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Questions and Answers
What is the role of acetylcholinesterase in the neuromuscular junction?
Which structures are involved in the conduction of action potentials from the soma of peripheral somatic motor neurons to the neuromuscular junction?
In skeletal muscle contraction, which component primarily interacts with calcium ions?
Which area is NOT a major component of the neuromuscular junction?
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What role do myosin heads play in muscle contraction?
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What is the primary function of the T-tubules in muscle fibers?
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Which component of the sarcomere primarily determines the muscle's ability to contract?
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Which element is essential for the contraction of skeletal muscle and must be released into the sarcoplasm?
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What is the correct sequence of events during the action potential phase of a neuron?
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What effect does the neurotransmitter binding to its receptors on the postsynaptic neuron typically have?
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Which component is crucial for the release of neurotransmitters at the axon terminal?
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Which part of a neuron is primarily responsible for receiving incoming signals?
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What is the primary function of the Na+/K+ pump during repolarization?
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What is indicated by the decussation of pyramids in the nervous system?
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What best describes the role of synaptic cleft in neuron communication?
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Which part of the central nervous system is primarily responsible for voluntary movement initiation?
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What is the role of calcium ions (Ca2+) during muscle contraction?
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Which statement describes a key process in muscle relaxation?
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What happens to the I-band during muscle contraction according to the sliding filament model?
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Which type of muscle fiber is characterized by slower contraction speed and higher endurance?
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How do sympathetic postganglionic neurons primarily affect the myocardium?
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What is a defining characteristic of type II muscle fibers compared to type I fibers?
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What neurotransmitter is primarily secreted by preganglionic parasympathetic neurons?
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Which of the following accurately describes the role of acetylcholine (Ach) in the autonomic nervous system?
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Study Notes
Central Nervous System Components
- The brain, brainstem (midbrain, pons, medulla oblongata), and spinal cord are the main components of the central nervous system.
- The motor cortex in the brain is crucial for voluntary movement initiation by sending signals to the body.
- Central motor neurons within the brain communicate with peripheral somatic motor neurons in the spinal cord through synapses.
Neurons (Nerve Cells)
- Structure: Neurons have a cell body (soma), dendrites that receive signals, an axon that conducts action potentials, and axon terminals that release neurotransmitters.
- Function: Dendrites have receptors for neurotransmitters, the axon conducts the action potential, and axon terminals release neurotransmitters.
Neurotransmitters
- Location, Release, and Effects: Neurotransmitters are chemical messengers released from axon terminals to transmit signals across synapses.
- Stimulatory or Inhibitory: Depending on the receptor they bind to, neurotransmitters can either excite (stimulatory) or inhibit (inhibitory) their target cells.
The Action Potential
- Resting Membrane Potential (RMP): The RMP of a neuron is approximately -70mV, influenced by the distribution of sodium (Na+) and potassium (K+) ions across the cell membrane.
- Depolarization: Sodium channels open, allowing Na+ to enter the cell, causing the membrane potential to become less negative and reach a positive value.
- Repolarization: Potassium channels open, allowing K+ to leave the cell, restoring the membrane potential to its resting state. The Na+/K+ pump actively moves sodium out and potassium into the cell to maintain the concentration gradients necessary for RMP.
The Synapse
- Communication: The synapse is the junction between two neurons, where communication occurs via neurotransmitters.
- Presynaptic and Postsynaptic Neurons: The presynaptic neuron releases neurotransmitters, while the postsynaptic neuron receives the signal.
- Role of Ca2+: Calcium ions (Ca2+) in the axon terminal trigger the release of neurotransmitters.
- Synaptic Cleft: The space between the presynaptic and postsynaptic neurons is called the synaptic cleft.
- Neurotransmitter Binding: Neurotransmitters bind to receptors on the postsynaptic neuron's membrane.
- Removal or Inactivation: Enzymes or reuptake mechanisms remove or inactivate neurotransmitters from the synaptic cleft to terminate the signal.
Nervous System: Motor Neurons
- Central and Peripheral Motor Neurons: Central motor neurons originate in the brain and conduct signals to peripheral motor neurons in the spinal cord. Peripheral motor neurons innervate muscles.
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Voluntary Movements:
- The motor cortex initiates action potentials.
- The action potential travels down the axon of the central motor neuron.
- The action potential crosses the synapse between the central and peripheral motor neurons in the spinal cord.
- The action potential travels down the axon of the peripheral motor neuron to the neuromuscular junction.
- Neurotransmitters are released from the axon terminal of the peripheral motor neuron into the synaptic cleft.
- Neurotransmitters bind to receptors on the muscle fiber, initiating muscle contraction.
- Decussation of Pyramids: The point where fibers of the motor cortex cross over to the opposite side of the body, ensuring that the left hemisphere controls the right side and vice versa.
Neuromuscular Junction
- Structure: Consists of the axon terminal (containing acetylcholine), synaptic cleft, and sarcolemma (muscle cell membrane) with acetylcholine receptors.
- Function: Acetylcholine (Ach) released from the axon terminal binds to receptors on the sarcolemma, triggering a muscle action potential. Acetylcholinesterase removes Ach from the synaptic cleft, ending the signal and allowing muscle relaxation.
Skeletal Muscle
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Structure:
- Sarcolemma: The muscle cell membrane.
