1. Cardiac and Smooth Muscle Expected Learning Outcomes –Describe the structural and physiological differences between cardiac muscle and skeletal muscle. –Explain why these differences are important to cardiac function. –Describe the structural and physiological differences between smooth muscle and skeletal muscle. –Relate the unique properties of smooth muscle to its locations and functions. 11-1

2. 5-2 Cardiac Muscle Limited to the heart Myocytes or cardiocytes are shorter, branched, and notched at ends Contain one centrally located nucleus surrounded by light- staining glycogen Intercalated discs join cardiocytes end to end –Provide electrical and mechanical connection Striated and involuntary (not under conscious control) Figure 5.26a Figure 5.26b,i Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) (b) Glycogen Striations Intercalated discs © Ed Reschke

3. 11-3 Cardiac Muscle Properties of cardiac muscle –Contraction autorhythmic (pacemaker) –Muscle cells of each chamber must contract in unison –Contractions must last long enough to expel blood –Must work in sleep or wakefulness, without fail, and without conscious attention –Must be highly resistant to fatigue

4. 5-4 Gap Junctions Gap (communicating) junction—formed by a ringlike connexon –Consists of six transmembrane proteins arranged like segments of an orange –Surrounding water-filled pores –Ions, glucose, amino acids, and other solutes pass from one cell to the next Figure 5.28 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cell-adhesion proteins Intercellular space (a) Tight junction Intermediate filaments of cytoskeleton Plaque (b) Desmosome Proteins Connexon Pore (c) Gap junction Basement membrane(d) Hemidesmosome Cell-adhesion proteins Plasma membrane Intercellular space

5. 11-5 Cardiac Muscle Damaged cardiac muscle cells repair by fibrosis –A little mitosis observed following heart attacks –Not in significant amounts to regenerate functional muscle

6. 11-6 Cardiac Muscle –Autonomic nervous system does send nerve fibers to the heart Can increase or decrease heart rate and contraction strength –Very slow twitches; does not exhibit quick twitches like skeletal muscle Gives the heart time to expel blood –Uses aerobic respiration almost exclusively Rich in myoglobin and glycogen Has especially large mitochondria

7. 5-7 Smooth Muscle Figure 5.27a (a) Muscle cellsNuclei (b) Figure 5.27b,i a: © The McGraw-Hill Companies, Inc./Dennis Strete, photographer Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lacks striations and is involuntary Relatively short, fusiform cells (thick in middle, tapered at ends) One centrally located nucleus Visceral muscle—forms layers of digestive, respiratory, and urinary tract: propels contents through an organ, regulates diameter of blood vessels

8. 11-8 Does smooth muscle have myofilaments? Thick and thin filaments are present, but not aligned with each other –No visible striations –Reason for the name “smooth muscle” –Many are in the cell membrane Z discs are absent and replaced by protein plaques or dense bodies –On the plasma membrane and sarcoplasm –Anchor myofilaments for contraction or transfer force from cell to cell

9. 11-9 Smooth Muscle Contraction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dense body Intermediate filaments of cytoskeleton Actin filaments (b) Contracted smooth muscle cells (a) Relaxed smooth muscle cells Plaque Myosin Figure 11.24a,b

10. 11-10 Smooth Muscle Some smooth muscles lack nerve supply, while others receive autonomic fibers, not somatic motor fibers as in skeletal muscle Capable of mitosis Injured smooth muscle regenerates well

11. 11-11 Excitation of Smooth Muscle Smooth muscle is involuntary and can contract without nervous stimulation –Can contract in response to chemical stimuli Hormones, carbon dioxide, low pH, and oxygen deficiency In response to stretch Single-unit smooth muscle in stomach and intestines has pacemaker cells that set off waves of contraction throughout the entire layer of muscle

12. 11-12 Excitation of Smooth Muscle Most smooth muscle is innervated by autonomic nerve fibers –Can trigger and modify contractions –Stimulate smooth muscle with either acetylcholine or norepinephrine –Can have contrasting effects Relax the smooth muscle of arteries Contract smooth muscles of the bronchioles

13. 11-13 Types of Smooth Muscle Multiunit smooth muscle –Occurs in some of the largest arteries and pulmonary air passages, in piloerector muscles of hair follicle, and in the iris of the eye –Autonomic innervation similar to skeletal muscle Terminal branches of a nerve fiber synapse with individual myocytes and form a motor unit Each motor unit contracts independently of the others Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Synapses Autonomic nerve fibers (a) Multiunit smooth muscle Figure 11.23a

14. 11-14 Types of Smooth Muscle Single-unit smooth muscle –More widespread –Occurs in most blood vessels, in the digestive, respiratory, urinary, and reproductive tracts –Also called visceral muscle Often in two layers: inner circular and outer longitudinal –Myocytes of this cell type are electrically coupled to each other by gap junctions –They directly stimulate each other and a large number of cells contract as a single unit Figure 11.23b Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Varicosities Gap junctions Autonomic nerve fibers (b) Single-unit smooth muscle

15. 11-15 Smooth Muscle Epithelium Mucosa: Muscularis externa: Lamina propria Muscularis mucosae Circular layer Longitudinal layer Figure 11.22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

