bio7: Our senses - the window into our environment (v1.0)

This is the last essay on biochemistry/molecular biology. 

The five senses are taste, smell, vision, hearing, and touch. In this essay we explore these. What we experience of our surroundings is wholly dependent on these senses. The brain interprets these sensory inputs and makes decisions.  

We sense 5 types of tastes. Sweet, Salty, Sour, Bitter, and Savory/Umami.  The sweet taste is related to energy rich carbohydrates. Salty relates to sodium and potassium. Sour taste is related to food past their prime and related to PH and organic acids. Savory taste is related to the amino acid glutamate and aspartate in protein rich foods. Much of the sensory information about foods also comes from our noses. Distinguishing smells is more finely tuned than distinguishing taste. The texture of food is related to the sense of touch. The temperature of foods also contributes. 

There are bumpy structures scattered all over the surface of the tongue called papillae. They hold the taste buds. The taste bud looks like a garlic. The taste molecule enters through a small opening at the top. The taste bud has about 50 to 100 sensors. Humans have about 2000 to 8000 taste buds. At the bottom is the dendrites or sensory nerves that carry the signal. Different sensor groups sense different tastes.  Sweet taste receptors are also in the intestines and is related to glucose regulation. Bitter taste receptors are found in the lungs and the airways and is related to opening and closing of airways and has implications for treatment of respiratory diseases like asthma. The bitter molecules must be in the form of an aerosol to be inhaled.  I will skip the biochemical reactions related to taste. 

Taste and smell are closely connected. The sense of smell called olfaction is enormously important not just for enjoyment of food but also warning of danger. It is also known to be a component in partner selection in animals for mating. There is an olfactory bulb in the nasal cavity with olfactory receptor sites. The primary pathway to the receptors is via the nostrils. The mouth is connected to the nasal cavity. Chewing food sends molecules up this connection to the nasal cavity and the olfactory receptors. Mucus is a viscous fluid full of glycoproteins that covers the lining of the nose.  It serves as both a solvent and protection against infection. Humans have only about 350 types of active olfactory receptors while animals like dogs have much more. Humans have about 10 to 20 million olfactory receptor neurons. A study put the number of smells humans can tell apart at 1 trillion!! I will skip the detailed biochemical reactions related to smell.

Our vision is an amazing combination of optics, light sensitive proteins, nerve signaling and brain processing. 130 million photo receptors in our retina absorb light and transmit signals to the brain.  The bandwidth of the human retina is about 9 megabits per second. Light passes through the Cornea, Pupil and Lens and focused on the retina at the back of the eyeball. The retina has the photo receptors (proteins called Opsins). A key property is Opsins hold a molecule of Vitamin A that gets altered when exposed to light. Photoreceptors pass signals to neurons called ganglion. The axons of the ganglions are bundled into the optic nerve which ultimately passes the signal to the visual cortex of the brain.  

There are two types of photo receptors - rods and cones. Rods detect light under very low light conditions and provides little color information. A rod cell can even detect a single photon of light. Cones detect color. There are three types of cone cells - red, green and blue. Seeing color requires input from all three types of cone cells. Cone cells operate best in bright light. Both rods and cones have opsin proteins to sense light. The amino acid sequence in different opsins determine the wavelength detected. Retinol and retinyl esters are often referred to as preformed vitamin A. Retinol can be converted by the body to retinal. Retinal bound to Opsins is the chemical basis of visual phototransduction. I will skip the detailed biochemical reactions involved. 

The sense of hearing is remarkable in its range spanning from 20 to 20,000 Hertz. Normal speech is between 250 to 6000 Hertz. The nervous system responds to the sound signal in tens of microseconds. The ear drum vibrations move through the ossicles in the middle ear (tiny, sound-conducting bones called the malleus, incus and stapes). Your ossicles carry sound vibrations to your inner ear. The ciliated cells that line the cochlea in the inner ear are the key. They have hair like extensions that move like riding a wave when the vibrations from the ear drum arrives traveling through the middle ear.  This movement triggers chemical reactions that converts to a signal to the brain. The human ear has about 15,000 ciliated cells which has to be positioned in just the right places to hear properly. I will skip the biochemical reactions involved. 

I will briefly touch on the most mysterious sense - touch. There are low threshold mechano-receptors (specialized nerve ending with adventitious tissue for mechanical sensation) and temperature receptors beneath the epidermis like the Pacinian corpuscle (pressure), Meissner's corpuscle (sensitive touch), Merkel's disk (touch), Ruffini's Corpuscle (touch and pressure), Krause's corpuscle (cold), hair follicle receptor (touch), and free nerve ending (pain and temperature). I will skip the details.