Introduction:

Without our sensory organs, you wouldn't be able to read this. The sensory organs, eyes, tongue, skin, nose, and ears are much more complex then most people would think. Everything that we perceive is through our senses. Waking up in the morning when our eyes adjust, tasting breakfast, feeling the warmth of your bed, and so much more are only capable with senses. Sensory receptors send a nerve impulse to the brain, and that's how we perceive everything. All of the actions of our senses happen so fast. There is so much more involved with the senses, and it's amazing at the speed that they work.

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Concept #1: Cutaneous senses

The skin is the largest organ in the body, and is the sensory organ for touch, pressure, heat, cold, and pain. There are many sensory neurons located in the skin. Free nerve endings which sense hot, cold, pain, and a light touch are located around the hair follicles and throughout the skin. Merkel's discs are located at the base of the dermis and sense pressure and a sustained touch. Located in the deep dermis and hypodermis are Ruffini corpuscles and they sense sustained pressure. Changes in texture, or slow vibrations found in the upper dermis are called Meissner's corpuscles. Pacinian corpuscles are deep within the dermis and sense fast vibrations and deep pressure. The nerve fibers for the skin don't synapse until they reach the brain. The medulla oblongota is where these synapses take place. Heat, cold, and pain sensations are sent along the spinal cord to the brain. A receptive field is an area of the skin that changes the rate that the neuron fires. The amount of receptors in one area affects the receptive field. For example the receptive field on a finger tip is going to be small, but the back will have a large receptive field because it covers more area. The two-point threshold is when two points on the skin can be felt as one point. For example, a calipers, which looks like a compass, has two metal points at the end that are the same length. When both ends touch the skin and they aren't in a distance that is too large, both ends will be felt only as one end. If you think about how many things you touch throught the day, and how complex each sense of touch is, it's pretty amazing.


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Concept #2: Taste and smell

The receptors for taste and smell are chemoreceptors because they respond to dissolved molecules. Interoceptors are chemoreceptors that respond to chemical changes in the internal environment of the body. Exteroceptors respond to the chemical changes in the external environment. Gustation is the sense of taste which are stimulated by taste buds. The sense of taste is also strongly influenced by the sense olfaction, which is smell. Within each taste bud are many specialized epithelial cells that go through a pore in the taste bud to the outside of the tongue where the saliva is. Action potenials have to happen within the sensory epithelial cells, and then release neurotransmitters that then stimulate the neurons in the taste buds. Taste cells are the name of the specialized epithelial cells inside the taste buds. Salty, sweet, sour, and bitter are the tastes that we are associated with. When different chemicals come in contact with the microvilli inside the taste cells. Umami is a new taste that was discovered which means savory. All of these taste cells are located all over the tongue, not just in one spot. The temperature and texture of the food also stimulate receptors on the tongue. Sodium ions and hydrogen ions are depolarized when the salty and sour tastes are activated. When the sodium ion goes through the channel of the taste cell salty it tasted. Hydrogen ions go through the channels in the taste cell, and sour is tasted. Sugars bind to the receptor membrane when sweet and umami are tasted. Quinine binds to the receptor protein in the taste cell when bitter is tasted. Smell, olfaction, and taste go hand in hand. The olfactory epithelium contains the receptors for olfaction. When the neurons are damaged by exposure to the environment, stem cells regenerate new receptor cells. Inside the olfactory epithelium the receptory neurons synapse with the neurons in the olfactory bulb of the cerebral cortex. In order to smell, an ordorant molecule must bind to a receptor protein. Before the odorant molecule can bind to the receptor, it first must bind with three G-protein subunits, alpha, beta, and gamma. When the odorant molecule binds to the G-protein subunits, the subunits dissociate and move into the plasma membrane. Enzymes are activated and change into different enzymes which then open ion channels and sodium ions and calcium ions flow into the cell. With these molecules rushing into the cell it causes depolarization, which then causes an action potential. Most other senses travel to different places in the brain before reaching the final destination, but the sense of smell goes directly to the cerebral cortex. The olfactory bulb is where the sense of smell begins. Taste and smell seem to be so simple for us everyday, but the path to receive the smell or taste it pretty complex.




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Concept #3: The ears and hearing

We hear all day everyday. When we are hearing noise, it's more then just the noise going into the ear. Sounds waves go into the ear and cause vibrations which allows us to hear. The waves that carry the sound are characterized by their frequency and intensity. The sound waves are funneled into ear by the pinna, or the auricle, and then into the external auditory meatus. Malleus, incus, and stapes are the ossicles that make up the inner ear. The tympanic membrane, or ear drum, is attatched to the malleus and then the waves are transfered to the incus and stapes. Vibrations of the tympanic membrane come from the oval window in the cochea, which is attached to the stapes. Sometimes when the noise is too loud, the stapedius muscle allows protection for the tiny bones. The inner ear holds the cochlea, which takes the shape of a snail and is the size of a pea. There are three chambers inside the cochlea. Perilymph is a fluid within the scala vestibuli that is displaced with the vibrations of the stapes and oval window. Scala tympani is the lower of the three chambers, while the scala media, or chochlear duct, is the middle chamber. Endolymph is the fluild that is found within the cochlear duct. If the sound frequency is too high, there isn't enough time for the waves to travel through the perilymph. When the sound is low, there is enough time for the waves to travel through the lymph. The basilar membrane which seprates the cochlear duct from the scala vestibuli contains hair cells. There are two kinds of hair cells, the inner hair cells, and the outer hair cells. Large microvilli arranged together are called stereocilia. Inside a gelatinous membrane where the hair cells are embedded is called the tectorial membrane. The functional unit called the organ of Corti is associated with the basilar membrane, inner hair cells with sensory fibers, and tectorial membrane. When the waves are going through the ear, is causes the stereocilia to bend which then opens up ion channels. Because the ion channels open up, depolarization of the hair cells happen, which then stimulates the sensory neurons. The nerve impulse is sent from the vestibulocochlear nerve which synapse with neurons in the medulla oblongata. The impulse is then sent to the midbrain, to the thalamus, and then to the auditory cortex of the temporal lobe. There are different areas of the cortex that represents different parts of the basilar membrane, and a different pitch. All of this must happen in order to hear. The rate that these impulses are sent is amazing.

