The dilator pupillae, the muscle of iris

  

 The dilator pupillae muscle is a ring of contractile cells within the iris. These cells are arranged radially, such that their contraction facilitates pupillary dilation (mydriasis). The dilator pupillae muscle receives innervation from the sympathetic nervous system.<script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-3738618711723990" crossorigin="anonymous"></script>

Gross anatomy

The dilator pupillae muscle is a circumferential, spoke-like arrangement of contractile myoepithelial cells located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium. Like the sphincter pupillae muscle, it is located posterior to the rich connective tissue stroma and neurovasculature of the anterior iris leaf 

Innervation

The dilator pupillae muscle is innervated by the sympathetic nervous system. Postganglionic sympathetic fibers project from the superior cervical ganglion to join the carotid plexus. These fibers then course with the ophthalmic artery, forming a number of long ciliary nerves that supply the dilator pupillae muscle

Action

Contraction of the dilator pupillae muscle facilitates dilation of the pupil (mydriasis). This increases the amount of light impinging on the retina

Clinical importance

• alongside endogenous sympathetic stimulation, sympathomimetic drugs (e.g. stimulants) may act on the dilator pupillae muscle, resulting in mydriasis

• prolonged significant mydriasis narrows the iridocorneal angle for aqueous humor drainage, increasing the risk of closed angle glaucoma

• the miosis seen in patients with Horner syndrome is mediated by inactivation of the dilator pupillae muscle due to a lesion interrupting sympathetic outflow

is a ring of contractile cells within the iris. These cells are arranged radially, such that their contraction facilitates pupillary dilation (mydriasis). The dilator pupillae muscle receives innervation from the sympathetic nervous system.

Gross anatomy

The dilator pupillae muscle is a circumferential, spoke-like arrangement of contractile myoepithelial cells located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium. Like the sphincter pupillae muscle, it is located posterior to the rich connective tissue stroma and neurovasculature of the anterior iris leaf 

Innervation

The dilator pupillae muscle is innervated by the sympathetic nervous system. Postganglionic sympathetic fibers project from the superior cervical ganglion to join the carotid plexus. These fibers then course with the ophthalmic artery, forming a number of long ciliary nerves that supply the dilator pupillae muscle

Action

Contraction of the dilator pupillae muscle facilitates dilation of the pupil (mydriasis). This increases the amount of light impinging on the retina

Clinical importance

• alongside endogenous sympathetic stimulation, sympathomimetic drugs (e.g. stimulants) may act on the dilator pupillae muscle, resulting in mydriasis

• prolonged significant mydriasis narrows the iridocorneal angle for aqueous humor drainage, increasing the risk of closed angle glaucoma

• the miosis seen in patients with Horner syndrome is mediated by inactivation of the dilator pupillae muscle due to a lesion interrupting sympathetic outflow

Sphincter pupillae

 Sphincter pupillae is a circular muscle, about 1 millimeter wide. It is located in the pupillary zone of stromal layer of the iris, attaching to and encircling the pupillary margin of iris. The muscle itself consists of six to eight circles of smooth muscle fibers, between of which are found the nerves and blood vessels that supply each fiber. 

Development : ectoderm 

Innervation

The sphincter pupillae receives parasympathetic innervation from the postganglionic fibers of ciliary ganglion, called short ciliary nerves, which are the branches of oculomotor nerve (CN III). Note that these particular nerve fibers originate from the accessory nucleus of oculomotor nerve (Edinger-Westphal) - a general visceral efferent nucleus - which provides autonomic (involuntary) control to this muscle.

Function

When contracting, the sphincter pupillae constricts the pupil, which is called miosis. This action happens during accommodation and pupillary light reflexes.

 

Accommodation

Accommodation is a reflexive event that adapts the eyes for observing close objects after being focused on distant ones. The reflex pathway includes optic nerve (CN II), visual and frontal cortex, oculomotor and accessory oculomotor nuclei and oculomotor nerve (CN III).

