THE HUMAN SENSORY SYSTEM

The Five Senses Hearing Sight Smell Taste Touch

HearingThree Sections: Inner, Middle and Outer.

Hearing When an object makes a noise, it sends

vibrations (better known as sound waves) speeding through the air. These vibrations are then funneled into your auditory canal by your outer ear (pinna).

Hammer, Anvil, and Stirrup Also know as the malleus, incus, and

stapes. Smallest bones in the human body. These tiny bones act as levers, to amplify

the vibrations created by the ear drum (tympanic membrane.

Hearing As the vibrations move into your middle

ear, they hit your eardrum (tympanic membrane) and cause it to vibrate as well. This sets off a chain reaction of vibrations. Your eardrum, which is smaller and thinner than the nail on your pinky finger, vibrates the three smallest bones in your body: first, the hammer, then the anvil, and finally, the stirrup. The stirrup passes the vibrations into a coiled tube in the inner ear called the cochlea.

The fluid-filled cochlea contains thousands of hair-like nerve endings called cilia. When the stirrup

causes the fluid in the cochlea to vibrate, the cilia move. The cilia change the vibrations into messages that are sent to the brain via the auditory nerve. The auditory nerve carries

messages from 25,000 receptors in your ear to your brain. Your brain then makes sense of the messages and tells you what sounds you are

hearing.

How do you keep your balance? Near the top of the cochlea are three loops

called the semi-circular canals. The canals are full of liquid also. When you move your head, the liquid moves. It pushes against hair like nerve endings, which send messages to your brain via the vestibular nerve. From these messages, your brain can tell whether or how your body is moving.

If you have ever felt dizzy after having spun around on a carnival ride, it was probably because the liquid inside the semicircular canals swirled around inside your ears. This makes the hairs of the sensory cells bend in all different directions, so the cells' signals confuse your brain.

Auditory Nerve Also called the cochlear nerve. Carries signals from the cochlea to the

brain to be processed.

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Sight How Does The Human Eye Work?

The individual components of the eye work in a manner similar to a camera. Each part playsa vital role in providing clear vision.

The Human Eye

Process of Vision Light waves from an object enter the eye

first through the cornea, which is the clear dome at the front of the eye.  The light then progresses through the pupil, the circular opening in the center of the colored iris.

Process of Vision Fluctuations in incoming light change the

size of the eye’s pupil.  When the light entering the eye is bright enough, the pupil will constrict (get smaller)

Process of Vision Initially, the light waves are bent or converged

first by the cornea, and then further by the crystalline lens (located immediately behind the iris and the pupil), to a nodal point (N) located immediately behind the back surface of the lens.  At that point, the image becomes reversed (turned backwards) and inverted (turned upside-down).

Process of Vision The light continues through the

vitreous humor, the clear gel that makes up about 80% of the eye’s volume, and then, ideally, back to a clear focus on the retina.

Process of Vision Within the layers of the retina, light impulses are

changed into electrical signals.  Then they are sent through the optic nerve, along the visual pathway, to the occipital cortex at the posterior (back) of the brain.  Here, the electrical signals are interpreted or “seen” by the brain as a visual image.

Process of Vision

Rods and Cones There are two types of photoreceptors in

the human retina, rods and cones. Rods are responsible for vision at low

light levels (scotopic vision). They do not mediate color vision, and have a low spatial acuity.

Cones are active at higher light levels (photonic vision), are capable of color vision and are responsible for high spatial acuity.

Rods and Cones

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Taste

Sense of Taste Taste belongs to our chemical sensing system

(chemosensation). The complicated process of tasting begins when molecules released by the substances around us stimulate special nerve cells in the nose, mouth, and throat. These cells transmit messages to the brain, where specific smells or tastes are identified.

Gustatory (taste nerve) cells are clustered in the taste buds of the mouth and throat. They react to food or drink mixed with saliva. Many of the small bumps that can be seen on the tongue contain taste buds. These surface cells send taste information to nearby nerve fibers, which send messages to the brain.

Sense of Taste There are 5 main taste

sensations: These are bitter, sweet, sour, salty, and

Umami (meat flavored) These sensations are located on different

parts of the tongue. Everything you taste is a combination of

these sensations.

Sense of Taste

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Sense of Touch Touch is actually a grab bag of various somatic senses, including the sensations of temperature, pressure, and pain, kinesthetic senses which give us a conception of our body in space, and visceral senses such as stomach aches or nausea.

Sense of Touch While your other four senses (sight, hearing,

smell, and taste) are located in specific parts of the body, your sense of touch is found all over. This is because your sense of touch originates in the bottom layer of your skin called the dermis.

The dermis is filled with many tiny nerve endings which give you information about the things with which your body comes in contact. They do this by carrying the information to the spinal cord, which sends messages to the brain where the feeling is registered.

Sense of Touch

Sense of Touch The nerve endings in your skin can tell

you if something is hot or cold. They can also feel if something is hurting you. Your body has about twenty different types of nerve endings that all send messages to your brain.

The most common receptors are heat, cold, pain, and pressure or touch receptors. Pain receptors are probably the most important for your safety because they can protect you by warning your brain that your body is hurt.

Some areas of the body are more sensitive than others because they have more nerve endings. Have you ever bitten your tongue and wondered why it hurt so much? It is because the sides of your tongue have a lot of nerve endings that are very sensitive to pain. However, your tongue is not as good at sensing hot or cold.