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Questions and Answers
Which type of muscle has a poorly developed sarcoplasmic reticulum?
Which type of muscle has a poorly developed sarcoplasmic reticulum?
Which muscle type can contract without nerve stimulation?
Which muscle type can contract without nerve stimulation?
What is the primary function of calmodulin in smooth muscle?
What is the primary function of calmodulin in smooth muscle?
What type of muscle contains intercalated discs?
What type of muscle contains intercalated discs?
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Which muscle type is characterized by striations and the presence of troponin?
Which muscle type is characterized by striations and the presence of troponin?
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What happens to skeletal muscle if denervation occurs?
What happens to skeletal muscle if denervation occurs?
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In smooth muscle, where can calcium ions enter the cytoplasm from?
In smooth muscle, where can calcium ions enter the cytoplasm from?
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Which statement about muscle fibers in skeletal muscle is true?
Which statement about muscle fibers in skeletal muscle is true?
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What shape are smooth muscle cells?
What shape are smooth muscle cells?
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Which of the following is true regarding the arrangement of myofilaments in smooth muscle?
Which of the following is true regarding the arrangement of myofilaments in smooth muscle?
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What is the primary factor that allows smooth muscle to contract when stretched?
What is the primary factor that allows smooth muscle to contract when stretched?
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How does smooth muscle contraction differ from skeletal muscle contraction?
How does smooth muscle contraction differ from skeletal muscle contraction?
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Which structural component is absent in smooth muscle but present in skeletal muscle?
Which structural component is absent in smooth muscle but present in skeletal muscle?
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What initiates or modulates smooth muscle contraction?
What initiates or modulates smooth muscle contraction?
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What type of ion channels are essential for smooth muscle contraction?
What type of ion channels are essential for smooth muscle contraction?
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What is the role of calmodulin in smooth muscle cells?
What is the role of calmodulin in smooth muscle cells?
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What is the main function of gap junctions in smooth muscle?
What is the main function of gap junctions in smooth muscle?
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Which type of smooth muscle exhibits pacemaker activity?
Which type of smooth muscle exhibits pacemaker activity?
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What initiates the contraction of smooth muscle cells?
What initiates the contraction of smooth muscle cells?
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How does the regulation of multiunit smooth muscle differ from single-unit smooth muscle?
How does the regulation of multiunit smooth muscle differ from single-unit smooth muscle?
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Which protein complex is formed when calcium combines with calmodulin in smooth muscle?
Which protein complex is formed when calcium combines with calmodulin in smooth muscle?
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What is primarily responsible for cross-bridging during smooth muscle contraction?
What is primarily responsible for cross-bridging during smooth muscle contraction?
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What happens during the relaxation phase of smooth muscle contraction?
What happens during the relaxation phase of smooth muscle contraction?
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Which of the following statements about the autonomic nervous system's role in smooth muscle is true?
Which of the following statements about the autonomic nervous system's role in smooth muscle is true?
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What is the approximate ratio of thin to thick filaments in smooth muscle cells?
What is the approximate ratio of thin to thick filaments in smooth muscle cells?
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How do smooth muscle cells differ in filament arrangement from striated muscle cells?
How do smooth muscle cells differ in filament arrangement from striated muscle cells?
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Which structure in smooth muscle is analogous to the Z discs found in striated muscle?
Which structure in smooth muscle is analogous to the Z discs found in striated muscle?
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What is a primary functional difference between smooth muscle and striated muscle during stretching?
What is a primary functional difference between smooth muscle and striated muscle during stretching?
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In which of the following locations is single-unit smooth muscle predominantly found?
In which of the following locations is single-unit smooth muscle predominantly found?
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What allows smooth muscle cells to function effectively when stretched?
What allows smooth muscle cells to function effectively when stretched?
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What biochemical role does Ca2+ play in smooth muscle contraction?
What biochemical role does Ca2+ play in smooth muscle contraction?
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What percentage of the volume of a smooth muscle cell do myofilaments and dense bodies occupy?
What percentage of the volume of a smooth muscle cell do myofilaments and dense bodies occupy?
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What is the primary role of sympathetic nerves in the fight-or-flight response?
What is the primary role of sympathetic nerves in the fight-or-flight response?
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How does the innervation of smooth muscles differ from that of skeletal muscles?
How does the innervation of smooth muscles differ from that of skeletal muscles?
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What are varicosities in the context of autonomic nerve fibers?
What are varicosities in the context of autonomic nerve fibers?
