E139 Lecture 7 (Muscle Mechanics I)

extensors

gastroc

flexors

peroneals

muscle: levels of organization

muscle (cm) → muscle fascicle (mm) → muscle fiber (100 um)

muscle fascicle

a bundle of skeletal muscle fibers (cells) wrapped in a connective tissue sheath

myofibrils

long contractile fibers in the muscle

t-tubules

deep invaginations of the sarcolemma that allow for rapid transmission of action potentials into the cell interior

SR

a specialized type of smooth endoplasmic reticulum found in smooth and striated muscle fibers that acts as the primary storage site for calcium ions

relationship between non-contractile material vs force capacity

inverse

sarcomere

a structural unit of a myofibril in striated muscle

• 1.5 - 4 um

z disc

end of sarcomere; delineate the lateral borders of sarcomeres and are the smallest functional units in striated muscle

• contractions move the z discs together

• consists of actin

sarcomere components

z disc, actin, myosin, nebulin, titin

nebulin

underlies actin filament (scaffold around actin); mutation will shorten or lengthen actin filament; maintains spacing between actin and myosin

titin

runs from one z disc to the next; controls distance of lattice; can change stiffness with Ca2+

steps of muscle contraction

motor command → neuromuscular junction → muscle AP → Ca2+ signal → contraction cycle (ATP → ADP) → force and motion

• excitation contraction coupling: NMJ → contraction cycle

mechanical work

involves force and motion

calcium in contraction

critical regulatory signal in muscle contraction

troponin

covers actin binding sites for the myosin heads; presence of Ca2+ uncovers

• Keeps tropomyosin covering actin; binding to calcium moves it

troponin C

binds calcium to initiate contraction

troponin I

inhibits actomyosin interaction at rest

troponin t

anchors the complex to tropomyosin

what hapens when troponin binds to calcium

troponin t, i, and c affinity strengthened

tropomyosin

moves once troponin binds to Ca2+; exposes actin for myosin binding

what is required for cross bridge detachment

atp, which is needed for x-bridge cycling

what does the myosin head look like before ca2+ binding

myosin head is in the high energy state because it is bound to ADP and Pi group

what is the energy level when the myosin head detaches

low energy

how is low energy level achieved

achieved by binding and ratcheting

• power stroke occurs, not a muscle contraction

• muscle contraction occurs from multiple power strokes

steps of myosin head binding and disengaging

1. calcium binds → myosin head (attached to ADP-Pi) binds to the actin filament

2. releases Pi group → myosin head goes into low energy state

3. 3 ATP go in → 3 ADP out = release myosin head from actin filament

during which steps is ATP necessary

Ca2+ signal = ATP is needed to sequester Ca2+ back in the SR

contraction cycle = needed for cross bridge detachment which is needed for crossbridge cycling

what is the contribution of each crossbrudge to overall force in a muscle

stress = (Fx nx)/A

• Fx = force per crossbridge

• nx = # of attached crossbridges

• A = cross sectional area

force per cross bridge (Fx)

about 2 - 3 pN (2E-12 to 3E-12 N)

pN to N conversion

1 pN = 1E-12 N

how many mysoin heads per thick filament

200 myosin heads/thick filament

myofibril diameter

1 um

um to m conversion

1 um = 1E-6 m

myofibril area (A)

0.78 um²

nm² to um² conversion

1 nm² = 1E-6 um²

calculating thick filament area

pi (distance between adjacent myosin filaments/2)²

calculating number of thick filaments

myofibril area/ thick filament area

• A = 0.78 um²

• thick filament area = pi (distance between adjacent thick filaments/2)²

arrangement of thick and thin filaments

hexagonal shapre

calculating nx

(# thick filaments)(200 myosin heads/thick filaments)

um² to cm² conversion

1 um² = 1E-8 cm²

Bz value

0.2 um

actin length (a) value

1 um

sarcomere length

subject to change

myosin length (m) value

1.6 um

relationship between sarcomere force and overlap (um)

proportional

calculating Hmax

Hmax = (m-Bz)

relating force and overlap

Fs/Fmax = H/Hmax

graph of tension vs sarcomere length

x axis = sarcomere length (um)

y axis = relative tension

• most optimal is intermediate overlap and sarcomere length

a muscles force-length relationship limits length changes to a max of ___% of its optimal length

80%

what is the graph of muscle length vs force

x axis = muscle length (% Lo)

y axis = force (%Fo)