Post on 25-Mar-2022
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