woodbridge physiotherapy
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SENSORY ORGANS
Hearing, Tasting, Touching, Seeing, SmellingCan you imagine what life would be like without your five senses? How would your life be different? What would you miss the most?

The sensory organs are specialized extentions of the nervous system that respond to specific stimuli and conduct nerve impules. Senses are classified according to the location, the structure of its receptors, or which stimulus it responds to.


The Sense of Hearing
Sound causes vibrations on the tympanic membrane, or ear drum, which causes movements of the middle-ear ossicles. These movements press against the oval window and produce pressure waves within the fluid of the cochlea, and then these waves cause movement in the basilar membrance. The hair cells in the membrane bend in response to the sound and stimulate action potentials that the brain interprets. Damage to any of these parts will cause a hearing deficit or loss. Complications from otitis media, ear infection, can cause conduction deafness due to an impairment of sound waves transmission of the middle ear to the oval window. Sensorneural deafness occurs when the transmission of nerve impulses anywhere from the cochlea to the auditory cortex is impaired. Loud noises that damage the hair cells in the inner ear can cause this type of deafness because the hair cells in the inner ear cannot regenerate.

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The basilar membrane seperates the cochlear duct from the scala tympani and detects frequency, or pitch. Sound frequency produces a maximum vibrations at a different region of the basilar membrane; for example, high pitch sounds cause maximum vibrations closer to the stapes and the lower pitched sounds cause maximum vibrations closer to the apex. The basilar membrane contains sensory hair cells called stereocilia, which are large, specialized microvilli arranged in bundles. The greater the displacement of the basilar membrane and the bending of the stereocilia, the greater the frequency of the sound.



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The exact pattern of basilar membrane deformation is important, because this membrane contains the sensitive receptor cells (hair cells) which transform sound energy (i.e. the pressure waves), into neural activity (receptor potentials). At the end of the cochlea, closest to the middle ear cavity, the basilar membrane is relatively stiff and narrow (approximately 0.1 mm wide). The membrane becomes more elastic and wider as it extends throughout the cochlea towards the apex (approximately 0.5 mm wide). The stiff portion of the membrane closest to the middle ear cavity (base) vibrates immediately in response to pressure changes transmitted to the oval window. The vibrations from the base then travel along the basilar membrane toward its apex (the wide end). However, the region of maximal displacement of the basilar membrane varies with sound frequency. The properties of the membrane nearest the oval window (base) are such that it resonates optimally (under goes the largest deformation) with high frequency tones; the more distant (wider) regions of the membrane (near the apex) vibrate maximally in response to low frequency sounds. Thus, the frequencies of incoming sound waves are "sorted out" along the basilar membrane: each frequency has its characteristic place (Fig. 5). Note, however that very low frequencies (< 200 Hz) are compressed on to a relatively limited section at the apical end of the membrane.





The Sense of Seeing
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Seeing is not as simple as most would think. The eyes do not 'see'. In order for you to see an image, the brain needs to interpret the image that is sent through the different parts of the eye, through nerves and fibers, and affected by the amount of light, visual acuity, neuron pathways, and other factors both internal and external.
As light enters the cornea of the eye, it passes through the pupil to the lens, and is projected to the retina in the back of the eye. The light rays are refracted by the cornea and the lens so that it is upside down and left-to-right on the retina. The retina contains photorecptor neurons called rod and cones that synapse with ganglion cells that transmit action potentials through the optic nerve to the cerebral cortex of the brain so that the light can be perceived.
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As a student nurse, care of patients with damage to any part of the eye needs to be done with knowledge. It is important to understand how the eye works and what affects each part. A common condition that nurses see is glaucoma. The increased pressure in the eye causes progressively worsening damage. Knowing what causes the increased pressure, how it affects the eye, and how to reduce it may prolong the time a patient is able to retain his sight. We are trained to know that to avoid increased pressure, we need to teach our patients not to bend, lift, or strain. We need to know that patients with glaucoma cannot take medications that cause vasoconstriction. We also need to monitor our patients for signs and symptoms of complications, so that corrective measures can be implemented if possible.


It's a little overwhelming to think of all the processes taking place as we enjoy a cheesecake, listen to your children tell a story, smelling the fresh cut grass, holding your husbands hand, or watching a sunrise. We experience these things without thinking much about them, but our bodies are designed to have receptors to receive stimulants, pathways to send the information, and centers that can interpret and respond appropriately.


SourcesFox, Stuart Ira. Human Physiology, Tenth Edition.

http://www.medicine.mcgill.ca/physio/cullenlab/Notes209.htm
http://www.mhhe.com/biosci/ap/vdghumananatomy/student/olc2/ch15summary.html
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