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HumanSenses

HearSight

TouchSmellTaste

By: Antonio Gonzalez

How do we incorporate each one in our daily lives?

What organs are involved?

How do they work?

January, 2012

The Five

Table Of Contents

Sense Title PageHearing Sound 3

- The Structure Of The Ear

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- The Processing Of Sound

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Sight The Eye 6

- How We See 7

- The Protective System 8

Taste Taste 9

- Taste Sensation 9

Smell Smell 10

Touch Touch 11

Built ForSurvival

Through generations of adaptation, humanity has evolved into a biological phenomena.

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Human beings are defec-tive, in the sense that they are biologically hampered by poor stamina, are structurally small in size, and are drastically tem-perature sensitive. With no doubts it has been our senses that have helped our commu-nity grow the most. Our sens-es 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

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.

40Million people around the world are blind.

9, 000Number of taste buds in a tongue.

2.5Length, in centimetres, of the auditory nerve.

“We live on the leash of our senses”

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 com-prehending their true importance. What would it be like living without one, or multiple, of theses major senses?

THE FIVE HUMAN SENSES

100Number, in billions, of nerve cells packed in the human brain.

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Hearing

Everything in our body has a PURPOSE, including our senses, so why do we hear?

To evade danger To navigateTo communicate

Tounderstandthescienceofhearing,onemustfirstlearnaboutthe concept of sound itself.Onemayidentifysoundassimplevibra-tions,thatmustpassthroughmediums.Sound,tobeperceivedbythebrain,musttheoretically‘bounceoff’matter.Toputitinscientificterms, sound is a sequence of rhythmic waves, composed of conden-sations and rarefactions.(Observefigurebelow)

Hearing is the ability to perceive sound by detecting vibrations.

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3) The Inner Ear The amplified sound waves now en-ter 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 electri-cal nerve impulses which are then directly transmitted to the brain.

The ear may be divided into three unique parts, each with their own responsibilities.

The outer, middle, and inner ear.

The 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 trans-mitted and amplified by 3 hinged inner ear bones; The Malleus, Incus, and Stapes. These bones are connected to a coiled fluid cham-ber, called the cochlea (see more on page 7).

The Processing of SoundOn 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.

Our brains interpret the pitch of sounds in a very particular way. It is in fact the hair cells in the coch-lea 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, fre-quency is the rate at which sound waves ‘bend’, whether they are in a state of rarefaction, or conden-sation. 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.

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 differ-ent 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, organ-isms 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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Sight

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.

Sight is the most structurally and anatomically complex sense, specially when considering the size of its organ, the eye.

The Eye

The eyeball may be divided in 3 layers

1) The Sclera This is first protective layer of the eyeball. It is fi-brous 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 de-liver 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 SeeSight 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 cor-nea 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 open-ing 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 jelly-like 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 stimu-lated. This stimulation causes for the retina to send nerve signals through the optic nerve, to the brain. Finally, the visual cor-tex of the brain interprets these signals into visual images.

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Protective SystemSince sight is a very delicate sense, it is protected

by various systems.

EyebrowsThe 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 mois-ture to flow sideways, away from the eye.

EyelashesThe 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.

EyelidsThe eyelids protect the eyeballs from injury by sweeping dirt on the surface of the eye

TearsTears are drops of clear salty that are secreted from glands in a person’s eyes. They help the eyes wash out any unwant-ed particles, such as dirt or dust.

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 respon-sible for a different motion of the eye.

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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 informa-tion 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 gustato-ry receptor cells and the food, creating a more accurate sense of taste.

To narrow things even further, each gustato-ry receptor cell has a hair (named the gusta-tory hair), which sticks through an opening called a taste pore. Saliva, along with mol-ecules of food(s), will enter through these pores and interact with the gustatory hair in that certain receptor cell. This will stimu-late the sense of taste. The stimulus activates an impulse, which in turn sends electrical impulses to the gustatory portion of the cer-ebral cortex.

Taste sensation is made up of 4 distinct cat-egories, which are: sweet, bitter, sour and salty. Different parts of the tongue are more or less sensitive to the 4 taste sensations. For exam-ple, 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.

Taste Sensation

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SmellSmell 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 inte-rior of the nasal cavity. Mucus acts as a bait for smell, as it essentially captures the odour mol-ecules. 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 extreme-ly 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 con-tact with a receptor cell, that cell will com-municate with nerve cells located in the olfactory bulb. This bulb carries information to the brain via the olfactory nerves.

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TouchOf 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 epider-mis and the hypodermis. The dermis is full of nerve endings which work to give you information on the ob-jects 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 responsi-ble 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 tem-perature. 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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