 

Once the eyes need to focus on a near object, the optic nerve sends signals to the visual cortex, which then stimulates the eye field within frontal cortex. The frontal neurons then send fibers to the ipsilateral oculomotor nucleus (unicortical innervation), and to both ipsi- and contralateral accessory oculomotor nuclei (bicortical innervation). The oculomotor nerve sends these signals to the ciliary ganglion, which then outsources the final stimuli via its two sets of branches;

 

Short ciliary nerves that cause the contraction of sphincter pupillae and miosis

Long ciliary nerves which stimulate the medial recti and ciliary muscles; the former causes the convergence of the eyeballs, while the latter thickens the lens increasing its refractive power

Pupillary light reflex

The pupillary light reflex happens when the eyes are exposed to bright light and the amount of light that falls onto the retina needs to be decreased in order to maintain clear vision. The reflex arc includes optic nerve (CN II), pretectal nucleus of midbrain, accessory oculomotor nucleus and oculomotor nerve (CN III).

 

Once the retinal photoreceptors detect light, the optic nerve sends a neural impulse to the pretectal nucleus. The pretectal nucleus stimulates the accessory oculomotor nucleus, which in turn innervates and constricts the sphincter pupillae via the short ciliary nerves. As pretectal nucleus connects to both accessory oculomotor nuclei, stimulation and miosis of one eye (direct pupillary light reflex) will always be followed by miosis on non-stimulated eye (indirect pupillary light reflex).

Anatomy of iris

 Anatomy of Iris 

Iris is a circular pigmented diaphragm which is situated between the cornea and the lens 

Parts : It has 

Two margins 

• Peripheral margin attached with anterior surface of ciliary body : 

• Central circular margin known as pupil c

Two zones : 

• Ciliary zone contain ciliary crypts 

• Pupillary zone  contain pupillary ruff the reflected area of posterior pigmented area 

• Two zone is separated by collaratte a smooth ridge of tissue  

Shape: flatten cone with central opening, iris is push forward due to the anterior surface of the lens 

Iris divide anterior segment of eyeball into anterior and posterior chamber , both the chamber filled by aqueous humor which circulate through the opening of pupil 

Histology of iris 

From before backwards , The iris is composed of three layers:

1. Anterior surface of the iris :  an outer discontinuous layer of melanocytes and fibroblasts; 

2. Stroma of the iris the intermediate fibrous layer (FL), housing pigment cells (Pc) and fibroblasts; 

3. Muscle layer : 

• smooth muscle near the pupil , 

• smooth muscle in the deeper part of stroma 

4. Anterior Pigmented epithelium

5. Posterior pigmented epithelium

Anterior surface of the iris : 

The anterior surface of the iris is not covered by a distinct endothelium. Instead, it is formed by layers of branched fibroblasts and melanocytes, which blend with the pectinate ligament derived from the Descemet’s membrane of the cornea.

Stroma of the iris 

It contains collagen fibers, fibroblasts, melanocytes and blood vessels and nerves , sphincter and dilator pupillae muscles. Stromal spaces are in free communication with the fluid of the anterior chamber 

Sphincter pupillae is a circular muscle, about 1 millimeter wide. It is located in the pupillary zone of stromal layer of the iris, attaching to and encircling the pupillary margin of iris. The muscle itself consists of six to eight circles of smooth muscle fibers, between of which are found the nerves and blood vessels that supply each fiber. 

Development : ectoderm 

Innervation

The sphincter pupillae receives parasympathetic innervation from the postganglionic fibers of ciliary ganglion, called short ciliary nerves, which are the branches of oculomotor nerve (CN III). Note that these particular nerve fibers originate from the accessory nucleus of oculomotor nerve (Edinger-Westphal) - a general visceral efferent nucleus - which provides autonomic (involuntary) control to this muscle.

Function

When contracting, the sphincter pupillae constricts the pupil, which is called miosis. This action happens during accommodation and pupillary light reflexes.

