Muscle Contraction: Definition, Role, Causes

Muscle contraction is a series of molecular processes in which bulky myosin filaments connect to and pull on thin actin filaments, causing them to glide over one another. The actin filaments are connected to Z discs, which each represent the end of a sarcomere.

Therefore, muscle contraction is triggered by a signal sent by the central nervous system via a motor neuron. 

Muscle Contraction Definition

When we do any activity then the tightening, shortening, or lengthening of muscles that occurs is known as muscle contraction. It might happen when we stretch or workout with weights, or when we hold or lift up anything.

 Muscle contraction definition states that it is frequently followed by muscle relaxation or the return of contracted muscles to their natural condition. 

Role of Sarcoplasmic Reticulum in Muscle Contraction

The sarcoplasmic reticulum accumulates calcium ions and releases them into the sarcoplasm when an action potential is generated during muscle contraction. Calcium ion binding to troponin eliminates the masking of active myosin sites on actin filaments.

Muscle contraction is caused by calcium ions. The action potential causes the sarcoplasmic reticulum to release calcium ions, which bind to troponin on actin filaments and exposes myosin-binding sites owing to conformational changes.

Role of Calcium in Muscle Contraction

The thin filaments glide across the larger filaments during muscular contraction. Muscle contraction definition clearly mentions that it is initiated by a signal delivered by the central nervous system via a motor neuron. When a brain signal reaches a neuromuscular junction and an action potential is formed in the sarcolemma, acetylcholine is released. Calcium ion is released in the sarcoplasm as this moves across the muscle fibre. Calcium then binds to troponin on actin filaments, exposing myosin active sites.

Sliding Filament Theory

The sliding filament theory is the established explanation for understanding muscle contraction. Muscle contractions, according to this view, are part of a cycle of molecular processes. The thin actin and bulky myosin filaments glide past each other during muscle contraction. The procedure is carried out by cross-bridges that are extended from myosin filaments. Sarcomere is the name given to the contractile unit of muscle. Actin filaments are joined to the Z disc at the sarcomere’s end. The Z disc is then dragged farther by the sliding filament, resulting in sarcomere shortening and muscular contraction.

Voluntary muscular contractions begin in the brain, whereas involuntary reflexes begin in the spinal cord.

Why and How Muscle Contract

Muscles perform numerous functions in the body. As per muscle contraction definition, muscles contract for a variety of causes, but the most common ones are as follows: ‌

• To offer stability to your joints and connective tissues -Muscles lengthen and shorten, voluntarily or sometimes involuntarily, as per the need of the body
• To produce heat to keep body temperature stable – Muscle work accounts for around 40% of body temperature. Shivering is the body’s response to feeling cold, and thereby skeletal muscles activate to warm the body
• To maintain posture – Muscles help to maintain a position like sitting or standing

Myosin fibres are found in muscles. Myosin fibres either constrict and shorten or loosen and stretch out depending on how one needs to use one’s muscles. Myosin is also in charge of muscular contractions, such as heartbeat, which occurs at regular intervals.

Muscle Contraction Example

A muscle contraction example is a concentric contraction. It is a sort of muscular activation that shortens your muscle by exerting strain on it. When you shorten your muscle, you produce enough force to move an object.  This form of muscle contraction is very common.  A bicep curl is an easy-to-recognize concentric action in weight training.

Another muscle contraction example is eccentric contractions. When muscles actively extend during typical activity, this sort of contraction occurs. Walking is an example of this contraction since our quadriceps muscles are activated when our heel strikes the ground, and our knee bends or straightens out in stride. ‌

When we drop something heavy, eccentric muscular contractions occur as well. To manage the weight, our muscles must remain tight, yet they must extend to move the weight into a different position.