Материал: Kaplan USMLE-1 (2013) - Anatomy

Section I • Histology and Cell Biology

Table 1-4-1. Red versus White Skeletal Muscle Fibers

Red Fibers (Type I)

Slow contraction

.J, ATPase activity

i Capacity for aerobic metabolism

iMitochondrial content

iMyoglobin (imparts red color)

Best for slow, posture-maintaining muscles, e.g., back (think chicken drumstick/thigh)

White Fibers (Type II)

Fast contraction i ATPase activity

i Capacity for anaerobic glycolysis

.J, Mitochondrial content

.J, Myoglobin

Best for fast, short-termed, skilled motions, e.g., extraocu­ lar muscles of eye, sprinter's legs, hands (think chicken breast meat and wings)

Chapter 4 • Muscle Tissue

Smooth

Nonstriated, fusiform fibers

No Z lines; have dense bodies

Single nucleus

Absent; have limited SR

Gap junctions

Electrical syncytium

Inward Na+ current

No plateau

AP opens voltage-gated Ca2+ channels in sarcolemma;

hormones and neurotransmittersopen IP3-gated Ca2+ channels in SR

Calmodulin

Anendomysium surrounds each skeletal muscle cell and provides insulation be­ tween adjacent cells. A perimysium surrounds a group or fascicle ofmuscle cells and is where capillaries are found. The epimysium surrounds the outside ofthe entire muscle, and is where larger bloodvessels are found. Satellite cells and fibro­ blasts that have small dark nuclei with tightly condensed chromatin lie adjacent to each muscle cell.

The Z lines mark the ends ofeach sarcomere. The A band is in the center ofthe sarcomere and is the zone including all ofthe myosin filaments. The H zone is the zone with only myosin filaments in the center ofthe A band. The M line is in the center ofthe sarcomere. The I band is the zonewith only actin filaments between the Z line and the start ofthe A zone. When a muscle cell contracts, the A band stays constant in width (the length ofthe myosin molecule). As the overlap ofthe actin and myosin filaments increases, both the I and H bands become smaller. The overlap region is the distance between the I and H bands, and grows as the I and H bands get smaller. During relaxation, the opposite happens.

Note

Desmin intermediate filaments link adjacent myofibrils at adjacent Z lines.

Clinical Correlate

Skeletal muscle has a limited ability to regenerate. Satellite cells are stem cells situated outside the plasma membrane that form new myoblasts.

Muscular dystrophies deplete the pool of satellite cells.

Actin thin filaments in muscle consist of 2 strands of F-actin forming a helix, around which tropomyosin filaments are wrapped. Tropomyosin serves as a site of attachment for troponin, which has 3 component parts: (which attaches to tropomyosin), troponin C (which binds calcium), and troponin I (which inhibits the interaction of actin and myosin). When cytosolic calcium increases, it binds to troponin C, which blocks the inhibition of actin/myosin binding by troponin I. The F-actin filaments have polarity. On each side of the sarcomere, the barbed or plus end inserts into a Z line (composed in part ofdes­ min and actinin), while the other end faces toward, but does not reach, the M­ line in the center ofthe sarcomere.

Myosin thick filaments are formed of a bipolar polymer of individual myosin molecules. Each myosin molecule consists of 2 identical heavy chains, which each contribute to the head and tail ofthe molecule, and 2 pairs oflight chains, with one ofeach type bound to each head ofthe molecule. The structure ofthe tail end ofthe heavy chains allows the individual molecules to assemble into fila­ ments, while the head ends can interact with myosin as well as hydrolyze ATP. When the myosin molecules self-assemble, they form a bipolar thick filament, with myosin heads at each end, and with a central region free ofmyosin heads. The central region is attached to the M line at the center ofthe sarcomere, and the myosin heads extend toward (but do not reach) the Z lines at the end ofthe sarcomere. The polarity ofactin and myosin heads is reversed in the 2 halves of the sarcomere on either side of the M line, so that both halves pull toward the centerwhen they contract.

Nebulin is a scaffolding protein that binds to actin filaments along their entire length and also insert into the Z line. It serves as a template to help maintain constant length ofthe actin filaments.

Titin is a scaffolding molecule that binds to myosin, and which is not only anchored in the M line but also extends beyond the free end ofthe myosin molecule to the Z line. The part bound to myosin again helps to maintain the constant lengthofthe myosin molecule. Thepartthat extends from the end ofthe myosin molecule to the Z line may act as a spring, which is compressed during contrac­ tion and which helps to restore the sarcomere to its resting length when the sar­ comere relaxes, at the end ofactin/myosin interaction.

Contractions ofskeletal muscle produce force and movement by the interaction of thin actin (F-actin) and thick myosin (myosin II) filaments. The interaction uses energyderived fromATP and involves repetitive binding, sliding, release and reattachment ofthe head end of the myosin molecules to the adjacent actin fila­ ments. The contraction is ultimately startedby releasing calcium, which promotes binding and cocking ofthe myosin head, hydrolysis ofATP and movement ofthe myosin head back to its starting position. Relaxation ofskeletal muscle involves a reduction ofcytosolic calcium to halt the interaction ofmyosin and actin.