1. Misericordia University
Muscular System
General Physiology
Tony Serino, Ph.D.
Biology Dept.
Misericordia University

2. Muscular System Types of Muscle Cells Functions: Attributes:
Movement –generation of force and/or shortening
Maintenance of posture
Joint stabilization
Heat Generation
Attributes:
contractility, irritability, extensibility, and elasticity
Skeletal Muscle –voluntary, striated
Types of Muscle Cells
Cardiac Muscle –involuntary, striated
Smooth Muscle –involuntary, no striations

3. Antagonistic Muscle Arrangement
This arrangement plus the series elastic component allows the muscle to return to its original length.

4. Skeletal Muscle Cells Long, cylindrical, non-branching, multinucleated
mcm wide and up to 35 cm long
Voluntary, no spontaneous depolarization normally
Contractile proteins (myosin & actin) arranged in bundles called myofibrils
Each myofibril consists of overlapping thick and thin filaments arranged in units called sarcomeres.

5. H Band
M Line
Z Line
Myofibril Anatomy
Cross Section

6. Thick Filament Structure

7. Thin Filament Structure: Twisted bead chain of actin proteins

8. Calcium is Trigger Ion

9. Neuronal AP triggers release of ACh at neuromuscular junction (motor end plate).
ACh bind to the nicotinic receptor and triggers a MEPP

10. The MEPP triggers an AP that races along the sarcolemma and down the T-tubules. The depolarization affects the SR cisternae which releases Ca++ into the cytoplasm.
The rise of intracellular Ca++ triggers the mechanical events of contraction.

11. Muscle Cell Contraction (Excitation-Contraction Coupling)
A motor neuron is stimulated to fire an AP
AP reaches synaptic terminal triggering an influx of Ca++
The Ca++ stimulates the release of Ach
ACh diffuses across cleft and binds to nicotinic receptors in motor end plate. This causes Na+ channels to open; causing the generation of a MEPP
The MEPP triggers an AP along sarcolemma and into T-tubules
This depolarizes the SR cisternae which releases stored Ca++ into the cytoplasm

12. Muscle Cell AP Propagation
Action
potential
Sarcolemma
T tubule
Sarcomere
in myofibril
An action potential produced at the neuromuscular junction is propagated along the sarcolemma of the skeletal muscle, causing a depolarization to spread along the membrane of the T tubules.

13. AP link to Ca2+ Release
Ca2+
Sarcoplasmic
reticulum
T tubule
Sarcomere
in myofibril
The depolarization of the T tubule causes DHP receptors to open ryandine receptors that are linked to Ca2+ channels in the SR cisternae
to open, resulting in an increase in the permeability of the sarcoplasmic
reticulum to Ca2+. Ca2+ then diffuses from the SR into the sarcoplasm.

14. Summary
Ryanodine Receptors also known as “Foot” receptors.
09.12.jpg

15. Calcium Binds to TnC
Tropomyosin
Troponin
Ca2+ binds
to troponin
G actin
molecule
Action
potential
Sarcolemma
Sarcoplasmic
reticulum
Actin
myofilament
Myosin
T tubule
Ca2+
Sarcomere
in myofibril
Ca2+ released from the SR binds to troponin molecules in the actin myofilament.

16. Muscle Contraction: Mechanical Events (Sliding Filaments)
Calcium ions from SR flood the myofibrils
This causes the thick and thin filaments to bind to each other (generates tension) and may cause them to slide past each other
This causes the sarcomere to shorten

17. Fig
Attachment
Reset
09.08.jpg
Power Stroke
Detachment

18. Muscle Contraction Review

19. Fig
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20. Muscles are arranged as Motor Units
Motor Unit = 1 motor neuron + all the muscle fibers it controls (innervates)
The size of the motor unit depends on the degree of control needed in that particular whole muscle.

21. Isometric Contraction

22. Isotonic Contraction

23. Biomechanics of Force Production
Tension = force exerted on an object by a muscle
Load = force exerted on muscle by the weight of an object
Twitch = the mechanical response of a muscle to an AP
Types of Contractions:
Isometric = muscle increases tension without shortening
Isotonic = muscle shortens with no further increase in tension
Load
Tension
Bicep
Fulcrum (pivot point)
Weight of arm + object

24. Fig
At equilibrium, (load)(length of load arm)= (effort)(length of effort arm)
09.29.jpg
Biceps operates under mechanical disadvantage.

