PHYSIOLOGY OF NERVE AND MUSCLE CELLS lecture 16

O B J E C T I V E S :
AT THE END OF THE LECTURE YOU SHOULD BE ABLE TO
•Describe the components of the sarcotubular system.
•Explain the importance of the Dystrophin–Glycoprotein Complex.
•Distinguish between the electrical events in the nerve and skeletal muscle fibers.
Sarcotubular System
•The muscle fibrils are surrounded by membrane structures that appear in electron photomicrographs as vesicles and tubules.
•These structures form the sarcotubular system, which is made up of a T system and a sarcoplasmic reticulum.
•The T system of transverse tubules, which is continuous with the sarcolemma of the muscle fiber, forms a grid perforated by the individual muscle fibrils. The space inside T system is an extension of the extracellular space.
•The sarcoplasmic reticulum, which forms an irregular curtain around each of the fibrils, has enlarged terminal cisterns in close contact with the T system at the junctions between the A and I bands.
SKELETAL MUSCLES: Anatomical Considerations:
Sarcotubular System
•At these points of contact, the arrangement of the central T system with a cistern of the sarcoplasmic reticulum on either side has led to the use of the term triads.
•The T system, which is continuous with the sarcolemma, provides a path for the rapid transmission of AP from the cell membrane to all muscle fibrils.
•The sarcoplasmic reticulum is an important store of Ca+2 and also participates in muscle metabolism.
SKELETAL MUSCLES: Anatomical Considerations:
•A large rod shaped dystrophin protein (427 KDa) connects the thin actin filaments to a transmembrane protein called “?-dystroglycan”.
•dystroglycan is connected to laminins in the extracellular matrix by ?-dystroglycan.
•The dystroglycans are in turn associated with a complex of four transmembrane glycoproteins: sarcoglycan.
Anatomical Considerations: Dystrophin–Glycoprotein Complex
•Dystrophin – glycoprotein complex adds strength to the muscle by:
-providing support to the fibrils; and
- connecting them to the ECF.
•Disruption of this structure can lead to several different pathologies called muscular dystrophies.
Anatomical Considerations: Dystrophin–Glycoprotein Complex
•Muscular dystrophy is applied to diseases that cause progressive weakness of skeletal muscle.
•Mutations in the genes for the various components of the dystrophin–glycoprotein complex are a prominent cause.
•The dystrophin gene is one of the largest in the body, and mutations can occur at many different sites in it.