1. Chapter 9 Muscle Tissue

2. Three Types of Muscle Tissue
Skeletal muscle tissue:
Attached to bones and skin
Striated
Voluntary (i.e., conscious control)
Powerful
Primary topic of this chapter

3. Three Types of Muscle Tissue
Cardiac muscle tissue:
Only in the heart
Striated

4. Three Types of Muscle Tissue
Smooth muscle tissue:
In the walls of hollow organs, e.g., stomach, urinary bladder, and airways
Not striated
Involuntary
More details later in this chapter

5. Special Characteristics of Muscle Tissue
Excitability (responsiveness or irritability): ability to receive and respond to stimuli
Contractility: ability to shorten when stimulated
Extensibility: ability to be stretched
Elasticity: ability to recoil to resting length

6. Muscle Functions
Movement of bones or fluids (e.g., blood)
Maintaining posture and body position
Stabilizing joints
Heat generation (especially skeletal muscle)

7. Each muscle is served by one artery, one nerve, and one or more veins
Skeletal Muscle
Each muscle is served by one artery, one nerve, and one or more veins
All enter or exit near the central part of muscle
Rich blood supply
Give off large amounts of waste

8. Skeletal Muscle Connective tissue sheaths of skeletal muscle:
Epimysium: dense regular connective tissue surrounding entire muscle
Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers)
Endomysium: fine areolar connective tissue surrounding each muscle fiber

9. Myofibrils Densely packed, rodlike elements ~80% of cell volume
Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands
Contain the contractile elements of skeletal muscle

10. Sarcomere
Smallest contractile unit (functional unit) of a muscle fiber
The region of a myofibril between two successive Z discs
Composed of thick and thin myofilaments made of contractile proteins

11. Features of a Sarcomere
Thick filaments (myosin): run the entire length of an A band
Thin filaments (actin): run the length of the I band and partway into the A band
Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another
H zone: lighter midregion where filaments do not overlap
M line: line of protein myomesin that holds adjacent thick filaments together

12. Skeletal Muscle Fiber

13. Ultrastructure of Thick Filament
Composed of the protein myosin
Myosin tails contain:
2 interwoven chains
Myosin heads contain:
2 smaller chains that act as cross bridges during contraction
Link the thick and thin filaments together
Binding sites for ATP
ATPase enzymes-split ATP to generate energy
Each thick filament is about 300 myosin molecules bundled together with the tails forming the central part of the thick filament

14. Ultrastructure of Thin Filament
Composed of actin
Actin bears active sites for myosin head attachment during contraction
Tropomyosin and troponin: regulatory proteins bound to actin
Both help control the myosin-actin interactions involved in contractions

15. Thick (myosin) and Thin (actin) Filaments

16. Sarcoplasmic Reticulum (SR)
Network of smooth endoplasmic reticulum surrounding each myofibril
Pairs of terminal cisternae form perpendicular cross channels
Functions in the regulation of intracellular Ca2+ levels
Release Ca2+ when muscle contracts

17. Continuous with the sarcolemma
T Tubules
Continuous with the sarcolemma
Penetrate the cell’s interior at each A band–I band junction
Associate with the paired terminal cisternae to form triads that encircle each sarcomere
Increase the muscle’s surface area
Aid in conducting impulses to the deepest parts of the muscle and to every sarcomere

18. Contraction
The generation of force
Does not necessarily cause shortening of the fiber
Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening

19. Sliding Filament Model of Contraction
In the relaxed state, thin and thick filaments overlap only slightly
During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line
As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

20. 1 2 Figure 9.6 Z H Z I A I Fully relaxed sarcomere of a muscle fiber Z
Fully contracted sarcomere of a muscle fiber
Figure 9.6

21. Requirements for Skeletal Muscle Contraction
Activation: neural stimulation at a neuromuscular junction
Excitation-contraction coupling:
Generation and propagation of an action potential along the sarcolemma
Final trigger: a brief rise in intracellular Ca2+ levels

22. Events at the Neuromuscular Junction
Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles
Each axon forms several branches as it enters a muscle
Each axon ending forms a neuromuscular junction with a single muscle fiber

23. Events at the Neuromuscular Junction
Nerve impulse arrives at axon terminal
ACh is released and binds with receptors on the sarcolemma
Electrical events lead to the generation of an action potential
Situated midway along the length of a muscle fiber
Muscle fiber and axon terminal (nerve ending) seperated by space – synaptic cleft
Synaptic vesicles of axon terminal contain the neurotransmitter acetylcholine (ACh)
Junctional folds of the sarcolemma contain ACh receptors

24. Destruction of Acetylcholine
ACh effects are quickly terminated by the enzyme acetylcholinesterase
Prevents continued muscle fiber contraction in the absence of additional stimulation
Myasthenia gravis – shortage of Ach receptors; autoimmune disease
In people with myasthenia gravis the body produces antibodies that attack acetylcholine receptors on the muscle, preventing the signal from reaching the muscle properly and so weakening them.