25. Fig
Mechanical disadvantage produces great amplification of muscle velocity.
09.30.jpg

26. Single Muscle Twitch

27. Treppe –an increase in tension development with no summation present; due to enzyme warming, increase blood flow, more Ca2+ availability, etc.
treppe

28. Factors Affecting Muscle Fiber Performance
Load –affects velocity of contraction
Increasing load  decreases velocity
Frequency of stimulation
Initial Length of muscle fiber
Fatigue
Type of muscle fiber –fibers differ in strength, size, ATP splitting rate, and resistance to fatigue

29. Load Effect on Degree and Duration of Contraction

30. Load vs. Velocity of Contraction

31. Factors Affecting Muscle Fiber Performance
Load –affects velocity of contraction
Increasing load  decreases velocity
Frequency of stimulation
Initial Length of muscle fiber
Fatigue
Type of muscle fiber –fibers differ in strength, size, ATP splitting rate, and resistance to fatigue

32. Mechanical (Wave) Summation
Increase frequency of stimulation allows tension to add to previous contraction’s tension

33. Factors Affecting Muscle Fiber Performance
Load –affects velocity of contraction
Increasing load  decreases velocity
Frequency of stimulation
Initial Length of muscle fiber
Type of muscle fiber –fibers differ in strength, size, ATP splitting rate, and resistance to fatigue

34. Initial Length of Muscle Fiber: affects the maximum tension that can be developed due to degree of overlap between thick and thin filaments

35. Factors Affecting Muscle Fiber Performance
Load –affects velocity of contraction
Increasing load  decreases velocity
Frequency of stimulation
Initial Length of muscle fiber
Fatigue
Type of muscle fiber –fibers differ in strength, size, ATP splitting rate, and resistance to fatigue

36. Fatigue –inability to maintain contraction tension even while being stimulated. Two kinds:
Primary Fatigue –due to accumulation of lactic acid in sarcoplasm, this changes the cytoplasm pH and begins to change protein configurations which ends contraction.
Secondary Fatigue –related to the loss of energy reserves in the body, as seen in day after soreness. Why this triggers a low intensity pain signal (a dull ache) is unknown.

37. Fatigue and Recovery
09.23.jpg

38. Factors Affecting Muscle Fiber Performance
Load –affects velocity of contraction
Increasing load  decreases velocity
Frequency of stimulation
Initial Length of muscle fiber
Fatigue
Type of muscle fiber –fibers differ in strength, size, ATP splitting rate, and resistance to fatigue

39. Types of Muscle Fiber: each motor unit consists of only one type of muscle fiber
Slow twitch, red (oxidative) fibers (SO) –small diameter, weakest, slow ATPase, much myoglobin and mitochondria, abundant blood supply, fatigue resistant
Fast twitch, red (oxidative) fibers (FO) –medium diameter, moderate strength, fast ATPase, abundant mitochondria and myoglobin, good blood supply, moderate fatigue resistance
Fast twitch, white (glycolytic) fibers (FG) –largest diameter, great strength, fast ATPase, low amount of myoglobin or mitochondria, decreased blood supply, high in glycolytic enzymes, tire quickly

40. Muscle Fiber Fatigue Resistance
09.25.jpg

41. All muscles are blends of all fiber types
09.26.jpg

42. Fig
09.03.jpg

43. Control of Whole Muscle Tension dependent on:
Tension developed by each fiber
Dependent on fiber type, initial length and degree of wave summation, fatigue
Amount of fibers stimulated to contract
The number of motor units responding is directly related to amount of tension produced
If the body needs more power, it recruits more motor units to respond
Known as recruitment (motor unit summation)
Size of motor units

44. Fig
09.01.jpg

45. Energy Use: stored ATP in muscle used quickly so re-supply is crucial to function
Creatine Phosphate –quick re-supply, allowing time for aerobic respiration to gear up
Aerobic Respiration –oxidative phosphorylation dependent on adequate blood supply of oxygen, uses different sources for energy:
Stored glycogen
Glucose and fatty acids from blood
Fatty acids from blood
Anaerobic Respiration -becomes dominant as need for oxygen exceeds ability of blood to transport it into muscles
After exercise, energy continues to be consumed at increased levels to re-build reserves, etc., this is the oxygen debt incurred during the exercise

46. Creatine Phosphate Cycle
09.22.jpg

47. Anaerobic Threshold

48. Lactic Acid Cycle

49. Cardiac Muscle
Striated, single nucleus, branched cells, connected together by intercalated discs (with many gap junctions)
Spontaneously contracts, needs no innervation, involuntary
-source of cytoplasmic calcium is SR and extracellular fluid

50. Smooth Muscle
No sarcomeres, therefore, no striations, single nucleated, small spindle shaped cells
Spontaneously contracts, involuntary control, can remain contracted for long periods of time without fatiguing
Two types: Visceral (single unit) –united by gap junctions Multi-unit –needs innervations, behaves like skeletal muscle (Ex. Iris)

51. Fig
09.33.jpg

52. Visceral vs. Multi-unit
09.37.jpg
Visceral

53. 09.34.jpg

54. Fig
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55. Fig
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56. Fig
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