The contraction of the skeletal muscle is the result of a series of ultrastructural and biochemical events which are as follows:
Electron microscope studies by H.E. Huxley showed that as a muscle becomes shorter and shorter during contraction, the thick (myosin) and thin (actin) filaments slide past each other. It was also observed that during this sliding movement the filaments do not undergo any alteration in length. Therefore the width of the A -band remains constant. These observations led to the proposition of the sliding filament theory by H.E. Huxley and A.F. Huxley independently, in 1954, to explain the process of muscular contraction.
According to the sliding filament theory the shortening of muscles during contractions is due to the sliding movements of the thick and thin filaments past each other than from a change in the length of the filaments themselves. As the thin filaments slide past the thick filaments, the thin filaments (actin) meet in the centre of the sacromere. The I-band shortens and disappears when the sarcomere reaches the length of the thick filaments. It is thought that the cross bridges on the thick filaments might pull the thin filaments while muscle is contracted, but during relaxation these cross bridges disappear. This indicates the presence of active sites on the actin filaments into which the cross bridges temporarily hook to pull the filaments a short distance and then release them. It means that the contraction and relaxation of muscle are brought about by the repetitive formation and breakage of cross bridge respectively, between the thick filaments of A- band and the thin filaments of I - band.
The Sliding Filament Mechanism of Muscular ContractionChemical Events
Albert Szent Georgi and others worked out the chemical events associated with the muscle contraction. These chemical events can be summarised as follows:
- A motor nerve stimulates a muscle by the release of acetylcholine from the vesicles at the neuromuscular junction or motor end plate. The liberation of acetylcholine in turn evokes the release of calcium ions (Ca +)from the endoplasmic reticulum of the muscle into the interior of the muscle fibre.
- Myosin now binds with actin to form actomyosin in the presence of ATP (Adenosine triphosphate) and Ca++ ions.
- The immediate energy for muscle contraction comes from the breakdown of ATP into ADP (Adenosine diphosphate). The enzyme responsible for this reaction is myosin ATPase which is activated in the presence of Ca++ ions. Magnesium ions are also essential for ATPase activity and for contraction.
Neuro Muscular Junction
Thus the ATPase breaks up ATP into ADP and phosphate (p) plus energy which is used for the contraction of muscle fibre.
- At the end of the muscle contraction, the immediate reconversion of ADP into ATP takes place. The muscle is rich in glycogen which is broken down into lactic acid through a series of reactions together called glycolysis. Some of the energy liberated is used for the reformation of creatine phosphate and also for the conversion of 4 / 5th of lactic acid back into glycogen. The other 1 / 5th of lactic acid is oxidised to water and carbon dioxide (CO2). These reactions involving lactic acid which take place in muscle and liver are proposed by Cori and hence known as Cori's cycle.
When the muscle is repeatedly stimulated for prolonged time, the force of contraction decreases and eventualy falls to zero. This is known as muscle fatigue. This is due to the fact that during prolonged contraction the muscle uses up all its stored energy and the contractile proteins do not receive energy in the form of ATP. A muscle is able to contract even in the absence of oxygen for some time. Then it uses energy by the breakdown of ATP and from ATP resynthesised from ADP by CP. Therefore if muscles are deprived of oxygen, lactic acid accumulates as it is not reconverted to glycogen or oxidized due to insufficient oxygen supply. Muscle fatigue results due to accumulation of lactic acid. The muscles fatigue soon after a strenuous exercise than after a mild exercise.
The successive contraction and relaxation of the muscle depend up on the availability of energy in the form of ATP. Adenosine triphosphate performs the following functions:
- The energy released through the breakdown of ATP is readily used to move the cross bridges.
- The cross bridges become functional when the actomyosin bond is broken and for this the binding of ATP is necessary.
- The endoplasmic reticulum of the muscle, known as sarcoplasmic reticulum (SR) traps free calcium ions, and for this, ATP is utilised. When SR traps the Ca++ , the muscle undergoes relaxation. The Ca++ , ions are actively transported to SR, against the concentration gradient. The active transport is brought about by the utilisation of energy . For relaxation, the myosin ATPase activity has to be inhibited. The inhibition of myosin ATPase activity is due to the non -availability of Ca++ ions due to its active transport to SR. Relaxation can be represented as follows:
Since ATPase is inhibited due to the absence of Ca++ ions, ATP cannot be broken down to liberate energy. The non -availability of energy for contraction brings about relaxation.
Role of Muscles and bones in Movement
Movement of an organ occurs due to the pulling of the bones caused by the force generated by contracting muscles. Movement takes place along the joints which act as fulcrum of the lever. In fact, the bones and joints, function as lever, about which you have studied in physics. Functioning of all the three types of levers can be observed in the human skeleton. The joint between the first vertebra (atlas) and occipital bone of skull exhibits the example of first class lever, in which joint is the fulcrum, contraction of back muscle is the effort, and facial part of the skull on raised head acts as the resistance.
The example of second class lever, as the toe forms the fulcrum and contracting calf muscle provides effort distally. The body functions as resistance exerting in between the fulcrum and effort. The flexing movements of the elbow of forearm are based on the principle of third class lever. Here, the elbow-joint acts as fulcrum and the distal part of hand provides resistance. The contracting biceps muscles attached near the elbow joint exert the effort in between fulcrum and resistance.