It all starts with a stimulus
In the stone age, human beings were equipped with the sensory organs that they need for survival. Today, these fantastic organs are still relevant for us in navigating the challenges of urban environments.
By Roland Knauer
In the first part of ‘Ready to receive on multiple channels’, our author explained how our sense of smell works. The other sensory organs in humans and animals function in a similar way. A chain of biochemical reactions transforms a stimulus that reaches the sensory organ into an electrical nerve impulse which is then analysed in specialised areas of the brain. The differences are found only at the start of these reactions, where the receptors in different sensory organs are responsible for different stimuli.
How taste works
In the same way as the cells in the nose, a large number of cells on the tongue react to certain molecules which we register as ‘taste’. This sense, however, is not especially varied. Only five different types of taste cells provide information about what we’re tasting, whether it is a piece of chocolate or a roast. A range of other details on the aroma are provided by the sense of smell.
Light instead of chemicals
Instead of reacting to chemicals, the rods in the human eye are sensitive to light and deliver nerve impulses when the right light is received. This signal is processed in a different area of the brain, the visual cortex. The eye also has three types of cones, each of which converts a specific wave length of light into nerve impulses.
In humans these cones react to blue, green and red. By contrast, many birds and insects can also see ultraviolet light. This helps insects to pollinate specific flowers and allows the ultraviolet patterns on the plumage of certain types of birds to be recognised. Neither of these abilities is likely to be important for humans, so we don’t need to worry that we are unable to see ultraviolet light. In a similar way, many animals manage without being able to see in colour and probably see the world like an black-and-white film.
Reacting to pressure and heat
Other sensory organs react not to scent molecules or light but to pressure, which they convert into nerve impulses that are then analysed in other areas of the brain. In our sense of hearing, this pressure is caused by sound waves. In the vestibular system – which is responsible for our sense of balance – tiny crystals put pressure on sensory cells or minute hairs register the pressure of a liquid that moves as we turn our heads. Our sense of touch also reacts to pressure.
Another sense measures thermal radiation, causing us to feel heat and cold, as well as pleasant temperatures. When researchers compare our senses with those of other animals, they often find another species that outperforms us by far.
Human eyes versus eagle eyes
For example, human eyes often register an eagle in the sky as just a black dot as these birds of prey with a wing span of over two metres fly so high. However, from up there the eagle with its razor-sharp eyesight has no problem spotting a squirrel, which weighs just a few hundred grams and is much smaller than the eagle. Human eyes are nowhere near as sharp. After all, having eyes like a hawk would be of little use to us as we rarely need to spot things from such heights and would not be able to catch the squirrel anyway as we lack the eagle’s ability to swoop down through the air.
Instead, humans are able to recognise shapes incredibly well and can see in 3D, allowing us to easily determine whether the animal stalking through the grass a few metres away is tasty prey or a dangerous predator. Our ability to recognise shapes is so good that our senses sometimes deceive us. Find out how in the next part of ‘Ready to receive on multiple channels’.