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Mechanism Of Muscle contraction

Muscle Contraction

Motor Unit

• Single motorneuron & muscle fibers it innervates

• Eye muscles – 1:1 muscle/nerve ratio• Hamstrings – 300:1 muscle/nerve ratio

 Nicotinic

enlarged areas of the Sarcoplasmic reticulum surrounding the Transverse Tubules

deepInvaginatiom

of the sarcolemma,

allow depolarization of the membrane to quickly penetrate to the interior of

the cell

(RyRs)

Ryanodine receptors => RyRs (calcium-induced calcium release)

(physical coupling to the dihydropyridine receptor)

Dihydropyridine receptor => DHP

Ca2+unbinds from the calcium-binding protein called calsequestrin

H Band

Cross-Bridge Formation in Muscle Contraction

Z lineSarcom

ere 

Relaxed

Sarcomere Partially Contracted

Sarcomere Completely Contracted

 sarcoplasmic reticulum Ca2+-ATPase  actively pumps Ca2+ back into the sarcoplasmic reticulum where Ca2+ rebinds to calsequestrin.

Adenosine - PO4 ~ PO3 ~ PO3

11,000 calories of

11,000 calories of

•breakdown of ATP first to ADP and then to AMP

•Basic source of chemical energy for muscle contraction is adenosine triphosphate or ATP

Sources of ATP for Muscle Contraction

Phosphagen energy system

Glycogen-lactic acid system

Aerobic system

(Anaerobic)

(Anaerobic)

Creatine                 Creatine Phosphate(Molecule capable of storing ATP energy)

Creatine + ATP Creatine phosphate + ADP

(Molecule with stored ATP energy)

Creatine phosphate + ADP Creatine + ATP

Creatine ~ PO4 Creatine

+ PO4 + Energy

PO4 + Energy + ADP ATP

(Creatine phosphate recycles ADP back into ATP to provide fuel for our muscles)

Muscle Metabolic Energy Systems

Duration of Maximal Muscle Activity

Activity Types

Phosphagen System 3-5 seconds Power surges

Glycogen - Lactic acid system

30-40 seconds moreIntermediate athletic activities

Aerobic SystemUnlimited time (as long as nutrients last)

Prolonged athletic activities (marathon race)

The three different metabolic systems that we have at our disposal enable different degrees of muscle activation as      (different kinds of athletic events require different amounts of energy)

Sustaining Muscle contractions: ATP Sources/TimeSustaining Muscle contractions: ATP Sources/Time

Figure 25-2: Speed of ATP production compared with ability to sustain maximal muscle activity

athletic events requiring a quick "burst" of energy , amount of ATP needed only to be Cattered by Phosphagen System

Intermediate athletic activities require extra ATP to fuel the muscles. Now Glucose molecules comes to rescue. The extra ATP that is is provided by glucose break down in 

the absence of oxygen

each glucose molecule is split into two pyruvic acid molecules, and energy is released to form several ATPs, in addition phosphagen system

The pyruvic acid partly break down to lactic acid. If the lactic acid accumulates in the muscle, you will have muscle fatigue ( painful cramps)

aerobic system => for sports that require an extensive expenditure of energy Lots of ATP provided to your muscles to sustain the muscle power without excessive

production of lactic acid. (Oxidation- using glucose/glycogen/fatty acids)

Utilization of Energy Source for ATP Production

Figure 25-3: Use of carbohydrates and fats with increasing exercise

When loads are applied, the velocity of contraction becomes progressively less as the load increases. That is, when the

load has been increased to equal the maximum force that the muscle can exert, the velocity of contraction becomes zero and no contraction results, despite activation of the muscle

fiber.

This decreasing velocity of contraction with load is caused by the fact that a load on a contracting muscle is a reverse force

that opposes the contractile force caused by muscle contraction. Therefore, the net force that is available to cause

velocity of shortening is correspondingly reduced.

muscle contracts against a load => performs work.energy is transferred from the muscle to the external load to lift an object

to a greater height or

energy is transferred to overcome resistance to movement.

W = L X D

W is the work output, L is the load, and D is the distance of movement against the load.

energy required to perform the work is derived from the chemical reactions in the muscle cells during contraction,

Most of this energy is required to carry out mechanism by which the cross-bridges pull the actin filaments,

small amounts are required for

1. pumping calcium ions from the sarcoplasm into the sarcoplasmic reticulum after the contraction is over

2. pumping sodium and potassium ions through the muscle fiber membrane to maintain appropriate ionic environment.

rate of formation of ATP by the glycolytic process is about 2.5 times as rapid as ATP formation in response to cellular

foodstuffs reacting with oxygen

glycolytic reactions can occur even in the absence of oxygen,

muscle contraction can be sustained for sometimes up to more than a minute, even when oxygen delivery from the

blood is not available

final source of energy is oxidative metabolism.

combining oxygen with the end products of glycolysis and with various other cellular foodstuffs to liberate ATP.

>95% of all energy used by muscles for sustained, long-term contraction is derived from this source of foodstuffs

For extremely long-term maximal muscle activity-over a period of many hours-by far the greatest proportion of energy

comes from fats

for periods of 2 to 4 hours, as much as one half of the energy can come from stored carbohydrates.

Efficiency of Muscle Contraction

The percentage of the input energy to muscle (the chemical energy in nutrients) that can be converted into work, even under the best conditions, is less than 25 percent, with the

remainder becoming heat.

The reason for this low efficiency is that about half of the energy in foodstuffs is lost during the formation of ATP,

even then, only 40 to 45 percent of the energy in the ATP itself can later be converted into work

Maximum efficiency can be realized only when the muscle contracts at a moderate velocity. If the muscle contracts slowly or without any

movement, small amounts of maintenance heat are released during contraction, even though little or no work is performed,

Conversely, if contraction is too rapid, large proportions of the energy are used to overcome viscous friction within the muscle itself, and this,

too, reduces the efficiency of contraction.

Ordinarily, maximum efficiency is developed when the velocity of contraction is about 30 percent of maximum.