Podcast
Questions and Answers
What is the primary function of the axons of motor neurons?
How many neuromuscular junctions (NMJs) does each muscle fiber have?
What structure forms the NMJ with motor neuron terminals?
What is the role of the NMJ in muscle contraction?
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Which of the following best describes a synapse?
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What initiates the muscle contraction process when deciding to perform an action?
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What is the primary neurotransmitter involved in activating muscle cells for contraction?
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What type of ion channels are opened by chemical messengers such as acetylcholine?
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What is the role of voltage-gated ion channels in muscle contraction?
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Which of the following statements is true about somatic motor neurons?
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How do electrical signals spread across muscle cells?
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What characteristic do excitable cells, such as muscle cells, have in response to stimuli?
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What defines the role of the neuromuscular junction (NMJ)?
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What initiates the release of ACh neurotransmitter into the synaptic cleft?
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What is the primary result of ACh binding to its receptors on the sarcolemma?
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Which enzyme is responsible for breaking down ACh in the synaptic cleft?
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What happens to the membrane potential during the end plate potential (EPP)?
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Which ion primarily enters the muscle fiber after ACh binds to its receptors?
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During neuromuscular transmission, which sequence is correct?
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Which of the following describes a key function of acetylcholinesterase?
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What occurs during the repolarization phase of an action potential in a skeletal muscle fiber?
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What occurs after the end plate potential is generated?
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Which statement correctly describes excitation-contraction (E-C) coupling?
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What restores the ionic conditions of the resting state in a skeletal muscle fiber after an action potential?
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What is the significance of the refractory period following an action potential?
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During the propagation of an action potential, what happens after voltage-sensitive proteins are stimulated in T tubules?
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What occurs immediately after an action potential ends in a skeletal muscle fiber?
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What type of ion channel opens during the depolarization phase of an action potential?
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In the context of muscle contraction, what is the primary role of Ca2+ released from the sarcoplasmic reticulum?
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What occurs during the working (power) stroke in cross bridge cycling?
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What triggers the detachment of the cross bridge in the cross bridge cycle?
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During which phase does the myosin head return to a high-energy position?
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What physiological condition does rigor mortis describe?
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What is the primary factor that leads to cross bridge formation after death?
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What is the role of ATP in the muscle contraction cycle?
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Which structure do the thin filaments pull toward during muscle contraction?
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What happens to muscle fibers after ATP is completely depleted postmortem?
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Study Notes
Muscle fiber contraction
- The brain initiates muscle contraction
- Upper motor neurons in the brain transmit signals to the spinal cord
- The spinal cord transmits a signal to motor neurons
- Motor neurons activate skeletal muscle fibers at the neuromuscular junction
Action potentials
- Neurons and muscle cells can change their resting membrane potentials.
- Resting membrane potential changes lead to action potentials, which spread throughout the muscle cells
- Cells transmit electrical signals as chemical signals using neurotransmitters.
- The neurotransmitter acetylcholine signals muscle contraction.
- Acetylcholine opens specific ion channels on muscle cells
Ion Channels
- Ion channels are responsible for changing membrane potentials
- Chemically-gated ion channels open in response to chemical messengers such as neurotransmitters
- Neurotransmitters create local changes in membrane potential (small depolarization)
- Voltage-gated ion channels open or close in response to local changes in membrane potential
- Voltage-gated ion channels are responsible for action potentials
Anatomy of motor neurons and the neuromuscular junction (NMJ)
- Skeletal muscles are stimulated by somatic motor neurons
- Somatic motor neurons are part of the peripheral nervous system
- The peripheral nervous system controls conscious activities.
- The axons of motor neurons travel from the central nervous system to skeletal muscle.
- The axon divides into branches as it enters muscle
- Each axon branch ends on muscle fibers, forming the NMJ
- Each muscle fiber has one NMJ with one motor neuron
- An NMJ is a synapse
- Each synapse has a space across which impulses pass.
Overview of skeletal muscle contraction
- The NMJ is the region where motor neurons contact skeletal muscle
- The NMJ contains axon terminals and muscle fiber junctional folds
- Acetylcholine diffuses across the synaptic cleft and binds to sarcolemma receptors.
- Acetylcholine binding opens ion channels.
- The simultaneous movement of sodium into the muscle fiber and potassium out of the muscle fiber creates a local change in membrane potential (end plate potential)
- More sodium enters than potassium exits
- Acetylcholinesterase degrades acetylcholine, terminating the action potential.
Summary of events at the neuromuscular junction
- An action potential arrives at the axon terminal
- Voltage-gated calcium channels open, allowing calcium to enter the motor neuron.
- Calcium entry causes the release of acetylcholine into the synaptic cleft.
- Acetylcholine diffuses across the synaptic cleft and binds to acetylcholine receptors.
- Binding of acetylcholine opens chemically gated sodium channels on the sarcolemma
- Sodium entry causes local depolarization (end plate potential)
- Acetylcholinesterase degrades acetylcholine
Generating and propagating an action potential in a skeletal muscle fiber
- The end plate potential spreads to the neighboring membrane
- The spread of the end plate potential causes voltage-gated sodium channels to open
- Sodium flows down its concentration gradient, causing depolarization
- The action potential propagates in both directions along the sarcolemma
- Voltage-gated sodium channels close
- Voltage-gated potassium channels open
- Potassium flows down its concentration gradient, causing repolarization
Repolarization
- The resting membrane potential is restored
- Sodium channels close and potassium channels open, restoring the resting membrane potential
- During the refractory period, the muscle fiber can't be stimulated until repolarization is complete.
- The sodium-potassium pump restores the resting ionic conditions
Excitation-contraction (E-C) coupling
- E-C coupling involves transmitting an action potential along the sarcolemma (excitation) that leads to sliding myofilaments (contraction)
- The action potential travels along the sarcolemma and down the T tubules.
- Voltage-sensitive T tubule proteins stimulate calcium release from the sarcoplasmic reticulum
- Calcium release leads to contraction
- The action potential is brief and ends before contraction is seen
Transmission of the action potential along the T tubules of triads
- T tubule proteins change shape
- Shape changes cause calcium release channels in the terminal cisterns to release calcium into the cytosol
Cross bridge formation
- High-energy myosin heads bind to actin thin filament active sites
Muscle fiber contraction: cross bridge cycling
- Myosin head pivots and pulls the thin filament towards the M line
- ATP attaches to the myosin head, causing cross bridge to detach
- Hydrolysis of ATP "cocks" the myosin head into a high-energy position, ready for the next power stroke
Clinical - Homeostatic Imbalance: rigor mortis
- Rigor mortis begins 3-4 hours after death
- Peak rigidity occurs 12 hours postmortem
- Intracellular calcium levels increase because extracellular calcium enters the cell
- Increased calcium causes continued cross-bridge formation
- ATP continues being consumed, but is not synthesized
- Cross-bridge detachment is impossible, resulting in a state of sustained contraction
- Muscles remain contracted until muscle proteins break down and myosin heads detach
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Description
This quiz explores the mechanisms behind muscle fiber contraction, focusing on the roles of the brain, motor neurons, and neurotransmitters. Additionally, it delves into action potentials and the function of ion channels in muscle cells. Test your knowledge on these essential physiological processes!
