1. Muscular System Comparative Anatomy Tony Serino, Ph.D. Biology Dept.
Misericordia Univ.

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. Whole Muscle Structure

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

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

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

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

10. Unique Muscle Cell Structures
Sarcomere

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

12. Myofibril Structure

13. Each skeletal muscle cell must be innervated by a motor neuron to begin contracting.
A motor neuron plus all the muscle cells it innervates is a motor unit.

14. Motor End Plate
Neuronal AP triggers release of ACh at neuromuscular junction (motor end plate).
ACh bind to the nicotinic receptor and triggers a depolarization of sarcolemma which spreads across entire cell and down the T-tubules.

15. The depolarization affects the SR cisternae which releases Ca++ into the cytoplasm.
The rise of intracellular Ca++ triggers the mechanical events of contraction.

16. Muscle Cell Contraction (Excitation-Contraction Coupling)
A motor neuron is stimulated to fire an AP
It releases ACh at the motor end plate
ACh diffuses across cleft and binds to nicotinic receptors in motor end plate
This causes a depolarization of the sarcolemma and T-tubules
This deplorarizes the SR cisternae which releases stored Ca++ into the cytoplasm

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

18. Overlapping Arrangement

19. Thick Filament Structure

20. Thin filament

21. Calcium binding rolls tropomyosin

22. Accessory Proteins
Titin –large elastic protein, forming elastic lattice that attach to z-line and align thick filaments with respect to thin filaments α-actinin –anchoring protein of z-line Nebulin –helps to anchor thin filaments to z-line Tropomodulin –caps thin filament and regulates filament length Desmin- (not shown) form lattice that surrounds sarcomere at level of z-lines attaching them to other myofibrils and sarcolemma Myomesin and C-protein -attaches and aligns individual myosin molecules to one another Dystrophin –is a cell membrane associated protein which helps to connect the cytoskeleton to the extracellular matrix

23. Detachment

24. Detachment
Reset: energize myosin head

25. Detachment
Reset
Attachment

26. Detachment
Reset
Power Stroke
Attachment

27. Muscle Contraction: Review

28. 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.

29. 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

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

31. 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)

32. Electric organs
Electrocytes are modified skeletal muscle cells arraned in columns. Each column of cells are connected in series thus multiplying the voltage created, some eels can generate more than 600V.