25. Events in Generation of an Action Potential
Local depolarization (end plate potential)
Generation and propagation of an action potential
Repolarization
Remember the resting membrane potential is -70 mV
Local depolarization (end plate potential):
ACh binding opens chemically (ligand) gated ion channels
Simultaneous diffusion of Na+ (inward) and K+ (outward)
More Na+ diffuses, so the interior of the sarcolemma becomes less negative
Local depolarization – end plate potential
Generation and propagation of an action potential:
End plate potential spreads to adjacent membrane areas
Voltage-gated Na+ channels open
Na+ influx decreases the membrane voltage toward a critical threshold
If threshold is reached, an action potential is generated (propogated)
Repolarization:
Na+ channels close and voltage-gated K+ channels open
K+ efflux rapidly restores the resting polarity
Fiber cannot be stimulated and is in a refractory period until repolarization is complete
Ionic conditions of the resting state are restored by the Na+-K+ pump

26. Na+ channels close, K+ channels open Depolarization due to Na+ entry
Repolarization
due to K+ exit
Na+
channels
open
Threshold
K+ channels
close
Figure 9.10

27. Excitation-Contraction (E-C) Coupling
Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments
Latent period:
Time when E-C coupling events occur
Time between AP initiation and the beginning of contraction
The action potential is short and gone long before any sign of contraction has taken place.

28. Events of Excitation-Contraction (E-C) Coupling
AP is propagated along sarcomere to T tubules
Voltage-sensitive proteins stimulate Ca2+ release from SR
Ca2+ is necessary for contraction

29. Role of Calcium (Ca2+) in Contraction
At low intracellular Ca2+ concentration:
Tropomyosin blocks the active sites on actin
Myosin heads cannot attach to actin
Muscle fiber relaxes

30. Role of Calcium (Ca2+) in Contraction
At higher intracellular Ca2+ concentrations:
Ca2+ binds to troponin
Troponin changes shape and moves tropomyosin away from active sites
Events of the cross bridge cycle occur
When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends

31. Cross Bridge Cycle
Continues as long as the Ca2+ signal and adequate ATP are present
Cross bridge formation—high-energy myosin head attaches to thin filament
Working (power) stroke—myosin head pivots and pulls thin filament toward M line
ATP is needed for the movement of the myosin cross bridge – new ATP is needed after each stroke

32. Cross Bridge Cycle Rigor mortis Dying cells can not remove calcium
This promotes myosin cross bridging
After breathing stops, ATP synthesis stops but it is still used
Cross bridging detachment is impossible
Only thing that stops it is muscle protein breakdown
Rigor begins to settle in after 3-4 hours of death, peaks at 12 hours and then dissipates over the next hours

33. Review Principles of Muscle Mechanics
Same principles apply to contraction of a single fiber and a whole muscle
Contraction produces tension, the force exerted on the load or object to be moved

34. Review Principles of Muscle Mechanics
Contraction does not always shorten a muscle:
Isometric contraction: no shortening; muscle tension increases but does not exceed the load
Isotonic contraction: muscle shortens because muscle tension exceeds the load
Force and duration of contraction vary in response to stimuli of different frequencies and intensities

35. Motor Unit: The Nerve-Muscle Functional Unit
Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies

36. Motor Unit
Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle
Motor units in a muscle usually contract asynchronously; helps prevent fatigue
Asynchronously means not all at once

37. Graded Muscle Responses
Variations in the degree of muscle contraction
Required for proper control of skeletal movement
Responses are graded by:
Changing the frequency of stimulation
Changing the strength of the stimulus
Frequency
A single stimulus results in a single contractile response—a muscle twitch
Increase frequency of stimulus (muscle does not have time to completely relax between stimuli)
Ca2+ release stimulates further contraction  temporal (wave) summation
Further increase in stimulus frequency  unfused (incomplete) tetanus

