Physiology of Eye Muscles and Ptosis

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Questions and Answers

What type of fibers stimulate dilation of the pupil in dim light?

Parasympathetic fibers

Motor fibers

Sensory fibers

Sympathetic fibers (correct)

Which type of muscle is involved in the constriction of the pupil?

Cardiac muscle

Smooth muscle (correct)

Skeletal muscle

Epithelial tissue

What is the function of the ciliary body in the eye?

To regulate blood pressure

To transmit nerve signals

To control the amount of light entering the eye

To focus on near objects (correct)

What happens to the lens when the ciliary muscle contracts?

It becomes rounder Signup and view all the answers

Which artery supplies the orbit and eye?

Ophthalmic artery Signup and view all the answers

What is the function of the central artery of the retina?

To supply the retina Signup and view all the answers

Which of the following statements about the oculomotor nerve (CN III) is true?

It provides parasympathetic fibers to the ciliary ganglion. Signup and view all the answers

Which branch of the ophthalmic artery supplies the eyelids?

Medial palpebrae arteries Signup and view all the answers

Where do the postganglionic parasympathetic fibers from the ciliary ganglion course to?

The eyeball via short ciliary nerves Signup and view all the answers

What is the function of the postganglionic parasympathetic fibers that innervate the ciliary muscle?

To facilitate accommodation Signup and view all the answers

What is the function of the dorsal nasal arteries?

To supply the lateral nose and lacrimal sac Signup and view all the answers

Where do sympathetic preganglionic fibers ascend to synapse?

The superior cervical ganglion Signup and view all the answers

What is the origin of sympathetic innervation to the eyeball?

The thoracic spine (T1-T2) Signup and view all the answers

Which muscle is innervated by postganglionic sympathetic fibers?

Superior tarsal muscle Signup and view all the answers

What is the function of the postganglionic sympathetic fibers that innervate the dilator pupillae?

To regulate pupil size Signup and view all the answers

How many dural layers cover the optic nerve (CN II)?

Three Signup and view all the answers

What is the primary function of the bony orbit?

To contain and protect the eyeballs and their associated structures Signup and view all the answers

Which of the following structures is NOT primarily located in the orbit?

Central Retinal Artery Signup and view all the answers

What is the direction of the orbital openings?

Anterolaterally Signup and view all the answers

Which of the following is a clinical correlation related to head anatomy?

Orbital Infections Signup and view all the answers

What is the location of the apices of the orbits?

Posteromedially Signup and view all the answers

What is the primary function of the lacrimal apparatus?

To lubricate and protect the eye Signup and view all the answers

Which cranial nerve is responsible for mediating the special sense of sight?

CN II Signup and view all the answers

What is the primary function of the postganglionic sympathetic fibers from the superior cervical ganglion?

Innervating the smooth muscle responsible for eyelid movement Signup and view all the answers

Which nerve is responsible for conveying general sensory information from the orbit and eye?

CN V1 Signup and view all the answers

What is the primary function of the lacrimal branch of CN V1?

Sensory to conjunctiva, skin of the upper eyelid, and lacrimal gland Signup and view all the answers

Which muscle is responsible for eyelid movement and is innervated by postganglionic sympathetic fibers?

Smooth muscle of the eyelid Signup and view all the answers

What is the primary function of the H pattern test?

To test the convergence of the medial rectus muscles of both eyes Signup and view all the answers

Which nerve is responsible for innervating the extraocular skeletal muscles?

CN III Signup and view all the answers

What is the result of interruption of the sympathetic pathway to the smooth muscle of the eyelid?

Moderate ptosis Signup and view all the answers

What is the function of the trabecular meshwork?

To absorb aqueous humor into the scleral venous sinus Signup and view all the answers

What type of cells are responsible for low-light conditions and night vision?

Rods Signup and view all the answers

What is the function of the ora serrata?

To separate the neural from the nonvisual retina Signup and view all the answers

What is the purpose of the fovea centralis?

To facilitate color vision and acute discrimination Signup and view all the answers

What is the function of the ciliary body?

To produce aqueous humor Signup and view all the answers

What is the purpose of the optic disc?

To convey photosensory information to the optic nerve Signup and view all the answers

What is the function of the iris?

To separate the anterior and posterior chambers of the eyeball Signup and view all the answers

What is the characteristic of the photoreceptor layer in the fovea centralis?

