1. MECHANICAL PROPERTIES OF SKELETAL MUSCLE

2. Motor Unit
A motor unit consists of a somatic motor neuron plus all the skeletal muscle fibers it stimulates
A single somatic motor neuron makes contact with an average of 150 skeletal muscle fibers, and all of the muscle fibers in one motor unit contract in unison

4. Motor Unit
A twitch contraction is the brief contraction of all the muscle fibers in a motor unit in response to a single action potential in its motor neuron

5. Note that a brief delay occurs between application of the stimulus (time zero on the graph) and the beginning of contraction. The delay, which lasts about two milliseconds, is termed the latent period.
During the latent period, the muscle action potential sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum.
The second phase, the contraction period, lasts 10–100 msec. During this time, Ca2 binds to troponin, myosin-binding sites on actin are exposed, and crossbridges form. Peak tension develops in the muscle fiber.
During the third phase, the relaxation period, also lasting 10–100 msec, Ca2is actively transported back into the sarcoplasmic reticulum, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases

6. Q. In muscle physiology, the latent period refers to
a. the period of lost excitability that occurs when two stimuli are applied immediately one after the other.
b. the brief contraction of a motor unit.
c. the period of elevated oxygen use after exercise.
d. an inability of a muscle to contract forcefully after prolonged activity.
e. a brief delay that occurs between application of a stimulus and the beginning of contraction

7. Note that a brief delay occurs between application of the stimulus (time zero on the graph) and the beginning of contraction. The delay, which lasts about two milliseconds, is termed the latent period.
During the latent period, the muscle action potential sweeps over the sarcolemma and calcium ions are released from the sarcoplasmic reticulum.
The second phase, the contraction period, lasts 10–100 msec. During this time, Ca2 binds to troponin, myosin-binding sites on actin are exposed, and crossbridges form. Peak tension develops in the muscle fiber.
During the third phase, the relaxation period,also lasting 10–100 msec, Ca2is actively transported back into the sarcoplasmic reticulum, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases

8. PROPERTIES OF SKELETAL MUSCLE
Extensibility & elasticity
Excitability
Conductivity
Contractility
Tonicity
Refractory period
Fatigue
Muscle is a excitable tissue like nerve fibre/cell
When therashold stimulus applied it produce contraction/response.

9. Extensibility:Ability of the muscle to elongate when stretched.
Elasticity: Ability of the muscle to return to its original length when stretch is removed.
Excitability: Ability of the muscle to respond to a stimulus.(electrical ,thermal ,chemical,mechanical).
Contractility: Ability of the muscle to shorten or contract in response to a stimulus.

10. Conductivity: Ability of the muscle to transmit an impulse (AP) from one part of the fibre to another part.
The condution velocity of an impulse in skeletal muscle fiber is slow. it is about 5 meters/sec
Tone:The state of partial sustained contraction seen in all muscles.

11. REFRACTORY PERIOD: It is about 30-50millisec
REFRACTORY PERIOD: It is about 30-50millisec . It is a period of action potential in which another stimulus applied will not produce a response in a muscle.
Types: Absolute
Relative

12. The refractory period of a cardiac muscle fiber lasts longer than the contraction itself .
As a result, another contraction cannot begin until relaxation is well underway. For this reason, tetanus (maintained contraction) cannot occur in cardiac muscle as it can in skeletal muscle.
The refractory period is short in skeletal muscle, but very long in cardiac muscle – 250 msec
This means that skeletal muscle can undergo summation and tetanus, via repeated stimulation
Cardiac muscle CAN NOT sum action potentials or contractions and can’t be tetanized

13. Isometric contraction
CONTRACTILITY: Ability to contract in response to a stimulus.
Types :-
Isotonic contraction
Isometric contraction

14. Isotonic and Isometric Contractions
Isotonic contraction:The contraction that occurs when the muscle is allowed to freely shorten,so that tension in the muscle is kept constant.
Isometric contraction: the contraction that occurs without any shortening of the muscle, so that the tension increases, but the length of the muscle remains constant.

16. In a twitch, isometric force develops relatively rapidly, and subsequent
isometric relaxation is somewhat slower. The durations
of both contraction time and relaxation time are related
to the rate at which calcium ions can be delivered to
and removed from the region of the crossbridges, the actual
sites of force development. During an isometric contraction,
no actual physical work is done on the external environment
because no movement takes place while the force
is developed. The muscle, however, still consumes energy
to fuel the processes that generate and maintain force.

