1. Introductory Skeletal Muscle – Histology Flash Cards

2. Structural Organization in Skeletal Muscle

3. Part of a Multinucleate Skeletal Muscle Fiber (Cell)
Sarcolemma
Striations
Nuclei
Myofibrils
Mitochondria b
A diagrammatic view and a micrograph part of one skeletal muscle fiber (cell).

4. Structure and Internal Organization of a Skeletal Muscle Fiber
Myofibril
Muscle fiber
Sarcolemma
Nuclei
Sarcoplasm
Mitochondria
Terminal cisterna
Sarcolemma
Sarcolemma
Sarcoplasm
Myofibril
Myofibrils
Thin filament
Thick filament
Triad
Sarcoplasmic reticulum
T tubules

5. Levels of Functional Organization in a Skeletal Muscle
The Sarcomere
I band
A band
Contains:
Thick filaments
Thin filaments
Z line
M line
Titin
Z line
H band

6. The Sarcomere

7. The contraction cycle Z line distance: reduced
I band
H band
Z line distance: reduced
I band distance: smaller
H band distance: smaller

8. Neuro-Muscluar Junction

9. The Steps of Skeletal Muscle Contraction
Neuromuscular Junction
Skeletal muscle activity is controlled at the neuromuscular junction (NMJ)
This is a special intercellular connection between the nervous system and the skeletal myofiber
It controls calcium ion release into the sarcoplasm

11. Arriving action potential Junctional fold of motor end plate
The cytoplasm of the synaptic terminal contains vesicles filled with molecules of acetylcholine, or ACh. Acetylcholine is a neurotransmitter, a chemical released by a neuron to change the permeability or other properties of another cell’s plasma membrane. The synaptic cleft and the motor end plate contain molecules of the enzyme acetylcholinesterase (AChE), which breaks down ACh.
The stimulus for ACh release is the arrival of an electrical impulse, or action potential, at the synaptic terminal. An action potential is a sudden change in the transmembrane potential that travels along the length of the axon.
Arriving action potential
Vesicles
ACh
The synaptic cleft, a narrow space, separates the synaptic terminal of the neuron from the opposing motor end plate.
Junctional fold of motor end plate
AChE 11

13. Excitation–Contraction Coupling
In EC Coupling, an action potential reaches a triad, releasing Ca2+, triggering contraction
This process requires the myosin heads to be in the “cocked” position (loaded by ATP)

14. 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. 14

15. The Contraction Cycle Tropomyosin shifts Active-site is exposed
Cross-Bridge forms
Myosin head pivots (advances)
Cross-bridge detaches
Myosin reactivates

16. Contraction Cycle Begins Active-Site Exposure
The contraction cycle, which involves a series of interrelated steps, begins with the arrival of calcium ions within the zone of overlap.
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
Myosin head
Troponin
Active site
Tropomyosin
Actin 16

17. Cross-Bridge Formation Myosin Head Pivoting
Once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges.
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. 17

18. Cross-Bridge Detachment Myosin Reactivation
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.
Myosin reactivation occurs when the free myosin head splits ATP into ADP and P. The energy released is used to recock the myosin head. 18

19. Steps Involved in Skeletal Muscle Contraction and Relaxation
Steps in Initiating Muscle Contraction (ACTIVE)
Steps in Muscle Relaxation (PASSIVE)
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
Actin
Active site exposure, cross-bridge formation
Active sites covered, no cross-bridge interaction
Myosin
Contraction ends
Contraction begins
Relaxation occurs, passive return to resting length 19