1. Muscle Function

2. Functional Anatomy of skeletal muscle
3 layers of connective tissue
Epimysium – surrounds entire muscle
Perimysium – divide muscle into compartments
Endomysium – surrounds individual muscle cells
Blood vessels and nerves
Skeletal muscle fibers

3. Skeletal Muscle Fibers
Multinucleate – Each contains hundreds of nuclei just internal to membrane
Large
Sarcolemma – Cell membrane
Sarcoplasm - cytoplasm

4. During Contraction The whole fiber must contract simultaneously
Signal to contract must distribute quickly via Transverse Tubules
Transverse Tubules (T Tubules) – narrow tubes that are continuous with the sarcolemma and extend into the sarcoplasm at right angles to the cell surface
Like a belt

5. During Contraction
Myofibril – Cylindrical structures. Each muscle fiber contains hundreds to thousands of myofibrils
Contain a thick and thin filament
Thin Filament – Actin
Thick Filament – Myosin

6. During Contraction
Sliding Filament Theory – Thick and thin filaments slide across each other therefore shortening and causing a contraction.

7. Important structures Neuromuscular Junction Sarcoplasmic Reticulum
Ca (Calcium Ions)
Sarcomere (~10,000/myofibril)
Actin
Myosin
Acetylcholine (ACh)
Troponin
Tropomyosin 2+
Rope around Actin

8. Steps of Contraction ACh is released at Neuromuscular junction
Production of action potential along surface of muscle fiber/T tubules
Sarcoplasmic reticulum (SR) releases calcium ions that were stored into sarcoplasm
Calcium bind to troponin changing oritentaion and exposing active sites of actin. Cross-bridges form when myosin head binds to active site

9. Steps of contraction
Bind, pivot, detach via hydrolysis of ATP (therefore filament sliding and fiber shortens)
Action potential ends with ACh break down
SR resorbs calcium
Active sites recover to prevent cross-bridge
Muscle relaxation occurs

10. Figure 10-10 The Exposure of Active Sites
SARCOPLASMIC RETICULUM
Calcium channels open
Myosin tail (thick filament)
Tropomyosin strand
Troponin
G-actin (thin filament)
Active site
Nebulin
In a resting sarcomere, the tropomyosin strands cover the active sites on the thin filaments, preventing cross-bridge formation.
When calcium ions enter the sarcomere, they bind to troponin, which rotates and swings the tropomyosin away from the active sites.
Cross-bridge formation then occurs, and the contraction cycle begins. 10

11. Figure 10-12 The Contraction Cycle
Contraction Cycle Begins
The contraction cycle, which involves a series of interrelated steps, begins with the arrival of calcium ions within the zone of overlap.
Myosin head
Troponin
Tropomyosin
Actin 11

12. Figure 10-12 The Contraction Cycle
Active-Site Exposure
Calcium ions bind to troponin, weakening the bond between actin and the troponin– tropomyosin complex. The troponin molecule then changes position, rolling the tropomyosin molecule away from the active sites on actin and allowing interaction with the energized myosin heads.
Sarcoplasm
Active site 12

13. Figure 10-12 The Contraction Cycle
Cross-Bridge Formation
Once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges. 13

14. Figure 10-12 The Contraction Cycle
Myosin Head Pivoting
After cross-bridge formation, the energy that was stored in the resting state is released as the myosin head pivots toward the M line. This action is called the power stroke; when it occurs, the bound ADP and phosphate group are released. 14

15. Figure 10-12 The Contraction Cycle
Cross-Bridge Detachment
When another ATP binds to the myosin head, the link between the myosin head and the active site on the actin molecule is broken. The active site is now exposed and able to form another cross-bridge. 15

16. Figure 10-12 The Contraction Cycle
Myosin Reactivation
Myosin reactivation occurs when the free myosin head splits ATP into ADP and P. The energy released is used to recock the myosin head. 16

17. Table 10-1 Steps Involved in Skeletal Muscle Contraction and Relaxation
Steps in Initiating Muscle Contraction
Steps in Muscle Relaxation
Synaptic terminal
Motor end plate
T tubule
Sarcolemma
Action potential reaches T tubule
ACh released, binding to receptors
ACh broken down by AChE
Sarcoplasmic reticulum releases Ca2
Sarcoplasmic reticulum recaptures Ca2
Ca2
Active site exposure, cross-bridge formation
Actin
Active sites covered, no cross-bridge interaction
Myosin
Contraction ends
Contraction begins
Relaxation occurs, passive return to resting length 17