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Questions and Answers
What is the primary role of skeletal muscles during body movement?
What is the primary role of skeletal muscles during body movement?
- They connect bones to each other.
- They contract and relax to mechanically move the body. (correct)
- They create a rigid framework for the body.
- They store energy for immediate use.
What initiates the contraction of skeletal muscle fibers?
What initiates the contraction of skeletal muscle fibers?
- Direct contact from other muscle fibers.
- Stimulation from surrounding connective tissue.
- Excitation signals from motor neurons. (correct)
- Signals from the endocrine system.
What is the function of the neuromuscular junction (NMJ)?
What is the function of the neuromuscular junction (NMJ)?
- It connects two muscle fibers.
- It increases the voltage of electrical impulses.
- It stores calcium ions for muscle contraction.
- It allows for synaptic contact between a motor neuron and a muscle fiber. (correct)
What substance is broken down by acetylcholinesterase at the synaptic cleft?
What substance is broken down by acetylcholinesterase at the synaptic cleft?
What is contained in the junctional folds of the motor end plate?
What is contained in the junctional folds of the motor end plate?
How do voltage-gated sodium channels contribute to neuromuscular transmission?
How do voltage-gated sodium channels contribute to neuromuscular transmission?
What structural adaptation of the motor end plate increases its surface area?
What structural adaptation of the motor end plate increases its surface area?
What gap exists between the motor neuron and muscle fiber in the neuromuscular junction?
What gap exists between the motor neuron and muscle fiber in the neuromuscular junction?
What is the primary role of titin in muscle contraction?
What is the primary role of titin in muscle contraction?
How many actin filaments can one myosin filament interact with?
How many actin filaments can one myosin filament interact with?
Which band in the sarcomere does not shorten during muscle contraction?
Which band in the sarcomere does not shorten during muscle contraction?
What defines the region of a sarcomere?
What defines the region of a sarcomere?
What happens to the I band during contraction?
What happens to the I band during contraction?
Which of the following statements about sarcomeres is correct?
Which of the following statements about sarcomeres is correct?
What is the central portion of the A band that does not contain actin filaments called?
What is the central portion of the A band that does not contain actin filaments called?
How many myosin filaments can one actin filament interact with?
How many myosin filaments can one actin filament interact with?
What is the primary cause of Eaton-Lambert syndrome?
What is the primary cause of Eaton-Lambert syndrome?
What type of bacteria is responsible for botulism?
What type of bacteria is responsible for botulism?
Which receptors are responsible for sensing action potentials in muscle fibers?
Which receptors are responsible for sensing action potentials in muscle fibers?
What is the role of the sarcoplasmic reticulum in skeletal muscle?
What is the role of the sarcoplasmic reticulum in skeletal muscle?
Which neuromuscular disorder is characterized by muscle fatigue and weakness that fluctuates with activity?
Which neuromuscular disorder is characterized by muscle fatigue and weakness that fluctuates with activity?
What structural feature gives skeletal muscle its striated appearance?
What structural feature gives skeletal muscle its striated appearance?
What is the role of calcium ions (Ca$^{2+}$) during excitation-contraction coupling?
What is the role of calcium ions (Ca$^{2+}$) during excitation-contraction coupling?
How does the sarcoplasmic reticulum release calcium ions in response to an action potential?
How does the sarcoplasmic reticulum release calcium ions in response to an action potential?
What initiates the contraction of a skeletal muscle fiber?
What initiates the contraction of a skeletal muscle fiber?
What is the primary function of calcium ions (Ca+2) during muscle contraction?
What is the primary function of calcium ions (Ca+2) during muscle contraction?
Which of the following components does not play a role in the contraction cycle of skeletal muscle?
Which of the following components does not play a role in the contraction cycle of skeletal muscle?
During muscle relaxation, what is the primary action taken regarding calcium ions?
During muscle relaxation, what is the primary action taken regarding calcium ions?
What is the role of ATP in muscle contraction?
What is the role of ATP in muscle contraction?
What primarily happens during the contraction of muscle fibers at the sarcomere level?
What primarily happens during the contraction of muscle fibers at the sarcomere level?
Which statement correctly describes the M-line in the A-band of a sarcomere?
Which statement correctly describes the M-line in the A-band of a sarcomere?
