Transcript Resting Length of a muscle

Mechanics of Muscle
Contraction
12.4.12
Muscles
• The term muscle refers to a
number of muscle fibres
bound
together
by
connective tissue
• A single muscle cell is
known as muscle fiber.
Each muscle fiber consist of
many myofibrils.
The
myofibrils are divided into
functional
units
i.e.
sarcomeres by two Z lines
• Myofibrils consist of thick
and
thin
filaments
composed of contractile
proteins
• Electron microscopic studies have revealed light and
dark bands in the myofibrilss. The lighter areas on either
side of the Z—line are I bands and they contain thin
filaments. The dark area in the center of sarcomere are
called A bands.
• A bands are anisotropic while I bands are isotropic
Muscles
• Bones provide place for
the attachment of skeletal
muscle and maintain the
optimum length of the
resting muscle
• The muscles always begin
and end in the bones that
touch one another and
they never begin and end
on the same bone
• Resting Length of a muscle:
If a muscle is removed from the
body, it assumes a length slightly
shorter than the length of the muscle
at rest in the body (resting length).
The muscle is fixed to its bony
attachments in a slightly stretched
state
Mechanical Model of a Muscle
• Muscle has elastic and contractile components
1. Contractile component: produces force, composed of myofibers
2. Elastic component: absorbs some of muscle's force by stretching.
Has two subsets:
Parallel elastic component: membranes running along myofibers
(e.g. plasma membrane, sarcoplasmic reticulum)
Series elastic component: tendons and fascia that binds muscle to
insertion on bone
Active Tension
• Tension purely due to contraction (of sarcomeres) is
known as “active tension”
• This tension is due to the forces generated by many
cross-bridge formations. In the cross-bridge theory
the force generated is proportional to the number of
cross-bridges formed.
• It is the pulling of the actins by myosin heads towards
each other that exerts this tension. The magnitude of
the tension depends on the frequency of the
stimulation and the initial resting length of muscle
fibres
• The length of the sarcomeres dictates the overall
length of a muscle fibre.
Length-Tension Relation of an
Individual Muscle Fiber
• The best way to relate length-tension
changes to contractile machinery is
to perform this experiment with a
single muscle fibre
• An unstimulated muscle fibre is held
between a movable clamp and a
force transducer. The muscle fibre is
then stimulated at various lengths
and the force generated is measured
• The force generated is seen to be
related to the degree of overlap of
actin and myosin filament.
Length-Tension Relation of an Individual
Muscle Fiber
The active length-tension curve of a sarcomere is shown as an
ascending limb, a plateau and a descending limb. The length-tension
curve can be explained by the cross-bridge theory.
Length-Tension Relation of an
Individual Muscle Fiber
• Ascending limb:
In the regions of sarcomere lengths between 1.5 to 2.0
µm, there is a double overlap of thin filaments with thick
filaments. This double overlap can be seen as a darker
region in the A band at short sarcomere lengths under
electron microscope. Tension is low due to the pause in
cross-bridge cycling and formation. The actin filaments
from opposite ends of the sarcomere begin to interfere
with each other and at shortest lengths the Z disc may
block or otherwise impede movement of the thick
filament
Tension rises as the length is increased from 1.7 to 2.2
µm. This rise has been attributed to progressive
decrease in the extent of double overlap of actin
filaments
Length-Tension Relation of an
Individual Muscle Fiber
• Plateau:
Maximum active tension is produced when
sarcomeres are about 2.0 to 2.2 μm long, as
seen in 2. This is the optimal resting length for
producing the maximal tension. At the plateau of
the curve, there is an optimal overlap of actin
and myosin filaments and thus the opportunity
for maximal cross-bridge formation
• Descending Limb:
By increasing the muscle length beyond the
optimum, the actin filaments become pulled
away from the myosin filaments and from each
other.
Length-Tension Relation of an
Individual Muscle Fiber
Descending limb:
The force is reduced due to reduction in overlap of
the thin and thick filaments of sarcomere.
At 3, there is little interaction between the filaments.
Very few cross-bridges can form. Less tension is
produced.
Force tends to approach zero as the overlap is
eliminated When the filaments are pulled too far from
one another, as seen in 4, they no longer interact and
cross-bridges fail to form. No tension results.
