skeletal muscle and the molecular basis of contraction
TRANSCRIPT
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Skeletal Muscle and the
Molecular Basis of Contraction
Lanny Shulman, O.D., Ph.D.
University of Houston College of
Optometry
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• 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
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• 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
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Skeletal Muscle
• Movement of bones and joints
• Cross-striations
• Neural control
• Voluntary control
• Twitch responses
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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
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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
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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
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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
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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
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Anatomy of the Extraocular
Muscles
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Anatomy of the Extraocular
Muscles
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Anatomy of the Extraocular
Muscles
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Extraocular Muscles Controlled
by CN III• Medial Rectus Muscle
• Superior Rectus Muscle
• Inferior Rectus Muscle
• Inferior Oblique Muscle
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Overview of the Oculomotor
Nerve
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CNIII coursing through the area
of the right cavernous sinus
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Oculomotor Nuclear Complex &
Innervation of EOMs
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Primary Actions of the Extraocular
Muscles Innervated by CNIII
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CN III Innervates the Levator
Palpebrae Superioris Muscle
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Parasympathetic Innervation of the Iris
Sphincter Muscle & Ciliary Muscle
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Parasympathetic Innervation of the Iris
Sphincter Muscle & Ciliary Muscle
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Direct and Consensual Light
Reflex
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Superior Oblique Muscle
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Trochlear Nerve (IV) Innervates the
Superior Oblique Muscle
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Trochlear Nerve (IV) Innervates the
Superior Oblique Muscle
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Actions of the Superior Oblique Muscle
https://www.youtube.com/watch?v=eqV_t1-kP5c
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Lateral Rectus Muscle
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CN VI-Abducens Nerve
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CN VI-Abducens Nerve
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CN VI Through the Cavernous Sinus
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CN VI-Final Innervation
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Lateral Rectus Palsy
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Actions of the Extraocular
Muscles• https://www.youtube.com/watch?v=vd7OOJ7c1q4
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Orbicularis Muscle
• Innervated by Facial Nerve (CN VII)
• Function: closes the eyelid, involuntary reflex
blinking, voluntary wink
• Antagonist: Levator Palpebrae Superioris
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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
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