M1-C2-L18 --> Smooth Muscle
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- What are the two types of smooth muscles and which one is predominant?
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1. Multiunit Smooth muscle
2. Unitary Smooth muscle (predominant) - Describe the composition of multiunit smooth muscles.
- Discrete smooth muscle fibers
- Describe the composition of unitary smooth muscles.
- Large conglomerate of smooth muscle fibers.
- Describe the innervation of multiunit smooth muscle.
- Each muscle fiber is individually innervated, thus FINE control.
- Describe the innervation of unitary smooth muscle.
- A single neuromuscular junction serves the entire bundle/sheet of smooth muscles. The (outer) cells that receive stimulus will transmit it to other cells via gap junction.
- Unitary smooth muscle is also known as what?
- Syncitial smooth muscle.
- Examples of where you might find multiunit smooth muscle.
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Ciliary muscle of eye
Iris of eye
Piloerector - Examples of where you might find unitary smooth muscle.
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Blood vessels
GI tract
Uterine wall - What is the ratio of actin to myosin in smooth muscles?
- 15 actin to 1 myosin
- Describe the organizational structure of actin and myosin in smooth muscles.
- Irregular structure
- In smooth muscle, what is analogous to the Z disc in skeletal muscle?
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Dense bodies - Describe the sarcoplasmic reticulum in smooth muscle in comparison to skeletal muscles.
- Much reduced in smooth muscles
- Where does the calcium predominantly come from in smooth muscles?
- Outside the cell (ECF)
- Upon depolarization of smooth muscle cell membrane, the calcium comes in (from outside) and does what?
- Binds to calmodulin
- What activates the light chain kinase to phosphorylate myosin?
- Calcium / calmodulin complex
- What needs to happen to myosin so that contraction can occur?
- Needs to be phosphorylated by light chain kinase and needs the presence of actin.
- Why does smooth muscle stay contracted involuntarily?
- Because an enzyme is needed for relaxation to occur.
- Describe the four steps of contraction termination in smooth muscles.
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1. Calcium channels close
2. Cytosolic Ca levels fall below "critical level"
3. Myosin light chain is dephosphorylated by myosin phosphatase.
4. Cross-bridge cycling stops. - Compare speed of cross-bridge cycling in smooth muscle to skeletal muscle.
- Slower in smooth muscle
- Compare onset of contraction and relaxation after stimulus in smooth muscle vs. skeletal muscle
- Slower in smooth muscle
- Compare energy of sustained contraction in smooth muscle vs. skeletal muscle.
- Less energy required in smooth muscle.
- Compare maximum force of contraction in smooth muscle vs. skeletal muscle.
- Greater in smooth muscle.
- Compare degree of shortening in smooth muscle vs. skeletal muscle.
- Greater degree in smooth muscle.
- Compare the quantitative energy of sustained contraction between smooth muscle and skeletal muscle.
- Smooth muscle uses 1/300th the energy of skeletal muscle to sustain a contraction
- Describe the "Latch" mechanism.
- Once smooth muscle in fully contracted, it will sustain it at 1/300 the energy of skeletal muscle.
- What occurs slower in smooth muscle than in skeletal muscle?
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1. Cycling of cross bridges
2. Relaxation
3. Contraction - What are the two mechanisms of control in smooth muscle contraction?
- Neural and hormonal mechanism.
- Describe the receptors of muscle contraction in smooth muscles in comparison to skeletal muscles.
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1. Smooth muscle membrane has many different receptor types (rather than just Na+/K+).
2. Smooth muscles also have receptors that can inhibit contraction (not seen in skeletal muscles) - Do smooth muscles have discrete neuromuscular junctions?
- No. Nerves terminate over muscle fibers, not in them.
- Describe the actions of diffuse junction in smooth muscles.
- Secretes transmitters into fluid surrounding muscle fibers.
- What are the transmitting substances of autonomic nerves?
- Acetylcholine and Norepinepherine
- T/F - Acetylcholine and norepinephrine are released by the same nerve.
- False - released by different nerves, where one is stimulatory and the other is inhibitory.
- What is the general rule of ACh/NE effects on organs.
- Action varies with organ type, but in general: if ACh excites, NE inhibits. and vise versa.
- What is the resting potential in smooth muscles?
- -50 mV.
- Action potentials are seen in what type of smooth muscles?
- In unitary smooth muscles, but not much in multiunit smooth muscles.
- What are the two types of action potential seen in unitary smooth muscles?
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1. Spike potential
2. Action potential with plateaus - Describe the duration of spike potentials
- 10 - 50 milliseconds
- Describe the polarization character of spike potentials with plateaus.
- Very slow repolarization.
- Where would you find spike potentials?
- Places of peristalsis (approx. 8 contractions per minute)
- Where would you find action potentials with plateaus.
- In muscles requiring extended period of contraction usch as ureter, uterus, and vascular smooth muscle.
- Describe the number of calcium channels in smooth muscle cells.
- Calcium channels far exceed sodium channels in smooth muscle cells.
- The flow of what is responsible for Action potential in smooth muscles.
- Calcium
- Describe the opening of calcium channels.
- Opening is much slower.
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What are the characters of these slow waves? X 3 -
1. Self excitatory
2. No extrinsic stimulus needed
3. Source of slow waves unknown -
What are the possible reasons for explaining slow waves? -
1. Changes in Na pumping
2. Ion conductance changes
3. Leak of ions (either Na or Ca) - Action potential is directly COUPLED with what in smooth muscles?
- Contraction
- What causes excitation by stretch in smooth muscles? X 2
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1. Normal slow waves
2. Increase in membrane potential (less negative) - Half of smooth muscle contraction is caused by what two factors?
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1. Local tissue factors
2. Hormones - What are the five local tissue factors?
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1. Lactic acid
2. Acidity
3. Carbon Dioxide
4. Oxygen
5. Prostanoids - How does hormones effect smooth muscle contraction?
- Alters the ion conductance
- T/F - Troponin - Tropomyosin complex seen in smooth muscles.
- False
- What are three major factors of contraction stimulation in smooth muscles.
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1. Stretch
2. Hormones
3. Local factors - What exhibits intrinsic contraction without external stimuli?
- Slow waves