1. Histology and Physiology of Muscles
Chapter 8
Histology and Physiology of Muscles
Skeletal Muscle Fibers
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2. Functions of the Muscular System
Body movement (Skeletal)
Maintenance of posture (Skeletal)
Respiration (Skeletal)
Production of body heat (Skeletal)
Communication (Skeletal)
Constriction of organs and vessels (Smooth)
Heartbeat (Cardiac)

3. Functional Characteristics of Muscle
Contractility:
ability to shorten forcibly
Excitability:
ability to receive and respond to stimuli
Extensibility:
ability to be stretched or extended
Elasticity:
ability to recoil and resume original resting length

4. 3 Types of Muscle Tissue skeletal, smooth, cardiac
These types differ in
Structure
Location
Function
Means of activation
Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue

5. Types of Muscle Tissue Skeletal muscles Smooth muscle Cardiac muscle
responsible for most body movements
Maintain posture, stabilize joints, and generate heat
Smooth muscle
found in walls of hollow organs and tubes; moves substances through them
Helps maintain blood pressure
Squeezes or propels substances (i.e., food, feces) through organs
Cardiac muscle
found in heart; pumps blood throughout body

6. Tab. 8.1

7. Skeletal Muscle Structure
Cells elongated, often called skeletal muscle fibers
Each cell contains several nuclei located around periphery of fiber near plasma membrane
Fibers appear striated due to actin and myosin myofilaments
A single fiber can extend from one end of a muscle to the other!!!
Contracts rapidly but tires easily
controlled voluntarily (i.e., by conscious control)

8. Skeletal Muscle Structure
Fascia: general term for connective tissue sheets
3 muscular fascia separate and compartmentalize individual muscles or groups of muscles
Epimysium: overcoat of dense collagenous connective tissue that surrounds entire muscle
Perimysium: fibrous connective tissue that surrounds groups of muscle fibers called fascicles (bundles)
Endomysium: fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber

9. Skeletal Muscle Structure
connective tissue provides a pathway for blood vessels and nerves to reach muscle fibers
Fig. 8.1

10. Skeletal Muscle Structure
Muscle connective tissue blends with other connective tissue based structures, such as tendons
connect muscle to bone
Fig. 8.2

11. Skeletal Muscle Structure
Muscle Fibers
Sarcolemma: muscle cell plasma membrane
Sarcoplasm: cytoplasm of a muscle cell
Myo, mys, and sarco: prefixes used to refer to muscle
Muscle contraction depends on 2 kinds of myofilaments: actin and myosin
Myofibrils - densely packed, rod-like contractile elements
make up most of muscle volume

12. Fig. 8.2

13. Skeletal Muscle Structure: ACTIN
(thin) myofilaments
2 helical polymer strands of F actin (composed of G actin), tropomyosin, and troponin
G actin contains active sites to which myosin heads attach during contraction
Tropomyosin and troponin are regulatory subunits bound to actin
Fig. 8.2

14. Skeletal Muscle Structure: MYOSIN
(thick) myofilaments
consist of myosin molecules
Each myosin molecule has
A head with an ATPase, which breaks down ATP
A hinge region, which enables head to move
A rod
A cross-bridge is formed when a myosin head binds to the active site on G actin
Fig. 8.2

15. Skeletal Muscle Structure
Sarcomeres
smallest contractile unit of a muscle
bound by Z disks that hold actin myofilaments
6 actin myofilaments surround a myosin myofilament
Myofibrils appear striated because of A bands and I bands
Fig. 8.2

16. Skeletal Muscle Structure
Thick filaments: extend the entire length of an A band
Thin filaments: extend across the I band and partway into A band
Z-disc: coin-shaped sheet of proteins (connectins) that anchors thin filaments and connects myofibrils to one another
Thin filaments do not overlap thick filaments in lighter H zone
M lines appear darker due to presence of protein desmin
arrangement of myofibrils within a fiber is so organized a perfectly aligned repeating series of dark A bands and light I bands is evident

17. Fig. 8.3bc

18. Sliding Filament Model Summary
Actin and myosin myofilaments do not change in length during contraction
Thin filaments slide past thick ones so actin and myosin filaments overlap to a greater degree
Upon stimulation, myosin heads bind to actin and sliding begins
Each myosin head binds and detaches several times during contraction (acting like a ratchet to generate tension and propel the thin filaments to the center of sarcomere)
In relaxed state, thin and thick filaments overlap only slightly
As this event occurs throughout sarcomeres, muscle shortens
I band and H zones become narrower during contraction, and A band remains constant in length

19. Fig. 8.4

20. Sliding Filament Model
Actin and myosin myofilaments in a relaxed muscle (below) and a contracted muscle are the same length.
Myofilaments do not change length during muscle contraction!
Fig. 8.4

21. Sliding Filament Model
During contraction, actin myofilaments at each end of the sarcomere slide past the myosin myofilaments toward each other. As a result, the Z disks are brought closer together, and the sarcomere shortens
Fig. 8.4

22. Sliding Filament Model
As the actin myofilaments slide over the myosin myofilaments, H zones (yellow) and I bands (blue) narrow.
A bands, equal to length of myosin myofilaments, do not narrow because length of myosin myofilaments does not change
Fig. 8.4

