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Questions and Answers
What is the primary distinction of single-unit smooth muscle compared to multiunit smooth muscle?
What is the primary distinction of single-unit smooth muscle compared to multiunit smooth muscle?
Which statement correctly describes the role of gap junctions in single-unit smooth muscle?
Which statement correctly describes the role of gap junctions in single-unit smooth muscle?
What mechanistic advantage do varicosities provide in the autonomic nervous system?
What mechanistic advantage do varicosities provide in the autonomic nervous system?
Which of the following is NOT a characteristic of visceral smooth muscle?
Which of the following is NOT a characteristic of visceral smooth muscle?
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In what way do multiunit smooth muscles primarily differ in structure from single-unit smooth muscles?
In what way do multiunit smooth muscles primarily differ in structure from single-unit smooth muscles?
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What characterizes unitary smooth muscle?
What characterizes unitary smooth muscle?
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What is the typical duration of spike action potentials in unitary smooth muscle?
What is the typical duration of spike action potentials in unitary smooth muscle?
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How do slow wave potentials affect action potentials in smooth muscle?
How do slow wave potentials affect action potentials in smooth muscle?
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Which of the following is NOT a form of action potential observed in visceral smooth muscle?
Which of the following is NOT a form of action potential observed in visceral smooth muscle?
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What primarily causes the action potential in smooth muscle cells?
What primarily causes the action potential in smooth muscle cells?
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What role do gap junctions play in smooth muscle contraction?
What role do gap junctions play in smooth muscle contraction?
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Which protein is critical in regulating the interaction between actin and myosin in smooth muscle?
Which protein is critical in regulating the interaction between actin and myosin in smooth muscle?
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What role does myosin light chain kinase (MLCK) play in smooth muscle contraction?
What role does myosin light chain kinase (MLCK) play in smooth muscle contraction?
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What can trigger spike action potentials in smooth muscle?
What can trigger spike action potentials in smooth muscle?
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Which statement best explains why bowel sounds are generally irregular?
Which statement best explains why bowel sounds are generally irregular?
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How does smooth muscle contraction begin at the cellular level?
How does smooth muscle contraction begin at the cellular level?
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What must happen for smooth muscle relaxation to occur?
What must happen for smooth muscle relaxation to occur?
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Why are action potentials generally not observed in multi-unit smooth muscle?
Why are action potentials generally not observed in multi-unit smooth muscle?
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What is the effect of myosin phosphatase in smooth muscle relaxation?
What is the effect of myosin phosphatase in smooth muscle relaxation?
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What distinguishes smooth muscle contraction from striated muscle contraction?
What distinguishes smooth muscle contraction from striated muscle contraction?
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Which of the following statements about calcium channels in smooth muscle is true?
Which of the following statements about calcium channels in smooth muscle is true?
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What accounts for the prolonged contraction observed in certain smooth muscles?
What accounts for the prolonged contraction observed in certain smooth muscles?
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How is multiunit smooth muscle primarily characterized?
How is multiunit smooth muscle primarily characterized?
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Which of the following is a feature of graded potentials in multiunit smooth muscle?
Which of the following is a feature of graded potentials in multiunit smooth muscle?
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Where is multiunit smooth muscle NOT typically found?
Where is multiunit smooth muscle NOT typically found?
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Which of the following describes the resting membrane potential (RMP) of multiunit smooth muscle?
Which of the following describes the resting membrane potential (RMP) of multiunit smooth muscle?
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Which type of smooth muscle contraction is primarily controlled through the autonomic nervous system?
Which type of smooth muscle contraction is primarily controlled through the autonomic nervous system?
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Which of the following structures do NOT contain multiunit smooth muscle?
Which of the following structures do NOT contain multiunit smooth muscle?
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What effect does neurotransmitter release have on multiunit smooth muscle?
What effect does neurotransmitter release have on multiunit smooth muscle?
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What primarily causes contraction in smooth muscle cells?
What primarily causes contraction in smooth muscle cells?
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How does the latent period before contraction in smooth muscle compare to that in skeletal muscle?
How does the latent period before contraction in smooth muscle compare to that in skeletal muscle?
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Which characteristic distinguishes the sarcoplasmic reticulum in smooth muscle from that in skeletal muscle?