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Application:

Not everyone has all five senses. There are many people in the world who are blind and deaf. Understanding how the senses work as a nurse is very important. The process for each sense is complicated, but important. The sensory organs play some what of a personal role for me. My father, with a traumatic brain injury, does not have the same sensations for all of his senses as he did before his accident. Also my grandmother has macular degeneration which runs in my family. Most people when they get older lose some of their hearing. If I see a patient with conduction deafness, being able to explain the help of hearing aids is crutial. Being able to have all of your senses work is a blessing. Not being able to hear, see, smell, or taste would be horrible. Understanding why my father doesn't have the full working power of his sensory organs is good for me as his daughter. But more important is being educated on why those senses don't work to their fullest extent. Also with my grandmother, I can understand why she can't see very well.

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Essential question:

1. Sound travels from the outside world into the outer ear which contains the auricle and the external meatus. The external meatus sends the sound waves into the tympanic membrane. The sound waves then vibrate against the tympanic membrane, also known as the ear drum. Between the tympanic membrane and the outside of the cochlea is the middle ear which has the ossicles. These tiny bones, the malleus, stapes, and inncus are called the middle-ear ossicles. The vibrations of the tympanic membrane are sent to the malleus, then the incus, and lastly the stapes. The stapes are attatched to a membrane inside the cochlea called the oval window. The cochlea contains three chambers. Perilymph is a fluid inside the scala vestibuli which is the top chamber of the cochlea. The middle chamber is the scala media which is a membranous labyrinth that has endolymph inside of it. The lowest of the three chambers is the scala tympani. When the sounds waves are going through the perilymph they are traveling through all the chambers in the cochlea. The basilar membrane is the area between the scala tympani from the cochlear duct. The basilar membrane is the place where pitch discrimination occurs. Hair cells which are on the basilar membrane are projected into the endolymph. Stereocilia, which are actually hair cells, are moved in the direction of the tallest member of the cilia. When this movement happens, it's allows ion channels to open up which causes a depolarization of the hair cells. Glutamate is the neurotransmitter that is released because of the depolarization. An action potential is sent along the vestibulocochlear nerve to synapse with the neurons in the medulla oblongata. After the medulla oblongata the impulse travels to the midbrain, then into the thalamus, to the auditory cortex, and finally to the temporal lobe. The auditory cortex represents the different parts of pitches and the basilar membrane.

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2. The outermost part of the eye is called the sclera, which is a tough coat of connective tissue. Continuous with the sclera is the clear, or transparent cornea. After light passes through the cornea it enters the anterior chamber of the eye. Light will then go through an opening called the pupil. The iris, which is a colored, or pigmented muscle, surrounds the pupil. To bring in more or less light, the iris can adjust the diameter of the pupil. After the light goes through the pupil, it reaches the lens. The ciliary body is a muscular process that connects the sclera and encircles the lens. Aqueous humor is the fluid that fills that anterior and posterior chambers of the eye. The part of the eye that is behind the lens is filled with vitreous body, which is a viscous substance. After light passes through the lens, then into the vitreous body, and into the back of the eye which contains the photoreceptors. The retina, or neural layer, has the photoreceptors. While the light is passing through the retina, it's stimulating photoreceptors, which activates other neurons. The optic disc is where the neurons gather their fibers from the retina, and exit as the optic nerve. Because the eye contains different densities, the light ray has to be bent, or refracted, which sends the right half of the visual field to the left half of the retina of both eyes. The left half of the visual field is then projected into the right half of the retina of both the eyes. The left retina is the temporal half, while the right retina is the nasal half. Left lateral geniculate nucleus of the thalamus is the place where the left half of the left retina pass through. The right lateral geniculate body receives the input from eyes that relate to the left half of the visual field. Striate cortex ,of the occiptal lobe in the cerebrum, is where both lateral geniculate bodies send their message to. The pathway of this impulse is needed for visual perception.

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References from top to bottom:

http://1.bp.blogspot.com/_ho2Zi96OCDM/SEWfEvpD5aI/AAAAAAAAASg/1fT-43ufgOM/s320/image.jpg

http://www.mediaspin.com/fivesenses/images/fivesenses_banner.jpg

http://www.exploringnature.org/graphics/anatomy/sensory%20organs.jpg

http://www.wyoming.k12.mi.us/014/academics/science/Rathsack/physiology/IntegumentaryWebquest/sensoryreceptors.jpg

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http://www.youtube.com/watch?v=pM7H0Wud_Y0

http://www.unmc.edu/physiology/Mann/pix_4b/gust_receptors.gif

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