Skeletal Muscle

Question 1

Skeletal Muscle definition

Answer 1

. Attached to bones
. Contraction responsible for support and movement of skeleton
. Consists of many muscle fibers lying parallel to each other and bundled together by CT

Question 2

Muscle fiber

Answer 2

. Single skeletal muscle cell (myocyte)
. Large (up tp 30 cm)
. Elongated, cylinder shaped
. Fibers run entire length of muscle

Question 3

Myofibril

Answer 3

. Specialized contractile elements of muscle fibers of 2 um in diameter
. Many myofibrils make up one muscle fiber
. Each one consists of repeating units (sarcomeres)

Question 4

Myofilaments

Answer 4

. Ultramicroscopic filamentous structures in which contractile proteins are arranged into
. 2 types: thick (myosin) and thin (actin)

Question 5

Sarcomeres

Answer 5

. Functional unit of muscle
. Limited by 2 Z lines
. Lined up giving muscle fiber banded/striated appearance

Question 6

Z line

Answer 6

. Constitutes limits of sarcomeres

. Made of proteins that thin filaments are anchored to

Question 7

Alpha-actinin

Answer 7

. Main protein of Z line
. Cytoskeletal actin-binding protein that acts to anchor thin filaments to Z-line
. Forms anti-parallel rod-shaped dimer w/ one actin-binding domain at each end of rod
. Localized at z-line where it forms lattice-like structure to stabilize the contractile apparatus

Question 8

Myosin

Answer 8

. Dimers, each monomer has golf-club shape w/ 1 myosin heavy chain (MHC) and 2 myosin light chains (MLC)
. MHC has long tail that intertwines to make dimer and globular head hat has actin binding site and ATPase site

Question 9

Thick filament

Answer 9

. Myosin molecules lying lengthwise parallel to each other
. Half in one direction, half the opposite so tails from 2 halves line up end to end in middle of filament (polar arrangement)
. Globular heads protrude at regular intervals along filament to form cross bridges

Question 10

Titin

Answer 10

. Spring-like protein
. Links Z-line to free end of myosin
. Controls length and elasticity of sarcomeres
. Each molecule spans half the sarcomere from Z-line to M-line

Question 11

Myomesin

Answer 11

. Found in M-line

. Helps titin and myosin maintain 3D structure

Question 12

Creating kinase

Answer 12

. Enzyme found in M-line
. Transfers phosphate from creatine phosphate to ADP
. Vital to energetics of muscle contraction

Question 13

C-protein

Answer 13

. Maintains width of thick filaments by restricting them to 200-400 molecules each

Question 14

Myosin light chain kinase (MLCK)

Answer 14

. In fast-twitch skeletal m.
. Binds to thick filament to phosphorylate MLC
. Not as important in smooth muscle
. In fast fibers it contributes to low-frequency potentiation of peak isometric contraction

Question 15

Actin

Answer 15

Spherical-shaped protein that has binding site for myosin attachment
. Molecules of actin bind together to polymerize
. Makes 2-stranded actin helix string

Question 16

Nebulin

Answer 16

. Giant (600-900kDa) filamentous protein
. Wraps around thin filament
. Contributes to appropriate actin align
. Specifies thin filament length
. Regulates tension development by modifying myofilament Ca sensitivity

Question 17

Tropomyosin

Answer 17

. Thread-like regulatory protein
. In groove btw 2 actin strands
. In relaxed conditions, blocks myosin binding site on actin molecules

Question 18

Troponin (Tn)

Answer 18

. Regulatory protein bound to tropomyosin
. 3 subunits: Tn-I, Tn-T, and Tn-C
. Binding of Ca to Tn-C initiates conformational change that displaces Tn-I, exposing myosin binding sites on actin

Question 19

Arrangement of thick and thin filaments in transverse cut of A band

Answer 19

. Regular arrangement of interdigitating thick and thin filaments
. Each thick filament has hexagon of thin filaments around i
. Each thin filament surrounded by triangle of thick filaments
. Only thin filaments in I band and only thick filaments in H zone

Question 20

Dystrophin

Answer 20

. Actin binding protein
. Anchors entire myofibrillar array to cell membrane
. Participates in transfer of force from contractile system outside of cells
. Defective of absent in muscular dystrophy patients

Question 21

Sliding filament model

Answer 21

. Muscle contraction occurs by sliding thin filaments over thick filaments
. Actual length of individual filaments remains unchanged
. Relaxed muscle: low rate cross-bridge cycling bc myosin binding site covered by troponin-tropomyosin complex
. Excited muscle: Ca binds to troponin pulling complex aside for cross-bridge formation
. Actin binding to myosin elicits power stroke to pull thin filaments inwards then detaches and returns to resting conformation

Question 22

Changes in banding pattern during shortening

Answer 22

. Contraction: thin filaments slide closer together btw thick filaments, z-lines pulled closer together
. Width of A bands doesn’t change as fiber shortens
. I band and H zones become shorter

Question 23

Cross bridge cycling steps

Answer 23

. Myosin head activated: ATP used, myosin binds ADP to activate
. Cross-bridge formation: Ca binds to troponin, complex moves exposing myosin binding site, active myosin head binds to actin
. Power stroke: release fo ADP from myosin head, myosin undergoes conformational change, thin filament slides along thick filament, each cycle its pulled 10 nm
. Cross bridge detachment: ATP binds myosin head, myosin dissociates from actin, crossbridge breaks

Question 24

Muscle load

Answer 24

Force imposed on muscle

Question 25

Tension

Answer 25

. Resulting muscle force determined by difference btw cross bridges formed minus those that are broken