Which characteristic distinguishes the sarcoplasmic reticulum in smooth muscle from that in skeletal muscle?
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What role do calcium channels play in smooth muscle contraction?
What role do calcium channels play in smooth muscle contraction?
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What occurs immediately after the myosin head detaches from the actin filament in smooth muscle?
What occurs immediately after the myosin head detaches from the actin filament in smooth muscle?
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Study Notes
Excitation-Contraction Coupling in Smooth Muscle
- Smooth muscle mechanism differs from striated muscle.
- Smooth muscle cell membranes have more voltage-gated Ca2+ channels and fewer voltage-gated Na+ channels than skeletal muscle.
- Ca2+ influx is primarily responsible for smooth muscle action potentials.
- Smooth muscle lacks troponin; Ca2+-calmodulin regulates myosin light chain kinase (MLCK) and cross-bridge cycling.
Mechanism of Smooth Muscle Contraction
- Intracellular Ca2+ increases via entry through membrane channels or release from the sarcoplasmic reticulum (SR).
- Ca2+ binds calmodulin (CaM), activating MLCK.
- Active MLCK phosphorylates the myosin light chain.
- Myosin heads bind actin, initiating contraction.
Mechanism of Smooth Muscle Relaxation
- Intracellular Ca2+ decreases below a critical level through pumping out of the cell or into the SR.
- Ca2+ dissociates from CaM.
- Myosin phosphatase dephosphorylates the myosin light chain.
- Myosin detaches from actin, causing relaxation.
Smooth Muscle Innervation
- No specialized neuromuscular junctions exist.
- Autonomic neurons synapse with groups of cells, not individual cells.
- Neurotransmitters release at varicosities along axons, causing neighboring cell groups to contract together.
- Gap junctions allow electrical signal spread between cells.
Types of Smooth Muscle
- Classified based on electrical coupling: multiunit, single-unit (unitary), and combinations of both.
Multiunit Smooth Muscle
- Little to no coupling between cells.
- Each cell acts mostly independently, often innervated by a single nerve ending.
- Covered by a basement membrane-like substance for insulation.
- Fewer gap junctions.
- Found where fine control of contraction is needed (e.g., ciliary muscle, iris muscle).
Single-Unit (Unitary) Smooth Muscle
- Functions as a syncytium; stimulation of one cell spreads to adjacent cells.
- Sparse innervation; fewer neurons innervate groups of connected cells.
- Myogenic; contracts regularly without motor neuron input.
- Present in the gastrointestinal tract, ureter, bladder, and uterus.
- Gap junctions facilitate coordinated contraction.
Single-Unit Smooth Muscle: Slow Wave Potentials
- Exhibit spontaneous pacemaker activity or slow waves (unstable resting membrane potentials).
- Slow wave frequency determines the pattern and frequency of action potentials and contractions.
- Not every slow wave cycle reaches threshold, resulting in irregular contractions (e.g., bowel sounds).
Action Potentials in Unitary Smooth Muscle
- Similar to skeletal muscle action potentials but not always present in multiunit smooth muscle.
- Two forms: spike potentials (10-50 milliseconds) and action potentials with plateaus (hundreds to 1000 milliseconds).
- Plateaus account for prolonged contractions (e.g., uterine contractions during labor).
Electrical Characteristics: Multiunit Smooth Muscle
- Neurogenic; neural regulation via the autonomic nervous system is significant.
- Stable resting membrane potential; few voltage-gated channels.
- Graded potentials; typically no action potentials, only local potentials upon stimulation.
- Junctional potentials cause graded contraction responses.
Calcium in Excitation-Contraction Coupling
- Most Ca2+ for contraction enters from the extracellular fluid during action potentials or other stimuli.
- SR is less developed compared to skeletal muscle.
- A latent period (200-300 milliseconds) exists before contraction due to Ca2+ diffusion. This is much longer than in skeletal muscle.
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Description
This quiz explores the mechanisms of excitation-contraction coupling in smooth muscle. It covers the roles of calcium in muscle action potentials and the unique pathways for contraction and relaxation compared to striated muscle. Test your understanding of the intricacies involved in smooth muscle physiology.
