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Questions and Answers
What role do varicosities play in smooth muscle innervation?
What role do varicosities play in smooth muscle innervation?
Which of the following ions primarily enters smooth muscle cells to initiate contraction?
Which of the following ions primarily enters smooth muscle cells to initiate contraction?
How does the binding of a ligand affect smooth muscle cell function?
How does the binding of a ligand affect smooth muscle cell function?
What is a characteristic difference between smooth muscle and skeletal muscle control?
What is a characteristic difference between smooth muscle and skeletal muscle control?
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Which neurotransmitter is typically associated with smooth muscle excitation?
Which neurotransmitter is typically associated with smooth muscle excitation?
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What supplements calcium storage in smooth muscle besides the sarcoplasmic reticulum?
What supplements calcium storage in smooth muscle besides the sarcoplasmic reticulum?
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What effect does norepinephrine have on smooth muscle?
What effect does norepinephrine have on smooth muscle?
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Which factor does not directly influence smooth muscle contraction?
Which factor does not directly influence smooth muscle contraction?
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Which of the following statements about varicosities is correct?
Which of the following statements about varicosities is correct?
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What is the primary difference between neuromuscular junctions in skeletal muscle and junctions in smooth muscle?
What is the primary difference between neuromuscular junctions in skeletal muscle and junctions in smooth muscle?
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Which neurotransmitter is identified as an excitatory substance for smooth muscle in certain organs?
Which neurotransmitter is identified as an excitatory substance for smooth muscle in certain organs?
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How do acetylcholine and norepinephrine primarily affect smooth muscle?
How do acetylcholine and norepinephrine primarily affect smooth muscle?
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What structure keeps calcium ions close to the membrane in smooth muscle?
What structure keeps calcium ions close to the membrane in smooth muscle?
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What determines whether smooth muscle is excited or inhibited?
What determines whether smooth muscle is excited or inhibited?
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Which neurotransmitter works opposite to acetylcholine in terms of smooth muscle activity?
Which neurotransmitter works opposite to acetylcholine in terms of smooth muscle activity?
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Which of the following statements about smooth muscle innervation is true?
Which of the following statements about smooth muscle innervation is true?
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What primarily causes smooth muscle relaxation when there is a lack of oxygen in local tissues?
What primarily causes smooth muscle relaxation when there is a lack of oxygen in local tissues?
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Which hormone is least likely to facilitate smooth muscle contraction?
Which hormone is least likely to facilitate smooth muscle contraction?
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Which of the following correctly describes the cell structure of smooth muscle?
Which of the following correctly describes the cell structure of smooth muscle?
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What is the primary source of calcium ions for smooth muscle contraction?
What is the primary source of calcium ions for smooth muscle contraction?
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How do calmodulin and troponin differ in their roles concerning calcium ions in muscle cells?
How do calmodulin and troponin differ in their roles concerning calcium ions in muscle cells?
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What type of control governs smooth muscle activity?
What type of control governs smooth muscle activity?
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Which of the following factors does NOT contribute to smooth muscle contraction?
Which of the following factors does NOT contribute to smooth muscle contraction?
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What characterizes the contraction speed of smooth muscle in comparison to skeletal muscle?
What characterizes the contraction speed of smooth muscle in comparison to skeletal muscle?
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What characterizes unitary smooth muscle?
What characterizes unitary smooth muscle?
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What is a distinguishing feature of multi-unit smooth muscle?
What is a distinguishing feature of multi-unit smooth muscle?
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Where is unitary smooth muscle primarily located?
Where is unitary smooth muscle primarily located?
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What type of smooth muscle is also known as syncytial smooth muscle?
What type of smooth muscle is also known as syncytial smooth muscle?
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In what way do fibers in multi-unit smooth muscle interact?
In what way do fibers in multi-unit smooth muscle interact?
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What anatomical feature is present in unitary smooth muscle that facilitates electrical communication?
What anatomical feature is present in unitary smooth muscle that facilitates electrical communication?
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How do the fibers of multi-unit smooth muscle differ structurally from those of unitary smooth muscle?
How do the fibers of multi-unit smooth muscle differ structurally from those of unitary smooth muscle?
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What is the primary function of the basement membrane in multi-unit smooth muscle?
What is the primary function of the basement membrane in multi-unit smooth muscle?
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What correctly describes varicosities in smooth muscle?
What correctly describes varicosities in smooth muscle?
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Which factor influences whether smooth muscle is excited or inhibited?
Which factor influences whether smooth muscle is excited or inhibited?
