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Can a human smell through the ears?

Can a human smell through the ears?


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The title does, prima facie, appear absurd.

Performing a swallowing action whilst pinching the nostrils shut gives the impression of listening through a bucket filled with water around the head; audible sound levels are perceptibly reduced. Some humans may encounter a similar sensation blowing their nose - hard.

This makes sense give the ears connect to the mouth/nose through the eustachian tubes. Can an odour make its way to the olfactory sensors through the ears?


A quick diagram to point out to people who may not know what Eustachian tubes are (#2).

In order for the aromatic molecule to reach the olfactory bulb, it would first have to get through the Tympanic Membrane (#22) [a.k.a. - Eardrum]. The Tympanic Membrane is water/airtight unless pierced.

So, while it's plausible that an aromatic molecule could travel through the Eustachian tubes and enter the pharynx areas, there isn't any way to get into the middle-ear in the first place unless the Tympanic Membrane has been compromised.

In a healthy and intact individual, the answer should be No.


@MCM gave a succinct and accurate description of how a healthy and "normal" person will not be able to smell via olfactory sensing trough the Eustachian tube.

Here is an interesting concept in which the brain is able to confuse senses, or alternatively, use sensory input as a metaphor for interpretation via another sensory output. This is a condition known as synesthesia. The majority of known synesthetes have cross-talk between audio-visual sensory pathways. That is, one may interpret sounds as shapes or colours, while another may interpret specific objects (or symbols) as colours or sounds. There were a handful of synesthetes studied in the late 1980's (referred to in this article by Day S.) and one patient was able to interpret sounds as smells. In the review article, smell-interpretation of sound accounts for ~0.1% of known synesthetes, while sound-interpreting olfaction is more common at ~0.6% incidence. On some higher level, the neural information for two senses is confused, and synesthetes that may confuse smelling for some other sense, can not technically perceive scent (using olfactory receptors) through other sensory organs (like the eyes), so the answer is technically no.

  1. Day S. Psyche 1996, 2(32).

Modified 6 Dec 2012: One approach to this question is to perform a sequence similarity search for genes encoding genes with a functioning in sensing external stimuli, be they olfactory receptor, taste receptor or other such genes that are expressed in cDNAs or RNA-Seq data from library preparations of the appropriate ear tissue. While there is not likely at this point in time to be much data available in RNA-Seq format, one might find cDNA libraries from cochlea or neighboring tissue.

A "hit," meaning there is evidence that, for example, an olfactory receptor gene is expressed in cochlea, eg, only means that there is potential to smell through the ears. Taste receptors are expressed throughout the alimentary canal in humans, but tasting the contents - food or other - is not readily perceived as de facto taste outside the oral cavity. Nonetheless, there is no reason not to expect such sensors are expressed to relay information to the brain as to what is or is not present. "Smelling" in/by the ear for the purpose of detecting the presence of a given entity or molecule could serve a similar function.


Electrical stimulation in the nose induces sense of smell in human subjects

Physicians at Massachusetts Eye and Ear have, for the first time, induced a sense of smell in humans by using electrodes in the nose to stimulate nerves in the olfactory bulb, a structure in the brain where smell information from the nose is processed and sent to deeper regions of brain. Reporting online today in International Forum of Allergy & Rhinology, the research team describes their results, which provide a proof of concept for efforts to develop implant technology to return the sense of smell to those who have lost it.

"Our work shows that smell restoration technology is an idea worth studying further," said corresponding author Eric Holbrook, MD, Chief of Rhinology at Mass. Eye and Ear and associate professor of otolaryngology at Harvard Medical School. "The development of cochlear implants, for example, didn't really accelerate until someone placed an electrode in the cochlea of a patient and found that the patient heard a frequency of some type."

Smell loss, or anosmia, has an estimated prevalence of 5 percent of the general population. While some cases of anosmia may be treated by caring for an underlying cause (often nasal obstruction, in which odors can't reach the nerves of the olfactory system due to swelling, polyps or sinus disease), other cases involving damage to the sensory nerves of the nose (i.e. head injury, viruses and aging) are much more complex. There are currently no proven therapies for these cases.

