Every time you move, your muscles contract and relax. Although, it just takes our body a few seconds, contracting or relaxing a muscle is quite a complex process, and this Bodytomy article details the various steps involved in contracting a muscle.
Every single activity in our body is controlled by muscles. Whether it is a vital function like breathing, circulation of blood, or digestion; or something simpler like smiling, frowning or winking – muscles make it possible. But it’s not just the movement that muscles control, they also play a role in keeping our body warm, maintaining its posture, and stabilizing the joints and bones.
A muscle is basically a bundle of fibers wrapped in a connective tissue. Each fiber is made of a single, elongated muscle cell, and each cell is composed of myofibrils, which in turn are made of myofilaments. The thick myofilaments are made of myosin, and thin myofilament of Actin, Troponin, and Tropomyosin.
The primary mode of action for muscle is by contraction. What are the steps in muscle contraction? When the CNS sends a signal, the thick and thin myosin filaments form a “crossbridge” pattern by sliding past each other. This makes the sarcomeres shorter and thicker, contracting the muscle.
Muscle Contraction Steps in Detail
- A signal is sent from the brain or the spinal cord to the muscle via neurons
- An action potential is generated in the neuron, releasing Ca++ in the neuromuscular junction
- The influx of caalcium ions causes acetylcholine (AcH) to be released in the synaptic cleft
- AcH binds to the AcH receptors present in the sarcolemma, increasing its permeability
- Na++enter the sarcolemma, changing its polarity, and creating an action potential
- Ca++ are released by the sarcoplasmic reticulum, as the action potential travels down the T-tubules in the muscle fiber
- Ca++ bind with troponin C, causing the tropomyosin to shift, and expose the myosin binding sites on actin
- ATP is hydrolyzed into ADP and phosphorus, releasing energy for myosin power stroke
- Myosin binds to actin
- Myosin head bends and actin slides over the myosin surface
- Myosin releases the ADP molecule
As the myosin head swivels, another ATP molecule binds to myosin, breaking the actin-myosin bridge. The ATP is again hydrolyzed, and last four steps of the process are repeated, making the sarcomere shorter and shorter, until adequate Ca++ and ATP are present. Many myosin heads move in the same direction and a number of times, to contract a single muscle.
When the nervous impulse stops, the calcium gates close, and the sarcoplasmic reticulum is no longer permeable. The Ca++ return to the sarcoplasmic reticulum, and troponin and tropomyosin are reverted to their original positions. With the binding sites blocked, myosin cannot form cross-bridges with actin, and the muscle relaxes.
Here is a list of few structures, to help you have a better understanding of the process –
Myofibrils – Thin fibers in the muscle cells
Sarcomere – A structural unit of myofibril
Sarcoplasmic Reticulum – Tubules surrounding myofibrils responsible for storing and diffusing Ca ions
Sarcolemma – Cell membrane of muscle cells
T-tubules – Tubules running through the muscle fibers through which Ca++ flow
Troponin – A complex of proteins, which combine with Calcium ions, and shift tropomyosin
Tropomyosin – Protein component of muscle fiber, which in its natural state, blocks myosin-actin binding sites
In short, when a stimulus reaches a muscle, its sarcoplasmic reticulum releases calcium ions, which bind troponin and shift the tropomyosin, which are blocking the myosin-binding sites on actin. Once the binding sites are free, myosin binds with actin, shortening the sarcomere, and contracting the muscle. This mechanism is also known as the sliding-filament theory. As the movement of myosin head resembles a ratchet, the process is also referred as ratchet mechanism.