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How do neurotransmitters affect smooth muscle contractions?
How do neurotransmitters affect smooth muscle contractions?
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What is the primary differentiating factor in the structure of smooth muscle compared to skeletal muscle?
What is the primary differentiating factor in the structure of smooth muscle compared to skeletal muscle?
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What is a characteristic feature of diffuse junctions in smooth muscle?
What is a characteristic feature of diffuse junctions in smooth muscle?
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What is the primary reason why acetylcholine can have both excitatory and inhibitory effects on smooth muscle?
What is the primary reason why acetylcholine can have both excitatory and inhibitory effects on smooth muscle?
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Why are varicosities significant in the function of smooth muscle?
Why are varicosities significant in the function of smooth muscle?
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What distinguishes smooth muscle contraction from skeletal muscle contraction?
What distinguishes smooth muscle contraction from skeletal muscle contraction?
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What structure in smooth muscle serves a similar function to Z lines in skeletal muscle?
What structure in smooth muscle serves a similar function to Z lines in skeletal muscle?
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Which characteristic distinguishes smooth muscle from skeletal muscle in terms of contraction control?
Which characteristic distinguishes smooth muscle from skeletal muscle in terms of contraction control?
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Which of the following describes a function of adherens junctions in smooth muscle?
Which of the following describes a function of adherens junctions in smooth muscle?
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How does the arrangement of actin and myosin filaments in smooth muscle differ from skeletal muscle?
How does the arrangement of actin and myosin filaments in smooth muscle differ from skeletal muscle?
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What is the primary energy molecule utilized for contraction in smooth muscle?
What is the primary energy molecule utilized for contraction in smooth muscle?
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What primarily facilitates the propagation of action potentials between smooth muscle cells?
What primarily facilitates the propagation of action potentials between smooth muscle cells?
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In smooth muscle, what is the relationship between myosin and actin filament sizes?
In smooth muscle, what is the relationship between myosin and actin filament sizes?
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What distinguishes the duration of contraction in smooth muscle compared to skeletal muscle?
What distinguishes the duration of contraction in smooth muscle compared to skeletal muscle?
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What is the resting potential of most smooth muscles?
What is the resting potential of most smooth muscles?
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Which type of action potential occurs in unitary smooth muscle with a delay in repolarization?
Which type of action potential occurs in unitary smooth muscle with a delay in repolarization?
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What primarily determines the resting potential of smooth muscle?
What primarily determines the resting potential of smooth muscle?
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Which of the following can NOT elicit action potentials in typical spike action potentials?
Which of the following can NOT elicit action potentials in typical spike action potentials?
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How long does a typical spike action potential last in smooth muscle?
How long does a typical spike action potential last in smooth muscle?
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In which type of smooth muscle is prolonged contraction primarily associated with action potentials with plateau?
In which type of smooth muscle is prolonged contraction primarily associated with action potentials with plateau?
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What is a characteristic of spike potentials in smooth muscle compared to skeletal muscle?
What is a characteristic of spike potentials in smooth muscle compared to skeletal muscle?
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What primarily accounts for the less negative resting potential in smooth muscle compared to other muscle types?
What primarily accounts for the less negative resting potential in smooth muscle compared to other muscle types?
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What initiates smooth muscle relaxation in local tissues when oxygen levels are low?
What initiates smooth muscle relaxation in local tissues when oxygen levels are low?
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Which hormone is primarily involved in smooth muscle contraction?
Which hormone is primarily involved in smooth muscle contraction?
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How do the speed of contractions compare between smooth muscle and skeletal muscle?
How do the speed of contractions compare between smooth muscle and skeletal muscle?
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Which condition contributes to local vasodilation through smooth muscle relaxation?
Which condition contributes to local vasodilation through smooth muscle relaxation?
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What type of control primarily governs smooth muscle contraction?
What type of control primarily governs smooth muscle contraction?
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Calcium in smooth muscle contraction primarily comes from which source?
Calcium in smooth muscle contraction primarily comes from which source?
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What distinguishes the cell structure of smooth muscle from skeletal muscle?
What distinguishes the cell structure of smooth muscle from skeletal muscle?
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Which chemical factor is NOT associated with causing local vasodilation in smooth muscle?
Which chemical factor is NOT associated with causing local vasodilation in smooth muscle?
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Study Notes
Smooth Muscle Control
- Smooth muscle regulation is complex compared to skeletal muscles
- Smooth muscle responds to nerve stimulation, hormones, pharmacological stimuli, metabolites, temperature, pressure, stretch, and touch.