Our sense of smell not only contributes to our enjoyment of life, but also to our daily safety and well being. We rely on our sense of smell to make us aware of smoke in detecting a fire, natural gas leaks and to avoid eating rotten food. In the elderly, of whom there are estimates that greater than 50 percent of the population over the age of 65 has experienced smell loss, it can be difficult to get proper nutrition, as the sensation of flavor is closely tied to the sense of smell, and as flavor diminishes, appetite decreases in this population.

A Cochlear Implant for the Nose

Motivated by work conducted by research colleagues at Virginia Commonwealth University's School of Medicine, Mass. Eye and Ear physicians wanted to address the question of whether electrical stimulation of the olfactory bulb could induce the sense of smell in human subjects.

The findings described in the International Forum of Allergy & Rhinology report demonstrate this feasibility. In the report, the researchers describe endoscopic procedures to position electrodes in the sinus cavities of five patients with an intact ability to smell. Three patients described sensations of smell (including reports of onions, antiseptic, sour and fruity aromas) as a result of the stimulation.

This breakthrough in human patients opens the door for a "cochlear implant for the nose" to be developed -- though the study authors caution that the concept of an olfactory stimulator is more challenging than existing technologies. The most successful neuroprosthesic device in the world, cochlear implants have been on the market for more than three decades to electrically stimulate the auditory nerve to restore hearing in people with profound hearing loss.

"There's currently so little that we can do for these patients, and we hope to eventually be able to reestablish smell in people who don't have a sense of smell," Dr. Holbrook said. "Now we know that electrical impulses to the olfactory bulb can provide a sense of smell -- and that's encouraging."

In addition to Dr. Holbrook, authors on the International Forum of Allergy & Rhinology study include co-first author Sidharth V. Puram, MD, PhD, of Washington University School of Medicine, Reiner B. See, MD, and Aaron G. Tripp, of Massachusetts General Hospital, and Dinesh G. Nair, MD, of Brigham and Women's Hospital.


What causes a smell behind the ear?

Most people do not give the area behind their ears much attention. However, some people may notice a smell that originates there. Sometimes, poor hygiene can cause the smell, but minor infections are also a common cause.

Because people cannot see the area behind their ears, they may not think to wash it or check for skin irritation or signs of infection.

Many relatively minor issues can cause a smell behind the ears. These include:

  • seborrheic dermatitis, a type of eczema
  • poor hygiene
  • piercing infections
  • yeast infections
  • cut or injury infections

In most cases, a smell behind the ears is not a sign of a serious problem. Finding the right treatment and paying a little more attention to the area can usually clear it up.

There are also many effective methods of prevention, which we also discuss in this article.

Share on Pinterest Some types of dermatitis can trap sweat and odors.

It can affect any area of the body, including the back of the ears. In some cases, fungi that live on the skin cause seborrheic dermatitis.

The condition does not usually cause a bad smell. However, the scaly, oily flakes it produces can trap sweat and odor. Also, the condition can sometimes be painful, which may cause people to avoid thoroughly washing behind their ears.

Using antifungal treatments can usually clear symptoms. Many people with seborrheic dermatitis on the skin also have this condition on the scalp, so it may be helpful to wash the scalp with antifungal shampoo. Many antifungal shampoos are available to purchase online.

The area of skin directly behind the ears can very easily trap sweat and oils. The back of the ear can trap residue from skin and hair care products. Having long hair may also make it easier to trap oil and other residues.

The area behind the ears is also impossible to see without a mirror, so most people do not pay much attention to it. They may not wash the area very much or pay close enough attention when they do.

So, if the area does not hurt and just smells bad, the most simple solution is to thoroughly wash with warm water and soap.

An ear piercing is an open wound until it fully heals.

For this reason, it is easy for bacteria to enter the wound. Bacteria can also infect healed ear piercings, especially if the piercing is unclean.

