Muscle
Specialized for:
Types:
Single contraction characteristics of the three muscle types
Skeletal Muscle
• Striated (sarcomere is functional unit);
multinucleate
• Voluntary (activated by a motor
neurons)
• Each fiber is a cell; many fibers to a
muscle
• Different types of fibers
• Each fiber has a nerve connection
Typing based on succinate
dehydrogenase activity
Type I = low activity
Type IIa = moderate activity
Type IIb = high activity
Typing based on speed of myosin (myosin
ATPase activity
Type I = low activity
Type II = high activity
Basic Structure of a Skeletal Muscle Fiber
Sarcolemma = cell membrane
Fiber (cell) > myofibril (bundles of actin and myosin) > myofilaments (actin and myosin)
sarcomere • Functional unit of a myofibril is the
sarcomere
• Major filamentous proteins are
actin and myosin, z proteins (in the
z disk)
• Many structural proteins; alpha
actinin, myomesin, C protein, titin,
nebulin
• Cytoskeletal proteins; desmin,
vimentin, filamin
Myosin (thick) filaments
• Each filament contains about 400 individual myosin
molecules
• Bundled so that half of the molecules have their head
facing one direction, the other half, the opposite direction
Myosin
heads are in
a staggered
arrangement
along the
filament
• Myosin is composed of 6 polypeptide
chains, 2 H or heavy chains, and two L or
light chains
• Myosin head has ATPase activity
• Myosin has a hinge region where the
molecule is flexible
• The myosin head has a high affinity for g
actin
• In smooth muscle, light chains regulate
myosin action; in cardiac and skeletal
muscle, light chains partially determine
the speed of the myosin ATPase activity
Actin Filaments (thin filaments)
• Monomers of globular or g actin combine to form long fibers of f actin.
Two f actin molecules twist around one another to form a single thin
filament
• All actin myofilaments are anchored to the proteins of the z disk
• Each g actin molecule has a binding site for the myosin head
•Actin
•Tropomyosin – covers active sites on g actin molecules
•Troponin – regulates tropomyosin; three subunits – troponin c, troponin I,
and troponin m
• Troponin c has a binding site for calcium and is bound to the other two
subunits
• Troponin I keeps the tropomyosin over the myosin binding sites on G
actin (inhibits actin/myosin binding)
• Troponin m anchors the three subunits to the tropomyosin molecule
Ryanodine receptor
Excitation-contraction coupling
1. Motor neuron releases Ach onto the surface of the skeletal muscle fiber. The fiber’s
nicotinic receptors are activated, opening K+ and Na+ channels. The cell membrane is
depolarized.
2. The action potential moves away from the motor end plate in all directions, including
down the t-tubule system.
3. Receptors in the t-tubules called dihydropyridine receptors (DHP) are activated by the
change in voltage. They are connected to the ryanodine receptors in the lateral sacks
of the sarcoplasmic reticulum (SR).
4. The ryanodine receptors are opened by the change in conformation of the DHP
receptors and Ca2+ is released from the SR.
5. Ca2+ diffuses across the myofilaments.
6. The Ca2+ binds to troponin C, causing it to change conformation, pulling on troponin
I, which in turn pulls on tropomyosin. With the altered conformation in tropomyosin,
the myosin binding sites on g actin molecules are exposed.
7. Myosin heads can now bind to g actin molecules and cross-bridge cycling begins,
shortening the sarcomere by pulling on the actin filaments and drawing the z disks
closer together
Cross-Bridge Cycling
Cross-bridge cycling
• Continues as long as nerve
depolarizes muscle
membrane; also dependent
on fuel supply as myosin
power stroke is dependent
on ATP
• Pulls z lines closer
together; shortens
sarcomere
• Myosin heads do not pull
in a synchronized manner;
random rowing
Why?
Relaxation
What must happen for a muscle to relax:
1. _
2. _
3. _
4. _
Factors Affecting Strength of Muscle Contraction
• Energy supply
• Anaerobic glycolysis; aerobic glycolysis; FA metabolism; phosphocreatine
metabolism
ATP + creatine phosphocreatine + ADP ATP + creatine
• Muscle length
• Frequency of stimulation
• Summation; tetanus
Why does the force of contraction increase? What factors are
changing at the cellular level?
Frequency of stimulus determines the overall cellular [Ca2+]. The more Ca2+, the
greater the binding to troponin. The greater the binding, the more movement of
tropomyosin. The more active sites exposed, the greater the number of myosin heads that
can bind. The more myosin heads performing a powerstroke, the greater the strength of
contraction.
• Number of motor units
activated (recruitment)
• Slow twitch, fatigue
resistant activated first;
weak stimulus activates
them
• Fast twitch, fatigue-
resistant (intermediate)
activated as stimulus
intensity increases
• Fast twitch, glycolytic
activated at stimuli of
highest intensity
What will happen if you maximally contract a muscle for an extended
period of time? Why?
Types of Contractions
1. Isotonic = same stretch;
moves a load; as muscle
contracts, it shortens
• Concentric = muscle
shortens as it moves a
load
• Eccentric = muscle
lengthens as it moves a
load
2. Isometric = same length; load
is not moved; muscle
contracts but doesn’t shorten
Smooth muscle
• Small, not striated
• Involuntary
• Found in hollow organs; other
organs important to homeostasis
• Can be activated by stretch,
neurons, hormones
• Little SR; caveolae
• Myosin has heads down entire
length
• Slow ATPase activity
• Regulated by myosin light
chains
• Dense bodies anchor actin
• Actin has no troponin or
tropomyosin components