Chapter 6 - Guyton Flashcards
General skeletal muscle fiber characteristics.
numerous fibers ranging from 10-80 micrometers, most fibers extend the length of entire muscle, each fiber is usually innervated by one nerve ending near the middle of each fiber
The membrane of a skeletal muscle fiber?
sarcolemma (fuses with tendon fiber at the end)
General characteristics of myofibrils.
each muscle fiber consists of several thousand, composed of myosin and actin interdigitated causing alternating light and dark bands (2:1 actin to myosin)
I bands
light bands that contain only actin, isotropic to polarized light (appear the same from all directions)
A bands
dark bands that contain only myosin filaments at the ends of actin filaments where they overlap myosin, anisotropic to polarized light (appear different in different directions)
Cross-bridges
small projections from the sides of the myosin filaments that connect the actin and myosin allowing for skeletal muscle contraction
M-line
thick, mid region of myosin
Z-disc
where the ends of actin filaments attach, extend in both directions to connect myosin (creating striations)
Sarcomere
portion that lies between two z-discs
When is the muscle fiber capable of producing its strongest contraction force?
actin filaments completely overlap myosin filaments and actin filaments are beginning to overlap one another
What is the filamentous molecule of protein that keeps the actin and myosin in place?
titin (attaches Z-discs to M-line)
What is one of the largest protein molecules in the body (molecular weight of ~3 million)?
titin
What is the sarcoplasm?
intracellular fluid between the myofibrils, contains large amounts of potassium, magnesium, phosphate, and some protein enzymes
What lies parallel to the myofibrils in the sarcoplasm?
mitochondria to supply the ATP to the contracting myofibrils
Explain the general mechanism of muscle contraction (deep breath here we go…).
- AP travels along motor neuron to its ending on muscle fibers 2. Ach released at ending 3. ligand-gated Ach channels are opened through protein molecules floating in the membrane 4. Na diffuses into muscle fiber leading to depolarization and opening of voltage-gated Na channels initiating AP at the membrane 5. AP depolarizes muscle membrane, SR releases stored Ca 6. calcium ions cause actin and myosin to slide alongside in contractile process 7. Ca pumped back into SR by pump until new AP
What is responsible for the muscle contraction stopping?
removal of calcium ions from the myofibrils
Describe the positioning of actin filaments in a relaxed state.
the ends of actin filaments extend from two successive z-discs and are barely beginning to overlap the A-band
Describe the positioning of actin filaments in a contracted state.
the actin filaments have pulled inward over the myosin heads, Z-discs have been pulled by actin to the ends of the myosin filaments
Molecular Characteristics of Myosin
total molecular weight ~480,000, composed of 6 polypeptide chains, golf club
What is the significance of the heavy and light chains of myosin?
2 heavy chains form the head (site of ATPase activity) and tail, 4 light chains also form the head and help control the movement
Molecular Characteristics of Actin
F-actin: double-stranded protein molecule that acts as backbone to actin filament, each strand is composed of polymerized G-actin molecules, 1 ADP is attached to each G-actin, which is the active site of actin filaments with which the cross-bridges of myosin interact
What is tropomyosin?
protein component of actin filament; wrapped spirally around the sides of the F-actin helix, in resting state lies on top of actin so that actin and myosin cannot produce contraction
What is troponin?
protein molecule attached to the sides of tropomyosin, believed to initiate contraction process
What are the three protein subunits of troponin?
Troponin I - strong affinity for actin; Troponin T - strong affinity for tropomyosin; Troponin C - strong affinity for calcium