Muscle Contraction
Outline
• Steps of contraction
• Sliding filament model
• Sarcomeres
• Skell mm = troponin. Smooth mm = no troponin, Yes =calmodulin
• Tone
• Types of Contraction (Eccentric, concentric, isometric)
Contraction Steps
• AP arrives at NMJ
• Ach (NT) released, causes another AP
• AP travels along T-Tubule to SR
• SR releases Ca
• Troponin-tropomysin is blocking (Shielding) the
actin. Ca binds to Trop-Trop results in
unshielding of Actin
• Myosin heads bind to actin filament. (This is
called a cross bridge)
• The myosin heads drag the actin thin filament
past the thick filament one time (Power
stroke)
• The myosin heads must be “re-cocked” using
ATP so they can drag the actin thin filament.
Skeletal Muscle Contraction
• Troponin-tropomysin is blocking (Shielding) the actin. Ca
binds to Trop-Trop results in “unshielding” of Actin
• Myosin heads bind to actin filament. (This is called a cross
bridge)
• The myosin heads drag (Power stroke) the actin thin
filament past the thick filament one time (toward center of
sarcomere)
• The myosin heads must be “re-cocked” using ATP so they
can drag the actin thin filament.
• Myosin is an ATPase (it also needs Mg2+ to be active)
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• Can only happen when actin is
“unshielded”
• Actin is shielded by troponin-
tropomysin guard
• Ca++ unshields actin by binding
to trop-trop
• Actin has a binding site for
myosin heads but the binding
sites are shielded by troponin-
tropomyosin complex
Sarcomere - Lines
• I band
• Actin
• Troponin & Tropomysin shields
actin
• Ca binds to Trop-Trop to uncover
actin
• I is a THIN letter so it is only the
THIN filament
• H band
• Myosin (ATPase, needs Mg2+)
• H is a THICK letter so it is only the
THICK filament
SARCOMERE MEMORY AIDS
• Z-line denotes the BoundareeZ of the sarcomere
• H is a THICK letter so it is only the THICK filament
• I is a THIN letter so it is only the THIN filament
• A band is made up of the thick And thin filament
• A is also the Abstinence band, It abstains from shortening during contraction
• H, I, Z, shorten during contraction.
Skelly mm = Troponin. Smooth mm = no troponin
• In Skeletal muscle Ca++ binds to Troponin-Tropomysin Complex to begin
contraction
• Ca++ binds to Troponin C NOT troponin I (think C for Ca++)
• In Smooth Muscle Ca++ binds to calmodulin-Tropomysin Complex to begin
contraction
Calmodulin
• Ca+ binds to calmodulin à which activates myosin light chain kinase à which then
activates an enzyme called myosin kinase à that phosphorylates myosin heads so
they can form the cross-bridges with actin and then pull on the thin filaments.
• What regulates contraction in smooth muscle? Calmodulin
• Smooth muscle has calmodulin instead of troponin.
• All muscles have tropomyosin
Muscle Tone (Tonus)
• Muscle Tone = At rest your muscles are never floppy/fully relaxed. Always
have a low level of resting tension/contraction. Allows you to react
quicker . If muscles were completely loose/floppy, it would take more
neuronal firing and time to contract
• Gama fibers set Muscle tone
• A muscle devoid of tone(tonus) is atonic
Types of contraction
• Concentric Contraction = muscles
contract/shorten
• Eccentric Contraction = muscles elongate
• Isometric Contraction = contracting with No
Movement (holding the pose, eg bodybuilders
posing)
Duchenne Muscular Dystrophy (DMD)
• Defect in contraction pathway
• Caused by lack of a protein named dystrophin which helps thin filaments
bind to the sarcolemma
• During contraction the sarcolemma tears (thin filament isnt connected to
sarcolemma. So when contraction occurs, thin fillament is like a firehose
laying on the floor that’s turned on. When turned on it goes wild and
destroys the sarcolemma. One by one the myofibrils inside a muscle fiber
dystrophy. Results in muscle dystrophy
Review
• When an AP gets to the NM jxn, what molecule triggers the next AP along the sarcolem? à release Ach
• Which enzyme recylces AcH in the post synaptic terminal? = Acetylcholine esterase stops neuromuscular
transmission
• The depolarization of skeletal mm membrane by motor nerves is directly produced by change in end plate
potential level to a critical value
• Transmission of impulses from the motor nerve to the muscle cell regularly produces an action potential in
the mm cell
• Ca release form the S.R. into the cytoplasm is the 1st measurable event in the myofiber following generation
of an AP in the sarcolemma
• Ca is stored in the sarcoplasmic reticulum
• Length-tension diagram shows that the maximum active tension of a mm occurs when there is maximum
overlap of crossbridges
• Exitation-contraction coupling is measured by release of Ca from the of calcium from the SR
• Ca binds to troponin C on the thin filaments. (C for Calcium)
• Ca ions trigger contraction of mm when they bind to troponin
• The stimulation of mm contraction is mediated via troponin C (Not mediated by actin, tropomysin, troponin 1 or SR) (in
other words Ca acts on mm via troponin C , not the other things
• ATP is attached to which component of skeletal muscle? Myosin head
Review
• What ion will start contraction of cardiac muscle?
