.............................. THE ROLE OF RELATION III. THE SENSE ORGANS

Activities to do. Read the explanation of the mechanoreceptors and thermoreceptors and perform the first Multiple Choice Test and the first Connect Pictures With Names. Then read the explanations of the other types of receptors and perform the second Multiple Choice Test, the Connect Pictures With Names and the Crossword.

1 . The sensory receptors and the sense organs. The sensory receptors are the structures containing cells specialized in detecting specific types of the environmental variations, when those variations get past a certain value (threshold) they originate a nerve impulse that is transmitted through nerve cells. These types of variations are called "stimulus". The sensory receptors can be seperated by the body, as with temperature sensory receptors, or they may be grouped together forming the "sense organs", such as those which constitute the eye or the ear.

2 . The types of the sensory receptors. The sensory receptors are classified depending in the type of stimulus that they capture, in:

• Mechanoreceptors (they capture the mechanical effects), as touch receptors in the skin (sense of touch), the inner ear balance and those of the cochlear of the ear (sense of hearing).
• Thermoreceptors (they capture the temperature) as the skin thermoreceptors.
• Chemoreceptors (they capture the chemical substances) as the mucous membranes of the nose olfactory (sense of smell) and taste buds of the tongue (sense of taste).
• Photoreceptors (they capture the light) as the retina of the eye (sense of sight).

3 .The mechanoreceptors of the sounds or the fonorreceptores. They are responsible for the sense of hearing, ie capturing the sounds. Are the ears. The air vibrations move the eardrum and are transmitted through the ossicular chain to the oval window membrane which is connected with inner ear chambers and canals filled with a fluid called endolymph. From the first chamber called the utricle leave three semicircular canals and from the second chamber called the saccule leave a long spiral-shaped conduit called cochlear duct or cochlea. All these cameras take up the cavities of the temporal bone filled with a fluid called perilymph. When there is a sound the endolymph, that fills the cochlea, moves and this stimulate the cilia internal sensitive cells, which communicate with the acoustic nerve which informs the brain from where the sound comes.

4 . The equilibrium mechanoreceptors. The responsible for the sense of static equilibrium or the "still body" are the sensitive cells that are inside of the utricle and the saccule. They act in response to the pressure variations of the internal endolymph. The responsible for the dynamic equilibrium or "body in motion" are the internal sensitive cells of the semicircular canals which are also filled with endolymph.

5 . The mechanoreceptors of the skin. They are responsible for the sense of touch, ie capturing the pression on the skin. Are the Meissner's corpuscles and Vater-Pacini corpuscles, which consist of nerve endings and conjuctive tissue.

6 . The termoreceptors of the skin. They are responsible for detecting the temperature of the bodies. Are the Krause's corpuscles (sensitive to the heat output or cooling sensation) and Ruffini corpuscles (sensitive to heat input or burning sensation), which as well consist of nerve endings and conjuctive tissue.

7 .The chemoreceptors of the nostrils. They are responsible for the sense of smell, ie uptake of the substances dispersed in the air. There are neurons that are intercalated in the olfactory mucosa or the yellow pituitary on the roof of the nasal passages.

8 .The chemoreceptors of the tongue. They are responsible for the sense of taste, ie uptake of the substances dissolved in liquids. They are the sensitive cells that form the taste buds and are located in protuberances of the tongue known as the papillae.

9 . Photoreceptors. They are responsible for the sense of sight, ie the capture of light. Are the eyes. The light passes through the cornea, which is the transparent front part of the sclerotic (the white front part of the eye), enters through the pupil, crosses through the crystalline lens (lens that focuses the image) and is projected onto the retina, the layer that has light-sensitive cells (the cones and the canes) which passes the received stimulus to the optic nerve that goes to the brain. The place of the retina where the optic nerve passes through is called the blind spot because there is no visual sensitivity. Near of it, there is a depression called the fovea, surrounded by a ring called the macula lutea, with a high concentration of cones and, therefore, there is more visual efficiency.

10 . The diseases of the sense organs. The main diseases are:

• Myopia. The defect of the ocular refraction that places the images in front of the retina The main cause is an abnormal size of the eyeball. Other causes are the anomalies of the cornea or crystalline lens. The consequence is that the distance vision is blurry.
• Hyperopia. The defect of the ocular refraction that places the images behind the retina. The main cause is an abnormal size of the eyeball. Other causes are the anomalies of the cornea or crystalline lens. The consequence is that the near vision is blurred.
• Astigmatism. The defect of the ocular refraction due to a change of curvature of the cornea. It causes a distorted and blurred vision that varies depending on whether vertical, horizontal or inclined.
• Cataracts. It is a partial or total clouding of the crystalline lens.
• Presbyopia or tired vision. Progressive inability of the eye to focus on nearby objects usually occurs after the age of 40.
• Conjunctivitis. The inflammation of the conjunctiva of the eye due to an infection, allergic reaction or injury. The conjunctiva is a mucous, transparent and wet layer with many vessels that covers the inside of the eyelids and the front part of the eyeball except the cornea.
• Blindness. The inability to see.
• Otitis. The inflammation of the tympanic region due to the infection.
• Otosclerosis. The ossification of the oval membrane which is the membrane that receives the vibrations from the stapes. It usually causes the deafness.
• Deafness. The inability to perceive the sounds.

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