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Muscle:
Contractions and Neural
Control
More than ___ muscles in the animal body
Characteristics of muscle dictate its function
Mammalian and avian – long, unbranched,
and threadlike that taper at both ends
Muscle Fiber = ___________________
Function
• Movement of skeleton
• Blood pressure/supply
• Transport of ingesta
• Generation of Body heat
• Circulation of blood
Muscle general anatomy
• Tendon (tough connective tissue)
• Muscle
• Tendon
Embryonic development Myoblasts
• Muscle: Myotome (myoblast) cells
migrate to various places in the
embryo
– Chemotaxis – ____________________
– Morphogens – chemicals produced in
one area which effect distant cells
Growth
• Hypertrophy: cells increase in _____
• Hyperplasia: addition of more cells
• Growth: increase in muscle and bone
• Fattening: accumulation of fat
Skeletal Muscle Types:
Slow- and Fast-Twitch Fibers
• Divided on basis of contraction speed:
– Slow-twitch (type I fibers).
• ______________________:
• Muscles which get used a lot, for long periods of time
• Fowl that fly often, have dark breast (pectoral) meat
– Fast-twitch (type II fibers).
• _________________: fast twitch, easy to fatigue, lots of
___________________ present
• ________________________: light colored pectorals
– Intermediate: have a combination of the two
• Most muscles
• Differences due to different myosin ATPase
isoenzymes that are slow or fast.
Upper Motor Neuron Control
of Skeletal Muscles
• Cerebellum:
– Receives sensory input from muscle
spindles
• No descending tracts from the
cerebellum.
– Influences motor activity indirectly.
• All output from cerebellum is inhibitory.
– Aids motor coordination.
Upper Motor Neuron Control
of Skeletal Muscles
(continued)
• Basal nuclei:
• Profound inhibitory effects on the
activity of lower motor neurons.
– Damage to basal nuclei result in
increased muscle tone.
Anatomy of Skeletal Muscle
Figure 12.1
• Epimysium – CT that surrounds
______________
• Perimysium – CT that surrounds muscle
bundle
• Endomysium – CT that surrounds
____________________
• Sarcolemma – “____________”
– Elastic CT that surrounds the myofibrils
– Invaginations form a network of tubules
called the transverse tubules “T-tubules”
Muscle types
• Striated muscle
– ___________
– Autonomic control
• Smooth muscle
– _________________________
– No striations
• Cardiac muscle
– ____________________
– ______________________ control
Skeletal Muscle
• Sarcoplasm – cytoplasm
• Nuclei – multinucleated
• Myofibrils – unique to muscle tissue
– Long, thin, cylindrical rods 1 to 2 µm in
diameter
– Bathed in sarcoplasm
– Consists of: ________________________
• Thick & Thin
Structure of Skeletal Muscle
Sarcomere
Skeletal Muscle
• Myofilaments
– Thick are aligned parallel to each other
– Thin , parallel to thick
– These “bands” of parallel fibers give SM striated
appearance
• A and I bands
– Called this due to light refractions
– _________band is denser than ___________
• Shows up darker in pictures
– Both are bisected by thin, dense lines
Skeletal Muscle
• A and I bands
– I band is bisected by line called: _______
– Unit of myofibril between 2 different Z disks
is called: ____________________
• Includes the A band and parts of the I band
• Is the repeating structural unit of the myofibril
• Basic unit in _____________ & ________________
• Length of sarcomere is not constant and will
differ depending on relaxation or contraction
Myofilaments
• Thick and thin differ in dimensions and
chemical composition
– Thick
• 14 to 16 nanometers (nm = 1 billionth of a
meter)
• Constitute the ______________ of sarcomere
• ____________________ is the predominant protein
– held in position by other proteins some of which are
located in the M line
Myofilaments
• Thin
– 6-8 nm in diameter
– Extend 1.0 µm on either side of the Z disk
– Filaments constitute the ________ of the
sarcomere
– Extend into the _________________
– I band only contains the ________
filaments
Myofilaments
• ________________ – only thick filaments
are present
• ________________ – thick and thin are
located
– Shows 6 thin filaments surrounding each
thick
Z- Disk
• Z disk
– Comprised of z-filaments which
connect actin molecules
– Z line is composed of 4 z-filaments
which attach actin molecules from
each sarcomere
Proteins of myfofibril
• 20 different proteins associated with
myofibrils
– 6 proteins account for 90% of total myofibrillar
protein
– Decreasing order of abundance:
• Myosin
• Actin
• Titin
• Tropomyosin
• Troponin
• Nebulin
Proteins of myofibril
• Major Contractile Proteins
– Actin – 20% of myofibrillar protein
Globular –– G
G -- actin
actin
Globular
Form aa “super
“super helix”
helix”
Form
Actin myofilaments are made of
globular actin (G-actin) monomers
and other proteins.
