The Five Human Senses
January, 2012
How do we incorporate each one in our daily lives? What organs are involved? How do they work?
Hear Sight Touch Smell Taste By: Antonio Gonzalez
Table Of Contents Sense
Title
Page
Hearing
Sound
3
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The Structure Of The Ear
4
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The Processing Of Sound
5
Sight
The Eye
6
-
How We See
7
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The Protective System
8
Taste
Taste
9
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Taste Sensation
9
Smell
Smell
10
Touch
Touch
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Built For
Survival Through generations of adaptation, humanity has evolved into a biological phenomena. Human beings are defective, in the sense that they are biologically hampered by poor stamina, are structurally small in size, and are drastically temperature sensitive. With no doubts it has been our senses that have helped our community grow the most. Our senses are everywhere at all times without us noticing. We have restaurants playing on our sense of taste, advertisements playing on our sense of sight and even music, playing on our sense of hearing. In fact, even the cosmetics industry works hard at targeting our
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smell and beauty. Moreover, our senses are crucial to our survival. Through them, we learn to evade danger or harm. Each sense is special in its own way, yet they are all of great value to us. Smell allows for us to anticipate food, which prompts our digestive system to produce digestive enzymes. Taste also enables the creation of enzymes. Sight impacts our state of mind, and even the way that we think about the world. Hearing allows for our species to communicate and grow.
THE FIVE HUMAN SENSES 40
Million people around the world are blind.
9, 000
Number of taste buds in a tongue.
2.5
Length, in centimetres, of the auditory nerve.
100 “We live on the leash of our senses”
Number, in billions, of nerve cells packed in the human brain.
We, as human beings, constantly fail to perceive the complexity of our bodies. Every part has a crucial importance, that aids us in accomplishing what our bodies were built for; to survive. More commonly known and taught, however, are the 5 human senses. Hear, touch, smell, taste, and see. These senses are commodities that we take granted for without comprehending their true importance. What would it be like living without one, or multiple, of theses major senses?
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Hearing Hearing is the ability to perceive sound by detecting vibrations. Everything in our body has a PURPOSE, including our senses, so why do we hear?
To communicate
To evade danger
To navigate
To understand the science of hearing, one must first learn about the concept of sound itself. One may identify sound as simple vibrations, that must pass through mediums. Sound, to be perceived by the brain, must theoretically ‘bounce off’ matter. To put it in scientific terms, sound is a sequence of rhythmic waves, composed of condensations and rarefactions. (Observe figure below)
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The Ear The ear may be divided into three unique parts, each with their own responsibilities. The outer, middle, and inner ear.
1) The Outer Ear
First, the pinna (1) acts as a funnel that, through its helical shape, assists in redirecting sound into the auditory canal (2). The pinna also eliminates resistance to some extent, by regulating the differences in pressure inside and outside of the ears. Sound moves through the auditory canal for 2-3 centimetres, before hitting the eardrum (3), also known as the tympanic membrane.
2) The Middle Ear
The moving air causes the eardrum to vibrate. These weak vibrations are then transmitted and amplified by 3 hinged inner ear bones; The Malleus, Incus, and Stapes. These bones are connected to a coiled fluid chamber, called the cochlea (see more on page 7).
3) The Inner Ear
The amplified sound waves now enter the cochlea. The cochlea, located in the inner ear, is considered are the ‘organ of hearing’. In it are thousands of sensory hair cells, which convert mechanical movement (caused by sound vibrations), into electrical nerve impulses which are then directly transmitted to the brain.
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The Processing of Sound On a larger scale, sound hits the inner ear, where it is then converted into neuronal signals. This process enables for humans and other organism to both interpret and understand a wide range of sounds.
Interpreting the Pitch of a Sound
This figure shows which parts of the cochlear duct respond to which sound frequencies
Perceiving Sound Signals Scientific studies have concluded that there are areas of the temporal lobe on both sides of the brain, that are responsible for interpreting the many different aspects of sound. The brain also learns to identify and recognized sounds based on factors such as their tonality, volume, or even duration. Over time, organisms learn to connect sounds with a certain response. For example, a person might know that lunch is ready when they hear the beeping of the microwave.
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Our brains interpret the pitch of sounds in a very particular way. It is in fact the hair cells in the cochlea that convey these variations in tonality, by responding to different frequencies at different locations along the cochlear duct. Looking back at the figure on page 5, frequency is the rate at which sound waves ‘bend’, whether they are in a state of rarefaction, or condensation. The faster they bend, the higher the pitch of the sound will be. The end of the duct responds to low-frequency sounds, whilst the entrance to the duct responds to high-frequency sounds.
Sight Sight is the most structurally and anatomically complex sense, specially when considering the size of its organ, the eye.
The Eye Sight is a human being’s most developed sense. With a single glance, our eyes will send nervous impulses to our brains, which contain information regarding the shapes, sizes, textures and colours of the objects that we see. Additionally, our eyes have are able to read movement, in the sense that we can tell whether an object is stationary or not. Eyes can also perceive depth, enabling us to understand distance.
The eyeball may be divided in 3 layers 1) The Sclera This is first protective layer of the eyeball. It is fibrous tissue that essentially surrounds the eyeball, and attaches itself to the cornea. The sclera may be identified as the white portion of the eye.
2) The Choroid The choroid is the middle layer of the eyeball, and contains various blood vessels which work to deliver a constant supply of oxygen and nutrients to the inside of the eye.