- Smooth muscle may be spontaneously active.
### Innervation of Smooth Muscle
- Smooth muscle often receives innervation from both divisions of the autonomic nervous system.
- Autonomic nerve axons run throughout smooth muscle tissue.
- Varicosities along axons release neurotransmitters in response to action potentials.
- Neurotransmitters can be excitatory or inhibitory and diffuse to nearby smooth muscle cells.
### Calcium Sources in Smooth Muscle
- Smooth muscle has limited sarcoplasmic reticulum (SR).
- Caveolae supplement SR for calcium storage.
- Caveolae membranes contain gated calcium channels opening in response to:
- Changes in membrane potential
- Ligand binding
- Calcium enters the cell from extracellular fluid and the SR.
Smooth Muscle Contraction
- Chemical factors act by:
- Opening or closing membrane ion channels
- Production of second messengers that diffuse to the cell interior
- Changes in intracellular calcium concentration control smooth muscle contraction.
Relaxation of Smooth Muscle
- The relaxation of smooth muscle is the result of calcium removal from the cytoplasm
- The calcium is either transported back into the SR or out of the cell through the plasma membrane.
### Effect of Local Tissue Factors and Hormones on Smooth Muscle
- Local tissue chemical factors and hormones can affect smooth muscle contraction without action potentials.
Local Tissue Factors
- Lack of oxygen: causes smooth muscle relaxation and vasodilation.
- Excess carbon dioxide, increased hydrogen ion concentration, adenosine, lactic acid, increased potassium ions, diminished calcium ion concentration, and increased body temperature all cause local vasodilation.
#### Hormones
- Many circulating hormones and neurotransmitters affect smooth muscle contraction including:
- Norepinephrine
- Epinephrine
- Acetylcholine
- Angiotensin
- Endothelin
- Vasopressin
- Oxytocin
- Serotonin
- Histamine
Comparison of Smooth Muscle and Skeletal Muscle Contraction
- Smooth muscle cells are small, spindle-shaped, and lack striations. Skeletal muscle cells are large, cylindrical, multinucleated and striated.
- Smooth muscle is involuntary and controlled by the autonomic nervous system, hormones, and local factors. Skeletal muscle is voluntary and controlled by the somatic nervous system.
- Smooth muscle contractions are slow and sustained. Skeletal muscle contractions are fast and designed for short bursts of activity.
- Smooth muscle calcium comes from both the extracellular fluid and the SR. Skeletal muscle calcium is primarily released from the SR.
- Smooth muscle uses calmodulin to bind calcium and activate myosin light chain kinase (MLCK). Skeletal muscle uses troponin to bind calcium, causing a conformational change in tropomyosin that allows actin-myosin interaction.
- Smooth muscle lacks troponin complex and neuromuscular junctions, relying on caveolae instead of T-tubules. Skeletal muscle uses troponin complex and neuromuscular junctions.
- Smooth muscle uses varicosities instead of neuromuscular junctions.
Varicosities
- Expanded axon parts or bulbous swellings in smooth muscle
- Release neurotransmitters into a wide synaptic cleft
- Varicosities are considered diffuse junctions.
### Excitatory and Inhibitory Transmitters in Smooth Muscle
- Smooth muscle can be excited, inhibited, or both, unlike skeletal muscle which is only excited.
- The excitatory or inhibitory nature of the response depends on which neurotransmitter is released by the varicosites.
- Acetylcholine and norepinephrine are the most important neurotransmitters released at smooth muscle neuromuscluar junctions.
- The same nerve fiber never releases both acetylcholine and norepinephrine.
- Acetylcholine can be excitatory or inhibitory depending on the smooth muscle target. When acetylcholine excites, norepinephrine usually inhibits and vice versa.
- Acetylcholine and norepinephrine bind to receptor proteins on the muscle cell membrane.
- Some receptor proteins are excitatory, others are inhibitory.
Smooth Muscle Types
- Two main types: Multi-unit and Unitary
-
Unitary (syncytial or visceral): Consists of fibers that contract together as a single unit.
- Cells are connected by gap junctions, allowing ion flow for efficient communication.
- Found in the walls of many organs: gastrointestinal tract, bile ducts, ureters, uterus, and blood vessels.
-
Multi-unit: Composed of individual fibers that operate independently.
- Each fiber is innervated by a single nerve ending.
- Cells are not linked electrically.
- Examples: ciliary muscle of the eye, iris muscle of the eye, and piloerector muscles.
Smooth Muscle Contractile Mechanism
- Contains actin and myosin filaments, similar to skeletal muscle.