Proteins of myofibril
• Major Contractile Proteins
– Myosin – 45% of myofibril protein
Elongated Rod
Thickened end: Head – 2 of them
Thin end: Tail
In-between: neck
Proteins of myofibril
• Major Contractile Proteins
Myosin
– 2 fractions: ________ and
__________meromyosin
– Center of A band – myosin contains only
rods, no heads
• Known as pseudo H zone
– Heads are functionally active during
contraction
• Form a cross-bridge with ______________
Proteins of myofibril
• Tropomyosin
5% of myofibrillar protein
– Lies in close contact with ____________
– Lies in grove with __________
Proteins of myofibril
• Troponin: 5% of myofibril protein
– In groove of _________________
– Lies astride the tropomyosin strands
Proteins of myofibril
• Titin: 10% of myofibrillar protein
– Scaffold for alignment of filaments during
myofibril and sarcomere formation
Proteins of myofibril
• Nebulin: 4% of myofibril protein
– Anchors thin filaments to Z disks
– Serves as a template for assembly /
scaffold for stability of thin filaments
Muscle Contraction
• Each myofibril contains myofilaments.
• 2 major ones involved with contraction
– Thick filaments aka ________________:
• A bands contain thick filaments (primarily composed of
myosin).
• Myosin initiates the contraction
• There an enzyme that converts ATP to ADP and
Phosphate
– Thin filaments aka ___________________:
• I bands contain thin filaments (primarily composed of
actin).
• Most abundant protein
• Major constituent of muscle
– Center of each I band is Z disc.
Mechanisms of Contraction
(continued)
• Sarcomere:
– _____ disc to ________ disc.
– M lines:
• Produced by protein filaments in a sarcomere.
– Anchor myosin during contraction.
• Titin:
– Elastic protein that runs through the
myosin from ____ line to ____________.
• Contributes to elastic recoil of muscle.
Mechanisms of Contraction
(continued)
• Tropomyosin:
– Part of the “___________” filament
• Continuous stand that sits on actin
• Exposes the actin binding site
• Troponin:
– Inhibits actin-activated myosin ATPase
Activity
Sliding Filament Theory of
Contraction
• Sliding of filaments is produced by the
actions of cross bridges.
– Cross bridges are part of the myosin
proteins that extend out toward actin.
• Form arms that terminate in heads.
– Each myosin head contains an ATPbinding site.
• The myosin head functions as a myosin
ATPase.
Sliding Filament Theory of
Contraction (continued)
• Muscle contracts:
– Occurs because of sliding of thin filaments
over and between thick filaments towards
center.
• Shortening the distance from Z disc to Z disc.
• A bands:
– Contain ______________.
• Move closer together.
– Do not shorten.
Sliding Filament Theory of
Contraction (continued)
• I bands:
– Distance between A bands of successive
______________________.
– Decrease in length.
• H bands shorten.
– Contain only _____________.
– Shorten during contraction.
Contraction
• Myosin binding site splits ATP to ADP
and Pi.
• ADP and Pi remain bound to myosin
until myosin heads attach to actin.
• Pi is released, causing the power stroke
to occur.
• Power stroke pulls actin toward the
center of the A band.
• ADP is released, when myosin binds to
a fresh ATP at the end of the power
stroke.
Contraction (continued)
• Release of ADP upon binding to
another _______, causes the cross
bridge bond to break.
• Cross bridges detach, ready to bind
again.
• Synchronous action:
– Only 50% of the cross bridges are
attached at any given time.
Regulation of Contraction
• Regulation of cross bridge attachment
to actin due to:
– Tropomyosin:.
• Lies within grove between double row of Gactin.
– Troponin:
• Attached to tropomyosin.
• Serves as a switch for muscle
contraction and relaxation.
– In relaxed muscle:
• Tropomyosin blocks binding sites on actin.
