Lecture 10 - EOMs & Oculomotor Control

What is the embryologic origin of extraocular muscles?

Mesenchymal origin (condenses bilaterally around week 4–5).

Which embryologic tissue forms extraocular muscle cells?

Mesoderm forms the muscle cells.

What structures do neural crest cells form in relation to EOMs?

Connective tissue components associated with muscles.

Neural crest cells form which structures that innervate extraocular muscles?

Cranial nerves.

What additional developmental role do neural crest cells play in EOM formation?

They induce muscle cell formation.

Into what two masses does mesenchyme differentiate during EOM development?

• Superior mass
• Inferior mass

Which muscles arise from the superior mass?

• Superior rectus
• Superior oblique
• Levator palpebrae superioris

Which muscles arise from the inferior mass?

• Inferior rectus
• Medial rectus
• Lateral rectus
• Inferior oblique

When do cranial nerves begin growing toward the orbit to innervate EOMs?

Around week 4–6 of development.

What is the relationship between EOM differentiation and cranial nerve growth?

They are interdependent processes (one does not develop properly without the other).

Which EOMs develop first during embryology based on innervation?

Muscles innervated by CN III.

How do muscle origin, belly, and insertion likely develop during EOM development?

They develop simultaneously.

What earlier theory existed regarding EOM development stages?

Previous research suggested they developed through separate stages.

Where are scleral insertions located early in development relative to adults?

More posteriorly than adult insertions.

Why do EOM insertions migrate anteriorly during development?

As the globe enlarges, insertions move anteriorly to adult locations (forming the spiral of Tillaux).

When does the common tendinous ring form during EOM development?

Around month 6 of development.

Which pulley system develops late during EOM embryology?

The superior oblique trochlea pulley system.

Do extraocular muscles fully mature at birth?

No — they continue to mature after birth as binocular vision develops.

What is a myofiber?

A skeletal muscle fiber that is long, cylindrical, and multinucleated.

What are myofibrils?

Contractile structures inside muscle fibers composed of actin (thin) and myosin (thick) filaments.

What is the sarcolemma?

The muscle cell membrane.

What are transverse (T) tubules?

Extensions of the sarcolemma into the muscle fiber that allow rapid ion passage.

What is the sarcoplasmic reticulum?

Specialized smooth endoplasmic reticulum that stores and releases Ca²⁺.

What is a sarcomere?

The repeating contractile unit of skeletal muscle.

What is the Z disc?

Boundary of the sarcomere that anchors actin filaments.

What is the I band?

Region containing only actin (thin) filaments.

What is the H zone?

Region containing only myosin (thick) filaments.

What is the A band?

Region spanning the entire length of the myosin filament, including overlap with actin.

What is the M line?

Protein structure that anchors and aligns myosin filaments.

What is the neuromuscular junction?

Synapse connecting motor neurons to skeletal muscle fibers.

What is the primary neurotransmitter at the neuromuscular junction?

Acetylcholine (ACh).

Where is acetylcholine released from at the neuromuscular junction?

Presynaptic synaptic vesicles.

What receptors are found on the motor end plate?

Nicotinic acetylcholine receptors (ligand-gated ion channels).

What enzyme breaks down acetylcholine in the synapse?

Acetylcholinesterase (AChE).

What does the sliding filament model explain?

Skeletal muscle contraction at the molecular level.

What occurs during the nerve signal stage of muscle contraction?

• Motor neuron propagates action potential
• Nerve impulse reaches neuromuscular junction
• Triggers ACh release.

What occurs during muscle fiber activation?

• ACh binds sarcolemma receptors
• Causes depolarization and action potential propagation
• Action potential travels along sarcolemma and through T-tubules.

What triggers Ca²⁺ release during contraction?

Action potential opens ionic channels → Ca²⁺ released from sarcoplasmic reticulum into sarcoplasm.

What occurs during cross-bridge formation?

• Ca²⁺ binds troponin-tropomyosin complex
• Conformational change exposes actin binding sites
• ATP on myosin head → hydrolyzed to ADP + Pi + energy
• Myosin head attaches to actin.

What occurs during the power stroke?

• Phosphate released from myosin head
• Myosin head pulls actin filament
• ADP dissociates.

What occurs during cross-bridge release?

• New ATP binds myosin head
• Myosin detaches from actin
• Repositions for next contraction cycle.

What structural change results from sliding filament contraction?

Sarcomere shortening along the length of the muscle fiber.

How long does the sliding filament contraction cycle continue?

As long as Ca²⁺ and ATP are present.

What type of disease is myasthenia gravis?

An autoimmune disease affecting the neuromuscular junction.

What happens to acetylcholine receptors in myasthenia gravis?

Antibodies block or destroy ACh receptors.

How does myasthenia gravis affect neuromuscular transmission?

It prevents transmission of nerve impulses to muscle fibers.

What is often the first clinical symptom of myasthenia gravis?

Ptosis.

What other visual symptom is common in myasthenia gravis?

