Iris dilator muscle
|Iris dilator muscle|
Iris, front view. (Muscle visible but not labeled.)
The upper half of a sagittal section through the front of the eyeball. (Iris dilator muscle is NOT labeled and not to be confused with "Radiating fibers" labeled near center, which are part of the ciliary muscle.)
|Latin||Musculus dilatator pupillae|
|outer margins of iris|
|inner margins of iris|
|Long ciliary nerves (sympathetics)|
|iris sphincter muscle|
|Anatomical terms of muscle|
The iris dilator muscle (pupil dilator muscle, pupillary dilator, radial muscle of iris, radiating fibers), is a smooth muscle of the eye, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When stimulated, the cells contract, widening the pupil and allowing for more light to pass through the eye.
It is innervated by the sympathetic system, which acts by releasing noradrenaline, which acts on α1-receptors. Thus, when presented with a threatening stimuli that activates the fight-or-flight response, this innervation contracts the muscle and dilates the iris, thus temporarily letting more light reach the retina.
The dilator muscle is innervated more specifically by postganglionic sympathetic nerves arising from the superior cervical ganglion as the sympathetic root of ciliary ganglion. They will follow both short ciliary and long ciliary nerves to reach the dilator muscle.
The pupillary dilator acts as an antagonist to the pupillary constrictor which narrows the pupil and admits less light to the eye. It has its origin from the anterior epithelium. Pupillary dilation occurs in two situations: when light intensity changes and when we shift our gaze between distant and nearby objects.
The English name dilator pupillae muscle as currently used in the list of English equivalents of the Terminologia Anatomica, the reference-work of the official anatomic nomenclature, can be considered as a corruption of the full Latin expression musculus dilatator pupillae. The full Latin expression exhibits three words that each can be traced back to Roman antiquity. The Classical Latin name musculus is actually a diminutive of classical Latin, mus, and can be translated as little mouse. In the medical writings of Aulus Cornelius Celsus we can also find this specific name to refer to a muscle instead of its literal meaning. Latin musculus can be explained by the fact that a muscle looks like a little mouse that moves under the skin. In the writings of Greek philosopher Aristotle the Ancient Greek word for mouse, i.e. μῦς is also used to refer to a muscle.
Dilatator in the Latin expression musculus dilatator pupillae is derived from the classical Latin verb dilatare, to dilate, to spread out. Two possible explanations exist concerning the etymological derivation of this verb. The first explanation considers dilatare as frequentative of differere. The Latin verb differe can mean, to carry different ways, to spread abroad, to scatter, but also to delay. The other explanation considers dilatare as a compound from di- and latus, with the latter word meaning, broad or wide, hence the German name Erweiterer for Latin dilatator.
The expression dilator pupillae muscle, as used in the list of English equivalents of the Terminologia Anatomica, is actually partly Latin, i.e. dilator pupillae, with pupillae (=of the pupil), a noun in the genitive case modifying dilator, a noun in the nominative case, and partly English, i.e. muscle. In previous editions (Nomina Anatomica) this muscle was officially called the musculus dilator pupillae, The Nomina Anatomica as authorized in 1895 in Basle and in 1935 in Jena used the full Latin expression.
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- Histology image: 08010loa — Histology Learning System at Boston University