Chemical Senses

Definition of the Chemical Senses

Our ability to smell and taste is important. Without it, humans would have had a difficult time making it this far. From an evolutionary perspective, these two senses helped us stay safe in dangerous environments. They helped us detect spoiled food and the toxic substances produced by plants when they start to rot.

We also use our sense of smell to detect smoke, indicating that fire is nearby. Can you remember a time when your sense of smell kept you safe from your environment? Though smell and taste obviously still play a big role in helping us interact safely with our environment, they also play a role in enjoying our lives. Certain smells can trigger strong memories, and certain foods can remind us of home. So what makes smell and taste so special, and how do these senses work?

Our chemical senses are the sense of smell and the sense of taste. These are called chemical senses because they rely on detecting chemical compounds. The sense of touch receives information through pressure receptors, sight receives information through photoreceptors, and sound receives information through specialized hair cell receptors; taste and smell, on the other hand, receive information through chemical receptors, or chemoreceptors. Chemical senses use specialized receptors that transmit impulses to the brain.

Examples of the Chemical Senses

You may think taste only comes from your taste buds, but biting into an apple and knowing that you're eating an apple requires both chemical senses, i.e., smell and taste. Our gustatory receptors (taste buds) will be able to determine if what we just ate was sweet, and our olfactory receptors (smell) will be able to determine if what we're eating smells like an apple. Our brain then uses the smell and taste chemical information provided to it to tell you that you're eating an apple!

What would happen if you bit into the apple and it had no taste? You would look at it, smell it, and try to figure out what is wrong with it.

Taste

Our ability to taste starts with the tongue and the tastebuds found on it. These tastebuds are located throughout the tongue and are found in structures called papillae. Though it may not seem intuitive, different nerves pick up taste on different areas of the tongue! The nerves involved in taste include cranial nerves 7 (the facial nerve), 9 (the glossopharyngeal nerve), and 10 (the vagus nerve)!

Smell

Our ability to smell starts with the nose and the olfactory nerves that sit within it. These nerves contain smell receptors that transmit chemical information to our brain. The cranial nerve involved in smell is cranial nerve 1, the olfactory nerve.

Another example of chemical senses is our ability to detect gas. A special odor is added to household gas for the sole purpose of making it detectable by scent. In the case of a gas leak, it is important to quickly identify the spill in order to prevent fires or explosions. Our sense of smell is so powerful that we can pick up on even small amounts of gas.

Fg. 1 Smell helps to detect fire, pixabay.com

The Chemical Senses and Body Senses

We are able to smell because of the olfactory system, including cranial nerve 1, and the olfactory section of our brains. The olfactory nerve (cranial nerve 1) has fibers called olfactory fibers that reach all the way into the nasal cavity. These fibers sit on the surface of the inside of our nose and pick up aromatic chemicals (chemicals that we can smell) and transmit them through nerves to our brains. This system is so specialized that there are roughly 450 scent receptors! An average healthy human can tell the difference between around 1 trillion different odors.

But what about taste? The ability to taste comes from another super-specialized neurologic system called the gustatory system. The ability to taste is split up on the tongue and the pharynx. The chemoreceptors that detect different tastes are located in the taste buds, which are found in papillae.

Fg. 2 Taste and smell are important to cooking, pixabay.com

There are five categories of tastes: sweet, sour, bitter, salty, and savory. Savory is also known as umami. The ability to taste is affected by our ability to smell. While both a strawberry and an apple are sweet, the ability to tell which is which comes from the olfactory system. This is because while both strawberries and apples have sugar, they have different scent chemicals!

Sweetness

Sweetness is usually one of the tastes people find pleasurable. Sugar is important for energy, and we find eating sweet things to be enjoyable.

Sourness

The sour flavor is the flavor of acidity. These compounds are common in citrus fruits.

Bitterness

Bitterness is one of the most sensitive tastes. Many of the toxic compounds are bitter. One of the most loved beverages around the world is high in bitter compounds, i.e., coffee.

Salty

Salt is found in abundance in modern society, but it can lead to hypertension and kidney disease. Ancient humans didn’t have this luxury, and the ability to taste salt helped them make sure they ate enough of this essential mineral.

Savory

Taste buds that detect savory flavor usually bind to glutamate. One common compound used in cooking is Monosodium Glutamate (MSG), which produces a savory flavor.

The Psychology of the Chemical Senses

The connection between smell and memory is immensely strong due to the location of scent processing in the limbic system of the brain. It is also due to the fact that smell is the only sense not processed through the thalamus. Certain smells can bring back past memories more effectively than any other sense. You have held a pencil many times, but the feel of the pencil may not bring back strong memories for you. I bet if you smelt pencil shavings, though, you might have a strong memory of being in class as a young kid! Research has shown that smell may have a part in attraction and mating as well.

The loss of the chemical senses can be devastating. Some people may even report feeling anxious or depressed after a period of not being able to smell. In some instances, losing the sense of smell can be a sign of a more serious condition such as Parkinson's disease, or Alzheimer's.