Chapter 50: Sensory and Motor Mechansims

1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 50.5: Muscles move skeletal parts by contracting • The action of a muscle is always to contract • Skeletal muscles are attached in antagonistic pairs, with each member of the pair working against each other

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 50.5: Muscles move skeletal parts by contracting • The action of a muscle is always to contract • Skeletal muscles are attached in antagonistic pairs, with each member of the pair working against each other

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biceps contracts Human Triceps relaxes Forearm flexes Biceps relaxes Triceps contracts Forearm extends Extensor muscle relaxes Flexor muscle contracts Grasshopper Extensor muscle contracts Flexor muscle relaxes Tibia extends Tibia flexes

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Vertebrate Skeletal Muscle • Vertebrate skeletal muscle is characterized by a hierarchy of smaller and smaller units • A skeletal muscle consists of a bundle of long fibers running parallel to the length of the muscle • A muscle fiber is itself a bundle of smaller myofibrils arranged longitudinally

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The myofibrils are composed to two kinds of myofilaments: – Thin filaments consist of two strands of actin and one strand of regulatory protein – Thick filaments are staggered arrays of myosin molecules

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Skeletal muscle is also called striated muscle because the regular arrangement of myofilaments creates a pattern of light and dark bands • Each unit is a sarcomere, bordered by Z lines • Areas that contain the myofilments are the I band, A band, and H zone

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bundle of muscle fibers Single muscle fiber (cell) Plasma membrane Nuclei Muscle Myofibril Dark band Sarcomere Z lineLight band I band TEM A band I band 0.5 µm M line Thick filaments (myosin) Sarcomere H zoneZ line Thin filaments (actin) Z line

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Sliding-Filament Model of Muscle Contraction • According to the sliding-filament model, filaments slide past each other longitudinally, producing more overlap between thin and thick filaments • As a result of sliding, the I band and H zone shrink Sarcomere 0.5 µm Z H A Relaxed muscle fiber I Contracting muscle fiber Fully contracted muscle fiber

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The sliding of filaments is based on interaction between actin and myosin molecules of the thick and thin filaments • The “head” of a myosin molecule binds to an actin filament, forming a cross-bridge and pulling the thin filament toward the center of the sarcomere

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE 49-30–4 Thin filaments Thick filament Thin filament Thick filament Myosin head (low-energy configuration) Cross-bridge binding site Myosin head (high- energy configuration) Actin Cross-bridge Myosin head (low- energy configuration) Thin filament moves toward center of sacomere.

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Role of Calcium and Regulatory Proteins • A skeletal muscle fiber contracts only when stimulated by a motor neuron • When a muscle is at rest, myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE 50-28 Myosin-binding sites blocked. Myosin-binding sites exposed. Tropomyosin Ca2+ -binding sites Actin Troponin complex Myosin- binding site Ca2+

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • For a muscle fiber to contract, myosin-binding sites must be uncovered • This occurs when calcium ions (Ca2+ ) bind to a set of regulatory proteins, the troponin complex • The stimulus leading to contraction of a muscle fiber is an action potential in a motor neuron that makes a synapse with the muscle fiber

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 49-32 Ca2+ released from sarcoplasmic reticulum Mitochondrion Motor neuron axon Synaptic terminal T tubule Sarcoplasmic reticulum Myofibril Plasma membrane of muscle fiber Sarcomere

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The synaptic terminal of the motor neuron releases the neurotransmitter acetylcholine • Acetylcholine depolarizes the muscle, causing it to produce an action potential

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Action potentials travel to the interior of the muscle fiber along transverse (T) tubules • The action potential along T tubules causes the sarcoplasmic reticulum to release Ca2+ • The Ca2+ binds to the troponin-tropomyosin complex on the thin filaments • This binding exposes myosin-binding sites and allows the cross-bridge cycle to proceed

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Neural Control of Muscle Tension • Contraction of a whole muscle is graded, which means that the extent and strength of its contraction can be voluntarily altered • There are two basic mechanisms by which the nervous system produces graded contractions: – Varying the number of fibers that contract – Varying the rate at which fibers are stimulated

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In a vertebrate skeletal muscle, each branched muscle fiber is innervated by one motor neuron • Each motor neuron may synapse with multiple muscle fibers

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE 49-34 Motor unit 1 Motor unit 2 Nerve Synaptic terminals Motor neuron cell body Spinal cord Motor neuron axon Muscle Tendon Muscle fibers

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A motor unit consists of a single motor neuron and all the muscle fibers it controls • Recruitment of multiple motor neurons results in stronger contractions • A twitch results from a single action potential in a motor neuron • More rapidly delivered action potentials produce a graded contraction by summation

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE 49-35 Tetanus Summation of two twitches Single twitch Tension Action potential Pair of action potentials Time Series of action potentials at high frequency

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Tetanus is a state of smooth and sustained contraction produced when motor neurons deliver a volley of action potentials • The bacterial tetanus toxin blocks the release of inhibitory neurotransmitters. When nervous impulses cannot be checked by normal inhibitory mechanisms, the generalized muscular spasms characteristic of tetanus are produced.