Infected ear piercings sometimes smell bad. Pus, dead skin, and other drainage from the infection can stick to earring posts and backs. This can cause a bad smell to linger.

Cleaning the ears and earring posts with either rubbing alcohol or a special ear piercing solution might help. Ear piercing solution is available to purchase online.

If the infection is painful, if there is a fever or swollen lymph nodes, or if home treatment does not work, see a doctor. As with other infections, those of ear piercings can travel to other areas of the body, potentially becoming very serious.

People should see a doctor if the infection is in the cartilage of the ear. These infections can be more difficult to treat and may require stronger antibiotics.

Candidiasis , which people tend to call a yeast infection, is an infection with the fungus Candida albicans.

Yeast tends to grow in warm and moist areas. As a result, people who sweat a lot or those who do not regularly clean the area behind their ears may develop a yeast infection.

Yeast infections tend to itch and may produce a beer- or bread-like smell.

Rarely, a person may develop a serious yeast infection that doctors call invasive candidiasis. This occurs when yeast gets into the bloodstream and spreads through the body. When this happens, a person may develop signs of a yeast infection in several areas of the body.

People with weak immune systems, such as those with HIV or AIDS, are more vulnerable to this infection.

Most yeast infections respond well to over-the-counter antifungal remedies. If the infection is severe, if a person with a weak immune system develops the yeast infection, or if home remedies do not work, a doctor can prescribe a pill or cream to clear the infection.

Sometimes, yeast or other fungi infect the inside of the ear, usually in the outermost part. Doctors call this otomycosis.

This infection may become invasive, spreading deep into the ear or even into the bone. Otomycosis can cause intense pain and itching in the ear. A doctor can prescribe medication to treat it.

Infected injuries sometimes smell unpleasant. It is possible not to notice an injury behind the ear, such as a cut, scrape, or pimple, until it becomes infected.

If there is swelling, pain, or discharge, the infection probably requires antibiotics.

If the pain is minor, try cleaning the injury with soap and water and applying a triple antibiotic ointment. See a doctor if symptoms do not go away in 1–2 days.

If there is a fever or intense pain, or if the injury is very flushed, seek immediate medical attention.

Preventing a bad smell behind the ear is typically as simple as keeping the area clean. Good hygiene may also help prevent infections and skin irritation.

People can try the following strategies to prevent developing a smell behind the ears:

  • Wash behind the ears during every bath or shower. People with sensitive skin or eczema should use sensitive skin soap, which is available online.
  • Wipe the area behind the ears with a warm, wet washcloth after intense physical activity.
  • Keep ear piercings clean. Twist and rotate the piercings in a circle several times each day. Do not take new posts out until at least 6 weeks after piercing. Children who are too young to clean their ears must receive adult help.
  • Gently exfoliate the area behind the ears once or twice per week. This prevents dead skin from building up. An exfoliating wash or rough washcloth can help with this task. People with skin conditions should discuss exfoliation with a doctor before trying it.
  • Do not ignore a bad smell, even if there is no pain. A bad smell may be a warning sign of an infection or other problem, so it is best to see a doctor.

Noticing a bad smell behind the ears can be alarming. Finding the right treatment can help remove the smell as well as resolve the underlying cause.

In many cases, treatment is a simple matter of taking the time to wash this often neglected area. Even when an infection or other serious issue is the cause, a doctor can usually prescribe a quick-acting treatment.

Discussing unusual smells in the body may feel uncomfortable, but people should not hesitate to seek help from a healthcare professional. They can offer reassurance that the problem is common and a quick path to relief.


The human nose can sense 10 basic smells

We’ve got categories to describe our perceptions of taste, colors, and sounds. But things aren’t as clear-cut when it comes to our sense of smell. Looking to overcome this surprising limitation, a team of researchers have proposed a list of 10 basic smells.