Diplopia.

How do extraocular muscles differ from noncranial skeletal muscle regarding blood supply?

EOMs have a denser blood supply.

How do EOM movements compare with other skeletal muscle movements?

They have higher velocity and more precise movements.

How do motor units in EOMs differ from other skeletal muscles?

EOMs have fewer muscle fibers per motor unit. (More fine-tuned innervation)

What molecular characteristic differs in extraocular muscles compared to other skeletal muscles?

They contain additional isoforms of myosin.

Do individual extraocular muscles contain the same myosin isoforms?

No — individual EOMs have different isoforms.

How long are extraocular muscle fibers compared to typical skeletal muscle fibers?

They have short myofiber lengths.

Do EOM fibers extend the entire distance from origin to insertion?

No — they do not run all the way from origin to insertion.

How are EOM fibers arranged structurally?

In parallel, series, and branching arrangements.

Do branching EOM fibers lead to summative contractile forces?

No — branching does not produce summative contractile forces.

How do mitochondria levels in EOMs compare to other skeletal muscles?

EOMs have higher amounts of mitochondria for efficient handling and storage of Ca²⁺.

What effect does mitochondrial abundance have on EOM calcium regulation?

Provides resistance to pathologic elevations of intracellular Ca²⁺.

How does aerobic metabolism in EOMs compare with other muscles?

EOMs have higher levels of aerobic metabolism.

Which muscle type has higher blood flow than extraocular muscles?

Cardiac muscle (the only one with higher blood flow).

How do antioxidant levels in EOMs compare to other muscles?

EOMs have higher antioxidant activity → resistance to injury and oxidative stress.

How does fatigue resistance of EOMs compare to other skeletal muscles?

EOMs are more fatigue-resistant.

What metabolic adaptation allows EOMs to sustain contraction?

Ability to utilize lactate metabolic byproduct during contraction.

How does myofiber remodeling differ in EOMs compared to other skeletal muscles?

EOMs have higher levels of myofiber remodeling.

What cellular mechanism supports EOM repair and growth?

Increased myonuclear addition (formation of new nuclei).

Does myonuclear addition significantly change EOM size or function?

No — it does not impact overall size and function.

Which cranial nerve innervates the superior rectus?

CN III (superior division).

Which cranial nerve innervates the inferior rectus?

CN III (inferior division).

Which cranial nerve innervates the medial rectus?

CN III (inferior division).

Which cranial nerve innervates the inferior oblique?

CN III (inferior division).

Which cranial nerve innervates the lateral rectus?

CN VI (abducens nerve).

Which cranial nerve innervates the superior oblique?

CN IV (trochlear nerve).

What are the primary, secondary, and tertiary actions of the superior rectus?

Primary → Elevation
Secondary → Intorsion
Tertiary → Adduction

What are the primary, secondary, and tertiary actions of the inferior rectus?

Primary → Depression
Secondary → Extorsion
Tertiary → Adduction

What are the actions of the medial rectus?

Primary → Adduction
Secondary → None
Tertiary → None

What are the actions of the lateral rectus?

Primary → Abduction
Secondary → None
Tertiary → None

What are the primary, secondary, and tertiary actions of the superior oblique?

Primary → Intorsion
Secondary → Depression
Tertiary → Abduction

What are the primary, secondary, and tertiary actions of the inferior oblique?

Primary → Extorsion
Secondary → Elevation
Tertiary → Abduction

What is the X-axis in Fick’s axes?

Horizontal axis running nasal → temporal.

What movements occur around the X-axis?

Elevation and depression.

What is the Y-axis in Fick’s axes?

Sagittal axis running anterior → posterior.

What movements occur around the Y-axis?

Intorsion and extorsion.

What is the Z-axis in Fick’s axes?

Vertical axis running superior → inferior.

What movements occur around the Z-axis?

Adduction and abduction.

What are ductions?

Movements involving one eye only.

What are the duction movements?

Elevation, depression, intorsion, extorsion, adduction, abduction.

What is the purpose of forced duction testing?

To determine the cause of a muscle restriction.

How is a forced duction test performed?

• Apply topical anesthesia (proparacaine)

• Grasp conjunctiva with forceps

• Attempt to move the globe

What does resistance during forced duction indicate?

Fibrosis or muscle entrapment → physical restriction.

What does free movement during forced duction indicate?

Nerve palsy or decreased innervation (no mechanical restriction).

What are versions?

Binocular conjugate eye movements where both eyes move in the same direction.

What is dextroversion?

Right gaze (both eyes move to the right).

What is levoversion?

Left gaze (both eyes move to the left).

What is supraversion?

Upward gaze of both eyes.

What is infraversion?

Downward gaze of both eyes.

What are vergences?

Binocular disjunctive eye movements where both eyes move in opposite directions.

What is convergence?

Both eyes move medially toward the midline.

What is divergence?

Both eyes move laterally away from the midline.

Why are vergence movements important?

They are important for stereopsis and localization within space.