17. When the muscle is stimulated,
it will begin to develop force without shortening,
since it takes some time to build up enough force to begin
to lift the weight. This means that early on, the contraction
is isometric (phase 1; Fig. 9.8). After sufficient force has
been generated, the muscle will begin to shorten and lift
the load (phase 2). The contraction then becomes isotonic
because the force exerted by the muscle exactly matches
that of the weight, and the mass of the weight does not
vary. Therefore, the upper tracing in Figure 9.8 shows a flat
line representing constant force, while the muscle length
(lower tracing) is free to change. As relaxation begins
(phase 3), the muscle lengthens at constant force because it
is still supporting the load; this phase of relaxation is isotonic,
and the muscle is re extended by the weight. When
the muscle has been extended sufficiently to return to its
original length, conditions again become isometric (phase
4), and the remaining force in the muscle declines as it
would in a purely isometric twitch. In almost all situations
encountered in daily life, isotonic contraction is preceded
by isometric force development; such contractions are
called mixed contractions (isometric-isotonic-isometric

18. 1 Same Tension In The Muscle
Isotonic Contraction
1 Same Tension In The Muscle
2 Muscle Length Decreases
3 Work Is Done –Weight Lifted
4 Extra Heat produced.Relaxation Heat produced After Contraction
5 Greater Energy Is Used
6 Eg-1. Muscles of upperlimb while lifting weight ,lifting the leg while walking
Isometric Contraction
1same Length-muscle Length Remains Constant
2 Increase In Tension
3 Work Performed Is Not Seen
4 Less Heat Produced
5 Less Energy Is Used
6 Eg:- Calf muscles on standing

19. How do we control the strength of contraction?
Large Motor unit involved
More motor units recruited
More fast type II b types of fiber
Increasing the rate of stimulation
All of these will increase the force of contraction

20. SUMMATION
When the strength of the stimulus is increased the contractile response also increases.The increase in response is due to recruitment of motor units. This is called quantal summation or multimotor unit summation

21. In muscles adapted
for fine and precise control, only a few muscle fibers are associated
with a given motor axon; in muscles in which high
force is more important, a single motor axon controls many
more muscle fibers. The total force produced by a muscle is
determined by the number of motor units active at any one
time; as more motor units are brought into play, the force
increases. This phenomenon, called motor unit summation,
is illustrated in Figure 9.6. The force of contraction of
the whole muscle is further modified by the degree

22. Twitch, Summation, and Tetanus
Muscle is stimulated with a single electrical shock (above threshold).
Quickly contracts and then relaxes.
Summation:
If second electrical shock is administered before complete relaxation of muscle.

23. Temporal summation of muscle twitches
Temporal summation of muscle twitches. A,The first contraction is in response to a single
action potential.
B,The next contraction shows the summed re-sponse to a second stimulus given during relaxation; the two indi-vidual responses are evident.
C,The last contraction is the resultof two stimuli in quick succession. Though measured force was still rising when the second stimulus was given, the fact that there could be an added response shows that internal activation had be-gun to decline. In all cases, the solid line in the lower graph rep-resents the actual summed tension.

24. Twitch, Summation, and Tetanus (continued)
Incomplete tetanus:
Stimulator delivers an increasing frequency of electrical shocks.
Relaxation period shortens between twitches.
Strength of contraction increases.
Complete tetanus:
Fusion frequency of stimulation.
No visible relaxation between twitches.
Smooth sustained contraction.
Treppe:
Staircase effect.
Electrical shocks are delivered at maximum voltage.
Each shock produces a separate, stronger twitch (up to maximum).
Due to increase in intracellular Ca2+.
Represents “warm-up.”

26. Simple Twitch, Summation, and Tetanus

27. TETANUS
When the number of stimuli are more than two but less than tetanizable frequency then partial tetanus is obtained –clonus 30/sec
Tetanus- when stimuli are applied at a higher frequency, tetanus-40/sec (continuous contraction) is produced.
Infection by clostridium tetani is due to synchronous discharge from all the nerve fibers-leading to tetanic contraction of all skeletal muscles .

