I. An increase in cytosolic calcium leads to the binding of calcium by calmodulin. The calcium-calmodulin complex then binds to myosin light-chain kinase, activating the enzyme, which uses ATP to phosphorylate smooth-muscle myosin. Only phosphorylated myosin is able to bind to actin and undergo cross-bridge cycling.
II. Smooth-muscle myosin has a low rate of ATP splitting, resulting in a much slower shortening velocity than is found in striated muscle. However, the tension produced per unit cross-sectional area is equivalent to that of skeletal muscle.
III. Two sources of the cytosolic calcium ions initiate smooth-muscle contraction: the sarcoplasmic reticulum and extracellular calcium. The opening of calcium channels in the smooth-muscle plasma membrane and sarcoplasmic reticulum, mediated by a variety of factors, allows calcium ions to enter the cytosol.
IV. The increase in cytosolic calcium resulting from most stimuli does not activate all the cross bridges. Therefore smooth-muscle tension can be increased by agents that increase the concentration of cytosolic calcium ions.
V. Table 11-5 summarizes the types of stimuli that can initiate smooth-muscle contraction by opening or closing calcium channels in the plasma membrane or sarcoplasmic reticulum.
PART TWO Biological Control Systems
VI. Most, but not all, smooth-muscle cells can generate action potentials in their plasma membrane upon membrane depolarization. The rising phase of the smooth-muscle action potential is due to the influx of calcium ions into the cell through open calcium channels.
Some smooth muscles generate action potentials spontaneously, in the absence of any external input, because of pacemaker potentials in the plasma membrane that repeatedly depolarize the membrane to threshold.
Smooth-muscle cells do not have a specialized endplate region. A number of smooth-muscle fibers may be influenced by neurotransmitters released from the varicosities on a single nerve ending, and a single smooth-muscle fiber may be influenced by neurotransmitters from more than one neuron. Neurotransmitters may have either excitatory or inhibitory effects on smooth-muscle contraction.
IX. Smooth muscles can be classified broadly as single-unit or multiunit smooth muscle (Table 11-6).
CHAPTER 11 THOUGHT QUESTIONS
SECTION B KEY TERMS
SECTION B REVIEW QUESTIONS
Compare the mechanisms by which a rise in cytosolic calcium concentration initiates contractile activity in skeletal- and smooth-muscle fibers. What are the two sources of calcium that lead to the increase in cytosolic calcium that triggers contraction in smooth muscle?
What types of stimuli can trigger a rise in cytosolic calcium in smooth-muscle fibers? What effect does a pacemaker potential have on a smooth-muscle cell?
In what ways does the neural control of smooth-muscle activity differ from that of skeletal muscle? Describe how a stimulus may lead to the contraction of a smooth-muscle cell without a change in the plasma-membrane potential. Describe the differences between single-unit and multiunit smooth muscles.
CHAPTER 11 CLINICAL TERMS
(Answers are given in appendix A.)
1. Which of the following corresponds to the state of myosin (M) under resting conditions and in rigor mortis? (a) M • ATP, (b) M* • ADP • P,, (c) A • M* • ADP • P., (d) A • M.
If the transverse tubules of a skeletal muscle are disconnected from the plasma membrane, will action potentials trigger a contraction? Give reasons. When a small load is attached to a skeletal muscle that is then tetanically stimulated, the muscle lifts the load in an isotonic contraction over a certain distance, but then stops shortening and enters a state of isometric contraction. With a heavier load, the distance shortened before entering an isometric contraction is shorter. Explain these shortening limits in terms of the length-tension relation of muscle. What conditions will produce the maximum tension in a skeletal-muscle fiber?
A skeletal muscle can often maintain a moderate level of active tension for long periods of time, even though many of its fibers become fatigued. Explain. If the blood flow to a skeletal muscle were markedly decreased, which types of motor units would most rapidly have their ability to produce ATP for muscle contraction severely reduced? Why? As a result of an automobile accident, 50 percent of the muscle fibers in the biceps muscle of a patient were destroyed. Ten months later, the biceps muscle was able to generate 80 percent of its original force. Describe the changes that took place in the damaged muscle that enabled it to recover. In the laboratory, if an isolated skeletal muscle is placed in a solution that contains no calcium ions, will the muscle contract when it is stimulated (1) directly by depolarizing its membrane, or (2) by stimulating the nerve to the muscle? What would happen if it were a smooth muscle? The following experiments were performed on a single-unit smooth muscle in the gastrointestinal tract.
a. Stimulating the parasympathetic nerves to the muscle produced a contraction. Applying a drug that blocks the voltage-sensitive sodium channels in most plasma membranes led to a failure to contract upon stimulating the parasympathetic nerves. c. Applying a drug that binds to muscarinic receptors (Chapter 8), and hence blocks the action of ACh at these receptors, did not prevent the muscle from contracting when the parasympathetic nerve was stimulated. From these observations, what might one conclude about the mechanism by which parasympathetic nerve stimulation produces a contraction of the smooth muscle?
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