Anatomy And Physiology
The five human senses

The senses: Touch, taste, sight, sound and smell



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The five human senses
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"The senses: Touch, taste, sight, sound and smell"
Caption: The five human senses
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Image by: Alicia M Prater, hand image by Evan Amos (Creative Commons), other images public domain
© Creative Commons http://commons.wikimedia.org/wiki/File:Human-Hands-Front-Back.jpg

The senses are a concept taught very early in human education. They define the environment, allowing people to act and react to their surroundings as well as survive and adapt. Sight, smell, sound, taste and touch are the five senses – all attached to the brain via nerve endings called receptors, which function in specialized organs meant to obtain specific information related to the sense. Though some people survive without one of the senses (and occasionally more than one), human existence is defined by the following five physiological sets of equipment.

Vision – the eyes

Sight is the act of interpreting light waves and creating a picture in the brain. Every object reflects and absorbs light wavelengths (red, blue, green, etc). Reflected light travels off the surface of the object until it meets another object. The clear cornea and open pupil of the eye allow these light waves to pass into the eyeball, where they are focused by a lens (part of the refractory system of the eye) onto the retina. The retina at the back of the eye is lined with light receptors, called rods and cones. These receptors are specialized nerve endings that send information to the brain about the brightness and color of the light waves. The brain interprets this information and you “see” a picture of what the eyes are focused on.

Auditory – the ears

Hearing is the act of receiving and interpreting sound waves. These waves cause movement of the air around objects, called vibration. Propagation of these waves creates sound, but you need equipment capable of interpreting the waves, such as the human ear. Sometimes these sound waves can be felt, such as with a loud bass in a car, because they are simply the movement of air. The outer ear is shaped in a way that helps direct sound waves into the ear canal. The waves then vibrate the eardrum, also known as the tympanic membrane. The waves are propagated through the three bones of the middle ear to the oval window, another membrane. The waves then propagate in the fluid of the cochlea. Special hair receptors in the inner ear pick up on the vibrations, sending the information to the brain where it is interpreted for pitch, volume and tone - creating sound.

Olfactory – the nose

Smelling (known as olfaction) is the act of recognizing airborne molecules, called odor molecules. Only objects that contain volatile molecules have a “smell”. These volatile molecules travel in the air, potentially making their way into the nostrils. Molecules that do make it inside the nose come into contact with specialized odor receptors on the roof of the nasal cavity, the olfactory epithelium. Which of the thousands of receptors the molecule activates tells the brain what the molecule is, triggering the awareness of scent.

Taste – the tongue

Tasting is the act of recognizing chemicals in the mouth, particularly from ingested food items. Chemicals and ions dissolved in the saliva, called tastants, are detected by the taste buds on the tongue. Five sets of taste buds recognize five different chemical signatures: salty, sweet, sour, bitter and umami (or savory). These buds contain nerve endings that transmit to the brain. What you taste depends on which nerve endings are activated.

Touch – the skin

Touch is the act of recognizing contact with an object or surface. The skin is the largest organ in the human body, and within its layers are several different types of nerve endings that detect pressure and temperature. Some of the nerve endings are superficial, others are deeper. Which nerves are stimulated in the context of other nerve endings being activated determines what is felt, and it may be different for different people (what “wet” feels like to you may not be exactly what it feels like for someone else). These sensations are learned as a person develops and grows – they are told something is “wet” and equate that sensation with that stimulus.

Humans as a sensory organism

The five senses work together to form a picture of the world. Research has shown that people have emotional responses to color, body language and smells. They also serve to protect the human body. Extremes in pressure or temperature alert the brain to react before damage is done to the body. For example, something that is hot results in pain, alerting the brain to withdraw the affected body part before it can be severely burned. The bitter taste of toxins also alert the body to not eat a certain item, preventing poisoning. The senses also allow verbal and non-verbal communication. A human simply wouldn’t be human without them.

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