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PART 2 Support and Movement

increases, the contractions that move food through them do notdramatically change in amplitude.

ASSESS YOUR PROGRESS

59. Describe a typical smooth muscle cell. How do its structureand its contraction process differ from those of skeletalmuscle fibers?

60. What ion is key to smooth muscle contraction? What arethe functions of this ion?

61. What is the role of calmodulin? Of myosin phosphatase?

62. Compare visceral smooth muscle and multiunit smoothmuscle as to locations and structure.

63. Explain why visceral smooth muscle contracts as a single unit.

64. How do smooth muscle cells differ from skeletal musclefibers in their electrical properties?

65. How are spontaneous contractions produced in smoothmuscle?

66. List four functional properties of smooth muscle that are notseen in skeletal muscle. Can smooth muscle develop anoxygen deficit?

67. How do the nervous system and hormones regulatesmooth muscle contraction?

68. How are ion channels affected by receptors that stimulatesmooth muscle contractions? That inhibit smooth musclecontractions?

Regulation of Smooth Muscle

The autonomic nervous system innervates smooth muscle, whereasthe somatic motor nervous system innervates skeletal muscle (seechapter 11). The regulation of smooth muscle is therefore involun-tary, and the regulation of skeletal muscle is voluntary.The most important neurotransmitters released from the nervesthat innervate smooth muscle cells are acetylcholine and norepi-nephrine. Acetylcholine stimulates some smooth muscle types tocontract and inhibits others.Hormones are also important in regulating smooth mus-cle. Epinephrine, a hormone from the adrenal medulla, stimu-lates some smooth muscles, such as those in the blood vesselsof the small intestine, and inhibits other smooth muscles, suchas those in the intestinal wall. Oxytocin stimulates contrac-tions of uterine smooth muscle, especially during childbirth.These and other hormones are discussed more thoroughly inchapters 17 and 18. Other chemical substances producedlocally by surrounding tissues —such as histamine, prosta-glandins, and by-products of metabolism —also influencesmooth muscle function. For example, blood flow throughcapillaries is dramatically influenced by these substances (seechapter 21).The type of receptors present on the plasma membrane towhich the neurotransmitters or hormones bind determines theresponse of the smooth muscle. Some smooth muscle types havereceptors to which acetylcholine binds, and the receptor respondsby stimulating contractions; other smooth muscle types have recep-tors to which acetylcholine binds, and the receptor responds byinhibiting contractions. A similar relationship exists for smoothmuscle receptors for norepinephrine and certain hormones.The receptor molecules that stimulate smooth muscle + contractions often open either Na + or Ca 2 channels. When these + 2+ channels open, Na and Ca pass through their respective channelsinto the cell and cause depolarization of the plasma membrane. It + is also possible for the receptor to open Ca 2 channels in the + plasma membrane and sarcoplasmic reticulum. As a result, Ca 2 can diffuse into the cytoplasm of the smooth muscle cells withoutdepolarization of the membrane potential to its threshold level andtherefore not produce action potentials.The receptor molecules that inhibit smooth muscle contractions + often close Na + and Ca 2 channels or open K + channels. The resultis hyperpolarization (membrane potential below resting) of thesmooth muscle cells and inhibition. It is also possible for the recep- + + tors to increase the activity of the Ca 2 pump that transports Ca 2 out of the cell or into the sarcoplasmic reticulum. As a result,relaxation may occur without a change in the resting membranepotential.The response of specific smooth muscle types to either neuro-transmitters or hormones is presented in the chapters dealing withthe smooth muscle types.

9.9 Cardiac Muscle

LEARNING OUTCOME

After reading this section, you should be able to

A. Discuss the structural and functional characteristics ofcardiac muscle.

Cardiac muscle, which is found only in the heart, is discussed indetail in chapter 20. Like skeletal muscle tissue, cardiac muscle tis-sue is striated, but each cell usually contains one nucleus locatednear the center. Adjacent cells join to form branching fibers byspecialized cell-to-cell attachments called intercalated (in-ter ′ k ă -l ā -ted) disks, which have gap junctions that allow action potentialsto pass from cell to cell. Some cardiac muscle cells are autorhyth-mic, and one part of the heart normally acts as the pacemaker. Theaction potentials of cardiac muscle are similar to those of nerveand skeletal muscle but have a much longer duration and refractory(unresponsive) period. The depolarization of cardiac muscle results + from the influx of both Na + and Ca 2 across the plasma membrane. + The regulation of contraction in cardiac muscle by Ca 2 is similar tothat of skeletal muscle.

ASSESS YOUR PROGRESS

69. Compare the structural and functional characteristics ofcardiac muscle with those of skeletal muscle.

70. How is cardiac muscle similar to smooth muscle?