Stars for Eyes – The Neurological Wonder of the Star-Nosed Mole

Beneath the eastern wetlands of Canada and the United States, there lives underground a bizarre and unique animal with an impressive list of evolutionary adaptations. This creature holds the world record as fastest eater among mammals [1], can smell underwater [2], and has a very unique sensory organ that basically operates as its eyes [3]. Meet Condylura cristata, otherwise known as the star-nosed mole.

The star-nosed mole is a small animal, weighing about 30-50 grams. It rarely comes above ground, instead using its shovel-like front paws to burrow through extensive tunnels to forage for prey. Although it has eyes, they are extremely small, as are the optic nerves that carry visual information from the eyes to the brain. It’s likely that its eyes are only good enough to detect small variations in lightness and darkness; better eyesight wouldn’t do it much good underground, anyway. Nonetheless, this lack of vision does very little to hold it back, as it has evolved to use another sense highly efficiently to navigate its world — touch. It does this with its unusual star-like “nose”.

Actually, the star isn’t a nose at all. It does not carry any olfactory information. Nor is it used as an extra hand – it doesn’t have any muscles or bones. Instead, it is a purely mechanosensory structure dedicated to detecting the sensation of touch. About 1 centimeter across, the star contains over five times as many nerve fibers as the human hand!

Anatomy of the star

The star is composed of 11 pairs of appendages which fan out radially and are labeled, beginning with the one on the very top of its head. On each of these appendages, there are between 1,000 to 2,000 special sensory structures called Eimer’s organs. Across the entire star, there are about 25,000 of these structures. Although most species of moles which have been studied have Eimer’s organs, the star-nosed mole has more than any other.

 

fig1

Star on the left, zoomed in image of one appendage on upper right, zoomed in image of Eimer’s organs on lower right. Adapted from Figs 1 & 2 of [3].

screenshot2019-01-16at11.06.50pmSo, what’s so special about Eimer’s organs? Each structure contains three types of receptors which detect different forms of sensory information. Just like the sensory receptors in our skin that allow us to feel pressure, vibrations, and texture, these receptors house the endings of neurons, or nerve cells. These nerve endings receive sensory information and then generate an electrical signal to send to the brain, allowing the moles to perceive and interpret the sensation. At the base of the organ, the first nerve ending is wrapped in Schwann cells, which are fatty glial cells that serve to insulate neurons just like the plastic coating around a wire. This receptor, called an encapsulated corpuscle, sends simple information to the brain about vibrations or when the organ first touches an object. The receptor directly above this is called a Merkel cell-neurite complex, and sends information to the brain about sustained pressure on the skin. Finally, at the top of the organ lies a receptor made up of a circular series of nerve endings that form a swelling. While the first two receptors are common in mammals, this last one is unique to moles, and provides the most high-resolution information about texture. In fact, the ring of skin this receptor receives sensory information from is about 15 micrometers across- the length of about 3 or 4 red blood cells. Because of this, these organs can detect microscopic details in texture [3]!

A star for eyes

star_nosed_moleAlthough the star itself doesn’t have muscles to allow for precise control of each appendage, the muscles in the head attach to the very base of the star, allowing the mole to move it around as it explores its environment. As it searches for food, its star is in constant motion that looks like a blur to the human eye, touching at least 10 different places per second. The longest appendages, pairs #1-9, typically find prey first. Once this happens, the mole shifts its star so that its smallest pair of appendages, #11, which is closest to its mouth and has the highest density of sensory receptors, can feel the object more carefully before eating it. Interestingly, scientists have compared the mole’s use of its star for touch to our use of eyes for vision.

You may appreciate this comparison by focusing on your vision right now. While the majority of your surroundings can be seen in the periphery, much of this picture is low-resolution. Only a small portion of the scene in front of you is being examined with the high-resolution center of your eye, the fovea. The information you get from your low-resolution surroundings let you know of objects or events to examine next, which will trigger rapid movements of your eyes from object to object, just as the star-nosed mole uses its star to rapidly explore its surroundings [3]. In fact, the mole moves its star so rapidly that it can detect and eat five separate items in a single second [4]. To capture this idea, researchers have referred to the star as a “tactile fovea”.

Cortical representation of the star

homunculusNow that we know about the unique complexity of the mechanosensory star, how is this information represented in the brain of the star-nosed mole? In humans, touch information from the outside world is sent to a strip of the brain called the somatosensory cortex. This information is organized like a map, where sensory details from the feet are represented near those from the legs, or details from the nose are near those from the lips. This type of map can be represented with a homunculus, which is a distorted representation of the body based on the proportion of the brain that is dedicated to processing information from that region. As an example, a diagram of the human somatosensory homunculus is shown here. Some parts of our body, such as our fingertips, are drawn much larger than others, such as the legs. This is because sensory receptors are distributed unevenly throughout our bodies; we use our hands and fingers for much more fine-tuned interaction with the world, and need to be able to discriminate textures at a higher resolution than we do with our legs. So our fingertips have many more sensory receptors than our legs, and the portion of our somatosensory cortex devoted to processing information that comes from our fingers is larger than the portion dedicated to our legs.

The brain of the star-nosed mole shows a similar kind of mapping, where distinct parts of the cortex are dedicated to processing sensory information from each individual appendage. For instance, cells in the cortex which respond to stimulation of appendage pair #1 are next to those that respond to appendage pair #2, and so on. Just as with human sensory processing, we can depict this topographic organization of the star-nosed mole’s cortex with a homunculus.

homunculusmole

from [3]

One particularly interesting aspect of this sensory brain mapping concerns the cortical representation of appendage pair #11. About 25% of the somatosensory cortex of the star-nosed mole is solely dedicated to processing information coming from this last pair of appendages, which is the one closest to the mouth. Even though these appendages have a higher density of Eimer’s organs (and thereby sensory receptors) than the others, this is still a much larger portion of the cortex than we would expect based on nerve innervation alone. Instead, researchers believe this enlarged representation is directly related to the behavior of the moles. The 11th pair of appendages is used more when searching for food – it makes more contact with the prey than the other appendages, and it is the last pair that feels the food before it gets eaten.

screenshot2019-01-17at12.35.02am

from [4]

So, there you have it! The star-nosed mole is a fascinatingly weird little creature. Aside from being cute and interesting, they also serve as a good model to help us learn more about how touch sensation is transmitted to and interpreted by the brain. As is often the case, we can learn a lot about ourselves by studying other animals with unique neurological systems.

mole

References

[1] http://www.guinnessworldrecords.com/world-records/fastest-eater-(mammals)

[2] https://doi.org/10.1038/4441024a

[3] https://www.jstor.org/stable/1312987

[4] https://www.jstor.org/stable/10.2307/26059888

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