Download Chapter 8a

Function depends on structure
Muscle classification
1. Striated muscle
A. Skeletal Muscle - voluntary muscles that allow for
movement
B. Cardiac Muscle - heart - specialized, involuntary
2. Non-striated muscle
Smooth Muscle such as blood vessels, digestive tract,
internal organs, involuntary
Muscle functions
Muscle perform four import functions:
1. Produce movement
2. Maintaining posture
3. Stabilizing joints
4. Generating heat
Functional characteristics of muscles
Excitability (irritability): the ability to receive and respond to a
stimulus
Contractility: the ability to shorten forcibly when adequately
stimulated
Extensibility: the ability to be stretched or extended
Elasticity: the ability of a muscle fiber to resume its resting
length after being stretched.
Sarcomere: the contractile unit
of a myofibril
contains actin thin filament and
myosin thick filament
I band
A band
I band
Cross bridge
Thick filament
Thin filament
Fig. 8-4, p.317
Contraction of sarcomeressliding-filament theory
muscle contraction- sarcomeres shorten,
actin and myosin move past each other
and increase overlap between actin and
myosin.
muscle stretched- Sarcomeres elongate.
Reduce overlap between actin and
myosin.
Note: Length of thick (myosin) and thin
(actin) filaments remains constant.
Length-tension relation
Total tension is
proportional to the total
number of cross-bridges
(overlap) between actin
and myosin filaments
Ideal resting length:
generate maximum
force.
Overlap to small: few
cross-bridges can attach.
No overlap: no crossbridges can attach to
actin.
3 pairs of molecules:
1. myosin heavy chains
2. essential light chains
3. regulatory light chains
Actin - thin filaments
1. comprised of protein dimers linked in "chains"
2. each actin monomer has a myosin binding site
3. thin filaments are anchored at one end to "Z-line"
proteins
4. thin filaments are free at other end
5. "sarcomere" is the name for unit between "Z-lines"
G-actin
polymerize
F-actin (filamentous)
Troponin is a complex of 3 protein subunits:
Troponin C binds Ca 2+
Troponin T binds tropomyosin
Troponin I binds both actin & tropomyosin
Troponin C binds Ca 2+
Troponin T binds tropomyosin
Troponin I binds both actin & tropomyosin
Cross-bridge chemistry
1. Attachable
2. Revisable
Transduction of chemical to mechanical energy
in muscle causes the filaments to slide:
Partial rotation of the actin-bound myosin head.
neuromuscular
junctions Each
muscle cell is directly
innervated by the
terminal branch of a
motor neuron. The
contact between
nerve and muscle
occurs at a small
specialized spot
termed the
neuromuscular
junction (NMJ).
Transverse tube (T tube not Z disk): transmit excitation into muscle
fibers
Frog
Crab
Sarcoplasmic reticulum (SR):
Ca2+ is stored and released as free
Ca2+ during excitation-contraction
Calsequestirin: Ca2+ binding
protein in SR
Ca2+/Ma2+ pump (ATPase):
proteins in SR actively transport
Ca2+ ions (requires ATP).
Ca++ regulation
a. neural activation >> muscle is electrically excited >> AP
AP ionic currents reach SR, open voltage sensitive Ca++
channels
Ca++ rushes out of SR, binds to troponin C,
actin-myosin permitted to interact >> contraction
b. AP stops, voltage sensitive Ca++ channels close,
Ca++ rapidly pumped into SR, tropomyosin returns,
actin-myosin interactions blocked >> relaxation
c. Ca++ is sequestered (pumped and stored) in Sarcoplasmic
Reticulum (SR)
- SR is the endoplasmic reticulum of muscle cells
- SR is intracellular Ca++ store
d. Ca++ is actively pumped into SR from muscle cytoplasm
Ryanodine receptor: located on SR membrane
Dihydropyridine receptor: located on T tubule membrane, no or
little Ca2+ passes through in skeletal muscle.
Releasing Ca2+ from SR into the myoplasm depends
1. interaction of activated dihydropyridine receptor and ryanodine
receptor-plunger model
2. Calcium-induced calcium release
Mechanisms of Contraction
• AP travels down the motor neuron to bouton.
• VG Ca++ channels open, Ca++ diffuses into the
bouton.
• Ca++ binds to vesicles of NT.
• ACh released into neuromuscular junction.
• ACh binds onto receptor.
• Chemical gated channel for Na+ and K+open.
Mechanisms of Contraction
• Na+ diffuses into and
K+ out of the
membrane.
• End-plate potential
occurs (depolarization).
• + ions are attracted to
negative membrane.
• If depolarization
sufficient, threshold
occurs, producing AP.
Mechanisms of Contraction
• AP travels down
sarcolema and T
tubules.
• Terminal
cisternae release
Ca++.
Mechanisms of Contraction
• Ca++binds to
troponin.
• Troponintropomyosin
complex moves.
• Active binding
site on actin
disclosed.
Sliding Filament Theory
• Sliding of filaments is produced by
the actions of cross bridges.
• Cross bridges are part of the myosin
proteins that form arms that terminate
in heads.
• Each myosin head contains an ATPbinding site.
• The myosin head functions as a
myosin ATPase.
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.
Contraction
• 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.
• Release of ADP upon binding to another ATP, causes
the cross bridge bond to break.
• Cross bridges detach, ready to bind again.
Contraction
•
•
•
•
ACh-esterase degrades ACh.
Ca++ pumped back into SR.
Choline recycled to make more ACh.
Only about 50% if cross bridges are
attached at any given time.
– Asynchronous action.
Contraction
• A bands:
– Move closer together.
– Do not shorten.
• I band:
– Distance between A bands of
successive sarcomeres.
– Decrease in length.
• Occurs because of sliding of thin filaments over and
between thick filaments.
• H band shortens.
– Contains only thick filaments.
Regulation of Contraction
• Regulation of cross-bridge attachment to
actin due to:
– Tropomyosin.
– Troponin.
Role of
++
Ca
• Relaxation:
– [Ca++ ] in sarcoplasm low when
tropomyosin block attachment.
– Ca++ is pumped back into the SR in the
terminal cisternae.
– Muscle relaxes.
++
Ca in
Role of
Muscle
Contraction
• Stimulated:
• Ca++ is released from
SR.
• Ca++ attaches to
troponin
• Tropomyosin-troponin
configuration change
Two major processes require ATP in muscle
contraction:
1. Hydrolysis ATP by myosin (70-80%)
2. Pumping of Ca2+ back into SR (20-30%)