3) The Retina The retina is a soft layer of nervous tissue which is responsible of carrying signals to the brain through the optic nerve, where they are then interpreted as images.
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How We See Sight begins with the cornea focusing large amounts of light into the eye. The cornea is see-through and so cannot be seen by the naked eye. Behind the cornea is a fluid called the aqueous humor. Along with the cornea, this fluid forms an outer lens that refracts light.
The iris, which is the coloured circular membrane, controls the amount of light that enters the eye through the pupil. The pupil is a circular opening in the centre of the iris. To control the amount of light that enters the pupil, the iris either widens or narrows to allow for more light or less light respectively.
Behind the iris is a lens which focuses light to create clear images. After light has been focused by the cornea and the aqueous humor, it hits this lens. The light is then sent through the vitreous humor, which is a jellylike material, and onto the retina.
The retina consists of over 30 million light receptors called rods and cones. Rods allow for us to see in dim light by detecting shades of grey. They are a lot more sensitive to light than cones. Cones allow for us to see in bright light, and help us distinguish between colours. When the retina is exposed to the focused light, the cones and rods are stimulated. This stimulation causes for the retina to send nerve signals through the optic nerve, to the brain. Finally, the visual cortex of the brain interprets these signals into visual images.
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To increase the efficiency of our eyes, they have to be able to move in all directions. There are 6 extra-ocular muscles that allow for easy movement (as picture on the left). The muscles on each eye move simultaneously, as this allows for the eyes to remain aligned. Each muscle is responsible for a different motion of the eye.
Protective System
Since sight is a very delicate sense, it is protected by various systems.
Eyebrows
Eyelashes
Eyelids
Tears
The eyebrows are strips of hair that grow on the ridge above the eye sockets. It mainly prevents moisture (in the form of sweat or water) or any debris from coming into contact with the eyes.. Its curved shape allows for the build up of moisture to flow sideways, away from the eye. The eyelids protect the eyeballs from injury by sweeping dirt on the surface of the eye
The eyelashes work to keep the eyeball clean by collecting small amounts of dust or dirt particles floating in the air. The eyelashes also protect the eye from lights.
Tears are drops of clear salty that are secreted from glands in a person’s eyes. They help the eyes wash out any unwanted particles, such as dirt or dust.
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Taste Our mouth and tongue is filled with around 9,000 taste buds. These numerous taste buds pick up on the various chemicals that different foods possess, and then through nerves, this information is passed on to the brain which in turn interprets that information as a taste. Each separate taste bud is made up of around 50 gustatory receptor cells. Taste buds also contain themselves in drop-shaped papillae, which can be seen as small bumps along the tongue, for example. These bumps allow for further friction between the gustatory receptor cells and the food, creating a more accurate sense of taste. To narrow things even further, each gustatory receptor cell has a hair (named the gustatory hair), which sticks through an opening called a taste pore. Saliva, along with molecules of food(s), will enter through these pores and interact with the gustatory hair in that certain receptor cell. This will stimulate the sense of taste. The stimulus activates an impulse, which in turn sends electrical impulses to the gustatory portion of the cerebral cortex.
Taste Sensation
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Taste sensation is made up of 4 distinct categories, which are: sweet, bitter, sour and salty. Different parts of the tongue are more or less sensitive to the 4 taste sensations. For example, the tip of the tongue is most sensitive to sweet and salty tastes, the sides of the tongue are most sensitive to sour tastes and the back of the tongue is most sensitive to bitter tastes. The centre of the tongue is not very sensitive to any of the taste sensations. This is because it has fewer taste receptors.
Smell Smell is very closely linked with the sense of taste, as both of these senses depend on receptor cells’ ability to react to the presence of distinct chemicals. Different signals will be created with the presence of different chemicals. In the olfactory system, the receptors, which are present in the nose, convert these chemical signals into electrical signals which are then sent to the brain.
When we inhale, odour is dissolved in the interior of the nasal cavity. Mucus acts as a bait for smell, as it essentially captures the odour molecules. Mucus is constantly being renewed to ensure an efficient sense of smell. The back of the nasal cavity is filled with over 40 million olfactory receptor cells. These specialized cells are extremely sensitive to smell.
Much like gustatory receptor cells, olfactory receptor cells have hairs cilia. These hairs float in mucus. Each receptor cell has around 20 of these hairs, which work to increase the surface area of the cell. An increase in surface area enhances our sense of smell.
When an odour molecule comes into contact with a receptor cell, that cell will communicate with nerve cells located in the olfactory bulb. This bulb carries information to the brain via the olfactory nerves.
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Touch Of all the senses, the sense of touch comes with the body’s largest organ, skin. Unlike all of the other senses, the sense of touch is all over the body. The sense originates from a layer of your skin called the dermis. This layer of skin is found between the epidermis and the hypodermis. The dermis is full of nerve endings which work to give you information on the objects that you come into contact with. The information is actually passed through the spinal cord, where it is then relayed to the brain. The brain then registers the sensation.
The dermis has around 20 types of nerve endings, each responsible for a specific receptor such as heat, cold, pain or pressure. Pain receptors are there as a safety measure as they warn your brain that your body is being hurt.
Different parts of the body are more sensitive to touch than others, and this all depends on the amount of each receptor type in that place. For example, the tongue has a lot of nerve endings and it is very sensitive to pain and temperature. The finger tips are also the most sensitive to touch. In fact, there are about 100 touch/pressure receptors on our fingertips.
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