Skeletal Muscle and the

Molecular Basis of Contraction

Lanny Shulman, O.D., Ph.D.

University of Houston College of

Optometry

• Like neurons, all muscle cells can be excited chemically, electrically, and mechanically to produce an action potential.

• Contractile proteins: actin and myosin (also troponin and tropomyosin)

• Actin-binding protein myosin is a molecular motor that converts energy from ATP hydrolysis into movement

• Three Types:

– Smooth

lacks cross striations

found in hollow viscera

functionally syncytial

– Cardiac

has cross striations

functionally syncytial

contracts rhythmically in the absence of

external innervation due to the presence of

pacemaker cells

– Skeletal

Skeletal Muscle

• Movement of bones and joints

• Cross-striations

• Neural control

• Voluntary control

• Twitch responses

Contractile Properties of Skeletal

Muscle

• Two mechanisms control the amount

of force generated by a muscle:

1. Recruitment of more motor units

2. Increase firing frequency

Fiber Types of Striated Muscle

(except ocular)

1. Slow (Red) fibers:

– Moderate power output

– Moderate ATP consumption

– Fatigue resistant

2. Fast (White) fibers:

– Maximum power output

– Maximum ATP consumption

– Fatigable

Extraocular Muscles Are Different From Other

Striated Muscles

1. Smaller diameter: 5-40mm

2. Smaller motor unit: 10 fibers / motor neuron

3. Higher discharge rates than spinal motor

neurons

4. Innervation pattern is different in oculomotor

muscles

5. Extraocular muscles have twitch and non-

twitch fibers

6. Fatigue resistance in extraocular muscles is

the highest of any skeletal muscle

7. Extraocular muscles differ in fiber type

Actions of the Extraocular MusclesMuscle Innervation Primary Action Secondary Action

Medial Rectus

Oculomotor N

(inferior

branch)

Adduction

Inferior Rectus

Oculomotor N

(inferior

branch)

Depression Excyclotorsion

Inferior Oblique

Oculomotor N

(inferior

branch)

Excyclotorsion Elevation

Superior Rectus

Oculomotor N

(superior

branch)

Elevation Incyclotorsion

Lateral RectusAbducens N

(CNVI)Abduction

Superior ObliqueTrochlear N

(CNIV)Incyclotorsion Depression

Levator Palpebrae

SuperiorisOculomotor N

Elevation/retraction

of the upper eyelid

Extraocular Muscles Mnemonic

• Extraocular muscles cranial nerve

innervation:

• LR6-SO4-R3

• Lateral Rectus is 6th CN

• Superior Oblique is 4th CN

• Rest are all 3rd CN

Anatomy of the Extraocular

Muscles

Anatomy of the Extraocular

Muscles

Anatomy of the Extraocular

Muscles

Extraocular Muscles Controlled

by CN III• Medial Rectus Muscle

• Superior Rectus Muscle

• Inferior Rectus Muscle

• Inferior Oblique Muscle

Overview of the Oculomotor

Nerve

CNIII coursing through the area

of the right cavernous sinus

Oculomotor Nuclear Complex &

Innervation of EOMs

Primary Actions of the Extraocular

Muscles Innervated by CNIII

CN III Innervates the Levator

Palpebrae Superioris Muscle

Parasympathetic Innervation of the Iris

Sphincter Muscle & Ciliary Muscle

Parasympathetic Innervation of the Iris

Sphincter Muscle & Ciliary Muscle

Direct and Consensual Light

Reflex

Superior Oblique Muscle

Trochlear Nerve (IV) Innervates the

Superior Oblique Muscle

Trochlear Nerve (IV) Innervates the

Superior Oblique Muscle

Actions of the Superior Oblique Muscle

https://www.youtube.com/watch?v=eqV_t1-kP5c

Lateral Rectus Muscle

CN VI-Abducens Nerve

CN VI-Abducens Nerve

CN VI Through the Cavernous Sinus

CN VI-Final Innervation

Lateral Rectus Palsy

Actions of the Extraocular

Muscles• https://www.youtube.com/watch?v=vd7OOJ7c1q4

Orbicularis Muscle

• Innervated by Facial Nerve (CN VII)

• Function: closes the eyelid, involuntary reflex

blinking, voluntary wink

• Antagonist: Levator Palpebrae Superioris

Corneal Reflex

• Lightly touching the cornea with a tissue

induces a rapid blink reflex

• It is a reliable evaluation of afferent CN V

(V1) and efferent CN VII fibers

• Corneal reflex is used as part of some

neurological exams.

– Reflex may be slowed or absent in some

conditions such as coma, stroke or a lesion

involving the trigeminal or facial nerve