The Special Senses

What we see, hear, taste, smell, and our balance are all part of our special senses.  They are called special senses because their sensory receptors are located within large sensory organs in our heads (the nose, eyes, ears, and tongue). 

Fun fact:  Touch is not considered a special sense but a generalized one.

Chemical receptors that respond to changes in chemical concentrations are call olfactory receptors.  In the nose, chemicals must be dissolved in the mucus in order to activate smell receptors.  The receptors are located in the olfactory organ which is found in the upper part of the nasal cavity.  Once the smell receptors are activated, they send the information to the olfactory nerves who send it to olfactory bulbs and tracts to different areas in the cerebrum.  From there, it is determined as to what particular smell it is.  

Fun facts: -Because there are fewer smell receptors in humans and because the chemicals have to diffuse all the way up the nasal cavity, our sense of smell is poor compared to an animal. 

                -The same chemical can only stimulate smell receptors for a short period of time and then they no longer respond to that odor (it can no longer be smelled).  This is called sensory adaption.

  

Our tongues consist of tiny little bumps (papillae) that are our taste receptors.  Taste cells and supporting cells make up each taste bud.  Taste cells are types of chemoreceptors like the olfactory cells of smell.  Chemicals that are in the foods and drinks we in ingest have to be dissolved in saliva in order to activate the taste cells.  There are 4 types of taste cells:

• Sweet- found on the tip of the tongue.
• Sour- found on the sides of the tongue
• Salty- found on the tip and sides of the tongue
• Bitter- found at the back of the tongue

Fun fact:  Spicy is not an actual taste sensation.  Spicy foods activate pain receptors that interpret the food as hot or spicy.

Our eyes are complex organs.  They process light in order to produce the images we see throughout the day.  There are 3 layers of the eye:

• The outer layer is the white of the eye called sclera.  Light cannot pass through this layer and covers everything except the front of the eye which is transparent and acts as a window to let light in called the cornea.  
• The middle layer is the choroid.  Most of the eyes blood vessels are located here.  It contains the iris or colored part of the eye and the ciliary body.  The iris is made of muscular tissue that contracts and relaxes to make the center (pupil) grow smaller or larger.  The purpose of this is to regulate the amount of light the eye takes in.  The ciliary body controls the shape of the lens, allowing us to view close or distant objects.  
• The inner layer consists of the retina.  The nerve cells of the retina sense light.  The optic disc is where the optic nerve enters the retina and there are no sensory nerves.  This area is also known as the blind spot.  There are 2 types of nerve cells which are rods and cones.  Rods are sensitive to light and function in dim light.  Cones work best in bright light and are sensitive to color and provide us with sharper images.

Accessory organs of the eye include the orbits, eyebrows, eyelids and eyelashes, conjunctivas, the lacrimal apparatus, and the extrinsic eye muscles. The eye orbits (eye sockets) are the protective shell around the eye.  Eyebrows, eyelashes and eyelids are all protective organs to help keep particles out of the eye as well as eyelids are to help prevent the mucous on the eye from drying out.  Conjunctivas are the mucous membranes that line the eyelids and cover the surface of the eye to keep the eyeball moist. The lacrimal apparatus contain lacrimal ducts that produce tears.  Tears contain enzymes that help to destroy bacteria and viruses that the eyeball may come in contact with.  Extrinsic eye muscles are what move the eyeball so that we can look different ways.  In order for us to see, light is reflected from an object and enters the eye through the cornea.  From the cornea, it goes through the pupil, lens, and fluids and onto the retina by bending it (refraction).  At this point the image is upside-down.  The retina takes the light and converts it into nerve impulses which are sent along the optic nerve to the brain where it is sent to the visual area in the occipital lobes of the cerebrum and interpreted.  Half of what we see in the right eye is interpreted in the left side of the brain and the left eye in the right side where it is brought together and turned right-side up to develop the image of object.

Fun fact:  Newborns do not produce tears.  We don't start producing tears until about 4 weeks old.

Our ears are what provides our sense of hearing.  Like the eye, the ear is divided into 3 parts

• The external ear which is composed of the auricle and external auditory canal.
• The middle ear which begins with the tympanic membrane and ends at the oval window.  It also contains the eustachian tube that helps maintain equal pressure on both sides of the eardrum.
• The inner ear consists of complex systems of communicating chambers and tubes known as labyrinth.

The auricle is where sound waves are collected.  They are carried through the auditory canal to the tympanic membrane.  The canal also contains hairs and glands the produce cerumen, (earwax) that traps dirt, dust, and other microbes.  Once the sound waves reach the tympanic membrane it causes vibrations.  When it vibrates, it causes the ossicles (3 tiny bones known as the malleus, incus, and stapes) to vibrate and hit a membrane called the oval window.   The labyrinth is divided into 3 portions:  semicircular canals, a vestibule, and a cochlea.  There are 3 semicircular canals.  These canals detect the body's balance.  The cochlea contains the hearing receptors including the organ of Corti, which is the largest organ of hearing.  The area between the semicircular canals and the cochlea is the vestibule and it also helps in equilibrium.  When the head moves, fluids in the canals and vestibule move to activate both the equilibrium and the hearing receptors.  The equilibrium receptors send the information along the vestibular nerves to the cerebrum to be interpreted.  Sound waves consists of different frequencies that move through the air and when it is collected they are channeled to the tympanic membrane.  The vibrations are amplified and cause the hairs of the cochlea to move triggering nerve impulses.  The auditory nerve transmits the impulses to the brain where they are recognized as sounds.  

Without our senses, we wouldn't have the privilege to experience all unique features the world has to offer.  Our brains are working constantly to allow our senses to be interpreted.  Even though they are unconscious acts to us, we don't realize how important they are until we no longer have the ability to enjoy them.