28. Preload
Preload is the load on a muscle in a relaxed state, that is, prior to contraction.
Applying preload to muscle does two things:
Causes the muscle to stretch. The greater the preload added, the greater the stretch of the muscle. Along with stretching the muscle, preload stretches the sarcomere. The greater the preload, the greater the pre- stretch of the sarcomere.
Causes the muscle to develop passive tension. If a 2-g weight is suspended from a muscle, a 2-g force also develops within that muscle. This force is the passive tension. The greater the preload, the greater the passive tension in the muscle.
Normally a muscle is
protected against overextension by attachments to the
skeleton or by other anatomic structures. If the muscle has
not been stimulated, this resisting force is called passive
Force or resting force

29. Afterload
Afterload is the load the muscle is working against or trying to move during stimulation.
If the muscle is trying to lift 100 lb. during stimulation, then the afterload is 100 lb.
During contraction, the muscle does not necessarily lift or move the afterload.

30. LENGTH-TENSION CURVES
Preload-length Tension Curve
resting skeletal muscle acts as a simple spring. As preload is added, the muscle stretches and develops a passive tension. The passive tension can be considered an internal force that opposes and equals the preload force.

32. ISOMETRIC CONTRACTION OF THE ISOLATED SKELETAL MUSCLE
During an isometric (same length) contraction, the overall muscle length will not change.
the cross-bridge cycling will produce active tension
Active Tension Development
The active tension developed during an isometric contraction is proportional to the number of these cross-bridges that cycle. The more cross-bridges that cycle, the greater the developed active tension.

33. the active tension is maximal when there is maximal overlap of thick and thin filaments and maximal possible cross-bridges which is at resting state.
When the muscle is stretched to longer lengths, the number of possible cross-bridges is reduced, and active tension is reduced.
When muscle length is decreased, the thin filaments don’t have enough space to slide on thick filaments so more active tension can’t be generated.
The amount of active force or
active tension a muscle can produce during an isometric
contraction depends on the length at which the muscle is
held. At a length roughly corresponding to the natural
length in the body, the resting length,the maximum force
is produced. If the muscle is set to a shorter length and then
stimulated, it produces less force. At an extremely short
length, it produces no force at all. If the muscle is made
longer than its optimal length, it produces less force when
stimulated. This behavior is summarized in the length-ten-sion curve

34. A length-tension curve for skeletal muscle
A length-tension curve for skeletal muscle. Contractions are made at several resting lengths, and the resting (passive) and peak (total)forces for each twitch are transferred to the graph at the right. Subtraction of the passive curve from the total curve yields the active force curve.
The muscle length is changed only
when the muscle is not stimulated, and it is held constant
(isometric) during contraction.

35. Total Tension
The preload creates a passive tension prior to contraction, and cross-bridge cycling during contraction adds an active tension component.
The total tension in the active muscle is the passive or preload tension plus the active tension.

37. In muscle mechanics, there are two types of tension:
l Passive tension: produced by preload prior to contraction
l Active tension: produced by cross-bridge cycling during the process of con-traction
Length-tension relationship in skeletal muscle. Maximal active tension occurs at muscle lengths where there is maximal overlap of thick and thin filaments.

38. Q. All of the following will occur when an unstimulated muscle is stretched except:
A. increased preload
B. increased afterload
C. increased muscle length
D. increased passive tension

39. Q. The figure depicts the isometric length-tension relationship of skeletal muscle. Identify the region where actin and myosin overlap is the least

41. FORCE-VELOCITY RELATIONSHIP
describes the velocity of shortening when the force against which the muscle contracts i.e. the afterload, is varied
Muscle A: a smaller, slower muscle (red muscle)
Muscle B: a larger, faster muscle (white muscle)
*Maximum velocity (Vmax) is determined by the muscle’s ATPase activity. It is the ATPase activity that determines a fast versus a slow muscle.
**Maximum force generated by a muscle (or maximum load lifted by a
muscle) is determined by muscle mass or, putting it another way, the num-ber of motor units activated during contraction. The greater the muscle
mass, the greater the maximum force generated.

42. Isotonic (same tone) contraction - The force, rather than the length, is fixed i.e. the muscle contracts with the same force
The velocity of shortening reflects the speed of cross-bridge cycling.
the velocity of shortening will be maximal (Vmax) when the afterload on the muscle is zero.
As the afterload on the muscle increases, the velocity will be decreased because cross-bridges can cycle less rapidly against the higher resistance.
As the afterload increases to even higher levels, the velocity of shortening is reduced to zero.

43. Classification of Fiber Types in Skeletal Muscles

44. Type I Red fibers: in postural muscles
Examples:
Type I Red fibers:  in postural muscles
Large myoglobin content and many mitochondria
Type IIa Red fibers: in muscles needed for activities like middle distance running, swimming, etc.
Type IIb White fibers: needed for activities like sprinting
Low myoglobin content and few mitochondria