Skeletal Muscle Mechanics-3

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2. OXYGEN DEBT

The body normally contains about 2 liters of oxygen:

0.5 litersAir in lungs

0.25 litersBody Fluids

1 literHb of Blood

0.3 litersMyoglobin

2. OXYGEN DEBT During muscular exercise, a lot more Oxygen is supplied

to the muscles than is present. After exercise, Oxygen is required for the recovery of the

energy systems. The total amount of oxygen required is: 2 liters of normally present oxygen must be replenished 9 liters extra must be provided for:

This Oxygen is needed for the following processes: 1) Resynthesis of the Creatine Phosphate to restore its

reserves.2) Conversion of lactate into pyruvate,3) Conversion of part of pyruvate by oxidative

phosphorylation system into ATP.

All this extra Oxygen (11.5 liters) that must be “repaid” to the body is called the Oxygen Debt.

This is the reason that a person must breathe rapidly even after the exercise is over!

3. MUSCLE TONEEven when muscles are at rest, a certain

amount of tension usually remains—This is called Muscle Tone.

Cause:Low rate of nerve impulses coming from the

spinal cord which are controlled by the: Signals from the brain to the spinal cord-

anterior motor neurons Signals that originate in the muscle

spindles located in the muscle itself-Intrafusal fibers

4. MOTOR UNIT

Definition: All the muscle fibers innervated by a

single nerve fiber are called a MOTOR UNIT.

When a motor neuron enters a muscle, it branches, with each axon terminal supplying a single muscle fiber. When this neuron is stimulated, all the muscle fibers supplied by it contract together.Each muscle consists of a number of mixed motor units.For a weak contraction of the whole muscle, only one or a few of its motor units are activated. The number of muscle fibers per motor unit and the number of motor units per muscle vary widely, depending on the specific function of the muscle.

4. MOTOR UNIT Number of muscle fibers in a motor unit vary in

different muscles from 2 or 3 to more than 1000. Average: 80-100 muscle fibers to a motor unit. Muscles which have to perform fine grade,

intricate movements have motor units with as few as 3-5 muscle fibers to a unit .e.g. hand, eye.

Muscles with relatively crude movements, number of muscle fibers is quite large. E.g. muscles of lower limbs.

In one whole muscle, different motor units overlap.

5. Force of Contraction Summation:

Summation: is the addition of individual twitch contractions to

increase the intensity of whole muscle contraction.

There are 2 types of summation:1. Multiple Fiber Summation (No. of

motor units stimulated)2. Frequency Summation

5. a: Multiple Fiber SummationDefinition:

It is the summation of individual muscle fiber contractions by increasing the number of motor

units contracting simultaneously.

Initially, with a weak signal from the CNS-only smaller units are stimulated.

Later, when signal from CNS becomes stronger, larger motor units are excited----This is called SIZE PRINCIPLE.

Importance: It allows gradation of force to occur for weak & strong contractions.

5. b: FREQUENCY SUMMATIONDefinitions:

Force of contraction increases by increasing the frequency of contractions. Two action potentials in a muscle fiber add

together to produce greater tension in the fiber than produced by a single action potential. This is called twitch summation

or frequency summation. Twitch summation is possible only because the duration of

the action potential (1-2 msec) is much shorter than the duration of the resulting twitch (100 msec). Because the AP and refractory period are over long before the resulting muscle twitch is completed, the muscle fiber may be restimulated to produce summation of the mechanical response.

Remember we are talking about the summation of the muscle contractions and not muscle action potentials.

FREQUENCY SUMMATIONWhen APs come one after the

other after the relaxation of the muscle is complete, identical

muscle twitches are obtained…….

When APs come one after the other before relaxation of the muscle is complete, then the 2 twitches add

or “summate” to give greater tension…

5. b: FREQUENCY SUMMATION

If APs continue to stimulate the muscle repeatedly at short intervals, there is no time for complete

relaxation between contractions ↓

Individual twitches fuse into one continuous contraction↓

Whole muscle contraction appears to be smooth, sustained & of maximal strength

↓This is called TETANIZATION or TETANUS

(A tetanic contraction is usually three to four times stronger than a single twitch.)

Physiologic basis of twitch summation & Tetanus:

The main reason is the sustained elevation in cytosolic Ca2+ permitting greater cross-bridge cycling. As the frequency of action potentials increases, the duration of elevated cytosolic Ca2+ concentration increases, and contractile activity likewise increases until a maximum tetanic contraction is reached. With tetanus, the maximum number of cross-bridge binding sites remain uncovered so that cross-bridge cycling, and consequently tension development, is at its peak.

TYPES OF TETANUSINCOMPLETE/ UNFUSED

TETANUSIf repeated stimuli given at a slower rate, then muscle fiber contractions grow stronger as they summate but

the relaxation between the cont. remains incomplete.

COMPLETE or FUSED TETANUS

If repeated stimuli are applied at fast rate, then no relaxation occurs

between the stimuli, muscle reaches max. tension and a sustained contraction phase is obtained.

6. TREPPE

DEFINITION: When a series of maximal

stimuli are delivered to the muscle at a frequency just below tetanizing frequency (when muscle twitch due to previous stimulus has just completed), the tension developed during each

twitch increases till a max. height is reached & a

plateau is formed. This is called the Treppe/ staircase effect.

Because the tension rises in stages, like the steps in a staircase, this phenomenon is called treppe, a German word meaning "stairs."CAUSE: The rise is thought to result from a gradual increase in the concentration of calcium ions in the sarcoplasm.

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