Mydriasis in the context of Smooth muscle cell

Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non-striated, so-called because it has no sarcomeres and therefore no striations (bands or stripes). It can be divided into two subgroups, single-unit and multi-unit smooth muscle. Within single-unit muscle, the whole bundle or sheet of smooth muscle cells contracts as a syncytium.

Smooth muscle is found in the walls of hollow organs, including the stomach, intestines, bladder and uterus. In the walls of blood vessels, and lymph vessels, (excluding blood and lymph capillaries) it is known as vascular smooth muscle. There is smooth muscle in the tracts of the respiratory, urinary, and reproductive systems. In the eyes, the ciliary muscles, iris dilator muscle, and iris sphincter muscle are types of smooth muscles. The iris dilator and sphincter muscles are contained in the iris and contract in order to dilate or constrict the pupils. The ciliary muscles change the shape of the lens to focus on objects in accommodation. In the skin, smooth muscle cells such as those of the arrector pili cause hair to stand erect in response to cold temperature and fear.

The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation of vision to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict (miosis/myosis; thereby allowing less light in), whereas a lower intensity of light causes the pupil to dilate (mydriasis, expansion; thereby allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye. Light shone into one eye will cause both pupils to constrict.

Ophthalmoscopy, (from Ancient Greek ὀφθαλμός (ophthalmós), meaning "eye", and σκοπέω (skopéō), meaning "to look") also called funduscopy, is a test that allows a health professional to see inside the fundus of the eye and other structures using an ophthalmoscope (or funduscope). It is done as part of an eye examination and may be done as part of a routine physical examination. It is crucial in determining the health of the retina, optic disc, and vitreous humor.

The pupil is a hole through which the eye's interior can be viewed. For better viewing, the pupil can be opened wider (dilated; mydriasis) before ophthalmoscopy using medicated eye drops (dilated fundus examination). However, undilated examination is more convenient (albeit not as comprehensive), and is the most common type in primary care.

The adrenergic receptors or adrenoceptors are a class of G protein-coupled receptors that are targets of many catecholamines like norepinephrine (noradrenaline) and epinephrine (adrenaline) produced by the body, but also many medications like beta blockers, beta-2 (β₂) agonists and alpha-2 (α₂) agonists, which are used to treat high blood pressure and asthma, for example.

Many cells have these receptors, and the binding of a catecholamine to the receptor will generally stimulate the sympathetic nervous system (SNS). The SNS is responsible for the fight-or-flight response, which is triggered by experiences such as exercise or fear-causing situations. This response dilates pupils, increases heart rate, mobilizes energy, and diverts blood flow from non-essential organs to skeletal muscle. These effects together tend to increase physical performance momentarily.

Atropine is a tropane alkaloid and anticholinergic medication used to treat certain types of nerve agent and pesticide poisonings as well as some types of slow heart rate, and to decrease saliva production during surgery. It is typically given intravenously or by injection into a muscle. Eye drops are also available which are used to treat uveitis and early amblyopia. The intravenous solution usually begins working within a minute and lasts half an hour to an hour. Large doses may be required to treat some poisonings.

Common side effects include dry mouth, abnormally large pupils, urinary retention, constipation, and a fast heart rate. It should generally not be used in people with closed-angle glaucoma. While there is no evidence that its use during pregnancy causes birth defects, this has not been well studied so sound clinical judgment should be used. It is likely safe during breastfeeding. It is an antimuscarinic (a type of anticholinergic) that works by inhibiting the parasympathetic nervous system.

Cycloplegia is paralysis of the ciliary muscle of the eye, resulting in a loss of accommodation. Because of the paralysis of the ciliary muscle, the curvature of the lens can no longer be adjusted to focus on nearby objects. This results in similar problems as those caused by presbyopia, in which the lens has lost elasticity and can also no longer focus on close-by objects. Cycloplegia with accompanying mydriasis (dilation of pupil) is usually due to topical application of muscarinic antagonists such as atropine and cyclopentolate.

Belladonna alkaloids are used for testing the error of refraction and examination of eye.

Miosis, or myosis (from Ancient Greek μύειν (múein) 'to close the eyes'), is excessive constriction of the pupil. The opposite condition, mydriasis, is the dilation of the pupil. Anisocoria is the condition of one pupil being more dilated than the other.

Dilated fundus examination (DFE) is a diagnostic procedure that uses mydriatic eye drops to dilate or enlarge the pupil in order to obtain a better view of the fundus of the eye. Once the pupil is dilated, examiners use ophthalmoscopy to view the eye's interior, which makes it easier to assess the retina, optic nerve head, blood vessels, and other important features. DFE has been found to be a more effective method for evaluating eye health when compared to non-dilated examination, and is the best method of evaluating structures behind the iris. It is frequently performed by ophthalmologists and optometrists as part of an eye examination.