23. Sliding Filament Model
In a fully contracted muscle, ends of actin myofilaments overlap at center of the sarcomere and H zone disappears
Fig. 8.4

24. Physiology of Skeletal Muscle Fibers
Membrane Potentials!!!
nervous system stimulates muscles to contract through electric signals called action potentials
Plasma membranes are polarized, which means there is a charge difference (resting membrane potential) across plasma membrane
inside of plasma membrane is negative as compared to the outside in a resting cell
action potential: reversal of resting membrane potential so that inside of plasma membrane becomes positive

25. Physiology Ion Channels Assist with production of action potentials
Ligand-gated channels
Voltage-gated channels
Fig. 8.5

26. Fig. 8.6

27. Physiology of Skeletal Muscle Fibers
Action Potentials
Depolarization results from an increase in the permeability of the plasma membrane to Na+
If depolarization reaches threshold, an action potential is produced
The depolarization phase of the action potential results from the opening of many Na+ channels
Fig. 8.6

28. Physiology of Skeletal Muscle Fibers
Action Potentials
The repolarization phase of the action potential occurs when the Na+ channels close and K+ channels open briefly
Fig. 8.6

29. Fig. 8.6

30. Physiology of Skeletal Muscle Fibers
Action Potentials
Occur in an all-or-none fashion
stimulus below threshold produces no action potential
stimulus at threshold or stronger will produce an action potential
Propagate (travel) across plasma membranes

31. Physiology of Skeletal Muscle Fibers
Nerve Stimulus of Skeletal Muscle
Skeletal muscles are stimulated by motor neurons of the somatic nervous system
Axons of these neurons travel in nerves to muscle cells
Axons of motor neurons branch profusely as they enter muscles
Each axonal branch forms a neuromuscular junction with a single muscle fiber

32. Physiology of Skeletal Muscle Fibers
neuromuscular junction is formed from:
Axonal endings
small membranous sacs (synaptic vesicles)
Contain neurotransmitter acetylcholine (ACh)
Motor end plate of a muscle
Specific part of the sarcolemma
Contains ACh receptors
Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

33. Fig. 8.7

34. Neuromuscular Junction Physiology
action potential (orange arrow) arrives at presynaptic terminal and causes voltage-gated Ca2+ channels in the presynaptic membrane to open
Calcium ions enter the presynaptic terminal and initiate the release of the neurotransmitter acetylcholine (ACh) from synaptic vesicles
ACh is released into the synaptic cleft by exocytosis
Fig. 8.8

35. Neuromuscular Junction Physiology
ACh diffuses across the synaptic cleft and binds to ligand-gated Na+ channels on the postsynaptic membrane
Ligand-gated Na+ channels open and Na+ enters the postsynaptic cell, causing the postsynaptic membrane to depolarize. If depolarization passes threshold, an action potential is generated along the postsynaptic membrane
ACh is removed from the ligand-gated Na+ channels, which then close
Fig. 8.8

36. Neuromuscular Junction Physiology
enzyme acetylcholinesterase, which is attached to postsynaptic membrane, removes acetylcholine from synaptic cleft by breaking it down into acetic acid and choline
Choline is symported with Na+ into presynaptic terminal, where it can be recycled to make ACh. Acetic acid diffuses away from synaptic cleft
ACh is reformed within presynaptic terminal using acetic acid generated from metabolism and choline recycled from synaptic cleft. ACh is then taken up by synaptic vesicles
Fig. 8.8

37. Fig. 8.8

38. Excitation-Contraction Coupling
In order to contract, a skeletal muscle must:
Be stimulated by a nerve ending
Propagate an electrical current, or action potential, along its sarcolemma
Have a rise in intracellular Ca2+ levels, the final trigger for contraction
Linking the electrical signal to the contraction is excitation-contraction coupling

39. Excitation-Contraction Coupling
Invaginations of sarcolemma form T tubules, which wrap around sarcomeres and penetrate into cell’s interior at each A band –I band junction
Sarcoplasmic reticulum (SR) is an elaborate, smooth endoplasmic reticulum that mostly runs longitudinal and surrounds each myofibril
Paired terminal cisternae form perpendicular cross channels
Functions in regulation of intracellular calcium levels
A triad is a T tubule and two terminal cisternae
Fig. 8.9

40. Excitation-Contraction Coupling
An action potential produced at presynaptic terminal in the neuromuscular junction is propagated along sarcolemma of the skeletal muscle. depolarization also spreads along membrane of T tubules
depolarization of T tubule causes gated Ca2+ channels in SR to open, resulting in an increase in permeability of SR to Ca2+, especially in terminal cisternae. Calcium ions then diffuse from SR into sarcoplasm
Calcium ions released from SR bind to troponin molecules. troponin molecules bound to G actin molecules are released, causing tropomyosin to move, exposing active sites on G actin
Once active sites on G actin molecules are exposed, heads of myosin myofilaments bind to them to form cross-bridges

41. Fig. 8.11

42. Fig. 8.11

43. Fig. 8.11

44. Fig. 8.11

45. Fig. 8.11

46. Fig. 8.11