Parasympathetic And Strengths

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1) Describe the effects of the parasympathetic and sympathetic nervous systems on the smooth muscles of the eye

The smooth muscles of the eye are mainly controlled by the parasympathetic nervous system. The activation of the parasympathetic system will cause the release of acetylcholine (Ach), which will act on the m2 muscarinic receptors in the iris smooth muscle fibres. This will cause the pupil to constrict, reducing the amount of light able to enter the eye. The release of Ach will also act on the m2 muscarinic receptors in the ciliary muscle, causing them to contract, relaxing the shape of the lens in the eye (Rang and Dale, 2012). Therefore, the overall effect of the release of Ach will constrict the pupil, reducing the amount of light entering the eye, this is known as miosis (University of Sydney, 2017). This differs from the activation of the sympathetic nervous system which will cause the release of noradrenaline, which acts on the ⍺1-receptors in the radial smooth muscle fibres of the iris (dilator pupillae), causing it to contract. The release or noradrenaline will also act on the β2-receptors of the ciliary muscle causing them to relax, causing the lens to lengthen, however this is only a faint response(Rang and Dale, 2012). Hence, the contraction of the iris sphincter and slight relaxation of the ciliary muscles will cause the pupil of the eye to be dilated, increasing the amount of the light able to enter the eye, this is known as mydriasis (University
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