Molecular Mechanism of muscle contraction load velocity relationship

1. Molecular Mechanism of
Muscle Contraction
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
NRIIMS
Email: dr.goothy@gmail.com

2. Explain
How muscle tension is transmitted to bone
Beneficial effect
Causes of fatigue
First site of fatigue in intact human being
Why muscles are stiff during rigor mortis

3. The three primary types of contraction
Isotonic
Isokinetic
Isometric

4. The three primary types of contraction
Not all muscle contractions shorten muscles and move bones.
For a muscle to shorten during contraction, the tension developed in
the muscle must exceed the forces that oppose movement of the bone
to which the muscle’s insertion is attached
In the case of elbow flexion, the opposing force or load is the
weight of an object being lifted.

5. The three primary types of contraction
When you flex your elbow without lifting any external object,
there is still a load,
albeit a minimal one—
the weight of your forearm being moved against the force of gravity

6. The three primary types of contraction
In an isotonic contraction, the load remains constant
as the muscle changes length.

7. The three primary types of contraction
In an isokinetic contraction, the velocity of shortening remains constant
 as the muscle changes length.

8. The three primary types of contraction
In an isometric contraction, the muscle is prevented from shortening
 so tension develops at constant muscle length.

9. The three primary types of contraction
The same internal events occur in isotonic, isokinetic, and isometric contractions
Muscle excitation turns on the tension-generating contractile process;
 the cross bridges start cycling;
and filament sliding
shortens the sarcomeres,
which stretches the series-elastic components
to exert forces on the bones at the sites of the muscle’s origin and insertion.

10. Isotonic contraction
Considering your biceps as an example,
assume you are going to lift an object.
When the tension developing in your biceps becomes great enough to overcome
the weight of the object in your hand,
you can lift the object, with the whole muscle shortening in the process.
Because the weight of the object does not change as it is lifted,
this type of contraction is an isotonic (literally, “constant tension”) contraction.

11. Isokinetic contraction
Isokinetic (literally, “constant motion”) contractions occur when muscle fibers
shorten at a constant velocity or speed.
Isokinetic contractions do not take place normally
but can be achieved using special exercise machines set up to require muscle
contraction at a constant velocity throughout the entire range of motion.
One of the proposed advantages of isokinetic exercise is the more rapid
development of muscle strength

12. Isokinetic contraction
Isokinetic (literally, “constant motion”) contractions occur when muscle fibers
shorten at a constant velocity or speed.
Isokinetic contractions do not take place normally
but can be achieved using special exercise machines set up to require muscle
contraction at a constant velocity throughout the entire range of motion.
One of the proposed advantages of isokinetic exercise is the more rapid
development of muscle strength

13. Isometric contraction
What happens if you try to lift an object too heavy for you (that is, if the
tension you can develop in your arm muscles is less than required to
lift the load)?

14. Isometric contraction
isometric contractions are important
for maintaining posture (such as keeping the legs stiff while standing)
and
for supporting objects in a fixed position (such as holding a beverage
between sips).

15. Concentric and Eccentric Contractions
In concentric contractions the muscle shortens,
whereas with eccentric contractions the muscle lengthens

16. Other Contractions
Some skeletal muscles do not attach to bones at both ends but still
produce movement.
For example, tongue muscles are not attached at the free end.
 Contractions of tongue muscles maneuver the free, unattached
portion of the tongue to facilitate speech and eating.

17. Other Contractions
External eye muscles attach to the skull at their origin and to the eye,
 not another bone, at their insertion.
Contractions of these muscles produce eye movements that enable us
to track moving objects or read.

18. Load-velocity relationship
You can lift light objects requiring little muscle tension quickly,
whereas you can lift very heavy objects only slowly, if at all.
This relationship between load and shortening velocity is a
fundamental property of muscle

20. Load-velocity relationship
load and velocity for shortening are inversely related for concentric
contractions
 load and velocity for lengthening are directly related for eccentric
contractions

21. Work done
Muscle accomplishes work in a physical sense only when an object is
moved.
Work is defined as force multiplied by distance.
 Force can be equated to the muscle tension required to overcome the load
(the weight of the object).
The amount of work accomplished by a contracting muscle therefore
depends on how much an object weighs and how far it is moved.