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Types of Muscle Fibers • Skeletal muscle fibers are classified as slow oxidative, fast oxidative, and fast glycolytic • These categories are based on their contraction speed and major pathway for producing ATP

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other Types of Muscle • Cardiac muscle, found only in the heart, consists of striated cells electrically connected by intercalated discs • Cardiac muscle can generate action potentials without neural input

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In smooth muscle, found mainly in walls of hollow organs, contractions are relatively slow and may be initiated by the muscles themselves • Contractions may also be caused by stimulation from neurons in the autonomic nervous system

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Biceps contracts Human Triceps relaxes Forearm flexes Biceps relaxes Triceps contracts Forearm extends Extensor muscle relaxes Flexor muscle contracts Grasshopper Extensor muscle contracts Flexor muscle relaxes Tibia extends Tibia flexes

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Vertebrate Skeletal Muscle • Vertebrate skeletal muscle is characterized by a hierarchy of smaller and smaller units • A skeletal muscle consists of a bundle of long fibers running parallel to the length of the muscle • A muscle fiber is itself a bundle of smaller myofibrils arranged longitudinally

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The myofibrils are composed to two kinds of myofilaments: – Thin filaments consist of two strands of actin and one strand of regulatory protein – Thick filaments are staggered arrays of myosin molecules

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Skeletal muscle is also called striated muscle because the regular arrangement of myofilaments creates a pattern of light and dark bands • Each unit is a sarcomere, bordered by Z lines • Areas that contain the myofilments are the I band, A band, and H zone

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bundle of muscle fibers Single muscle fiber (cell) Plasma membrane Nuclei Muscle Myofibril Dark band Sarcomere Z lineLight band I band TEM A band I band 0.5 µm M line Thick filaments (myosin) Sarcomere H zoneZ line Thin filaments (actin) Z line

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Sliding-Filament Model of Muscle Contraction • According to the sliding-filament model, filaments slide past each other longitudinally, producing more overlap between thin and thick filaments • As a result of sliding, the I band and H zone shrink Sarcomere 0.5 µm Z H A Relaxed muscle fiber I Contracting muscle fiber Fully contracted muscle fiber

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The sliding of filaments is based on interaction between actin and myosin molecules of the thick and thin filaments • The “head” of a myosin molecule binds to an actin filament, forming a cross-bridge and pulling the thin filament toward the center of the sarcomere

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE 49-30–4 Thin filaments Thick filament Thin filament Thick filament Myosin head (low-energy configuration) Cross-bridge binding site Myosin head (high- energy configuration) Actin Cross-bridge Myosin head (low- energy configuration) Thin filament moves toward center of sacomere.

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Role of Calcium and Regulatory Proteins • A skeletal muscle fiber contracts only when stimulated by a motor neuron • When a muscle is at rest, myosin-binding sites on the thin filament are blocked by the regulatory protein tropomyosin

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE 50-28 Myosin-binding sites blocked. Myosin-binding sites exposed. Tropomyosin Ca2+ -binding sites Actin Troponin complex Myosin- binding site Ca2+

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • For a muscle fiber to contract, myosin-binding sites must be uncovered • This occurs when calcium ions (Ca2+ ) bind to a set of regulatory proteins, the troponin complex • The stimulus leading to contraction of a muscle fiber is an action potential in a motor neuron that makes a synapse with the muscle fiber

39. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings LE 49-32 Ca2+ released from sarcoplasmic reticulum Mitochondrion Motor neuron axon Synaptic terminal T tubule Sarcoplasmic reticulum Myofibril Plasma membrane of muscle fiber Sarcomere

40. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • The synaptic terminal of the motor neuron releases the neurotransmitter acetylcholine • Acetylcholine depolarizes the muscle, causing it to produce an action potential

41. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Action potentials travel to the interior of the muscle fiber along transverse (T) tubules • The action potential along T tubules causes the sarcoplasmic reticulum to release Ca2+ • The Ca2+ binds to the troponin-tropomyosin complex on the thin filaments • This binding exposes myosin-binding sites and allows the cross-bridge cycle to proceed

43. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Neural Control of Muscle Tension • Contraction of a whole muscle is graded, which means that the extent and strength of its contraction can be voluntarily altered • There are two basic mechanisms by which the nervous system produces graded contractions: – Varying the number of fibers that contract – Varying the rate at which fibers are stimulated

44. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In a vertebrate skeletal muscle, each branched muscle fiber is innervated by one motor neuron • Each motor neuron may synapse with multiple muscle fibers

45. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE 49-34 Motor unit 1 Motor unit 2 Nerve Synaptic terminals Motor neuron cell body Spinal cord Motor neuron axon Muscle Tendon Muscle fibers

46. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • A motor unit consists of a single motor neuron and all the muscle fibers it controls • Recruitment of multiple motor neurons results in stronger contractions • A twitch results from a single action potential in a motor neuron • More rapidly delivered action potentials produce a graded contraction by summation

47. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero LE 49-35 Tetanus Summation of two twitches Single twitch Tension Action potential Pair of action potentials Time Series of action potentials at high frequency

48. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • Tetanus is a state of smooth and sustained contraction produced when motor neurons deliver a volley of action potentials • The bacterial tetanus toxin blocks the release of inhibitory neurotransmitters. When nervous impulses cannot be checked by normal inhibitory mechanisms, the generalized muscular spasms characteristic of tetanus are produced.

49. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Types of Muscle Fibers • Skeletal muscle fibers are classified as slow oxidative, fast oxidative, and fast glycolytic • These categories are based on their contraction speed and major pathway for producing ATP

51. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other Types of Muscle • Cardiac muscle, found only in the heart, consists of striated cells electrically connected by intercalated discs • Cardiac muscle can generate action potentials without neural input

52. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings • In smooth muscle, found mainly in walls of hollow organs, contractions are relatively slow and may be initiated by the muscles themselves • Contractions may also be caused by stimulation from neurons in the autonomic nervous system

Chapter 50: Sensory and Motor Mechansims

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Chapter 50: Sensory and Motor Mechansims