MUSCLE CONTRACTION

MUSCLE CONTRACTION:

Introduction:

·         Muscles cells are specialized for contraction. Muscles help in motion such as walking, running, jumping and they also facilitative bodily processes such as respiration and digestion.

·         The body contains three types of muscle tissue; skeletal muscle, cardiac muscle and smooth or visceral muscle.

Conditions for skeletal muscle contraction:

         i.            Energy system fatigue:  There is more ATP left in the muscle cell so it cannot keep contracting.

       ii.            Nervous system fatigue: The nervous system is not able to create impulses sufficiently or uickly enough to maintain the stimulus and cause Ca+2 ion to release.

      iii.            Voluntary nervous system control: The nerves that tell the muscle to contract and stop sending that signal because the brain tells it to, so no more calcium ions will enter the muscle cell and the contraction stops.

     iv.            Sensory nervous system information: A sensory neuron provides feedback to the brain indicating that a muscle is injured while you are trying to lift a heavy weight and consequently the impulse to that muscle telling it to contract is stopped.

Stimulation of Muscle contraction:

Neurons or nerve cell are stimulated when the polarity across their plasma membrane change. The polarity change is called an action potential which travels along the neuron until it reaches the end of neuron. A gap called synapse or synaptic cleft separates the neuron from a muscle cells or another neuron. If a neuron stimulates muscle, then the neuron is motor neuron and this specialized synapse is called neuro-muscular junction.

Muscle contraction is stimulated through the following steps:

·         Action potential generates release of acetylcholine, when an action potential of a neuron secretes the neurotransmitter- acetylcholine which diffuses across synaptic cleft.

·         Action potential is generated on the motor end plate and throughout the T-tubules. Receptor on the motor end plate, a highly folded region of the sarcolemma, initiates an action potential. The action potential travels along the sarcolemma throughout the transverse system of the tubule.

·         As a result of the action potential throughout the transverse system of the tubule, Sarcoplasmic reticulum releases calcium ion.

·         Myosin cross bridges form. The calcium released by the sarcoplasmic reticulum binds to troponin molecule on the actin helix, prompting tropomyosin molecule to expose binding sites for myosin cross bridges formation if ATP is available, muscle contraction begins.

Phases of muscle contraction:

A muscle contraction in response to a single nerve action potential is called twitch contraction. A myogram, a group of muscle strength with time, shows several phases:

1.       The latent period is the time required for the release of calcium ion.

2.       Contraction period represents the time taken during actual muscle contraction.

3.       The relaxation period is the time during which the calcium ions are released from the sarcoplasmic reticulum by active transport.

4.       The refractory period is the time immediately following a stimulus. This is time period when a muscle is contracting and therefore will not respond to second stimulus. Since this is occurring at the same time as the contraction, it does not appear on the myogram as a separate event.

 SLIDING FILAMENT MODEL OF CONTRACTION:

For a muscle cells to contract, the sarcomere must shorten. However, thick and thin filaments- the component of sarcomere do not shorten. Instead, they slide by one another, causing the sarcomere to shorten while the filaments remain in same length. The sliding filament theory of muscle contraction was developed to fit the differences observed in the named bands on the sarcomere at different degrees of muscle contraction and relaxation. The mechanism of contraction is the binding of myosin to actin and forming cross bridges that generate filament movement.

When a sarcomere shortens, some of the regions shorten while others stay in the same length. A sarcomere is defined as the distance between two consecutive discs or Z- lines, when a muscle contracts, the distance between two discs is reduced. The H- zone, the center region of the A zone contains only thick filaments and is shortened during contraction. The I-bond contains only thin filaments and also shortens. The A band does not shorten, it remains in same length. But A- bands of different sarcomeres move closer together during contraction, eventually disappearing. The filaments are pulled by the thick filaments towards the centre of the sarcomere until the Z- discs approach the thick filaments occupy the same area, increases as the thin filament move inwards.

 

Process of muscle contraction:

The process of muscle contraction occurs over a number of key steps:

        I.            Depolarisation and calcium ion release:

·         An action potential from motor neurons triggers the release of acetylcholine into the motor end plate.

·         Acetylcholine initiates depolarisation within the sarcolemma which is spread throughout the muscle fibres like tubules.

·         Depolarisation causes the sarcoplasmic reticulum to release stores Ca+2 ion.

·         Ca+2 ion plays a pivotal role in initiating muscle contraction.

