Whenever a tension-generating factor affects the muscles, the muscle either contracts or relaxes. Muscle contraction causes a temporary increase or decrease of length in the muscles. For instance, when you hold a heavy object, you can feel the contraction of muscles in your arm. The types of muscle contraction depend upon the length of the muscle and the tension acting upon it. In the case of vertebrates, the contraction of muscles has a neurogenic effect on them. They require a synaptic input through motor neurons for the contraction. 

The muscles contain myosin fibres. These myosin fibres either tighten or loosen according to the body’s requirement. It also causes the contraction of cardiac muscles, which causes the heartbeat. The adenosine triphosphate (ATP) is hydrolysed from myosin filaments and typically interacts with the actin filaments that give energy for contraction.

The primary cause of the contraction of muscles is to provide stability to the joints and connective tissues. Muscle contraction and relaxation is essential for  proper locomotion and also help maintain the body’s posture.

Types of Muscles 

Vertebrates have three types of muscles: skeletal, cardiac and smooth muscles. Skeletal muscles are in the largest proportion. The mechanism of muscle contraction on the basis of type of muscles is as follows-

• Skeletal Muscles-These are attached to bones so are known as skeletal muscles. These muscles have light and dark coloured bands called striations which become prominent when stained with basic dyes like methylene blue.

 The light coloured bands have  actin filaments (made up of actin proteins), thin and elastic in nature which provide flexibility to the muscles. While dark coloured bands have myosin filaments (made up of myosin proteins),thick and less flexible, provide endurance to the muscles. Actin and myosin are alternately present  in the myofibrils, which is the structural and functional unit of  striated muscles. These muscles are also called voluntary muscles, i.e., we can control their mechanism. These muscle fibres are long, cylindrical and blunt at the ends, multinucleate with big, oval and peripherally arranged nuclei and are unbranched. Skeletal muscles contract as a neurogenic response that originates in the brain. The contraction of skeletal muscles can be either conscious or unconscious. 

• Smooth muscles- These muscles  do not work according to our will so are called involuntary muscles. These do not have light and dark coloured bands like skeletal muscles so are called smooth muscles. These muscle fibres are short, tapering at ends, uninucleate and unbranched.Smooth muscles are present in the gut, blood vessels, etc.. They are divided into two sub-groups, single-unit and multi-unit. Gap junctions unite these two units together to enable them to contract as a syncytium. 

The single-unit cells contract myogenically. The autonomic nervous system modulates its contraction. The multi-unit cells contract separately stimulated by units of the autonomic nervous system. Such muscles are present in the eye muscles and in the base of hairs. 

• Cardiac Muscles- The muscles present in the heart are called cardiac muscles. These muscles are involuntary as we cannot control their activity. These work continuously without getting fatigued. These muscles are branched, intercalated discs are present between the segments, each segment is uninucleate and faint striations are present. 

There are two types of cardiac muscles: autorhythmic and contractile. The autorhythmic muscles cannot contract themselves, but they help in contraction. In contrast, the contractile muscles can contract. Most of the heart is made of contractile muscles. The autonomic nervous system modulates the contraction of cardiac muscles.

Muscle Contraction Definition

Shortening of muscles with response to external stimuli or the wilful urge of moving is called muscle contraction. 

Mechanism of Muscle Contraction

The mechanism is best  explained by sliding filament theory –

• As the process is neurogenic, the nervous system generates an action potential. The motor neurons transmit the message by releasing a neurotransmitter (acetylcholine) from the nervous system to the muscle membrane. This message triggers chemical reactions.
• This causes release of calcium ions into the muscle cytoplasm (sarcoplasm). Increase in these ions help the ends of actin filaments to join the myosin filaments (these contractile proteins found in the myofibrils).

• The reactions cause the muscles to contract as the actin filaments are pulled closer to each other.
• As soon as the message of the nervous system is no longer present, the muscles return to their normal, relaxed position.

Types of Muscle Contraction 

The contraction of muscles is differentiated based on two factors- force and length. They are classified into four categories, which are as follows:

• Isometric Contraction- Isometric contraction of the muscles does not involve changing their length. Here is an isometric muscle contraction example. The muscles of the hand and forearm provide a grip for holding objects without changing their length. Although it does not change the length, it provides enough force to prevent the object from falling.
• Isotonic Contraction- In the isotonic contraction, the length of the muscles changes while the tension remains constant. In other words, the muscles remain in constant tone even while contracting. Examples of isotonic contraction include squats and stair climbing. 
• Concentric Contraction- In this type of contraction, the muscles get shorter when you lift something heavy. In other words, the muscles shorten to overcome the tension or force acting on them. For example, the upward phase of a bicep curl involves the concentric contraction of the muscles. Another example of such a type of contraction is when you squat down to pick up a heavy box. The muscles of the arm contract to hold the weight, and simultaneously, the leg muscles tighten to overcome the weight.
• Eccentric Contraction- In this type of contraction, the muscles of the part under tension are actively lengthened during activity. An example of eccentric muscle contraction is walking. In this movement, your quadriceps muscles become active when your heel touches the ground, and your knee bends or straightens. Another example is when you lower a heavy load; your muscles remain tight to manage the weight. It also lengthens for shifting the weight in a different position.

Muscles with heavy eccentric work are more prone to damage due to overload. Therefore, exercises which contain both concentric and eccentric contractions are necessary, rather than only eccentric, that can cause real harm to your muscles.

Conclusion 

Muscle contraction is the shortening, lengthening, and tightening of the muscles. Muscle contraction depends upon the force under effect. It creates tension among the muscles leading them to contract. Contraction is a neurogenic effect, i.e. the nervous system controls it. The nervous system sends signals in the form of electrochemical impulses and motor neurons. 

There are four basic types of muscle contraction depending upon the force and length. These are isometric, isotonic, eccentric and concentric. There are three basic muscles under contraction in vertebrates: skeletal muscles, smooth muscles and cardiac muscles. The skeletal muscles are voluntary, whereas the cardiac and smooth muscles are involuntary.