1. Anatomy and Physiology
Chapter 12:
Physiology of the Muscular System

2. General Functions Movement of the body and its parts Heat production
Posture 2

3. Function of Skeletal Muscle Tissue
Characteristics of skeletal muscle cells
Excitability (irritability)—ability to be stimulated
Contractility—to contract/shorten, & produce movement
Extensibility—to extend/stretch, to return to their resting length 3

4. Function of Skeletal Muscle Tissue
Muscle cells are called fibers because of their threadlike shape
Contain many mitochondria and several nuclei
Sarcomere
Each myofibril consists of many sarcomeres
Segment of myofibril between two successive Z disks
Contractile unit of muscle fibers 4

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6. Function of Skeletal Muscle Tissue
Sarcolemma—plasma membrane of muscle fibers
Sarcoplasmic reticulum (SR)
T tubules—network of tubules and sacs found within muscle fibers
Continually pumps calcium and stores the ions for later release 6

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9. Function of Skeletal Muscle Tissue
T tubules
Transverse tubules extend across the sarcoplasm
Membrane has ion pumps that continually transport Ca++ ions
Allow electrical impulses 9

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11. Myofilaments
Each myofibril contains thousands of thick and thin myofilaments
4 protein molecules make up myofilaments
Myosin
thick filament
Myosin “heads” attracted to actin
Form cross bridges when attached to actin 11

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13. 4 different kinds of protein cont..
2.) Actin—protein that forms thin filament
3.) Tropomyosin—protein that blocks the active sites on actin molecules
4.) Troponin—protein that holds tropomyosin molecules in place 13

14. Mechanism of contraction
Excitation and contraction
Skeletal muscle fiber remains at rest
Stimulated by a motor neuron
Neuromuscular junction(NMJ)—motor neurons connect to the sarcolemma at the motor endplate
“NMJ” is a synapse where neurotransmitter transmit signals 14

15. Excitation and contraction (cont)
Acetylcholine: neurotransmitter released into the synaptic cleft
Stimulates the receptors
Initiates an impulse in sarcolemma
Triggers the release of calcium ions
Calcium binds to troponin, which expose active sites on actin 15

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22. Sliding filament model/theory
Active sites on actin are exposed, myosin heads bind to them
Myosin heads bend and “pull” the thin filaments past them
Each head releases, binds to the next active site, and pulls again
The entire myofibril shortens 22

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24. Whole Muscle Contraction
A single muscle fiber has an “all or nothing” response
But a whole muscle can vary its force of contraction.
Two characteristics of a whole muscle allow this
Motor unit
Recruitment
This slide shifts the focus from the individual muscle cell to the whole muscle, which is comprised of muscle fibers.
When an individual muscle cell contracts, it must contract maximally, but a whole muscle can vary its strength of contraction.
Motor unit and recruitment are mechanisms that allow the force of contraction in the whole muscle to vary. Each concept is treated in more detail in future slides.

25. Function of Skeletal Muscle Organs
Motor unit
Motor unit= motor neuron + the muscle fibers it attaches
Could be a couple of fibers or an entire fascicle
Recruitment:
Smaller the number of fibers in a motor unit, the more precise are the available movements
Larger the number of fibers in a motor unit, the more powerful the contraction available 25

26. Energy sources for muscle contraction
ATP yields the energy required for muscular contraction
Muscle fibers resynthesize ATP from the breakdown of creatine phosphate (CP)
Catabolic pathways
Aerobic pathway
Presence of O2 from blood
Slower than anaerobic pathway
Supplies energy for the long term rather than the short term 26

27. Catabolic pathways (cont)
Anaerobic pathway
Little to no O2 is available
Very rapid, providing energy during first minutes of maximal exercise
Formation of lactic acid
Requires oxygen to convert back to glucose,
Soreness
producing of an “oxygen debt”
excess post-exercise oxygen consumption (EPOC) 27

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30. Responses of a Whole Muscle
Twitch: Single muscle response in which muscle contracts and then fully relaxes
Tetanus: Sustained muscle contraction caused by repeated stimulation
Smooth, sustained contraction
Tonus: Normal, continuous state of partial muscle contraction
Twitch is unimportant physiologically.
Tetanus, not to be confused with the disease, is essential to maintain posture or any other activity that involves a sustained muscle contraction.
In tonus, different groups of muscle fibers within a muscle take turns contracting while others relax.
Smooth muscle tonus in the blood vessels maintains blood pressure.

31. Skeletal Muscle Fiber Types
Speeds of response depends on which fibers are stimulated:
Type I: slow-twitch (weakest force, longest time)
Type II: fast-twitch (2 divisions)
type IIa: intermediate speed
type IIb: fastest (greatest force, shortest time)

32. Isotonic vs. Isometric contractions
Isotonic contraction: “same tension”
Contraction where tone/tension of muscle is the same as the length changes
Concentric—muscle shortens as it contracts
Eccentric—muscle lengthens while contracting 32

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34. Isometric contraction
Isometric: “same length”
muscle length stays the same while muscle tension increases
Most body movements occur as a result of both types of contractions 34