 

Accommodation

Accommodation is a reflexive event that adapts the eyes for observing close objects after being focused on distant ones. The reflex pathway includes optic nerve (CN II), visual and frontal cortex, oculomotor and accessory oculomotor nuclei and oculomotor nerve (CN III).

 

Once the eyes need to focus on a near object, the optic nerve sends signals to the visual cortex, which then stimulates the eye field within frontal cortex. The frontal neurons then send fibers to the ipsilateral oculomotor nucleus (unicortical innervation), and to both ipsi- and contralateral accessory oculomotor nuclei (bicortical innervation). The oculomotor nerve sends these signals to the ciliary ganglion, which then outsources the final stimuli via its two sets of branches;

 

Short ciliary nerves that cause the contraction of sphincter pupillae and miosis

Long ciliary nerves which stimulate the medial recti and ciliary muscles; the former causes the convergence of the eyeballs, while the latter thickens the lens increasing its refractive power

Pupillary light reflex

The pupillary light reflex happens when the eyes are exposed to bright light and the amount of light that falls onto the retina needs to be decreased in order to maintain clear vision. The reflex arc includes optic nerve (CN II), pretectal nucleus of midbrain, accessory oculomotor nucleus and oculomotor nerve (CN III).

 

Once the retinal photoreceptors detect light, the optic nerve sends a neural impulse to the pretectal nucleus. The pretectal nucleus stimulates the accessory oculomotor nucleus, which in turn innervates and constricts the sphincter pupillae via the short ciliary nerves. As pretectal nucleus connects to both accessory oculomotor nuclei, stimulation and miosis of one eye (direct pupillary light reflex) will always be followed by miosis on non-stimulated eye (indirect pupillary light reflex).

 The dilator pupillae muscle is a ring of contractile cells within the iris. These cells are arranged radially, such that their contraction facilitates pupillary dilation (mydriasis). The dilator pupillae muscle receives innervation from the sympathetic nervous system.

Gross anatomy

The dilator pupillae muscle is a circumferential, spoke-like arrangement of contractile myoepithelial cells located in the mid-periphery of the posterior leaf of the iris, anterior to the pigmented epithelium. Like the sphincter pupillae muscle, it is located posterior to the rich connective tissue stroma and neurovasculature of the anterior iris leaf 

Innervation

The dilator pupillae muscle is innervated by the sympathetic nervous system. Postganglionic sympathetic fibers project from the superior cervical ganglion to join the carotid plexus. These fibers then course with the ophthalmic artery, forming a number of long ciliary nerves that supply the dilator pupillae muscle

Action

Contraction of the dilator pupillae muscle facilitates dilation of the pupil (mydriasis). This increases the amount of light impinging on the retina

Clinical importance

• alongside endogenous sympathetic stimulation, sympathomimetic drugs (e.g. stimulants) may act on the dilator pupillae muscle, resulting in mydriasis

• prolonged significant mydriasis narrows the iridocorneal angle for aqueous humor drainage, increasing the risk of closed angle glaucoma

• the miosis seen in patients with Horner syndrome is mediated by inactivation of the dilator pupillae muscle due to a lesion interrupting sympathetic outflow

Anterior Cranial Fossa : anatomy and clinical anatomy

 

Anterior Cranial Fossa

Boundaries:

Anteriorly and laterally: by the inner surface of the frontal bone.

Median part of the posterior boundary: by the body of the sphenoid.

Lateral part of the posterior boundary: by the sharp posterior border of the lesser wing of the sphenoid.

The floor is formed by:

The lateral part of the floor is formed by the cerebral surface of the orbital parts of the frontal bone (forming the roof of the orbital cavity) and the lesser wings of the sphenoid bone.

The median area of the floor is formed by the cribriform plate of the ethmoid bone (forming the roof of the nasal cavity) and the anterior aspects of the body of the sphenoid.

 

Contents:

The frontal lobes of the brain,

The olfactory bulbs and tracts, and

The anterior cerebral arteries.