It consists only of cones Signup and view all the answers

Study Notes

Orbits

The orbits are pyramidal, bony cavities in the facial skeleton
Their bases (orbital openings) are directed anterolaterally, while their apices are located posteromedially
They contain and protect the eyeballs and their muscles, nerves, and vessels, along with most of the lacrimal apparatus

Clinical Testing of the Extraocular Muscles (EOM)

The examiner uses an H pattern to assess how each eye tracks movement of an object
The examiner can bring the finger directly to the midline to test convergence (medial rectus muscles of both eyes)
If an eye movement disorder is detected, a clinical specialist may be consulted for further evaluation

Nerves in the Orbit

Cranial nerves III, IV, and VI innervate the extra-ocular skeletal muscles
CN II mediates the special sense of sight
CN V1 (ophthalmic nerve) conveys general sensory information from the orbit and eye
The oculomotor nerve (CN III) provides parasympathetic fibers, which exhibit the following features:
- Parasympathetic fibers arise centrally from the nucleus of Edinger-Westphal (preganglionic fibers) and course along CN III and its inferior division to synapse in the ciliary ganglion on postganglionic parasympathetic neurons
- Postganglionic parasympathetic fibers then course via short ciliary nerves to the eyeball
- These postganglionic fibers innervate the sphincter muscle of the pupil (sphincter pupillae) and the ciliary muscle for accommodation
Sympathetic innervation to the eyeball is arranged as follows:
- Sympathetic innervation arises from the upper thoracic intermediolateral cell column of the spinal cord (T1-T2) and sends preganglionic fibers into the sympathetic trunk
- These fibers ascend to synapse in the superior cervical ganglion (SCG)
- Postganglionic sympathetic fibers course along the internal carotid artery, enter the orbit on the ophthalmic artery and ophthalmic nerve, and pass through the ciliary ganglion or along the long and short ciliary nerves to the eyeball
- These postganglionic fibers innervate the dilator muscle of the pupil (dilator pupillae) and the superior tarsal muscle of the upper eyelid

Anterior Segment of the Eye

The anterior segment of the eye consists of:
- The anterior chamber between the cornea and the iris
- The posterior chamber between the iris and the lens
- Filled with aqueous humor, which is produced by the ciliary body and circulates from the posterior chamber, through the pupil, and into the anterior chamber, where it is absorbed by the trabecular meshwork into the scleral venous sinus (canal of Schlemm), at the angle of the cornea and iris

Retina

The retina consists of:
- Nonvisual retina, which lines the internal surface of the ciliary body and iris
- Neural retina, composed of:
  - An outer retinal pigmented epithelium lying adjacent to the vascular choroid
  - A photosensitive region consisting of photoreceptive cells:
    - Rods (night vision) are more sensitive to light and the receptors for low-light conditions (gray tones)
    - Cones (daylight vision) are less sensitive to low light but very sensitive to red, green, and blue regions of the visual spectrum
  - Interspersed layers of conducting and association neurons and supporting cells lie more internally in the retina, closer to the vitreous body
The axons of ganglion cells ultimately convey the photosensory information to the optic disc, where the axons course in the optic nerve and are relayed centrally
The optic disc is the "blind spot" because no cones or rods are present in this region of the retina
The fovea centralis is the central focusing area and most sensitive portion of the retina, consisting only of cones, specialized for color vision and acute discrimination

Accommodation of the Lens

The ciliary body contains smooth muscle arranged in a circular fashion like a sphincter
When relaxed, it pulls a set of zonular fibers attached to the elastic lens taut and flattens the lens for viewing objects at some distance from the eye
When focusing on near objects, the sphincter-like ciliary muscle (parasympathetically innervated by CN III) contracts and constricts closer to the lens, relaxing the zonular fibers and allowing the elastic lens to round up for accommodation (near vision)

Blood Supply to the Orbit and Eye

Ophthalmic arteries arise from the internal carotid artery just as it exits the cavernous sinus, and supply the orbit and eye by the following branches:
- Central artery of the retina: travels in the optic nerve; occlusion leads to blindness
- Short and long posterior ciliary arteries: pierce the sclera and supply the ciliary body, iris, and choroid
- Lacrimal arteries: supply the gland, conjunctiva, and eyelids
- Ethmoidal arteries: supply the ethmoid and frontal sinuses, nasal cavity, and external anterior nose
- Medial palpebrae arteries: supply the eyelids
- Muscular arteries: supply skeletal muscles of the orbit and smooth muscles of the eyeball
- Dorsal nasal arteries: supply the lateral nose and lacrimal sac
- Supra-orbital artery: passes through supra-orbital notch and supplies the forehead and scalp
- Supratrochlear artery: supplies the forehead and scalp

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Description

Learn about the innervation of eye muscles, including the levator palpebrae superioris, and how interruptions can lead to ptosis. Understand the clinical testing of extraocular muscles.