How does the sarcoplasmic reticulum contribute to muscle contraction?
How does the sarcoplasmic reticulum contribute to muscle contraction?
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Study Notes
Skeletal Muscle Contraction
- Skeletal muscles contract and relax to move the body: walking, running, and manipulating objects.
- Muscle fibers contract and generate tension, transmitted through tendons to bones.
- This allows muscles to move bones, enabling a variety of movements.
Neuromuscular Control
- Excitation signals from motor neurons in the spinal cord or brainstem are the only way to activate skeletal muscle fibers to contract.
- The neuromuscular junction (NMJ) is the contact point between a motor neuron and the muscle fiber it controls.
- The NMJ contains a synaptic cleft, a 20-30 nm gap containing acetylcholinesterase (AChE).
- The motor end plate forms the postsynaptic part of the NMJ, a thickened portion of the sarcolemma with folded depressions called junctional folds.
- These invaginations increase the surface area of the postsynaptic membrane, containing ACh receptors and voltage-gated Na+ channels.
Acetylcholine Release
- Acetylcholine (ACh) is released from the motor neuron's axon terminal.
- Voltage-gated sodium channels (VGSCs) facilitate action potential generation during neuromuscular transmission.
Disorders of the Neuromuscular Junction
- Presynaptic membrane:
- Eaton-Lambert syndrome: Autoimmune disorder caused by antibodies to calcium channels in the presynaptic axon terminal.
- Botulism: Toxin produced by Clostridium botulinum bacteria inhibits ACh release from the presynaptic membrane.
- Postsynaptic membrane:
- Myasthenia Gravis: Autoimmune disorder caused by antibodies to cholinergic receptors in the postsynaptic membrane.
Excitation-Contraction Coupling
- Ca2+ triggers the contraction of skeletal muscle fibers.
- Ca2+ is released from the sarcoplasmic reticulum (SR).
Release of Ca2+ from Sarcoplasmic Reticulum
- Dihydropyridine (DHP) receptors in the muscle fiber membrane sense the action potential.
- DHP receptors are linked to Ca2+ release channels (ryanodine receptor channels; RyR) of the SR.
- Activation of DHP receptors triggers the opening of Ca2+ channels, releasing Ca2+ into the sarcoplasm.
- After repolarization, Ca2+ release channels close, and Ca2+ is pumped back into the SR.
Organization of Skeletal Muscle
- Skeletal muscle is organized from gross to cellular level:
- Whole muscle
- Muscle fascicles
- Muscle fibers
- Myofibrils
- Myofilaments (actin and myosin)
The Myofilaments
- Thin filaments are composed of actin.
- Thick filaments are composed of myosin.
- Titin fibers anchor thick myosin filaments in place.
- Z-lines anchor thin actin filaments.
- Each myosin interacts with 6 actin filaments.
- Each actin interacts with 3 myosin filaments.
The Sarcomere
- The sarcomere is the functional unit of contraction, extending from one Z-line to the next.
- Each sarcomere contains A and I bands.
- The I band meets at the Z-line.
- The central portion of the A band is the M-line, which doesn't contain actin.
- A band = Thick filaments
- I band = Thin filaments
The Sliding Filament Theory of Contraction
- Muscle fibers contract as myosin filaments pull actin filaments closer, shortening sarcomeres within a fiber.
- When all sarcomeres shorten, the fiber contracts.
- The sliding of actin past myosin generates muscle tension (force).
Ultrastructure of Skeletal Muscle
- This section details the molecular components involved in muscle contraction (not provided in the text)
Contractile Proteins of Skeletal Muscle
- Myosin is a thick filament, each myosin molecule has a flexible cross-bridge that binds ATP and actin.
The Molecular Basis of Skeletal Muscle Contraction
- This section details the molecular mechanism of contraction (not provided in the text)
The Contraction Cycle - Role of Ca2+ and ATP
- ATP hydrolysis by myosin energizes the cross-bridges, providing energy for force generation.
- ATP binding to myosin separates cross-bridges from actin, allowing the cycle to repeat.
- The cycle continues if ATP and Ca2+ are present.
Relaxation
- Relaxation occurs when nerve stimulation stops.
- The muscle fiber repolarizes, closing Ca2+ release channels in the SR.
- Ca2+ is pumped back into the SR by Ca2+ ATPase.
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