38. Response to Change in Stimulus Frequency
If stimuli are given quickly enough, fused (complete) tetany results

39. Response to Change in Stimulus Strength
Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs
Muscle contracts more vigorously as stimulus strength is increased above threshold
Motor unit summation – the more motor units recruited, the stronger the contraction

40. Isotonic Contractions
Muscle changes in length and moves the load
Isotonic contractions are either concentric or eccentric:
Concentric contractions—the muscle shortens and does work
Eccentric contractions—the muscle contracts as it lengthens

41. Muscle Metabolism: Energy for Contraction
ATP is the only source used directly for contractile activities
Supplies the energy needed cross bridge movement
Also operates the calcium pump
Available stores of ATP are depleted in 4–6 seconds

42. Muscle Metabolism: Energy for Contraction
ATP is regenerated by:
Direct phosphorylation of ADP by creatine phosphate (CP)
Anaerobic pathway (glycolysis)
Aerobic respiration
Anaerobic metabolism
At 70% of maximum contractile activity:
Bulging muscles compress blood vessels
Oxygen delivery is impaired
Pyruvic acid is converted into lactic acid
Lactic acid:
Diffuses into the bloodstream
Used as fuel by the liver, kidneys, and heart
Converted back into pyruvic acid by the liver
Aerobic Metabolism
Produces 95% of ATP during rest and light to moderate exercise
Fuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids

43. Physiological inability to contract Occurs when:
Muscle Fatigue
Physiological inability to contract
Occurs when:
Ionic imbalances (K+, Ca2+, Pi) interfere with E-C coupling
Prolonged exercise damages the SR and interferes with Ca2+ regulation and release
Total depletion of ATP rarely occurs
If it does, contractures occur and cross bridging detachment can not occur
E-C excitation contraction

44. Extra O2 needed after exercise for: Replenishment of
Oxygen Deficit
Extra O2 needed after exercise for:
Replenishment of
Oxygen reserves
Glycogen stores must be replenished
ATP and CP reserves must be resynthesized
Conversion of lactic acid to pyruvic acid, glucose, and glycogen
Conversion of lactic acid in the blood to glucose or glycogen takes place in the liver

45. Heat Production During Muscle Activity
~ 40% of the energy released in muscle activity is useful as work
Remaining energy (60%) given off as heat
Dangerous heat levels are prevented by radiation of heat from the skin and sweating

46. Force of Muscle Contraction
The force of contraction is affected by:
Number of muscle fibers stimulated (recruitment)
Relative size of the fibers—hypertrophy of cells increases strength
Frequency of stimulation
Length-tension relationship
Number
More fibers stimulated, greater the force of contraction
Hypertrophy
The larger the fiber, the greater the force
Frequency
 frequency allows time for more effective transfer of tension to noncontractile components
Length-tension relationship—muscles contract most strongly when muscle fibers are 80–120% of their normal resting length

47. Velocity and Duration of Contraction
Influenced by:
Muscle fiber type
Load
Recruitment
Muscle vary in how fast and how long the can contract before fatigue

48. Classified according to two characteristics:
Muscle Fiber Type
Classified according to two characteristics:
Speed of contraction: slow twitch or fast twitch, according to:
Speed at which myosin ATPases split ATP
Pattern of electrical activity of the motor neurons
Duration of contraction in these fibers can vary also based on how quickly Ca2+ is moved from the cytosol to SR

49. Metabolic pathways for ATP synthesis:
Muscle Fiber Type
Metabolic pathways for ATP synthesis:
Oxidative (slow) fibers—use aerobic pathways
Glycolytic (fast) fibers—use anaerobic glycolysis

50. Muscle Fiber Type Three types: Slow oxidative fibers
Fast oxidative fibers
Fast glycolytic fibers

51. Effects of Exercise Aerobic (endurance) exercise: Leads to increased:
Muscle capillaries
Number of mitochondria
Myoglobin synthesis
Results in greater endurance, strength, and resistance to fatigue
May convert fast glycolytic fibers into fast oxidative fibers

52. Effects of Resistance Exercise
Resistance exercise (typically anaerobic) results in:
Muscle hypertrophy (due to increase in fiber size)
Increased mitochondria, myofilaments, glycogen stores, and connective tissue

53. The Overload Principle
Forcing a muscle to work hard promotes increased muscle strength and endurance
Muscles adapt to increased demands
Muscles must be overloaded to produce further gains