Indeed, we’re all set when it comes to describing the way our other senses work. Our 100,000 taste buds elicit five different sensations , namely sweet, bitter, sour, salty, and umami (a Japanese word for a pleasant savory taste, but distinct from pure saltiness). When talking about vision, we’re able distinguish between wavelengths by referring to them by color, like red, green, and yellow. And when it comes to sound, we can speak of timbre, dynamic range, and frequency response .

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The sense of smell comes about through the stimulation of specialized cells in our nasal cavities — cells that are similar to the sensory cells of the antennae of invertebrates. The human olfactory system works when odorant molecules bind to specific sites on the olfactory receptors, which are used to detect the presence of smell.

And it all comes together at the glomerulus, a structure which transmits signals to the olfactory bulb — a part of the brain directly above the nasal cavity and below the frontal lobe. The end result is the subjective experience we call smell.

As we all know, odors can be rich and complex. And we have many ways of describing smells (e.g., smoky, sweet, pungent, etc.). But what we haven’t done is create a definitive list that organizes odors into their basic, or essential, categories.

Odor Profiling

To overcome this limitation, a research team consisting of Jason Castro, Arvind Ramanathan, and Chakra Chennubhotla analyzed 144 different odors to see if they could identify consistent odor profiles. These 144 odors were derived from an olfactory “atlas” created in 1985 at the Institute of Olfactory Sciences in Park Forest, Illinois.

To assist them with their task, the researchers used advanced statistical techniques (a mathematical technique called non-negative matrix factorization [NMF]) to develop an approach for the systematic description of smells. The researchers likened the process to digital data compression when a digital audio or image file is reduced in size the basic elements are retained at minimal expense to quality or essence.

Their analysis showed that olfactory space is highly dimensional — 10 dimensions to be exact.

  • Fragrant (e.g. florals and perfumes)
  • Fruity (all non-citrus fruits)
  • Citrus (e.g. lemon, lime, orange)
  • Woody and resinous (e.g. pine or fresh cut grass)
  • Chemical (e.g. ammonia, bleach)
  • Sweet (e.g. chocolate, vanilla, caramel)
  • Minty and peppermint (e.g. eucalyptus and camphor)
  • Toasted and nutty (e.g popcorn, peanut butter, almonds)
  • Pungent (e.g. blue cheese, cigar smoke)
  • Decayed (e.g. rotting meat, sour milk)

The last two items, pungent and decayed, get a kind of meta-category of their own, one the researchers describe as “sickening.”

Other aromas, like baked bread or fresh-brewed coffee, are amalgams of two or more of these 10 elements.

Smells Fishy?

This study is certainly interesting and helpful, but it’s lacking in several areas.

First, we’re talking about something that’s very subjective. Take pungent, for example, an odor the scientists placed into the “sickening” category. While strong and sharp, it’s not necessarily an unpleasant odor. What’s more, our appreciation and comprehension of smells are both culturally instilled and the result of such processes as developing an “acquired taste” for something.

Also, the 144 odors considered by the scientists comes from a very small sample pool. And indeed, the scientists acknowledge this in their paper, suggesting that future studies should broaden the scope of data.

Lastly, the study didn’t distinguish between perceptual and cognitive influences on the organization of human odor space. This would help alleviate some of the subjectivity problems inherent in the study by showing the various autonomous responses involved in olfaction.


Can humans smell sex pheromones?

You might have heard about pheromones in the news or in some advertisements claiming that a perfume will make you irresistible, however, many people don’t know what pheromone means. The definition of pheromone dates back to 1930s, when they were first discovered in insects . At the time it was found that a molecule, or a mix of molecules, released by an individual can affect the behaviour of another individual of the same species, often triggering a sexual response. These molecules normally function via the sense of smell. In insects the effect of a pheromone can be very straightforward, and even different for the two sexes, meaning that the same molecule can inhibit mating behaviour in a male fly, but promote mating in a female .

As one might imagine, the situation is more complicated in mammals, due to their higher brain complexity. For instance, scented filter paper was often used for insect’s copulatory behaviour studies, however it is difficult to imagine a mouse attempting copulation with a piece of paper. Although mammals’ behavioural responses are not as simple, the effect of a pheromone can be as striking as the ‘Bruce effect’, where a pregnant mouse can prematurely terminate the pregnancy if exposed to unfamiliar male pheromones .