16. 11-16 Types of Smooth Muscle Figure 11.21 Synaptic vesicle Mitochondrion Autonomic nerve fiber Varicosities Single-unit smooth muscle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

17. 11-17 Excitation of Smooth Muscle In single-unit smooth, each autonomic nerve fiber has up to 20,000 beadlike swellings called varicosities –Each contains synaptic vesicles and a few mitochondria –Nerve fiber passes amid several myocytes and stimulates all of them at once when it releases its neurotransmitter

18. 11-18 Contraction Contraction is triggered by Ca 2+, energized by ATP, and achieved by sliding thin past thick filaments Contraction begins in response to Ca 2+ that enters the cell mostly from ECF –Voltage, ligand, and mechanically gated (stretching) –Ca 2+ channels open to allow Ca 2+ to enter cell

19. 11-19 Contraction Calcium binds to calmodulin on thick filaments –Activates Myosin ATPase, hydrolyzing ATP Enables myosin similar power and recovery strokes like skeletal muscle –Thick filaments pull on thin ones, thin ones pull on dense bodies and membrane plaques –Force is transferred to plasma membrane and entire cell shortens

20. 11-20 Contraction and relaxation very slow in comparison to skeletal muscle –Latent period in skeletal 2 ms, smooth muscle 50 to 100 ms –Declines over a period of 1 to 2 seconds Contraction and Relaxation

21. 11-21 Smooth Muscle Contraction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dense body Intermediate filaments of cytoskeleton Actin filaments (b) Contracted smooth muscle cells (a) Relaxed smooth muscle cells Plaque Myosin Figure 11.24a,b

22. 11-22 Latch-bridge mechanism is resistant to fatigue –Heads of myosin molecules do not detach from actin immediately –Do not consume any more ATP –Maintains tetanus tonic contraction (smooth muscle tone) Arteries—vasomotor tone; intestinal tone –Makes most of its ATP aerobically Contraction and Relaxation

23. 11-23 Response to Stretch Stretch can open mechanically gated calcium channels in the sarcolemma causing contraction –Peristalsis: waves of contraction brought about by food distending the esophagus or feces distending the colon Propels contents along the organ

24. 11-24 Skeletal muscle cannot contract forcefully if overstretched Smooth muscle contracts forcefully even when greatly stretched –Allows hollow organs such as the stomach and bladder to fill and then expel their contents efficiently Response to Stretch

25. 11-25 Smooth muscle can be anywhere from half to twice its resting length and still contract powerfully Three reasons –There are no Z discs, so thick filaments cannot butt against them and stop contraction –Since the thick and thin filaments are not arranged in orderly sarcomeres, stretching does not cause a situation where there is too little overlap for cross- bridges to form –The thick filaments of smooth muscle have myosin heads along their entire length, so cross-bridges can form anywhere Response to Stretch

26. Constipation and Laxitives? Bulk-forming - cellulose (or mimic) swell in intestines stimulate stretch receptors Osmotic - increase the amount of water in intestines Stimulant – stimulate nerve plexus, habit forming Emollient – reduce straining, –Mineral oil - coat (prolonged use block absorption) –Docusates – water retension

27. Disorders

28. 11-28 Muscular Dystrophy Muscular dystrophy―group of hereditary diseases in which skeletal muscles degenerate and weaken, and are replaced with fat and fibrous scar tissue Duchenne muscular dystrophy is caused by a sex- linked recessive trait (1 of 3,500 live-born boys) –Most common form –Disease of males; diagnosed between 2 and 10 years of age –Mutation in gene for muscle protein dystrophin Actin not linked to sarcolemma and cell membranes damaged during contraction; necrosis and scar tissue result –Rarely live past 20 years of age due to effects on respiratory and cardiac muscle; incurable

29. 11-29 Muscular Dystrophy Facioscapulohumeral MD―autosomal dominant trait affecting both sexes equally –Facial and shoulder muscles more than pelvic muscles Limb-girdle dystrophy –Combination of several diseases of intermediate severity –Affects shoulder, arm, and pelvic muscles

30. 11-30 Myasthenia Gravis Autoimmune disease in which antibodies attack neuromuscular junctions and bind ACh receptors together in clusters –Disease of women between 20 and 40 –Muscle fibers then remove the clusters of receptors from the sarcolemma by endocytosis –Fiber becomes less and less sensitive to Ach –Effects usually first appear in facial muscles Drooping eyelids and double vision, difficulty swallowing, and weakness of the limbs –Strabismus: inability to fixate on the same point with both eyes

31. 11-31 Myasthenia Gravis Drooping eyelids and weakness of muscles of eye movement upon upward gaze Figure 11.25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

32. 11-32 Myasthenia Gravis Cont. Treatments –Cholinesterase inhibitors retard breakdown of ACh allowing it to stimulate the muscle longer –Immunosuppressive agents suppress the production of antibodies that destroy ACh receptors –Thymus removal (thymectomy) helps to dampen the overactive immune response that causes myasthenia gravis –Plasmapheresis: technique to remove harmful antibodies from blood plasma