      II.            Actin and myosin cross bridge formation:

·         On actin; the binding sites for the myosin heads are covered by a blocking complex.

·         Calcium ions bind to troponin and reconfigure the complex, exposing the binding sites for the myosin heads.

·         The myosin heads then form a cross bridge with the actin filaments.

    III.            Sliding mechanism of actin and myosin:

·         ATP binds to the myosin head, breaking the cross bridges between actin and myosin.

·         ATP hydrolysis causes the myosin heads to change position and swirl, moving them towards the next binding actin binding site.

·         The myosin heads bind to the new actin sites and return to their original confirmation.

·         This reorientation drags the actin along the myosin in a sliding mechanism.

·         The myosin heads move the actin filaments in a smilar fashion to the way in which an oar propels a row boat.

    IV.            Sarcomere shortening:

·         The repeated reorientation of the myosin heads drags the actin filament along the length of the myosin.

·         As actin filaments are anchored to the Z- lines, the dragging of actin pulls the Z-lines close together, shortening the sarcomere.

·         As the individual sarcomeres become shorter in length, the muscle fibers as whole contracts.

Quality of Muscle contraction:

The following factors contribute to the strength and mass duration of muscle contraction.

1)      Frequency of stimuli: If stimuli are repeatedly applied to a muscle fiber. Calcium may not be completely transported back into the Sarcoplasmic reticulum before the next stimulus occurs. Depending upon the frequency of stimulus, calcium ions may accumulate. In turn, the actual Ca+2 results in more power strokes and a stronger muscle contraction.                            Depending upon the frequency of stimulus, such effects are observed.

a)      Staircase effect (treppe): It is produced if excessive stimulus occurs after the relaxation period of the previous stimulus. Each successive muscle contraction is greater than the previous one upto some maximum value. In addition to the accumulation of calcium, other factors such as increase in temperature and change in Ph contribute to the warming up effect commonly employed by athelete.

b)      Wave (temporal) summation: It occurs if consecutive stimuli is applied during the relaxation period of each preceding muscle contraction. In this case, each subsequent contraction builds upon the previous contraction before its relaxation period ends.

c)       Incomplete tetanus:  It is also called as unfused tetanus. It occurs when the frequency the stimulus increases. Successive muscle contraction begin to blend which almost appearing as a single large contraction.

d)      Complete tetanus: It is also called as fused tetanus which occur when the frequency of stimuli increases. In this case individual muscle contraction completely fuse to produce one large muscle contraction.

2)      Strength of stimulus: Muscle contraction intensify when more motor neurons stimulate more muscle fibers.This effect called recruitment or multiple motor unit summation. IT is also responsible for fine motor coordination because by continually varying the stimulation of specific muscle fibers, smooth body movements are maintained.

3)      Length of muscle fiber contraction: Muscles are attached to bones, muscle contraction is restricted to length that are between 60% and 175% of the length limits myosin cross bridges and actin to positions only when they overlap and can generate contraction.

4)      Type of contraction: Muscle contraction implies that movement occur s between myosin cross bridges and actin. However, this movemnet does not necessari;y result in shortening of the muscle. As a result two kinds of muscle contraction are defined:

a)      Isotonic contraction: It occurs when the muscle do not change its length at the time of contraction.  For example, picking up a book.

b)      Isometric contraction: This tyoe of contaction occurs when muscle do not change its length during contraction. For example, when holding a book in midair. In this case muscle fibers produce a force but no motion is generated.

5)      Muscle fatigue: When inadequate amount of ATPs are available then muscle fibers stop the conytraction process. Some factors contribute to the muscle fatigue such are; Lack of oxygen and glycogen and the accumulation of lactic acid.

6)      Muscle tone: In any relaxed skeletal muscle, a small number of contraction continue to occur. Such factors are observed as firmness in a muscle; they maintain body posture and increases muscle readiness.

Conclusion:

·         Action potential in a motor neurons triggers the release of calcium ions from the sarcoplasmic reticulum.

·         Calcium ion binds to troponin (on actin) and cause tropomyosin to move, exposing binding sites for the myosin heads.

·         The actin filaments and myosin heads form a cross bridge that is broken by ATP.

·         ATP hydrolysis causes the myosin heads to swivel and change the orientation .

·         Swiveled myosin heads bind to the actin filament before returning to their original confirmation. (releasing ADP+Pi)

·         The reposition of the myosin heads moves the actin filaments towards the center of the sarcomere.

·         The sliding of actin along myosin shortens the sarcomere, causing muscle contraction.

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