 

Foramina:

The cribriform plate of the ethmoid, through which the olfactory nerves (cranial nerve I) pass.

Anterior ethmoidal foramen – through which the anterior ethmoidal artery, nerve, and vein pass.

Posterior ethmoidal foramen – through which the posterior ethmoidal artery, nerve, and vein pass.

Foramen caecum: it transmits a small emissary vein, known as the emissary vein of the foramen caecum, that connects the superior sagittal sinus and the veins of the nasal cavity.

 

Other important features:

Crista galli: The crista galli is situated at the midline of the skull, between the two cribriform plates of the ethmoid bone. The falx cerebri attaches to the crista galli, which lodges the superior sagittal sinus.

Jugum sphenoidale: The jugum sphenoidale, also known as the sphenoidal crest, is a ridge-like elevation located on the superior surface of the body of the sphenoid bone. It attaches to the tentorium cerebelli, a fold of dura mater that separates the cerebrum from the cerebellum, along its posterior edge.

Sulcus chiasmatis: The sulcus chiasmatis, also known as the optic groove or optic sulcus, is a shallow depression on the superior surface of the body of the sphenoid bone, located just posterior to the jugum sphenoidale. It lodges the optic chiasma.

Anterior clinoid process: It is a small, pointed process of the lesser wing of the sphenoid that extends anteriorly and medially from the body of the sphenoid bone. It serves as the attachment point for the anterior part of the tentorium cerebelli.

Which anatomical structures damage in fracture of the anterior cranial fossa ?

Sinuses: Fractures here can involve the frontal sinus, potentially leading to CSF leakage and sinusitis. It may also damage the ethmoidal and sphenoidal sinuses and be accompanied by bleeding from the nose or mouth.

Artery Tears: Tears in branches of the anterior cerebral arteries may result in intracranial hemorrhage or ischemic stroke affecting the frontal lobes.

Vein Tears: Damage to the superior sagittal sinus can cause venous hemorrhage and increased intracranial pressure.

Nerve Damage: Injury to the olfactory nerves (cranial nerve I) may result in anosmia, while damage to nearby structures can affect cognitive and emotional functions.

Foramen Damage: The cribriform plate of the ethmoid is the thinnest part of the anterior cranial fossa and is therefore most likely to fracture. There are two major consequences of a cribriform plate fracture:

Anosmia – loss of the sense of smell due to damage to the olfactory nerve fibers that run through the cribriform plate of the ethmoid bone.

CSF rhinorrhea – the leakage of cerebrospinal fluid into the nasal cavity due to a tear in the meningeal coverings of the brain by a fractured bone fragment.

Meningitis: Infection of the nasal cavity may pass into the meninges, causing meningitis.

Brain Damage: Fractures in this region can cause contusions or lacerations in the frontal lobes, leading to cognitive deficits, personality changes, or motor dysfunction.

Primary optic atrophy and blindness: Fracture of the optic canal.

Subconjunctival hemorrhage, which is associated with fractures involving the roof of the orbit.

A black eye does not always indicate a fracture of the anterior cranial fossa; direct contusion of the soft tissues may produce a black eye deep to the aponeurotic layer of the scalp.

Which anatomical structures damage in fracture of the anterior cranial fossa ?

 

Fractures of the anterior cranial fossa

1.     It may damage frontal, ethmoidal and sphenoidal sinuses and be accompanied by bleeding from nose or mouth

2.     Leakage of CSF may also occur due to tearing of the meninges, which increase risk of meningitis increases due to open communication with the exterior via nasal cavity

3.     Anosmia : fracture of cribriform plate rupture of fibers of the olfactory bulb

4.     Primary optic atrophy and blindness : fracture of optic canal

5.     Sub conjunctival haemorrhage which is associate with fractures involving the roof of the orbit  

6.     Black eye is not always indicate fracture of anterior cranial fossa, dirent contusion of the soft tissues may produce black eye deep to the aponeurotic layer of the scalp