Extraocular Muscles

• 6 muscles for movements of the globe
• 4 recti arising from the tendinous ring
• 2 oblique
• 1 further muscle for eyelid movement: levator palpebrae superioris
• The collagen tendons of the muscles blend with the scleral collagen

Histology

• The EOMs differ from other skeletal muscle
• Much higher ratio of nerve axons to muscle fibres
• 1:2 to 1:7 ratio of motor neuron to muscle fibres for very precise control
• Round/oval fibres which are smaller in the periphery and larger centrally
• The muscle sheath (epimysium) is thinner

• The muscle fibres are surrounded by more connective tissue (perimysium) so are less tightly packed

• EOMs are among the most vascularised muscles in the body second only to the myocardium

• They are poorly fasciculated

• All supplied by single motor endplates 
• Two types

• Fibrillinstruktur: type A thick, fast twitch type supplied by single motor endplates (‘en plaque’). These predominate in the outer, orbital zone of the muscles, and provide for saccadic movements

• Rich in mitochondria
• Well defined myofibrils
• Well developed sarcomeres

• Felderstruktur: type B, thin fibres, are innervated by multiple nerve endings (‘en grappe’) and predominate in the inner, zone adjacent to the globe; provide for slow/tonic, smooth pursuit movements

• Produce tonic contraction in response to ACh
• Poorly defined myofibrils and poorly developed sarcoplasmic reticulum
• Zig-zag Z lines
• Grape-like nerve endings
• Less blood supply than fibrillinstruktur fibres

• Mononuclear cell infiltrate
• Central nuclei
• Disorganised sarcolemma
• Disrupted Z-lines
• Mitochondrial clumping

Recti muscles

• The superior and inferior recti run at approx 25 degrees to the optical axis
• Their insertions form the spiral of Tillaux
• Superior branch of oculomotor:
• Superior rectus
• Levator
• Inferior branch of oculomotor:
• Medial rectus
• Inferior rectus
• Abducens nerve: lateral rectus

Note

The lateral rectus has a small second head arising from the greater wing of the sphenoid bone, lateral to the annulus of Zinn

Oblique muscles

• Superior oblique arises from the body of the sphenoid, superomedial to optic canal
• Rounded tendon that passes through the trochlea 

• Pulls at an angle of approximately 55 degrees from the visual axis in the primary position

• Only extraocular muscle to have a fusiform (spindle) shape

• Inferior oblique arises from the medial floor of the orbital rim (precisely, from the posterior lacrimal crest of the maxilla) but lateral to the nasolacrimal duct

• Shortest muscle 
• Travels in a posterolaterally direction
• Pulls at an angle of approximately 50 degrees from the visual axis
• Receives additional supply from the infraorbital artery

Note

Inferior oblique has no tendon

Nerve supply

• Inferior oblique: inferior branch of oculomotor
• Superior oblique: trochlear nerve (enters SOF outside the tendinous ring)

Relations

• Frontal nerve passes above the levator
• Lacrimal nerve passes above lateral rectus

• Nasociliary nerve (the continuation of the ophthalmic div.) passes along the superior border of medial rectus

• Ophthalmic artery travels above medial rectus
• Inferior div. of oculomotor runs above the inferior rectus
• Abducens nerve passes along the medial aspect lateral rectus
• Trochlear nerve runs along the upper surface of the superior oblique

Oculocardiac reflex

• Trigemino-vagal:

• Afferent limb formed by sensory fibres from the extraocular muscles travelling via the long and short ciliary nerves (through the ciliary ganglion but not synapsing), along the nasociliary nerve and then the ophthalmic

• Efferent limb is formed from the brainstem to the heart via the vagus nerve
• Primarily elicited by traction on the EOMs
• Eg. squint surgery
• Orbital fractures with muscle entrapment
• Can lead to severe bradycardia or even asystole