When it comes to humans, dealing with pheromones and behaviours is even more challenging, because of the complex nature of our social and sexual behaviours. In a famous study, researchers asked subjects to smell t-shirts that were previously worn by males and females for several days, and found that 70-80% of the subjects could identify the person’s sex based on the odour from the t-shirt . However, later it was found that people tend to associate stronger unpleasant odours with males, unveiling an unconscious bias in the ‘smelly t-shirt’ experiment . This shows how experiences can influence our choices or behaviours, making scientific testing and data interpretation extremely complicated.

Recently, a nipple secretion from lactating mothers, which induces suckling in neonates, has been proposed as a new human pheromone . In my opinion this is a good example of pheromone-induced behaviour, because there is no learning or experience bias involved. In other words, newborns have no idea how to get food from a nipple, but smelling this secretion will trigger a specific innate behaviour.

The big question is can humans smell sex pheromones? The answer is: we do not know yet. There are some candidate molecules, but none of them are widely accepted by the scientific community. Many studies proposing new putative pheromones have been largely criticised, mainly due to faults in the experimental approach. Some scientists are even reconsidering the very definition of pheromone, as the original one was based on how some chemicals trigger unequivocal behaviours in insects, and this might not be applicable to humans . Perhaps pheromones can affect our physiological state or mood in more subtle ways that are not easy to identify in a simple scientific test.

Follow Alfredo’s research on his webpage and twitter.


TimeHuman

It's been a fact for many years that dogs only see in black and white. I'm here to tell you it's a total myth. Dogs can actually see colors that are equivalent to red-green color blindness in humans. If you were to compare the vision of a human vs. a dog, the dog would win the contest for noticing the most in its surroundings because a dog has much better vision for catching motion, however, dogs can only see about half the brightness level than their human counterparts.

Now that you know dogs can see in color, let's take it sense by sense and compare the sensory systems of dogs and humans, starting with hearing:

Dogs can hear what?
Dogs can hear in frequencies ranging from around 40 Hz to 60 kHz, depending on the breed and age. Dogs have more than 18 muscles that enable them to move their ears so they can more precisely locate a sound. In addition, dogs can hear sound up to four times farther than us humans. So the next time you hear your dog barking at the wind, he/she may be hearing something quite interesting.

Humans can hear what?
Humans can hear in frequencies ranging from 12 Hz to 20 kHz (give or take). As we get older, that range can shrink, depending on the level of hearing loss you experience. Women tend to be more sensitive to higher frequencies than men. This is most likely due to the fact that women have to be more aware of their offspring.

Dogs can smell what?
Dogs have a brain that is built to sense smell. The olfactory cortex is the part of their brain that enables them to have superior smelling capabilities. The olfactory cortex is fourty times bigger in dogs than it is in humans and up to 100 million times more sensitive. Bloodhounds have an extremely superior sense of smell, hence the � million times more sensitive” figure. Other breeds don't have quite that capability. Dogs use their wet noses to detect what direction a smell is coming from. Dogs can use each nostril separately to further increase their smelling abilities. The sense of smell is the most highly evolved sense a dog has.

Humans can smell what?
The human nose can sense up to 4,000 to 10,000 different smells (dogs can sense around 30,000 to 100,000). Humans who do not have any sense of smell have a condition called Anosmia. Our sense of taste is largely influenced by our sense of smell. In fact, it can influence our sense of taste by up to 80 percent! Dogs can sense smells that are 100 million times less concentrated than what us humans can smell. So I guess that puts our sense of smell in perspective.

