The muscles in our body are made of huge number of muscle fibres. These muscle fibres are arranged in a number of small groups. Each of these groups comes under the control of a single motor neuron, the axon that sends a terminal branch to every fibre of the group. Now, the muscle fibres in a group contract whenever a nerve impulse goes to their motor neuron.
When a muscle gets stimulated it is followed by a short period during which it takes up the stimulus. Then it contracts and becomes short and thick. It relaxes and stretches after a while.
During the process of muscle contraction, the laterally projecting heads of the thick myosin myofilaments convene on the thin actin myofilaments and rotate on them. Due to this the thin myofilaments are pulled towards the middle of the sarcomere and move past the thick myofilaments. It marks the coming closer of the Z lines and the sarcomere becoming shorter. Length of the A band remains constant. Myofilaments remain the same length. The free end of actin myofilaments move closer to sacromere’s centre and bring the Z lines closer. H zone narrows and I bands shorten. When this action takes place in all the sarcomeres it gives way to the shortening of the myofibril and thus of the entire fibre and muscle. A contracted muscle gets thicker and shorter however its volume remains the same.
Changes During Muscle Contraction
Biochemical Changes
Various biochemical changes take place in a human body during muscle contraction. When a muscle fibre is in a resting position, sarcolemma is electronegative inside and electropositive outside. This difference across a membrane is known as resting potential. A membrane with this kind of resting potential is called polarised.
While the potassium ions are predominant in the inside of the sarcolemma, Sodium ions predominate its exterior. Because of the difference in concentration on these sides, the sodium ions entre the muscle fibre and the potassium ions leave. The process of potassium ions leaving the muscle fibre is faster than that of sodium ions entering the same and this develops a positive charge outside. As the motor nerve impulse moves near the neuro-muscular junction, the vesicles present at the motor end plate exude a neurotransmitter chemical known as acetylcholine (ach).
Physical Changes
The sarcomeres shorten to around 60-70% during muscle contraction. This is basically dependent on stimulus’ strength and the number of motor units involved. The tonicity increases during muscle contraction as more and more motor units come into the contraction phase. The energy exuded during muscle contraction is partly transformed to heat that aids in maintaining homeothermy. Also, the viscosity and density of sarcoplasm increases during the process.
Electrical Changes
The sarcolemma possesses a rest potential of 90mV. An action potential of +45 to +50 mv is created during contraction.
The Energy for Muscle Contraction
The energy for muscle contraction is obtained from the conversion of adenosine triphosphate (ATP) into inorganic phosphate and adenosine diphosphate (ADP) thereby releasing energy. An enzyme myosin ATPase catalyses the reaction while Ca2 and Mg2 ions are present. The used ATP needs to be restored for additional contractions. Hence phosphocreatine comes in and gives its energy-phospate bond to ATP thereby producing ATP. This reaction is known to be catalysed by an enzyme creatine kinase.
When the creatine phosphate is used a new ATP is created by aerobic respiration in muscle cells. In case the ATP is used up faster compared to the rate the muscle fibres can produce then the later begin anaerobic respiration to exude ATP. This results in the production of lactic acid which is then disseminated in to blood leaving a small segment to accumulate in the muscle fibre. Most of this lactic acid goes into the liver wherein it is oxidised to CO2 and H2O. The energy from this oxidation is then employed to change the rest of the lactic acid to glycogen.