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Questions and Answers
Which muscles are affected first in exacerbation and remission?
Which muscles are affected first in exacerbation and remission?
What happens to deep tendon reflexes in exacerbation and remission?
What happens to deep tendon reflexes in exacerbation and remission?
What is unique about neuromuscular junctions in smooth and cardiac muscle?
What is unique about neuromuscular junctions in smooth and cardiac muscle?
What is released from the varicosities in smooth and cardiac muscle?
What is released from the varicosities in smooth and cardiac muscle?
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How many varicosities can a neuron have in smooth and cardiac muscle?
How many varicosities can a neuron have in smooth and cardiac muscle?
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Where are adrenergic receptors located?
Where are adrenergic receptors located?
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What do adrenergic receptors bind to?
What do adrenergic receptors bind to?
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What type of receptors are adrenergic receptors?
What type of receptors are adrenergic receptors?
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What is the primary function of the sarcoplasmic reticulum in a muscle cell?
What is the primary function of the sarcoplasmic reticulum in a muscle cell?
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How does the action potential affect Na+ and K+ channels in a muscle cell?
How does the action potential affect Na+ and K+ channels in a muscle cell?
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What role does the T-tubule play in muscle cell physiology?
What role does the T-tubule play in muscle cell physiology?
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What is the resting membrane potential of a typical muscle cell?
What is the resting membrane potential of a typical muscle cell?
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What initiates the release of neurotransmitters at a chemical synapse?
What initiates the release of neurotransmitters at a chemical synapse?
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What type of synapse allows for direct electrical communication between neurons?
What type of synapse allows for direct electrical communication between neurons?
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What triggers the influx of calcium ions at the neuromuscular junction?
What triggers the influx of calcium ions at the neuromuscular junction?
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Which mechanism allows for the recycling of the membrane at the synapse?
Which mechanism allows for the recycling of the membrane at the synapse?
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What is a key feature of the motor endplate structure?
What is a key feature of the motor endplate structure?
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Which statement best describes myasthenia gravis?
Which statement best describes myasthenia gravis?
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What causes the release of calcium from the sarcoplasmic reticulum after the muscle action potential?
What causes the release of calcium from the sarcoplasmic reticulum after the muscle action potential?
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How is acetylcholine cleared from the synaptic cleft?
How is acetylcholine cleared from the synaptic cleft?
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Study Notes
Anatomy of a Muscle Cell
- A muscle cell is also known as a myocyte.
- The membrane of a muscle cell is called the sarcolemma.
- The cytoplasm of a muscle cell is called the sarcoplasm.
- The sarcoplasmic reticulum is a specific form of endoplasmic reticulum that contains Ca+ ion pumps and has no ribosomes.
- T-tubules are connected to the sarcoplasmic reticulum by protein complexes, which trigger the release of Ca+ from the sarcoplasmic reticulum when an action potential reaches down the T-tubule.
Resting Membrane Potential
- Every cell has a resting potential of around -70mV.
- The Na+ and K+ gradients across the membrane are maintained by Na/K-ATPase.
Ion Channels
- Ion channels are very fast and cannot be coupled to an energy source, making them passive transport only.
- Ion channels are ion selective and fluctuate between open and closed states.
- Ion channels are differently gated.
Action Potential
- The all-or-none principle applies to action potentials, which are always around 100mV and last for a few milliseconds.
- During an action potential, Na+ ions flow into the cell and K+ ions flow out of the cell through specific channels.
Synapses
- Synapses can be either electrical or chemical.
- Electrical synapses are formed by gap junctions between two neurons, allowing electrical current to flow directly.
- Chemical synapses transmit signals via neurotransmitters.
Chemical Synapse - Function
- Neurotransmitters are released from vesicles that are pre-docked to the membrane.
- When an action potential arrives at the synapse, voltage-gated Ca+ channels open, and Ca+ binds to synaptotagmin, initiating fusion with the plasma membrane.
Vesicle-Membrane Fusion
- Vesicle-membrane fusion is facilitated by SNARE proteins, which are anchored membrane proteins and targets of the proteolytic botulinum and tetanus toxin.
Recycling of Membrane at the Synapse
- Membrane fusion occurs through the "kiss-and-run" mechanism, clathrin-mediated endocytosis, and invagination.
- There is a pool of docked vesicles that can be exchanged and are quality controlled.
Synaptic Cleft
- The synaptic cleft is about 20nm wide and is shielded by basal lamina from the extracellular space.
- Neurotransmitters are released into the synaptic cleft, which is a specialized adhesive junction.
Neuromuscular Junctions
- One nerve fiber per endplate innervates the muscle.
- The impulse arriving at the neuromuscular junction triggers Ca+ influx, and neurotransmitter (acetylcholine) binds to nicotinic cholinergic receptors on the motor endplate.
- This generates an endplate potential, leading to muscle action potential and an increase in Ca+ concentration in the muscle.
Acetylcholine Receptor
- Acetylcholine receptors are classified according to their pharmacological profile as nicotinic or muscarinic.
- Nicotinic acetylcholine receptors are transmitter-gated Na+ channels with 5 subunits.
Motor Endplate
- The motor endplate is made up of junctional folds with thickened muscle membrane.
- Receptors are concentrated on the tops of the folds, and acetylcholine is cleaved by acetylcholinesterase into acetate and choline, which is transported back into the presynaptic terminal.
Muscle Action Potential
- Acetylcholine binds to Na+ channels, allowing Na+ influx, and Ca2+ channels on the sarcolemma allow Ca2+ influx.
- Ca2+ channels on the sarcoplasmic reticulum allow Ca2+ influx, and Ca2+ binds to troponin, triggering myosin contraction.
Pathophysiological Conditions
- Autoimmune disorders, such as myasthenia gravis, Lambert-Eaton myasthenic syndrome, and neuromyotonia, are associated with the neuromuscular junction.
- Antibodies against AChR and MuSK in myasthenia gravis, VGCC in Lambert-Eaton myasthenic syndrome, and VGKC in neuromyotonia.
- Myasthenia gravis is characterized by muscle weakness and fatigue, and the absence of deep tendon reflexes.
Smooth and Cardiac Muscle
- Nerve endings in smooth and cardiac muscle do not have recognizable endplates or other postsynaptic specializations.
- Acetylcholine, norepinephrine, or epinephrine-containing vesicles are released from varicosities along the length of the muscle cell.
- One neuron can innervate many effector cells.
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Test your knowledge of the structure and function of muscle cells, including the sarcolemma, sarcoplasm, and sarcoplasmic reticulum.