Role of Ca2+ in Muscle
Contraction
• Muscle Relaxation:
– [Ca2+] in sarcoplasm low when
tropomyosin blocks attachment.
• Prevents muscle contraction.
• Ca2+ is pumped back into the SR in the
____________________________.
– Muscle relaxes.
Sarcoplasmic Reticulum & T
Tubules
• Membranous system of tubules and
cisternae (reservoirs for calcium)
• Found around each _________________
• T-tubules:
– Associated with __________________
– T tubule runs transversely across the
sarcomere at the AI junction.
Sarcoplasmic Reticulum & T
Tubules
• SR: Longitudinal tubules of reticulum:
– Converge in H zone which forms a
fenestrated collar
– At AI junction:- the longitudinal tubules
join with terminal cisternae
– Longitudinal tubules extend in both
directions from fenestrated collar to
terminal cisternae
Contraction-Relaxation
• Plasma membrane is depolarized by an AP
• The AP is conducted deep into the muscle
fibers along the T-tubules In response to Ttubule depolarization, dihydropyridine
receptors in the muscle undergo a
confrontational change that directly link to
Ryanodine receptors in the SR membrane –
caused opening of Ca ++ channels in the SR
membrane
Contraction-Relaxation (cont.)
• Myosin cross bridged attach to the actin
filaments; ratchet through 4 steps ATP binds
the ATPase site on the myosin head, causing
myosin to detach from the thin (actin)
filament
• The cycle repeats
Muscle Fuel Consumption
During Exercise
Metabolism of Skeletal
Muscles
• Lactate threshold:
– % of max. 02 uptake at which there is a
significant rise in blood [lactate].
• During light exercise:
– Most energy is derived from aerobic respiration
of _______________________.
• During moderate exercise:
– Energy is derived equally from _________________
and _________________________.
• During heavy exercise:
– Glucose supplies 2/3 of the energy for muscles.
• Liver increases _____________________________.
Metabolism of Skeletal
Muscles (continued)
• Oxygen debt:
– Oxygen that was withdrawn from
___________ and __________________ during
exercise.
– Extra 02 required for metabolism tissue
warmed during exercise.
– 02 needed for metabolism of lactic acid
produced during anaerobic respiration.
• When person stops exercising, rate of
oxygen uptake does not immediately
return to pre-exercise levels.
– Returns slowly.
Metabolism of Skeletal
Muscles (continued)
• Phosphocreatine (creatine phosphate):
– Rapid source of renewal of ___________.
– ADP combines with creatine phosphate.
• [Phosphocreatine] is ________ times [ATP].
– Ready source of high-energy phosphate.
Smooth Muscle
• Characteristics
–
–
–
–
No striations
Centrally located nucleus
Autonomic control
Major proteins – ________ & ___________
• Proteins not arranged in any particular order =
no striations
Smooth Muscle
• Does not contain
sarcomeres.
• Contains > content
of actin than
myosin (ratio of
16:1).
• Myosin filaments
attached at ends
of the cell to dense
bodies.
• Contains gap
junctions.
Smooth Muscle
• Smooth Involuntary
– ________________ resistance
– Individual cells, centrally located (one)
nucleus, no striations
– Independent of higher nervous centers
– Autonomic NS (Para and Sym control)
– Includes: Digestive, Respiratory, Vascular
Smooth Muscle Contraction
• Depends on rise in free intracellular
Ca2+.
• Ca2+ binds with ________________.
– Ca2+ calmodulin complex joins with and
activates myosin light chain kinase.
• Myosin heads are phosphorylated.
– Myosin heads binds with actin.
• Relaxation occurs when Ca2+
concentration decreases.
Cardiac Muscle
• Contain actin and
myosin arranged in
sarcomeres.
• Contract via
sliding-filament
mechanism.
• Adjacent
myocardial cells
joined by gap
junctions.
– APs spread through
cardiac muscle
through gap
junctions.
• Behaves as one unit.
Cardiac Muscle
• Striated involuntary: Cardiac
– Fatigue resistance
– Don’t have to think about it (Autonomic NS)
• Branched: allows for nerve impulses to
branch as well
• Cells arranged sort of side by side. In each
cell is once nucleus. On each side is an
intercalated disk
• Usually just have one nucleus, sometimes 2
– Striated: proteins in them overlap