- Sarcoplasm: The cytoplasm of a muscle cell.
- Myofibrils: Contractile units within muscle fibers.
- T-Tubules: Invaginations of the sarcolemma that carry action potentials deep into the muscle fiber.
- Sarcoplasmic Reticulum: A network of internal membranes that stores calcium.
- Actin: A thin filament consisting of globular actin (G-actin), tropomyosin, and troponin.
- Myosin: A thick filament with head regions that bind to actin and contain ATPase activity.
Sarcomere
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Structure: The basic functional unit of a myofibril.
- Z-disc: The boundary of a sarcomere, anchoring the thin filaments (actin).
- I-band: The region of thin filaments only.
- A-band: The region where both thin and thick filaments overlap.
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Myofilaments:
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Actin:
- G-actin: Globular protein subunits that form the actin filament.
- Tropomyosin: A protein that wraps around the actin filament and blocks the myosin binding sites when the muscle is relaxed.
- Troponin: A complex of three proteins (TnI, TnC, TnT) that regulates tropomyosin's position.
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Myosin:
- Tails: The elongated portion of the myosin molecule that intertwines with other myosin molecules.
- Heads: The globular portion of the myosin molecule that binds to actin and contains an ATPase enzyme.
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Actin:
Contraction of Skeletal Muscle
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Role of Calcium:
- Action potentials travel down the T-tubules, triggering calcium release from the sarcoplasmic reticulum.
- Calcium binds to troponin C (TnC), causing tropomyosin to move and expose the active sites on actin.
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Excitation-Contraction Coupling:
- Ach Release: Acetylcholine is released from motor neuron axon terminals into the synaptic cleft.
- Sarcolemma: The action potential travels along the sarcolemma and into the T-tubules.
- Calcium Release: Calcium is released from the sarcoplasmic reticulum.
- Calcium Binding: Calcium binds to TnC, causing tropomyosin to move, exposing the actin active sites.
- Myosin Binding: Myosin heads bind to the active sites on actin.
- Power Stroke: Myosin heads pivot, pulling actin filaments towards the center of the sarcomere.
- ATP for Detachment: ATP binds to the myosin head, causing it to detach from actin.
- Muscle Relaxation: Calcium is pumped back into the sarcoplasmic reticulum, tropomyosin blocks the active sites, and the muscle relaxes.
Sliding Filament Model of Contraction
- Sliding Filament Model: Muscle contraction is a result of thin filaments (actin) sliding past thick filaments (myosin).
- Sarcomere Length: During contraction, the length of the I-band and the space between the Z-discs decreases while the length of the A-band remains constant.
- Power Stroke: The power stroke of the myosin head pulls actin filaments towards the center of the sarcomere, leading to muscle shortening.
Motor Units
- Components: A motor unit consists of one motor neuron and all the muscle fibers it innervates.
Skeletal Muscle Twitch
- Tension and Time: A muscle twitch is a single, isolated contraction caused by a single action potential.
- Speed of Contraction: Muscle fibers can be classified as Type I (slow-twitch) or Type II (fast-twitch), based on their speed of contraction, force production, and resistance to fatigue.
- Force Production: Type II fibers generate more force and contract more quickly than Type I fibers.
- Fiber Size: Type II fibers are typically larger than Type I fibers.
Skeletal Muscle Fiber Types
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Type I (Slow-twitch):
- Characteristics: Slower speed of contraction, lower force production, high fatigue resistance.
- Function: Adapted for endurance activities.
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Type II (Fast-twitch):
- Characteristics: Faster speed of contraction, higher force production, lower fatigue resistance.
- Function: Adapted for short bursts of high-intensity activities.
The Autonomic Nervous System (ANS)
- Sympathetic and Parasympathetic: The ANS is the part of the nervous system that controls involuntary body functions. It consists of the sympathetic and parasympathetic branches.
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ANS Anatomy:
- Preganglionic Parasympathetic Neurons: Located in the brainstem and sacral spinal cord.
- Postganglionic Parasympathetic Neurons: Located near or in the target organs.
- Preganglionic Sympathetic Neurons: Located in the thoracic and lumbar spinal cord.
- Postganglionic Sympathetic Neurons: Located in ganglia near the spinal cord.
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ANS Physiology:
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Neurotransmitters:
- Acetylcholine (Ach): Released by preganglionic neurons in both the sympathetic and parasympathetic systems.
- Norepinephrine (NE): Released by most postganglionic sympathetic neurons.
- Epinephrine (E): Released by the adrenal medulla.
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Effects (Stimulatory/Inhibitory):
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Ach:
- Heart: Decreases heart rate (inhibitory)
- Smooth Muscle of Digestive System: Increases digestive activity (stimulatory)
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NE and E:
- Heart: Increases heart rate and force of contraction (stimulatory)
- Smooth Muscle of Blood Vessels: Constriction (stimulatory) in most vessels, dilation (inhibitory) in skeletal muscle vessels.
- Adrenal Medulla: Releases NE and E into the bloodstream, causing widespread sympathetic effects.
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Ach:
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Neurotransmitters:
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Description
Test your knowledge on the components of the central nervous system, including the roles of neurons and neurotransmitters. This quiz covers essential information about the motor cortex, neuron structure, and neurotransmitter function. Perfect for students studying biology or neuroscience.