- Key difference: Lacks the troponin complex which is found in skeletal muscle.
- Calcium ions activate the contractile process.
- ATP is broken down to ADP to provide energy for contraction
-
Physical Organization:
- No striated arrangement of actin and myosin like in skeletal muscle.
- Actin filaments are attached to structures called dense bodies (functionally similar to Z lines in skeletal muscle).
- Some dense bodies are attached to the cell membrane, while others are distributed within the cell.
- Intercellular protein bridges connect membrane-dense bodies of adjacent cells to transmit force of contraction.
- Myosin: Located between actin filaments; twice the diameter of actin.
- Number: Five to ten times more actin filaments than myosin filaments.
Cell-to-Cell Contacts in Smooth Muscle
- Gap junctions: Low-resistance pathways allowing action potentials to propagate easily.
-
Adherens junctions (dense bodies): Provide mechanical linkage and contain dense granular material.
- Thin filaments extend into the junctions.
Comparison of Smooth Muscle and Skeletal Muscle
| Feature | Smooth Muscle | Skeletal Muscle |
|---|---|---|
| Cell Structure | Small, spindle-shaped, no striations | Large, cylindrical, multinucleated, striations |
| Control | Involuntary (autonomic nervous system, hormones, local factors) | Voluntary (somatic nervous system) |
| Speed of Contraction | Slow and sustained | Fast, short bursts of activity |
| Calcium Source | Extracellular fluid and sarcoplasmic reticulum | Primarily sarcoplasmic reticulum |
| Calcium-Binding Protein | Calmodulin | Troponin |
Smooth Muscle Excitation and Inhibition
-
- Neuromuscular junctions: Not present in smooth muscle; instead, varicosities (expanded axon parts) release neurotransmitter into a wide synaptic cleft.
Smooth Muscle: Excitation, Inhibition, and Transmitter Substances
- Acetylcholine and norepinephrine: Most common neurotransmitters.
- Different responses: Acetylcholine is excitatory in some organs and inhibitory in others; the opposite applies to norepinephrine.
- Receptors: The effect of these neurotransmitters depends on the type of receptor protein on the smooth muscle cell membrane: excitatory or inhibitory.
Smooth Muscle: Membrane Potentials and Action Potentials
- Resting potential: Approximately -50 mV (less negative than skeletal muscle).
-
Action potentials: Occur in unitary smooth muscle.
- Spike potentials: Similar to those in skeletal muscle; duration of 10-50 milliseconds.
- Action potentials with plateau: Repolarization is delayed for several hundred milliseconds to 1 second. This contributes to prolonged contractions in certain types of smooth muscle (e.g., ureters, uterus under specific conditions, vascular smooth muscle).
Relaxation of Smooth Muscle
- Factors affecting relaxation:
- Muscarinic receptor activation: Directly relaxes smooth muscle
- Calcium pumps: Move calcium from the cytoplasm into the sarcoplasm.
- Phosphorylation: Phosphorylation of myosin light chain kinase (MLCK) is reduced.
Local Tissue Factors and Hormones for Contraction
- Contraction can occur without action potentials through:
-
Local tissue chemical factors:
- Lack of oxygen: Leads to smooth muscle relaxation and vasodilatation.
- Excess carbon dioxide, increased hydrogen ion concentration, adenosine, lactic acid, increased potassium ions, diminished calcium ion concentration, and increased body temperature all result in vasodilatation.
-
Hormones:
- Norepinephrine, epinephrine, acetylcholine, Angiotensin, Endothelin, Vasopressin, Oxytocin, Serotonin, Histamine.
-
Local tissue chemical factors:
Smooth Muscle Contraction
-
Steps:
- Calcium entry: Calcium enters the cytoplasm from the extracellular fluid through voltage-gated calcium channels.
- Calmodulin activation: Calcium binds to calmodulin, a calcium-binding protein.
- MLCK activation: The calmodulin-calcium complex activates myosin light chain kinase (MLCK).
- Myosin phosphorylation: MLCK phosphorylates the myosin light chains, which enhances the interaction between actin and myosin filaments.
- Cross-bridge cycling: Phosphorylated myosin binds to actin and facilitates cross-bridge cycling. This process generates force and allows contraction.
- Relaxation: Inhibition of MLCK and dephosphorylation of myosin light chains contribute to relaxation.
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Description
Test your knowledge on the complex regulation and innervation of smooth muscles. This quiz covers important topics such as calcium sources and the response of smooth muscles to various stimuli. Perfect for students studying anatomy and physiology.
