Skeletal Muscle Contraction

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• Skeletal Muscle Contraction
• 8th ed 50.5
• 7th ed 49.6

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• In order to move all animals require muscle
  activity in response to nervous system input.
• Skeletal muscles responsible for voluntary
  movement.

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Biceps contracts

• Skeletal muscles are attached to bones by tendons
  and are responsible for their movement.

Forearm flexes
Triceps relaxes
Tendons
Biceps relaxes
Forearm extends
Triceps contracts
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• Organization of skeletal muscles Have a
  hierarchy of smaller and smaller units
• Bundles of long fibers running parallel to the
  length of the muscle
• Each fiber is made of longitudinally arranged
  myofibrils

Muscle
Bundle of muscle fibers
Nuclei
Single muscle fiber (cell)
Plasma membrane
Myofibril
Light band
Z line
Dark band
Sarcomere
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Muscle

• Myofibrils are composed of thin and thick
  filaments
• Thin filaments are made of two strands of
  regulatory proteins and two stands of actin
• Thick filaments are made of staggered myosin
  molecules

Bundle of muscle fibers
Nuclei
Single muscle fiber (cell)
Plasma membrane
Myofibril
Light band
Z line
Dark band
Sarcomere
TEM
0.5 µm
I band
A band
I band
M line
Thick filaments (myosin)
Thin filaments (actin)
H zone
Z line
Z line
Sarcomere
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• Skeletal muscles are striated arrangement of
  filaments create dark and light bands

0.5 µm
Z
H
I
A
Sarcomere
Skeletal muscle tissue under a light microscope
Skeletal muscle tissue under a electron microscope
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• Sarcomere repeating contractile unit of a
  muscle
• Thin filaments are attached at the Z line and
  proceed to the center of the sarcomere
• Thick filaments are attached at the M line in the
  center of the sarcomere
• Area near edge of sarcomere with only thin
  filament is the I band
• Region corresponding to the length of the thick
  filament is the A band
• Center of A band containing only thick filaments
  is called the H zone
• At a relaxed state thin and thick filaments
  partially overlap
• This overlapping arrangement is key to the
  contraction mechanism
• This regular arrangement produces dark and light
  bands and hence make the fibers look striated

Sarcomere
TEM
0.5 µm
I band
A band
I band
M line
Thick filaments (myosin)
Thin filaments (actin)
H zone
Z line
Z line
Sarcomere
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LE 49-29
0.5 µm

• Sliding-Filament model of muscle contraction
  (focus on a single sarcomere)
• During contractions the thin and thick filaments
  do not change in length but increase the overlap.
• This shortens the length of the sarcomere

Z
H
I
A
Sarcomere
Relaxed muscle fiber
Contracting muscle fiber
Fully contracted muscle fiber
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• Myosin molecule (thick filament) has a globular
  head and a long tail. Tail adheres to other
  tails.
• Thin filaments are actin molecules along with
  regulatory proteins

Thick filament
Thin filaments
Thin filament
Myosin head
Thick filament
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• Steps in muscle contraction
• Myosin head is bound to ATP in a low-energy
  configuration

Thick filament
Thin filaments
Thin filament
Myosin head (low-energy configuration)
Thick filament
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• ATP is hydrolyzed to ADP and Pi and the head is
  now in high-energy configuration

Thick filament
Thin filaments
Thin filament
Myosin head (low-energy configuration)
Thick filament
Cross-bridge binding site
Actin
Myosin head (high- energy configuration)
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Thick filament
Thin filaments
Thin filament
Myosin head (low-energy configuration)

• Head binds with actin filament at the myosin
  binding sites forms cross bridge

Thick filament
Cross-bridge binding site
Actin
Myosin head (high- energy configuration)
Cross-bridge
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• Head releases the ADP and Pi and returns to
  low-energy configuration Actin (thin) filament
  moves towards the center of the sarcomere

Thick filament
Thin filaments
Thin filament
Myosin head (low-energy configuration)
Thick filament
Cross-bridge binding site
Thin filament moves toward center of sacomere.
Actin
Myosin head (high- energy configuration)
Myosin head (low- energy configuration)
Cross-bridge
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• Myosin binds to new ATP molecule and resumes the
  cycle

Thick filament
Thin filaments
Thin filament
Myosin head (low-energy configuration)
Thick filament
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• Nerves conduct signals by changing the voltage on
  the membranes (action potential)
• Sensory neuron nerve cell that receives
  information from the internal or external
  environments and transmits the signal to the
  central nervous system (brain and spinal cord)
• Motor neuron transmits signals from brain or
  spinal cord to muscles or glands.

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(No Transcript)
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• Synapse junction where one neuron communicates
  with another neuron or with muscle/gland etc.
• Synaptic terminal A bulb at the end of the axon
  in which neurotransmitter molecules are stored
  and released.
• Synaptic cleft narrow gap separating synaptic
  knob of a transmitting neuron or its effector
  cell.

Synaptic terminal of motor neuron
Synaptic cleft
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Spinal cord
Motor unit 1
Motor unit 2

• Motor unit A single neuron and all the muscle
  fibers it controls
• When motor neuron produces action potential all
  the muscle fibers in its motor unit contract as a
  group.

Synaptic terminals
Nerve
Motor neuron cell body
Motor neuron axon
Muscle
Muscle fibers
Tendon
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• Regulation of skeletal muscle contraction
• Synaptic terminal receives action potential and
  releases neurotransmitter Acetylcholine (ACh)
• ACh binds to receptors in the muscle and triggers
  action potential in the muscle fiber.
• Action potential is propagated along the plasma
  membrane and down the T-tubule.

Synaptic terminal of motor neuron
PLASMA MEMBRANE
Synaptic cleft
T TUBULE
ACh
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Synaptic terminal of motor neuron
PLASMA MEMBRANE
Synaptic cleft
T TUBULE
SR
ACh

• Action potential triggers Ca2 release from
  sarcoplasmic reticulum

Ca2
CYTOSOL
Ca2
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• Myosin cross-bridges attach and detach, powered
  by ATP pulling the actin filament towards center
  of the sarcomere
• When action potential ends Ca2 is absorbed back
  into the sarcoplasmic reticulum
• Muscle contraction ends, muscle fibers relax

CYTOSOL
Ca2
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Synaptic terminal of motor neuron
PLASMA MEMBRANE
Synaptic cleft
T TUBULE
SR
ACh
Ca2
CYTOSOL
Ca2
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Motor neuron axon
Mitochondrion
Synaptic terminal
T tubule
Ca2 released from sarcoplasmic reticulum
Sarcoplasmic reticulum
Myofibril
Sarcomere
Plasma membrane of muscle fiber
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• Ca and regulatory proteins and their role in
  muscle fiber contraction
• Actin filaments have regulatory proteins on them.
• Tropomyosin, trponin complex and Ca2 regulate
  muscle contraction
• At rest tropomyosin covers the actin binding
  sites preventing actin and myosin from interacting

Tropomyosin
Ca2-binding sites
Actin
Troponin complex
Myosin-binding sites blocked.
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• When Ca2 is released into the cytosol from the
  sarcoplasmic reticulum it binds to troponin
  complex.
• This changes the alignment of the troponin
• That in turn shifts the position of the
  tropomyosin, exposing the myosin binding sites on
  the actin filament
• When Ca2 concentration drops the binding sites
  are covered and contraction stops.

Ca2
Myosin- binding site
Myosin-binding sites exposed.
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Tropomyosin
Ca2-binding sites
Actin
Troponin complex
Myosin-binding sites blocked.
Ca2
Myosin- binding site
Myosin-binding sites exposed.