Over the past few weeks in my neuroscience class (BIOL 470), we’ve been discussing the molecular mechanisms that underlie the senses of vision, taste (gustatory), smell (olfactory), and hearing (audition). Even though we’ve all experienced these senses and take their effects for granted, I was really amazed at how intricately our sensory systems are organized. As always, there’s no better way to appreciate new knowledge and understanding than to share it with others, which is what I’ll be doing in this blog.
First, a quick summary of how these four senses are organized. The sense of vision is generated by the passage of light through the eye. Light first enters the eye through the hole of the pupil, and it passes through two fluid-filled chambers called the anterior and vitreous chambers. Once light hits the retina at the back of the eye, it causes neural cells to fire and send signals to the brain. The sense of taste is generated when chemical compounds in food attach to tongue taste receptor cells, which are located in pits known as taste buds. The activation of these receptor cells sends neural signals to the brain, which can then perceive taste. Smells work in a similar fashion as taste. Chemical compounds from food enter the nasal cavity in two ways: when you smell food and while chewing, since the compounds can diffuse up the pharynx and into the nose as you exhale. These compounds bind to receptor cells in the nasal cavity, eliciting neural signals that allow us to perceive smells. Finally, hearing works by transducing sound from its physical form (differences in air pressure) into electrical signals produced by tiny hair cells deep within in the ear. The ear is divided into three parts: the outer ear, which serves to funnel sounds into the ear, the middle ear, which contains three bones that amplify the sound waves coming into the ear, and the inner ear, which is fluid-filled and contains the hair cells that produce electrical signals.
What’s fascinating about these senses that humans have evolved is how basic the underlying design is. Fundamentally, physical stimuli from our environment (whether it’s light, sound, or compounds in food) change the chemistry of sensory neurons, thus producing electrical signals that our brain interprets. I was amazed to learn that the complexity and accuracy of our senses can be explained by such a basic molecular mechanism.
Another interesting topic I learned about involves capsaicin, the chemical produced by chili peppers that causes us to sense spiciness. In mammals, capsaicin functions by binding to a receptor on our tongues called TRPV1. The TRPV1 receptor is normally activated (when no capsaicin is present) by high temperatures. Therefore, food that is hotter than 42°C would activate these receptors, allowing us to experience a burning sensation in our mouth and causing us to spit the hot food out. Thus, when we eat chili peppers, the capsaicin molecules activate the TRPV1 receptors, and we perceive the peppers as hot and spicy. What’s interesting is that capsaicin does not bind to TRPV1 receptors in birds, and they eat chili peppers without feeling spiciness. Evolutionarily, this is important because it means that birds can ingest chili pepper seeds and disperse them (via pooping) to areas that are miles away. But, squirrels and other small animals don’t eat the chili peppers (because to them, capsaicin is spicy), thus preventing more chili pepper plants from growing around the original plant and creating burdensome competition. For this reason, bird feeders should be filled with pepper flakes so that squirrels and other animals won’t steal food away from birds.
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