it also trips us up when we are searching for that less-used memory. It has even been shown to be responsible for what can seem like deliberate lapses of native-language words in novices after immersion in a foreign language.

Retrieval-induced forgetting shuts off when we are no longer trying to actively recall a memory. The thread of a related memory can then lead us to the forgotten memory. Memories are more like spiderwebs than file folders, because different aspects of a memory (such as someone’s name or image or an event involving the individual) are stored in different parts of the brain.

Mood also affects forgetting and recall. Studies have shown that positive moods enhance retrieval-induced forgetting, and negative moods inhibit it. This is because positive moods encourage global processing of information and connections between related ideas (connections that can lead to suppression of a memory during active attempts at retrieval), whereas negative moods encourage item-specific processing. Later though, when one is no longer trying to recall something and it is no longer being actively suppressed, a positive mood can enhance retrieval by making it more likely that a connection will be forged to the elusive memory.

A particularly intriguing insight into recall comes from people with synesthesia—a mixing of the senses. In one form of synesthesia, lexical-gustatory synesthesia, hearing, seeing, saying, or thinking about a word leads to specific, detailed food experiences, as well as activation of the brain region responsible for the perception of taste. For example, for one lexical-gustatory synesthete, the word “part” tasted like chicken noodle soup. When they have a word on the tip of their tongues, lexical-gustatory synesthetes can taste the word before they can retrieve it. This is consistent with the notion that memories have many components and connections, and access to an individual component can be blocked without affecting other connections.