Theories of Contraction

Question 1

AF Huxley’s 1957 ‘Sliding Filament’

Theory

Answer 1

Cross bridge (M) oscillates along the
myosin filament about its
equilibrium position (O)
o Each cross-bridge has the same force
potential, which is governed by an
elastic link at either end
o The actin filament has the crossbridge
binding site (A)
o Cross-bridge attachment to A, and
detachment from A are goverened by
rate constants (f,g) that are functions
of displacement (x).
o h is the maximum displacement,
where f is higher. g is very high when
x < 0
o Each cross-bridge cycle is associated
with the hydrolysis of one ATP

Question 2

Huxley’s 1957 Theory fitted to Hill’s data
Compared Huxley model with equation relating force and velocity
§ Demonstrated ___________correspondence

Answer 2

excellent

Question 3

Huxley’s 1957 Theory fitted to Hill’s data
It a predicted that the total rate of energy liberation (heat + work)
increases linearly as afterload is ________from maximum isometric force
in an isotonic contraction

Answer 3

reduced

Question 4

Huxley’s 1957 Theory fitted to Hill’s data
Predicted that the rate of heat production in an isotonic contraction
increases ________with speed of shortening in an isotonic contraction.

Answer 4

linearly

Question 5

Huxley’s 1957 Theory fitted to Hill’s data
predicted that the total rate of energy liberation in isotonic contraction
increased _____________with increasing velocity of shortening.

Answer 5

monotonically

Question 6

Huxley’s 1957 Model can’t explain

Answer 6

1. Predicts that the total rate of energy liberation (heat
   + work) increases monotonically with increasing
   velocity of shortening, fitted well to Hill’s 1938 data
   § But Hill’s model can’t predict Hill’s data in
   1964: The reduced energy liberation at high V
   presumably reflects the transition between
   crossbridge states that produces heat but
   doesn’t involve ATP hydrolysis, which
   followed, after shortning, by ATP hydrolysed
   but with a heat absorbing transition
2. Huxley’s 1957 model assumes that force generation occurs instantaneously
   when a cross bridge is formed, and hence fails to explain the rapid changes
   (declines then recovers) of tension when muscle length if suddenly changed
   during contraction
   o 3. Force enhancement (FE) and Passive force enhancement (PFE) property of
   skeletal muscle, as the model does not include the titin
   § i.e. both of these properties are explained by the presence of titin

Question 7

• Huxley-Simmons 1971 “Swinging Crossbridge” Model
  o Proposed a theory of contraction which differed substantially from the Huxley
  (1957) theory
  o Primary aim in setting up this model was to explain

Answer 7

the recovery of force afer a

quick isometric length step.

Question 8

Huxley-Simmons 1971 “Swinging Crossbridge” Model
incorporates elastic and stepwise shortening elements in the
generation of crossbridge tension, exemplified by three possible myosin head
positions 1, 2 and 3 of successively greater strengths of binding to actin in which
the myosin head can dissociate in position 1 without, but in position 3 only with
____utilisation

Answer 8

ATP