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In terms of neurotransmission, what does 'synapses en passant' refer to?
In terms of neurotransmission, what does 'synapses en passant' refer to?
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Which of the following statements accurately describes the roles of the sympathetic and parasympathetic nerves?
Which of the following statements accurately describes the roles of the sympathetic and parasympathetic nerves?
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Where are neurotransmitter receptor proteins primarily located in skeletal muscle fibers?
Where are neurotransmitter receptor proteins primarily located in skeletal muscle fibers?
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What physiological effect does sympathetic stimulation have on intestinal motility?
What physiological effect does sympathetic stimulation have on intestinal motility?
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What mechanism do smooth muscle cells use to receive signals from autonomic nerves?
What mechanism do smooth muscle cells use to receive signals from autonomic nerves?
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Study Notes
Smooth Muscle Structure
- Smooth muscle, also known as visceral muscle, is arranged in layers around hollow organs and blood vessels.
- Its contraction reduces the size of these structures.
- Smooth muscle cells are spindle-shaped.
- Actin-myosin myofilaments are not arranged into sarcomeres resulting in a non-striated appearance.
- Smooth muscle can contract even when significantly stretched, unlike skeletal muscle where contraction is limited by length.
- The sarcoplasmic reticulum is loosely arranged within the cells and lacks T tubules.
- Dense bodies, analogous to Z discs in skeletal muscle, are present.
Smooth Muscle Function
- Smooth muscle is involuntary.
- Contraction is slow and sustained due to slow ATPase activity.
- Circular and longitudinal layers produce peristaltic waves in the digestive tract, ureters, etc, to move contents.
- Smooth muscle responds to various stimuli such as hormones, local factors, and mechanical stretch.
- Its contraction is triggered by membrane depolarization that opens Ca2+ channels allowing influx of calcium (Ca2+).
- Ionotropic receptors or membrane oscillators can induce depolarization.
Types of Smooth Muscle
- Single-unit (visceral) smooth muscle: This is the predominant type found in the GI tract, bladder, uterus, and ureters. Its low-resistance channels between cells form a functional syncytium enabling coordinated contractions. Slow wave potentials generate spike potentials and trigger contraction. The activity is adjusted by the autonomic nervous system, and rhythmic fluctuations of membrane potential occur, serving as a pacemaker function.
- Multiunit smooth muscle: This type is located in the iris, ciliary muscle, arrector pili, and vas deferens. Each muscle fiber is innervated and functions independently. Gap junctions are absent, and activity is regulated by the autonomic nervous system.
Mechanism of Smooth Muscle Contraction
- Slow waves can stimulate spike potentials, triggering contraction in single-unit smooth muscle cells.
- Initial phase of contraction from intracellular Ca2+ released from the sarcoplasmic reticulum ( similar to skeletal ).
- Sustained contraction by Ca2+ influx via voltage-gated channels from interstitial fluid.
- Calcium combines with calmodulin to form a complex, activating myosin light-chain kinase (MLCK).
- MLCK phosphorylates myosin cross-bridges, exposing actin binding sites, allowing cross bridge formation and subsequent contraction.
- Relaxation occurs when calcium is pumped back to its storage sites, the complex dissolves, and the muscle relaxes.
Regulation of Contraction
- Smooth muscle has inherent pacemaker activity
- Sympathetic and parasympathetic autonomic nerves regulate smooth muscle activity in various organs simultaneously. Examples of actions include pupil dilation, coronary artery dilation, and reduced intestinal motility resulting from sympathetic stimulation.
- Parasympathetic effects are often the opposite of sympathetic for example increased intestinal motility.
Autonomic Innervation
- Smooth muscle cells have neurotransmitter receptors across their entire surface.
- Neurotransmitters are released along the length of autonomic nerve fibers.
- These fibers branch into varicosities (bulges) releasing neurotransmitters, each stimulating multiple smooth muscle cells in a localized way.
Smooth Muscle vs. Skeletal/Cardiac Muscle (Comparison)
- Key differences in contraction mechanisms, innervation, structure, and responsiveness to stimuli are noted in a comparison table (note: a table isn't requested but you should be able to piece one together).
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Description
This quiz explores the structure and function of smooth muscle, also known as visceral muscle. It discusses the unique characteristics of smooth muscle cells, their contraction mechanisms, and their role in various bodily functions. Test your knowledge on how smooth muscle differs from other muscle types and its importance in organ function.