22. Work done – Isometric contraction
In an isometric contraction when no object is moved, the muscle
contraction’s efficiency as a producer of external work is zero.
All energy consumed by the muscle during the contraction is
converted to heat.

23. Work done – Isotonic contraction
In an isotonic or isokinetic contraction,
the muscle’s efficiency is about 25%.
Of the energy consumed by the muscle during the contraction, 25% is
realized as external work,
whereas the remaining 75% is converted to heat

24. Homeostasis
Much of this heat is not wasted energy
because it is used in maintaining body temperature.
 In fact, shivering—a form of involuntarily induced skeletal muscle
contraction—is a well-known means of increasing heat production on a
cold day

25. Homeostasis
Much of this heat is not wasted energy
because it is used in maintaining body temperature.
 In fact, shivering—a form of involuntarily induced skeletal muscle
contraction—is a well-known means of increasing heat production on a
cold day

26. Similarities in isometric and isotonic contractions
Muscle excitation
Cross bridge formation
Filaments sliding

27. Isometric contraction
1. Muscle does not shorten
2. Contract against force with out
shortening ( fixed length)
3. Does not involve joint movement
4. Tension increases
5. No work done
6. Less release of heat
Isotonic contraction
1. Muscle shorten
2. Shorten against fix load
3. Involve joint movement
4. Tension constant
5. Work done
6. More release of heat

28. Motor unit
Motor neurons are present at anterior gray horn of spinal cord.
Each motor neuron can activate a group of the muscle fibers
through its axon terminals.
A single motor neuron and the muscle fibers it innervates,
together is called motor unit.
It is referred as physiological or functional unit of the muscle.

31. Types of motor units
There are three types of motor units.
Fast motor unit (IIB).
Slow motor unit (I).
Intermediate motor unit (IIA).
This classification is based on type of muscle fibers.
In a motor unit, only one type of muscle fibers are present.

32. Size of the motor unit
The size of motor unit depends on the number of muscle fibers that
a motor neuron innervates.
1. Small motor unit - muscles concerned with fine skilled
movements, number of muscle fibers in a motor unit varies from
2-6. example- middle ear muscles, ocular muscles.
2. Large motor unit – In muscles with gross and crude movements,
the number of muscle fibers in a motor unit varies from 100-600.
example- muscles of back and leg

33. Number of motor units in a muscle
The number of motor units in a muscle fiber vary from few
to few hundreds depending on the size of the muscle.

34. Recruitment of motor units
This depends on force of contraction required.
When there is a need for less force of contraction, fewer and smaller motor units
are activated.
When there is a need for more force of contraction, large and several motor units
are activated.
During prolonged low intensity activity, slow motor units are recruited
During short, high intensity activity, fast motor units are activated.
Motor units are activated asynchronously and this will help in smooth contractions
and delays early onset of fatigue.

36. Recruitment of motor units
It minimizes fatigue by allowing most fatigue-resistant muscle
fibers to be in use.
More fatigable fibers are kept in reserve until needed to
achieve higher forces
Cause smooth contraction and relaxation of muscle
The recruitment of motor units can be studied using
electromyography.

37. Force summation
Adding together individual twitch contractions to increase the
intensity of overall muscle contraction.
It can occur by
Increasing number of motor units contracting simultaneously-
multiple fiber summation
Increasing the frequency of contraction- frequency summation-
leads to tetanization.

38. Force summation
When weak signals were sent to the muscles to contract
from CNS, only small motor units are activated.
As there is an increase in the strength of the stimulus, larger
and larger motor units are recruited.
The phenomenon of activation of large motor units as the
strength of stimulus increase is size principle.

39. Frequency summation
As the frequency increases, there comes a point where each
new contraction occurs before the preceding one is over.
As a result, second contraction is added to first
Total strength of contraction raises progressively, with increase
in frequency.
Tetanization – at higher frequency muscle is only remains in
contraction and no relaxation.

40. Macromotor unit
Seen in poliomyelitis.
Motor neurons are destroyed and leads to paralysis of
corresponding muscle fibers
The neighboring nerve fibers give collateral branches that
supply paralyzed muscle fibers
Results in large motor unit-macromotor unit

41. Motor point
Point on the muscle at which the nerve enters the muscle
to establish contact with the individual muscle fiber.
A good response of the muscle is obtained when the
stimulus is applied at this point.

42. THANK YOU