Dogs can see what?
Dogs are not color blind. They can see in ranges that are similar to red-green color blindness in humans. As twilight hunters, they have a section of their eye called the tapetum lucidum, which gives them something like night vision. You can often see the tapetum by shining a light into a dog's eye. It's that reflective eye-shine you can see when light reflects in their eyes in the dark. Dogs don't have the greatest clarity of vision, but they can see motion much better than us humans. For example, dogs have been shown to be able to differentiate between their owners from distances of up to 900m. However, that's only the case if you are moving. If you were to stay in one place, the distance they can differentiate goes down to around 500m. Some researchers believe that dogs may see television as a flickering screen. The visual abilities of a dog varies by breed. Greyhounds have been touted as having the best eyesight compared to other breeds, however, it hasn't been thoroughly proven.


Sense of smell

One of the most important functions of the nose is its role in the sense of smell. Smell receptor cells are located in the upper part of the nasal cavity. These cells are special nerve cells that have cilia. The cilia of each cell are sensitive to different chemicals and, when stimulated, create a nerve impulse that is sent to the nerve cells of the olfactory bulb, which lies inside the skull just above the nose. The olfactory nerves carry the nerve impulse from the olfactory bulb directly to the brain, where it is perceived as a smell.

The sense of smell, which is not fully understood, is much more sophisticated than the sense of taste. Distinct smells are far more numerous than tastes. The subjective sense of taste while eating (flavor) involves taste and smell (see figure How People Sense Flavors) as well as texture and temperature. This is why food seems somewhat tasteless when a person has a decreased sense of smell, as may occur when the person has a cold. Because the smell receptors are located in the upper part of the nose, normal breathing does not draw much air over them. Sniffing, however, increases the flow of air over the smell receptor cells, greatly increasing their exposure to odors.


Humans can discriminate between hundreds, perhaps thousands, of different odorant molecules, each with its own structure. How can one kind of cell provide for this?

  • The mammalian genome contains a family of about 1000 related but separate genes encoding different odor receptors. (No more than 40% of these are functional in humans &mdash the rest are pseudogenes &mdash which may help to explain why dogs are better at detecting odors than we are.)
    • The olfactory epithelium of rats (which is more convenient to study than that of humans) expresses several hundred genes not expressed in other tissues.
    • Each gene encodes a transmembrane protein that resembles &mdash but is not identical to &mdash the others.
    • Each protein contains 7 regions of hydrophobic alpha helix that allow the molecule to pass back and forth 7 times through the plasma membrane.
    • In some cases, the portion of the molecule exposed outside the cell may be responsible for binding the odorant molecule.
    • However, many odorant molecules are hydrophobic and could easily enter the lipid bilayer and bind to the receptor there. This possibility is supported by the finding that much of the sequence variability from one receptor to another is found in the alpha helices.
    • Gene probes for a single type of receptor bind to only 1 in a 1000 sensory neurons in a normal olfactory epithelium.
    • However, rats made to express a single type of receptor in large numbers of their olfactory neurons responded much more vigorously to a single type of odorant than to any of the other 73 tested. (The procedure: solutions containing a recombinant virus carrying the receptor gene were inserted into the nasal cavities of living rats. Many of their olfactory neurons became infected and expressed that receptor gene.)
    • Cells taken from these rats and placed in tissue culture also responded to only that one type of odorant molecule.

    Although a single olfactory neuron contains over a thousand receptor genes, there is only a single enhancer capable of binding to the promoters of these genes and turning them on. (There are, of course, two alleles of the enhancer but only one is active [one is methylated the other is not]. Presumably, when the active enhancer encounters the promoter of an olfactory gene, it turns it on and ceases its search. Thus only one olfactory receptor gene gets to be expressed in a single cell, but which one is a matter of chance.

    In mice, the enhancer is on chromosome 11. Although several olfactory genes are also on that chromosome, many others are scattered over several other chromosomes. Nonetheless, it has been demonstrated (e.g., by FISH analysis) that the enhancer on chromosome 11 can find and bind to the promoter of an olfactory gene on other chromosomes not just those on #11. (Link to other examples of "kissing" chromosomes.)

