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Questions and Answers
Which of these statements is true regarding the composition of myofibrils?
What is the primary role of actin filaments during muscle contraction?
Which statement correctly describes the changes in muscle bands during contraction?
Which characteristic of cardiac muscle cells contributes to their unique function?
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Which statement about the role of calcium in muscle contraction is true?
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What distinguishes smooth muscle from skeletal muscle in terms of structure?
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How do gap junctions function in smooth muscle contraction?
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What happens to the I band during muscle contraction?
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What primarily distinguishes unitary smooth muscle from multi-unit smooth muscle?
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Which characteristic is true for skeletal muscle but not for smooth muscle?
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What is the main function of calmodulin in smooth muscle contraction?
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In Myasthenia Gravis, what is primarily attacked by antibodies causing muscle weakness?
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What causes the reduced muscle contraction seen in Lambert-Eaton Myasthenic Syndrome?
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Which of the following statements is incorrect regarding smooth muscle?
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Which process describes the formation of cross-bridges during muscle contraction?
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What phenomenon occurs when smooth muscle does not exhibit fatigue like skeletal muscle?
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What encapsulates a single muscle fiber?
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Which structure is responsible for transmitting action potentials into the muscle fiber?
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During skeletal muscle contraction, the activation of which component leads to calcium release from the terminal cisternae?
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What is the primary role of calcium in muscle contraction?
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What is the consequence of the removal of sarcoplasmic calcium in muscle fibers?
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What best describes the sliding filament theory?
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What is the main neurotransmitter that is released at the neuromuscular junction, initiating muscle contraction?
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Which connective tissue covers the entire skeletal muscle?
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Study Notes
Muscle Contraction
- The region between two Z lines is called a sarcomere.
- The physical lengths of the actin and myosin filaments do not change during contraction.
- The A band, which is composed of myosin filaments, does not change either.
- The distance between Z disks decreases, but the Z disks themselves do not change.
- The I band decreases in length as the muscle contracts.
- Only the I band and the H zone decreases in length as the muscle contracts.
Sliding Filament Theory
- Myosin filaments use ATP to walk along the actin filaments using cross bridges.
Types of Muscle
Skeletal muscle
- Voluntary movement, striated appearance.
- Multiple nuclei.
- Can become fatigued.
Cardiac muscle
- Involuntary movement, striated appearance.
- Single nuclei.
- Does not fatigue.
Smooth muscle
- Involuntary movement, lacks striations.
- Single nuclei.
- Does not fatigue.
Skeletal Muscle Structure
- Endomysium around single muscle fibre.
- Perimysium around a fascicle (bundle of fibres).
- Epimysium covers the entire skeletal muscle.
- Epimysium blends into a connective tissue attachment.
Neurological Impulse in Skeletal Muscle
- Somatic motor nerves branch from various levels of the spinal cord and control the contraction of skeletal muscle.
- The major communication site between the nerve and the muscle is the neuromuscular junction.
Neuromuscular Junction
- Action potential travels down motor neuron.
- Causes release of neurotransmitter acetylcholine.
- Acetylcholine binds to receptors on muscle.
- Depolarisation spreads across the muscle cell.
- Depolarisation triggers release of internal calcium stores.
- If sufficient ATP- and Ca2+-is present, muscle contraction.
Muscle Fibres
- The entire muscle fibre contracts simultaneously.
- Signal to contract is distributed evenly by T tubules.
T Tubules
- Tubes that extend from the sarcolemma into the sarcoplasm of the muscle fibre, and then around myofibrils.
Sarcoplasmic Reticulum
- The sarcoplasmic reticulum (SR) forms a tubular network around each myofibril, either side of each T-tubule.
- The SR enlarges and fuse forming large chambers, called the terminal cisternae.
- The T-tubule and a pair of terminal cisternae is called a triad.
Muscle Contraction
- Contraction of a skeletal muscle fibre begins with depolarisation of the muscle fibre membrane.
- This activates dihydropyridine (DHP) voltage sensors of the transverse tubules.
- Activation of the DHP voltage sensor leads to activation of the ryanodine receptor.
- This results in the release of calcium from the terminal cisterna.
- This release of calcium increases the calcium concentration in the sarcoplasm.
- Contraction of the muscle fiber follows.
- Removal of sarcoplasmic calcium terminates skeletal muscle contraction.
Sarcoplasmic Calcium
- Removal of sarcoplasmic calcium results in termination of muscle contraction.
Smooth Muscle
- Classified as multi-unit or unitary depending on whether the cells are electrically coupled.
- Unitary smooth muscle has gap junctions between cells which allow for the fast spread of electrical activity.
- Multi-unit have little or no coupling between cells.
- A combination of unitary and multi-unit is found in vascular smooth muscle.
Smooth Muscle: Excitation-Contraction
- In smooth muscle there is no troponin.
- The interaction of actin and myosin is controlled by the binding of calcium to another protein, calmodulin.
- Calmodulin regulates cross-bridge cycling.
Clinical Cases
- Lambert-Eaton Myasthenic Syndrome (LEMS)
- Myasthenia Gravis (MG)
Lambert-Eaton Myasthenic Syndrome (LEMS)
- LEMS is a rare condition that affects the signals sent from the nerves to the muscles.
- The muscles are unable to contract properly, resulting in muscle weakness.
- Antibodies attack the voltage-gated calcium channels on the presynaptic membrane.
- Without proper function of these channels, insufficient amounts of acetylcholine are released into the neuromuscular junction.
Myasthenia Gravis (MG)
- Myasthenia gravis (MG) is an autoimmune disease that occurs in 1 in 10,000 people.
- It occurs primarily in women between 20 and 40 years of age.
- Eye and facial muscles are often attacked first, producing double vision and drooping eyelid.
- These clinical features are often followed by dysphagia, limb weakness and decreased stamina.
- Can result in fatalities from paralysis of the respiratory muscles.
- Antibodies attack the acetylcholine receptors on the postsynaptic muscle fiber membrane.
- The abnormally clustered Ach receptors are removed from the muscle fibre sarcolemma by endocytosis, reducing the number of receptors in the sarcolemma
- Damage to the acetylcholine channels results in small endplate potentials that do not reach a threshold value required for generation of an action potential in the muscle fiber.
- This causes decreased muscle stimulation.
- This decreased stimulation results in fatigue and weakness.
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Description
Test your knowledge on muscle contraction, the sliding filament theory, and the different types of muscle tissue. This quiz covers key concepts such as sarcomeres, the A band, and the characteristics of skeletal, cardiac, and smooth muscles. Perfect for students in anatomy or biology courses.