• Muscle contraction causes shortening of which bands?
• T or F, smooth muscle has tropomysin.
• Myosin has ATPase action that turns ATP à ADP + phosphate, release of which ADP
or Phosphate causes the powrestroke?
• Inorganic pohosphate is released from ADP in myosin when?
• The ends of sarcomeres are anchored by which line?
• What needs to happen to allow myosin to be “re-cocked”?
• ATPase Myosin requires which element to complete the power stroke?
• Which comes first powerstroke or cross-bridging?
• Which protein in the sliding filament model is an ATPase?
• What is the main calcium binding protein in Skeletal mm?
• What is the main calcium binding protein in smooth mm?
• What ion will start contraction of cardiac muscle?
• Muscle contraction causes shortening of which bands? H, I, Z Band A is the Abstinant band… it Abstains
from contracting.
• T or F, smooth muscle has tropomysin. T (No troponin, yes calmodulin)
• Myosin has ATPase action that turns ATP à ADP + phosphate, release of which ADP or Phosphate causes the
powrestroke? the phosphate group causes the myosin powerstroke
• Inorganic pohosphate is released from ADP in myosin when? For a powerstroke
• The ends of sarcomeres are anchored by which line? Z lines
• What needs to happen to allow myosin to be “re-cocked”? ATP
• ATPase Myosin requires which element to complete the power stroke? Mg2+
• Which comes first powerstroke or cross-bridging? Cross Bridging
• Which protein in the sliding filament model is an ATPase? Myosin has ATPase action that turns
ATPàADP +phosphate, release of the phosphate group causes myosin powerstroke
• What is the main calcium binding protein in Skeletal mm? troponin-tropomysin
complex
• What is the main calcium binding protein in smooth mm? calmodulin-tropomysin
complex
• In relaxed muscle, the myosin-binding site on actin is blocked by
________. Muscle relaxation occurs when ________.
• titin
• troponin calcium ions are actively transported out of the sarcoplasmic reticulum
• myoglobin calcium ions diffuse out of the sarcoplasmic reticulum
tropomyosin
•
calcium ions are actively transported into the sarcoplasmic reticulum
• According to the sliding filament model, binding sites on actin open
calcium ions diffuse into the sarcoplasmic reticulum
when ________.
• creatine phosphate levels rise During muscle contraction, the cross-bridge detaches when ________.
• ATP levels rise
• acetylcholine levels rise the myosin head binds to an ADP molecule
• calcium ion levels rise
the myosin head binds to an ATP molecule
• The cell membrane of a muscle fiber is called ________. calcium ions bind to troponin
calcium ions bind to actin
• myofibril
• sarcolemma
• sarcoplasm
Thin and thick filaments are organized into functional units called _______
• myofilament
myofibrils
myofilaments
T-tubules
sarcomeres
• In relaxed muscle, the myosin-binding site on actin is blocked by
________. Muscle relaxation occurs when ________.
• titin
• troponin calcium ions are actively transported out of the sarcoplasmic reticulum
• myoglobin calcium ions diffuse out of the sarcoplasmic reticulum
tropomyosin
•
calcium ions are actively transported into the sarcoplasmic reticulum
• According to the sliding filament model, binding sites on actin open
calcium ions diffuse into the sarcoplasmic reticulum
when ________.
• creatine phosphate levels rise During muscle contraction, the cross-bridge detaches when ________.
• ATP levels rise
• acetylcholine levels rise the myosin head binds to an ADP molecule
• calcium ion levels rise
the myosin head binds to an ATP molecule
• The cell membrane of a muscle fiber is called ________. calcium ions bind to troponin
calcium ions bind to actin
• myofibril
• sarcolemma
• sarcoplasm
Thin and thick filaments are organized into functional units called _______
• myofilament
myofibrils
myofilaments
T-tubules
sarcomeres