    • Each receptor is probably capable of binding to several different odorants &mdash some more tightly than others. (The cells described above also responded &mdash although more weakly &mdash to 3 related odorants.)
    • Each odorant is capable of binding to several different receptors.

    This provides the basis for combinatorial diversity. It would work like this:

    • Odorant A binds to receptors on neurons #3, #427, and #886.
    • Odorant B binds to receptors on neurons #2, #427, and #743.

    Do snakes have ears?

    Snakes are unique animals, with their limbless bodies, flicking tongues and the ability to devour prey whole. They mostly rely on their sense of smell to hunt prey, although they do use sight and sound too. But do snakes have ears?

    Yes and no, Sara Ruane, a herpetologist at Rutgers University in New Jersey, told Live Science. Like many reptiles, snakes don't have an external ear structure. However, they do have ear bones in their heads that they use to hear.

    "When you think about animals, whether it's a dog or a jack rabbit, they hear a noise in a different direction and shift their external ear in order to better capture that sound in case it happens again," Ruane said. "An internal ear is the part where the actual nuts and bolts of hearing happen." Snakes only have the nuts and bolts part of the ear.

    Ears are typically made up of three major parts. The outer ear focuses sound on the eardrum, which separates the outer ear from the middle ear. The middle ear contains three bones that transmit sound from the eardrum to the inner ear via vibrations. The inner ear turns these vibrations into nerve impulses that travel to the brain.

    Snakes lack both an outer ear and middle ear, according to a 2012 study in the Journal of Experimental Biology. However, they have one middle ear bone that connects the inner ear to the jaw. This enables snakes to hear vibrations, such as a predator creeping closer on the forest floor. However, they're not as proficient at hearing sounds transmitted through the air.

    Due to this ear setup, snakes hear only a narrow range of frequencies. They can hear low frequencies but not high frequencies, because those sounds are mostly transmitted through the air. For example, royal pythons are best at hearing frequencies between 80 and160 Hertz, according to the 2012 study. For comparison, the normal human frequency range is 20 Hz to 20,000 Hz, according to "Neuroscience" (Sinauer Associates, Inc. 2001).

    "If you were swimming and went underwater, and somebody standing next to the pool shouted to you, you would hear them," Ruane said. "You might not be able to make out the details . That's sort of what snakes are hearing at higher frequencies."

    This narrow range of hearing isn't a problem for snakes, partly because they don't use vocalizations to communicate with each other. The vocalizations they make, such as hissing or growling, are at higher frequencies than they hear at and are probably intended for bird and mammal predators, according to the study.

    The bigger reason why snakes don't need sensitive hearing is because they rely on other senses. Their sense of smell is particularly useful. "Snakes are flicking their tongues out, picking up all the odor molecules that are in the air in the vicinity, bringing that back into a specialized organ they have for processing that, and to their brain," Ruane said. So although they don't have a chance at out-hearing most other animals, "snakes are the chemosensory kings."


    Can a human smell through the ears? - Biology

    Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration. Vestibular sensation, which is an organism’s sense of spatial orientation and balance, proprioception (position of bones, joints, and muscles), and the sense of limb position that is used to track kinesthesia (limb movement) are part of somatosensation. Although the sensory systems associated with these senses are very different, all share a common function: to convert a stimulus (such as light, or sound, or the position of the body) into an electrical signal in the nervous system. This process is called sensory transduction.

    There are two broad types of cellular systems that perform sensory transduction. In one, a neuron works with a sensory receptor, a cell, or cell process that is specialized to engage with and detect a specific stimulus. Stimulation of the sensory receptor activates the associated afferent neuron, which carries information about the stimulus to the central nervous system. In the second type of sensory transduction, a sensory nerve ending responds to a stimulus in the internal or external environment: this neuron constitutes the sensory receptor. Free nerve endings can be stimulated by several different stimuli, thus showing little receptor specificity. For example, pain receptors in your gums and teeth may be stimulated by temperature changes, chemical stimulation, or pressure.

