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There is a classic riddle that asks, if a tree falls in the middle of a forest, and nobody is there to hear it, did it actually make any sound?

The Five Senses

If a tree falls in the middle of a forest, and nobody is there to hear it, does it make any sound?

From a physics point of view, the falling tree creates sound waves that travel away from the tree as it crashes to the ground. From a biology perspective, a stimulus, such as sound, has to be received and perceived in order to be sensed. If nobody is in the area to hear the tree crash, the sound was not perceived.

What is perception? Perception is the recognition that a stimulus is present. In order to perceive a stimulus, the brain has to receive information that there is a stimulus: this information is called sensory information. The basic sensory pathway is the same for all types of sensation. The stimulus interacts with a receptor that receives the signal. The stimulus can be sight, sound, temperature, pressure, or any other stimulus recognized by sensory receptor. When a receptor is activated, the information is relayed to the central nervous system (the brain). Once there, the information is processed, and this is where perception happens. If the information is never perceived, thenthe body does not acknowledge that it exists. The human body has five senses: sight, sound, smell, taste, and touch. Each sense receives a different type of information at its sensory receptors. Although the types of stimuli and receptors differ, all five senses send information to the central nervous system for processing and perception. This basic information pathway is similar in the following discussion of the five senses. To keep things simple, the discussion focuses on the five senses in humans. Keep in mind that the details vary from one species to the next. Vision Visual perception, or sight, is the ability to perceive the surrounding environment visually. The organ of sight is the eye, which contains two typeof receptors, or photoreceptors, called rods and cones. Rods sense light and dark, while cones sense color.

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When a visual stimulus enters the eye through the

pupil, it crosses the interior chamber of the eye andhits the back surface. The back surface contains theretina, which is a sheet of light-sensitive tissue containing the rods and cones. When the stimulus activates the rods and cones, they send this information to the brain through theoptic nerve. The information travels to the occipitallobe in the back of the brain, where it is processed. When visual information hits the retina, the image is actually upside-down. The brain has to flip the image right-side-up when it is processed.

The Five Senses

The eye is responsible for vision.

The human ear is divided into the outer, middle, and inner ear.

Hearing The ability to perceive sound is called hearing. Sound travels as waves of compressed air through the air to the ear, the organ of hearing. The ear is divided into the outer, middle, and inner ear. The outer ear can be seen when looking at a person’s head. The outer ear is shaped so that sound is funneled into the ear canal. The sound waves travel through air in the canal until they come to the eardrum, also called the tympanic membrane. When the sound waves reach the eardrum, the eardrum vibrates in response. When the eardrum vibrates, it causes a series of very small bones called ossicles to vibrate in the middle ear. The ossicles transmit the sound waves

to the fluid-filled inner ear. The sound receptors, located in the inner ear, are aseries of small hairs called cilia that bend in responactivated, these receptors send signals to the brain.

se to sound waves. Once

The more intense the sound, the farther the cilia bend, and the stronger the signal sent to the brain. If the ear is exposed to an

The Five Senses

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Touching an object activates skin thermoreceptors, nociceptors, and mechanoreceptors. chemicals.

When olfactory receptors are activated by an odor, the information is communicated by olfactory bulb neurons to the olfactory cortex in the brain. A single receptor can respond to multiple odorants, yet the human brain can distinguish among hundreds of odors. Taste The fifth sense, taste, is also driven by the activation of chemoreceptors. These chemoreceptors are located in taste buds on the tongue. A small number of taste buds are also located along the sides and roof of the mouth, and in the throat, which means people can taste food in their throats. The chemoreceptors of taste detect specific chemical characteristics of food. This information is sent to the brain, where it is interpreted as one of five basic tastes: sweet, sour, bitter, salty, or umami (savory), or some combination thereof. The sense of taste is heavily dependent on the sense of smell. If people eat fruit-flavored candy with their noses plugged, they are

excessively loud sound, these cilia can become damaged, resulting in loss of hearing. Touch The sense of touch responds to several different stimuli. Touch is felt throughout the body. Each type of stimulus has a different type of receptor:

• Thermoreceptors respond to temperature. • Nociceptors respond to pain. • Chemoreceptors respond to chemical stimuli. • Mechanoreceptors respond to pressure.

Sensing touch often combines information from more than one of these receptors. Think about a hot object touching the skin. The person feels the pressure of the object, its heat, and pain, if it is too hot. Smell

The sense of smell has the scientific name olfaction. The sensory receptors for smell are in the nasal cavity. Olfactory receptors are chemoreceptors, which refers to their detection of odors, which are

The Five Senses

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often unable to tell the flavor of the candy. They may be able to perceive that the candy is sweet, but the subtleties of the flavor are lost without smell. All together, the five senses rely on the common pathway of a receptor receiving a stimulus, and sending this information to the brain for processing. Without this pathway, a tree could fall in the forest, producing sounds waves, but it would never be seen or heard.