1. Structure of a Single Muscle Fiber

2. Skeletal Muscle Fiber Structure Key Points An individual muscle cell is called a muscle fiber A muscle fiber is enclosed by a plasma membrane called the sarcolemma The cytoplasm of a muscle fiber is called a sarcoplasm Within the sarcoplasm, the extensive T- tubules allow transport of substances throughout the muscle fiber The sarcoplasmic reticulum (SR) stores calcium

3. The Myofibril Key Points Myofibrils are made up of sarcomeres A sarcomere is composed of protein filaments of myosin and actin Interactions between the filaments is responsible for muscle contraction Myosin, the thick filament, is composed of two protein strands, each folded into a globular head at one end The thin filament is composed of actin, tropomyosin, and troponin, with one end attached to a Z-disk

4. Alpha Motor Neurons One  -motor neuron innervates many muscle fibers, collectively called the motor unit The action potential arrives at the dendrites and travels down the axon to the axon terminal

5. Events Leading to Muscle Fiber Contraction 1.A motor neuron releases acetylcholine (ACh) at the neuromuscular junction 2.ACh binds to receptors on the sarcolemma 3.If enough ACh binds to receptors, an action potential is transmitted the full length of the muscle fiber 4.The action potential triggers the release of Ca 2+ from the sarcoplasmic reticulum 5.Ca 2+ binds to troponin on the actin filament, and the troponin pulls tropomyosin off the active sites, allowing myosin heads to attach to the actin filament

6. Muscle Fiber Contraction Key Points Muscle contraction is initiated by an  -motor neuron action potential The motor neuron releases ACh, which opens up ion gates in the muscle cell membrane Sodium enters the muscle cell depolarizing the plasmalemma The action potential travels throughout the plasmalemma and through the T-tubules, which releases stored Ca 2+ ions from the sarcoplasmic reticulum (continued)

7. Muscle Fiber Contraction (continued) Key Points Ca 2+ ions bind with troponin, lifting the tropomyosin molecules off the active sites on the actin filament The myosin head binds to the active actin site The myosin head binds ATP, and ATPase on the myosin heads splits ATP into ADP and P i, releasing energy to fuel the muscle contraction The myosin head tilts, pulling the thin filament past the thick filament—power stroke Muscle contraction ends when Ca 2+ is actively pumped out of the sarcoplasm back to the sarcoplasmic reticulum

8. Muscle Biopsy The muscle biopsy allows us to study muscle fibers and the effects of acute exercise and chronic training on muscle fiber composition – The skin is first anesthetized and then a small incision is made through the skin – A hollow Bergstrom needle is inserted into the muscle belly to take the sample – The sample is mounted, frozen, thinly sliced, and examined under a microscope

9. Muscle Biopsy

10. Muscle Fiber Types Slow-twitch fibers, Type I (~50%), oxidative Fast-twitch fibers, Type II –Type IIa (25%), fast oxidative/glycolytic (FOG) –Type IIx in humans (~25%) ~ IIb in animals, fast glycolytic (FG) –Type IIc (1-3%) The percentage of each fiber type is variable among muscles, among individuals, and with exercise training

12. Single Muscle Fiber Physiology Peak power is different between muscle fiber types All fiber types tend to reach their peak power at ~20% peak force

13. Muscle Fiber Types Key Points Skeletal muscle contains type I and type II fibers Different fiber types have different myosin ATPase activities Type II fibers have a more highly developed SR, delivering more Ca 2+ Type II motor units are larger compared to type I motor units Type II motor units have more muscle fibers to contract and produce more force than type I motor units The proportion of type I and type II fibers in an individual’s arm and leg muscles are usually similar (continued)

14. Muscle Fiber Types (continued) Key Points Type I fibers have higher aerobic endurance and are well suited to low-intensity endurance activities Type II fibers are better suited for anaerobic activity –Type IIa fibers play a major role in high intensity exercise –Type IIx fibers are activated when the force demanded of a muscle is high

15. Determination of Fiber Type Fiber type is genetically determined (twin studies) Fiber type is determined by the  -motor neuron that innervates the muscle fibers Endurance training, strength training, and muscular inactivity may cause a shift in myosin isoforms – Exercise training ↓ type IIx and ↑ type IIa Aging may shift the relative distribution of type I and type II fibers – ↓ type II and ↑ type I

16. Motor Unit Recruitment Principle of orderly recruitment Motor units are activated on the basis of a fixed order –type I → type IIa → type IIx

17. Size Principle Size principle The order of motor unit recruitment is directly related to the motor neuron size

18. Motor Unit Recruitment Key Points Motor units give an all-or-none response Activating more motor units and thus more muscle fibers produces more force Motor units are recruited in an orderly way to generate force or for long duration events –type I → type IIa → type IIx

20. Athletes and Fiber Type Key Points Muscle fiber composition differs in athletes by sport and event Speed and strength events are characterized by a higher percentage of type II fibers Endurance events are characterized by a higher percentage of type I fibers

21. Types of Muscle Contraction Concentric contraction: Force is developed while the muscle is shortening Isometric contraction: Force is generated but the length of the muscle is unchanged Eccentric contraction: Force is generated while the muscle is lengthening

22. Muscle Force Generation Key Points 3 types of muscle contraction –Concentric –Isometric –Eccentric Force production is increased by recruitment of more motor units and through increased frequency of stimulation Force production is maximized at the muscle’s optimal length Speed of contraction also affects the amount of force produced