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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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Myofibrils are composed of thin and thick filaments
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 Single muscle fiber (cell) Nuclei 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) Z line H zone Z line Sarcomere
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Skeletal muscles are striated – arrangement of filaments create dark and light bands
Z H A I 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) Z line H zone Z line Sarcomere
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LE 49-29 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 0.5 µm Z H A I Sarcomere Relaxed muscle fiber Contracting muscle fiber Fully contracted muscle fiber
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Thin filaments are actin molecules along with regulatory proteins
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 (low- energy configuration) Myosin head (high- 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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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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Motor unit: A single neuron and all the muscle fibers it controls
Spinal cord 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. Motor unit 1 Motor unit 2 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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Action potential triggers Ca2+ release from sarcoplasmic reticulum
Synaptic terminal of motor neuron PLASMA MEMBRANE Synaptic cleft T TUBULE ACh SR Action potential triggers Ca2+ release from sarcoplasmic reticulum Ca2+ CYTOSOL Ca2+
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Muscle contraction ends, muscle fibers relax
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 ACh SR Ca2+ CYTOSOL Ca2+
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Motor neuron axon Mitochondrion Synaptic terminal T tubule Sarcoplasmic reticulum Ca2+ released from 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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This changes the alignment of the troponin
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.
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