    Reception

    The first step in sensation is reception, which is the activation of sensory receptors by stimuli such as mechanical stimuli (being bent or squished, for example), chemicals, or temperature. The receptor can then respond to the stimuli. The region in space in which a given sensory receptor can respond to a stimulus, be it far away or in contact with the body, is that receptor’s receptive field. Think for a moment about the differences in receptive fields for the different senses. For the sense of touch, a stimulus must come into contact with the body. For the sense of hearing, a stimulus can be a moderate distance away. For vision, a stimulus can be very far away for example, the visual system perceives light from stars at enormous distances.

    Transduction

    The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. This takes place at the sensory receptor, and the change in electrical potential that is produced is called the receptor potential. How is sensory input, such as pressure on the skin, changed to a receptor potential? In this example, a type of receptor called a mechanoreceptor (as shown in Figure 1) possesses specialized membranes that respond to pressure. Disturbance of these dendrites by compressing them or bending them opens gated ion channels in the plasma membrane of the sensory neuron, changing its electrical potential. Recall that in the nervous system, a positive change of a neuron’s electrical potential (also called the membrane potential), depolarizes the neuron. Receptor potentials are graded potentials: the magnitude of these graded (receptor) potentials varies with the strength of the stimulus. If the magnitude of depolarization is sufficient (that is, if membrane potential reaches a threshold), the neuron will fire an action potential. In most cases, the correct stimulus impinging on a sensory receptor will drive membrane potential in a positive direction, although for some receptors, such as those in the visual system, this is not always the case.

    Figure 1. (a) Mechanosensitive ion channels are gated ion channels that respond to mechanical deformation of the plasma membrane. A mechanosensitive channel is connected to the plasma membrane and the cytoskeleton by hair-like tethers. When pressure causes the extracellular matrix to move, the channel opens, allowing ions to enter or exit the cell. (b) Stereocilia in the human ear are connected to mechanosensitive ion channels. When a sound causes the stereocilia to move, mechanosensitive ion channels transduce the signal to the cochlear nerve.

    Sensory receptors for different senses are very different from each other, and they are specialized according to the type of stimulus they sense: they have receptor specificity. For example, touch receptors, light receptors, and sound receptors are each activated by different stimuli. Touch receptors are not sensitive to light or sound they are sensitive only to touch or pressure. However, stimuli may be combined at higher levels in the brain, as happens with olfaction, contributing to our sense of taste.

    Encoding and Transmission of Sensory Information

    Four aspects of sensory information are encoded by sensory systems: the type of stimulus, the location of the stimulus in the receptive field, the duration of the stimulus, and the relative intensity of the stimulus. Thus, action potentials transmitted over a sensory receptor’s afferent axons encode one type of stimulus, and this segregation of the senses is preserved in other sensory circuits. For example, auditory receptors transmit signals over their own dedicated system, and electrical activity in the axons of the auditory receptors will be interpreted by the brain as an auditory stimulus—a sound.

    The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a more rapid train of action potentials, and reducing the stimulus will likewise slow the rate of production of action potentials. A second way in which intensity is encoded is by the number of receptors activated. An intense stimulus might initiate action potentials in a large number of adjacent receptors, while a less intense stimulus might stimulate fewer receptors. Integration of sensory information begins as soon as the information is received in the CNS, and the brain will further process incoming signals.

    Perception

    Perception is an individual’s interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system, in the brain. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. These neurons are dedicated to that particular stimulus and synapse with particular neurons in the brain or spinal cord.

    All sensory signals, except those from the olfactory system, are transmitted though the central nervous system and are routed to the thalamus and to the appropriate region of the cortex. Recall that the thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex (Figure 2) dedicated to processing that particular sense.

    How are neural signals interpreted? Interpretation of sensory signals between individuals is largely similar however, there are some individual differences. A good example of this is individual tolerances to a painful stimulus, such as dental pain, which certainly differ.

    Figure 2. In humans, with the exception of olfaction, all sensory signals are routed from the (a) thalamus to (b) final processing regions in the cortex of the brain. (credit b: modification of work by Polina Tishina)


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