Download Lecture 9: Muscle: Structure and Function

Muscle structure and
function
Animal Science 123
Animal Growth & Development
R. D. Sainz
Lecture 09
Types of muscle
• Skeletal muscle
– striated
– voluntary
• Smooth muscle
• Cardiac muscle
From: Frandson, RD. 1986. Anatomy and Physiology of Farm
Animals. Lea & Febiger, Philadelphia.
Smooth muscle
•
•
•
•
No visible striations
Spindle-shaped cells, central nucleus
Network arrangement of cells
Regulated by autonomous nervous system
(involuntary)
• Surrounding digestive tract, urogenital
system, blood vessels
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Cardiac muscle
• Found only in heart
• Network of cells, central nuclei
• Contraction inherent & rhythmic
– rate controlled by autonomic nervous
system
Skeletal muscle
• Striated (striped under low-power
microscopy)
• Multinuclear cells with peripheral nuclei (=
fibers)
• Fibers arranged in bundles or fasciculi,
separated by connective tissue:
– Endomysium – surrounds individual fibers
– Perimysium – surrounds fiber bundles
– Epimysium – surrounds entire muscle
Skeletal muscle
• Cell membrane = sarcolemma (plasma
membrane + basement membrane +
reticular fibers
• Attachments: fleshy or tendons
– Origin = least movable attachment (usually
proximal)
– Insertion = most movable attachment (usually
distal)
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From: Frandson, RD. 1986. Anatomy and Physiology of Farm Animals. Lea & Febiger, Philadelphia.
From: Romans, JR, Costello, WJ, Carlson, CW, Greaser, ML, and Jones, KW. 1994. The Meat We Eat. Interstate
Publishers, Danville, IL.
Histology of skeletal muscle: organelles
• Up to 100’s of nuclei, randomly distributed along fiber,
just beneath sarcolemma
• Mitochondria between contractile elements;
concentration varies with fiber type
• Ribosomes dispersed in sarcoplasm (very few on
sarcoplasmic reticulum; few secreted proteins)
• Sarcoplasmic reticulum ≈ endoplasmic reticulum, but
also regulates [Ca2+]
– action potential causes release of Ca2+ from sarcoplasmic
reticulum, stimulates contraction
• Transverse tubules (triads), continuous with sarcolemma
• Contractile machinery = myofibril
• Motor end-plate = neuromuscular junction
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From: Swatland, HJ. 1984. Structure and Function of Meat Animals. Prentice-Hall, Inc., Englewood Cliffs, NJ.
Source: Hill et al., 2004. Animal Physiology, Sinauer Associates, Sunderland, MA.
Myofibrils
• From 100s to 1,000s per fiber
• 12-14 aggregated proteins (ca. 55% of total
muscle protein)
• 2 main classes of filaments: thick and thin
• thick filaments
– 15 nm x 1,500 nm
– mainly myosin
• thin filaments
– 6 nm x 1,000 nm
– mainly actin, tropomyosin, troponin
– anchored at the Z-disk, overlap with the thick
filaments (A band)
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Sarcomere
• Fundamental unit of muscle fiber
• From Z line to Z line
From: Swatland, HJ. 1984. Structure and Function of Meat Animals. Prentice-Hall, Inc., Englewood Cliffs, NJ.
Source: Hill et al., 2004. Animal Physiology, Sinauer Associates, Sunderland, MA.
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Fiber types
• Based upon contraction speed and
metabolic pattern
• Slow-twitch, oxidative (SO)
– AKA Type 1, β red
– depend upon oxidative metabolism, ∴↑
mitochondria
– ↓ fiber diameters
– ↓ cell size (protein:DNA)
– slower contraction speeds (≤ 100 msec)
Fiber types
• Based upon contraction speed and
metabolic pattern
• Fast-twitch, glycolytic (FG)
– AKA Type 2b, α white
– depend upon anaerobic (glycolytic)
metabolism, ∴ ↓ mitochondria
– ↑ fiber diameters
– fast contraction speeds (↑ myosin ATPase); ≥
7.5 msec
Fiber types
• Based upon contraction speed and
metabolic pattern
• Fast-twitch, oxidative-glycolytic (FOG)
–
–
–
–
AKA Type 2a, intermediate, α red
both oxidative and glycolytic capacity
intermediate fiber diameters
fast contraction speeds
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Histochemical determination of fiber
types
• Myosin ATPase staining
– Acid or alkali
preincubation
– Fast – alkali-stable
– Slow – acid-stable
– Proportional to twitch
speed
• Succinate
dehydrogenase (SDH)
staining
– Proportional to
mitochondrial density
From: Swatland, HJ. 1984. Structure and
Function of Meat Animals. Prentice-Hall, Inc.,
Englewood Cliffs, NJ.
Functions of different fiber types
• Slow, oxidative
– Slow, repetitive movements, e.g. postural muscles
– Very resistant to fatigue
• Fast, glycolytic
– Rapid movements, heavy force generation
– Easily fatigued; can go into O2 debt for short periods
• Fast, oxidative-glycolytic
– Adapted for rapid, repetitive movements
– Recruited after SO fibers
– Intermediate fatigue; recover faster than FG fibers
• Also: tonic muscle fibers (unusual)
– Multiply innervated
– Graded response to stimulation frequency (vs. propagated action
potential)
– Efficient isometric tension (e.g., anterior latissimus dorsi, holds
wings against body)
From: Gerrard, DE & Grant, AL 2003. Principles of Animal Growth & Development. Kendall/Hunt Pub. Co., Dubuque, IA
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From: Hedrick, HB, Aberle, ED, Forrest, JC, Judge, MD, and Merkel, RA.
Kendall/Hunt Publishing Co., Dubuque, Iowa.
1994.
Principles of Meat Science.
Muscle contraction
1. Release of acetylcholine from nerve
ending across synaptic cleft (20-30 nm)
2. Twitch response to action potential
(ca. 2 msec later)
a) ↑ Resting membrane potential
(-90 mV → -10 mV)
b) Action potential (Na+ influx); 2-4 msec,
5m/sec
c) Refractory period 1-3 msec; repolarization
(K+ efflux)
Muscle contraction
1.
Intracellular events
a)
propagation of action potential
throughout plasma membrane
(including transverse tubules)
b)
release of Ca2+ from
sarcoplasmic reticulum into
sarcoplasm & myofibrils
c)
Ca2+ combines with troponin &
changes conformation of
tropomyosin, allowing attachment
of myosin heads to the actin
filament (at the ADP site)
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Muscle contraction
d) Attachment changes the angle
between the head & arm of myosin,
forcing the filament to slide,
shortening the sarcomere
e) Changed angle exposes ATP, which
is hydrolyzed, releasing energy and
detaching the myosin head
f) Myosin head bonds to the next ADP
site on actin, as long as Ca2+ is
present (ratchet theory)
g) When stimulation ends, Ca2+ is
sequestered by the sarcoplasmic
reticulum (calsequestrin =
transporter)
h) ATP is therefore required for muscle
relaxation
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