of the eye’s lens system allows the eye to produce sharply focused images. Par exemple, geometrical optics show that as a distant object is brought closer to the eye, the focus of the object becomes blurrier in the plane behind the retina; toutefois, as a result of the increase in the refractive power of the eye, this image becomes clear. The refractive power mainly resides in the cornea, but the overall refractive power is achieved by the actual lens changing its shape. In order to fixate on a near object, the ciliary muscle contracts around the lens to decrease its size. The suspensory ligaments relax and the radial tension around the lens is released. This causes the lens to form a more spherical shape achieving a higher level of refractive power. Focus on distant objects When the eye focuses on distant objects, the lens holds itself in a flattened shape by on the suspensory ligaments(zonal fibers). Ligaments pull the edges of the elastic lens capsule towards the surrounding ciliary body and by opposing the internal pressure within the elastic lens, keep it relatively flattened. Opposite of fixating on a near object, the ciliary muscle relaxes and the diameter of around the lens increases to increase the lens’ size. The tension along the suspensory ligaments is increased to flatten the lens and decrease the curvature and achieve a lower refractive power. Neural circuit Three regions make up the accommodation neural circuit, the afferent limb, the efferent limb and the ocular motor neurons that are between the afferent and efferent limb. 1. The afferent limb of the circuit This limb contains the main structures; the retina that contains the retinal ganglion axons in the optic nerve, chiasm and tract, the lateral geniculate body, and the visual cortex. 2.The efferent limb of the circuit This limb includes Edinger-Westphal nucleus and the oculomotor neurons. The main function of the Edinger-Westphal nucleus is to send axons in the oculomotor nerve to control the ciliary ganglion which in turn, sends its axons in the short ciliary nerve to control the iris and the ciliary muscle of the eye. The oculomotor neurons functions to send its axons in the oculomotor nerve, to control the medial rectus, and converge the two eyes. 3. Ocular motor control neurons Neurons that are interposed between the afferent and efferent limbs of this circuit and include the visual association cortex, which determines the image is "out-of-focus, ends corrective signals via the internal capsule and crus cerebri to the supraoculomotor nuclei. It also includes the supraoculomotor nuclei (located immediately superior to the oculomotor nuclei) that generates motor control signals that initiate the accommodation response and sends these control signals bilaterally to the oculomotor complex. See also Accommodation (eye) Pupillary reflex Ciliary muscle Presbyopia External links Accommodation Exam of cranial nerve II - accommodation is usually tested as a part of this exam. This page uses Creative Commons Licensed content from Wikipedia (voir les auteurs). ↑ Watson, Neil. Mind's Machine: Foundations of Brain and Behavior Ll. [S.l.]: Sinauer Associates, 2012. Print. Ser. 171. ↑ Bhatnagar SC. Neuroscience for the study of communicative disorders. p 185-6. | http (en anglais)://books.google.com/books?id=d0D0ghp2fSAC&pg=PA185&lpg=PA185&dq=accommodation+reflex+cranial+nerve+iii&source=bl&ots=JWF1lLVjYt&sig=-rQh6t8JPfSUapV8yiIvBuBcqNw&hl=en&ei=02_LS4ftEsP-8Ab60NX8BA&sa=X&oi=book_result&ct=result&resnum=10&ved=0CC4Q6AEwCQ#v=onepage&q=accommodation%20reflex%20cranial%20nerve%20iii&f=false ↑ "Ocular Motor System (Section 3, Chapter 7) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy - The University of Texas Medical School at Houston." Ocular Motor System (Section 3, Chapter 7) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy - The University of Texas Medical School at Houston. N.p., n.d. Web. 24 Oct. 2012.
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