1. Muscular System Anatomy & Physiology II Tony Serino, Ph.D.
Biology Dept.
Misericordia University

2. Muscular System Functions: Attributes:
Movement –generation of force and/or shortening
Maintenance of posture
Joint stabilization
Heat Generation
Attributes:
contractility, irritability, extensibility, and elasticity

3. Types of Muscle Cells Skeletal Muscle –voluntary, striated
Cardiac Muscle –involuntary, striated
Smooth Muscle –involuntary, no striations

4. Muscles wrapped with CT, that is continuous with tendon and periosteum

5. The elasticity of the CT sheaths, tendon and the muscle cells = the Series Elastic Component

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

7. Develop as a fusion of myoblasts, which accounts for multinucleated cells, extra myoblasts remain as satellite cells.

8. 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

9. Unique Muscle Cell Structures
Sarcomere

10. Each skeletal muscle cell must be innervated by a motor neuron to begin contracting.

11. Neuronal AP triggers release of ACh at neuromuscular junction (motor end plate).

12. ACh is released and diffuses across gap

13. ACh bind to the nicotinic receptor and triggers a MEPP

14. 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.

15. 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 deplorarizes the SR cisternae which releases stored Ca++ into the cytoplasm

16. Each myofibril consists of overlapping thick and thin filaments arranged in units called sarcomeres.

17. 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

18. H Band
Myofibril Anatomy
M Line
Z Line
(Cross section)

19. Overlapping Tick and Thin Filaments

20. Thick Filament Structure

21. Thin Filament Structure: Twisted bead chain of actin proteins
Thin Filament: Actin, Tropomyosin and Troponin

22. Calcium is trigger

23. Contraction Events
Detachment

24. Contraction Events
Detachment
Reset: energize myosin head

25. Contraction Events
Detachment
Reset
Attachment

26. Contraction Events
Detachment
Reset
Attachment
Power Stroke

27. The H Band shrinks as the filaments slide past each other.

28. Muscle Contraction Review

29. 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.

30. Single Muscle Twitch

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

32. 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

33. Isotonic Contraction

34. Isometric Contraction

35. Factors Affecting Muscle Fiber Performance
Load –affects duration, degree and 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

36. Load Effect on Degree and Duration of Contraction

37. Load vs. Velocity of Contraction
As load increases, velocity decreases.

38. 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

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

40. 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

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

42. 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

43. 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

44. Control of Whole Muscle Tension dependent on:
Tension developed by each fiber
Dependent on fiber type, initial length and degree of wave summation
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)

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 part of the oxygen debt incurred during the exercise (defined as the amount of energy required to rebuild supplies used during the exercise (glycogen, creatine, proteins, etc.))

46. Anaerobic threshold Aerobic threshold exceeded when the delivery of
Oxygen is not enough to maintain aerobic metabolism. The perfusion of oxygen maximum (VO2 max) is exceeded.

47. Lactic Acid Cycle

48. 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.

49. Cardiac Muscle
Striated, single nucleus, branched cells, connected together by intercalated discs (with many gap junctions)
Spontaneously contracts, needs no innervation, involuntary

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)

52. Smooth Muscle Contraction Control
Contraction: increased cytosolic Ca++, and activation of MLCK State of contraction can be maintained without further use of ATP
Relaxation: due to decrease cytosolic Ca++, deactivation of MLCK, and activation of MLCP

53. Visceral Smooth Muscle