Passive tension
• Parallel elastic elements of the muscle also generate
tension
• These elastic elements do not actively generate force
but if it is stretched beyond its resting length it acts
just like a rubber band and produces a passive,
elastic force
• Passive or resting tension is mainly due to stretching
of the series elastic elements i.e. tendons of the
muscle
• The force generated is passive as the contractile
machinery is not active
• The resting tension increases steeply with an increase
in the initial length of the muscle
Total Tension of a Muscle
The curve of total tension (N-shaped) is constructed
by stimulating the whole muscle at various lengths
and measuring the force generated
Total Tension of a Muscle
• Each of these forces will be the
sum of active forces (developed
by contractile machinery) and
passive
forces
(due
to
stretching of elastic elements)
• Forcibly stretching a muscle
well beyond its resting length
will generate a force higher
than that produced by active
contraction
Length-Tension Curve of Skeletal
Muscle Versus Cardiac Muscle
• The maximal active tension in skeletal muscle occurs when the
overlap of actin and myosin filaments is optimal and coincides
with the muscles natural resting length
• But in cardiac muscle the resting length is shorter than the
optimal length. Due to this the cardiac muscle normally operates
on the ascending part of the length-tension curve. Thus small
increase in initial length results in large increase in active and
total tension
• Unlike skeletal muscle, the decrease in developed tension at
high degree of stretch is not due to a decrease in number of
cross bridges between actin and myosin, because even severely
dilated hearts are not stretched to this degree. The descending
limb, if seen is due to the beginning of disruption of the
myocardial fibres
Types of Contraction
• Isometric contraction
When a muscle is stimulated such that it develops
tension but do not shorten. This is called an isometric
contraction (iso = same, metric = measurement or
length).
• Isotonic contraction
When a muscle is stimulated such that the muscle
shortens with a constant load but its tension remains
the same, the contraction is isotonic (iso = same, tonic
= tension)
Isometric Contraction:
Experimental setup
• A whole muscle is arranged in a muscle bath using a
lever arm. The muscle is first stretched to a resting
length by a given weight i.e. preload. Then a stop is
positioned to maintain the length of the muscle. After
this muscle is stimulated to contract, the tension or
force developed by the muscle is recorded
• Careful observation reveals that in the
isometric contraction, the sarcomeres
shorten and stretch the series elastic
component even though the muscle as
a whole does not shorten.
• Even though the muscle develops
tension, but because it does not
shorten, it does no external work (work
= force x distance moved) but there is
internal work being done
• The total tension is the sum of active
and passive tension (the curve of total
tension is the curve of isometric
contraction)
Isotonic Contraction:
Experimental setup
• The experimental setup is same. A preload is there to
stretch the muscle to a resting length
• There is no restriction on the length of the muscle now
and additional load is added to the lever (afterload)
• When the contractile elements shorten they must first
stretch the series elastic elements and develop a tension
equal to the load before the next increment in tension
causes the load to be lifted.
• All of the contraction that occurs before the load is lifted is
isometric. Even if the muscle carries no external load, it still
must develop a tension equal to its own weight before it
can shorten.
• When contractile forces exceed the load, shortening
begins; tension remains slightly larger than the load
throughout shortening. Shortening stops when active
tension drops to the point where it equals the load. At this
point, contraction again becomes isometric. The muscle
lengthens (is stretched) when the total tension in the
muscle falls below the load
Types of isotonic contraction
1. Conentric contraction
A concentric contraction is a type of
isotonic contraction in which the muscles
shorten while generating force
2. Eccentric contraction
During an eccentric contraction, the
muscle elongates while under tension
due to an opposing force (load) being
greater than the force generated by the
muscle
Types of contractions at a
glance
Force (tension)-velocity
relationship of a muscle
• The force a muscle can generate depends upon both
the length and shortening velocity of the muscle
• Force declines in a hyperbolic fashion relative to the
isometric force as the shortening velocity increases,
eventually reaching zero at some maximum velocity.
• The reverse holds true for when the muscle is
stretched – force increases above isometric
maximum, until finally reaching an absolute
maximum.
Force (tension)-velocity
relationship of a muscle
• This has strong implications for
the rate at which muscles can
perform mechanical work (power).
• Since power is equal to force
times
velocity,
the
muscle
generates no power at either
isometric force (due to zero
velocity) or maximal velocity (due
to zero force). Instead, the optimal
shortening velocity for power
generation is approximately onethird of maximum shortening
velocity