Human performance

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2.4 Hearing
There receptors in our ears are mechanoreceptors responsible for hearing or balance. Hearing receptors are called auditory cells sensitive to vibrations in the air (sound weaves) balance receptors called balance cells sensitive to movement.                                                                                                                                                        
The human ear                                                                                                                                                                                     
Is divided into 3 parts:
The outer ear formed by the ear or auricle and the ear canal.
The middle ear made up of the eardrum and a chain tiny bones ossicles, (malleus, incus and stapes) which link the eardrum to the inner ear.
The inner ear is formed by the cochlea, which contains the hearing receptors cells, and the semicircular canals and the cavities at their base, which house the balance cells.
How our ears work
Hearing. Sounds waves enter the outer ear and reach the eardrum, which begins to vibrate. This vibration is transmitted along the ossicles to the fluid inside the cochlea. The movement of the fluid stimulates the hearing receptor cells which send nerve impulses through the auditory or cochlear nerve to the brain, where they are transformed into sounds.
Balance. Sensory balance cells are located in the semicircular canals and the cavities found at their base. When we move, the fluid inside these structures also moves and stimulates the balance cells, which sends nerve impulses through the vestibular nerve to the brain, which provides us with information about our body´s position.
2.5 Sight
Sight receptors are located in the eyes and are photoreceptors sensitive to light.
The human eye
- The eyeball is slightly flattened sphere filed with transparent fluids(the aqueous and vitreous humours), it has three layers:
  - The sclera or other layer, which is white. The front part of this layer is transparent and is called the cornea. Is covered by a thin protective membrane called the conjunctiva.
  - The choroid. Or intermediate layer, which is darker and contains blood vessels. The front of this layer contains in the iris, which has an orifice in it, the pupil. The pupil opens or closes depending on amount of light reaching the eye. Behind the iris is a lens which changes shape in order to focus the image.
  - The retina is the internal layer photoreceptors, rods and cones
- The auxiliary structures are the eyebrows, eyelids, eye muscles and lacrimal glands. They protect the eyeball and enable it to move.
How our eyes work sight
Light reaches the cornea, which directs it towards the pupil .The pupil opens or close depending on the intensity of the light. The light then travels through the lens, which focuses in onto the retina. In the retina, photoreceptor cells generate nerve impulses which they then send through the optic nerve to the brain.

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