In order for muscles to move and support our body and organs, two muscular actions must be carried out, contraction and relaxation. Relaxation is when the first one stops or is interrupted.
Contraction, in simple terms, is produced when an impulse (signal) coming from the central nervous system orders the fibers that make up the muscle to shorten (decrease their size). This, at a first glance, seems easy, but it involves a complex process which we will detail next: First, the signal travels through the nerve networks (brain and spine) through the motor neurons. Each of these neurons approaches a muscle, branches out into projections called axons, which end up, one by one, in a different muscle fiber.
Each motor neuron and the muscle fibers they stimulate (innervate) know each other, as a whole, as a motor unit.
A motor neuron and a muscle fiber meet in a neuromuscular junction (see From impulse to contraction infography). But at this point they are separated by a small space. Therefore, for the nerve impulse to cross it, the synaptic bulb (terminal end of the axon) releases a neurotransmitter (transmitting chemical substance), acetylcholine. It crosses the space until it reaches the sarcolemma (cellular membrane of the fiber) and, specifically, the receptors of the motor plaque (folded part of the sarcolemma). Here, the transformation from chemical impulse to electric impulse takes place.
The electric activity begun extends to the sarcoplasmic reticulum (membranous system that wraps the myofibrils), causing the release of calcium ions previously accumulated there.
The secretions of these ions reach the myofilaments (myosin and actin). Here, the molecule heads of myosin bond to the actin and with the energy of the ATP (triphosphate adenosine), they turn towards the center of the sarcomere, pulling the myofilaments of actin towards the inside and shortening the sarcomere (until the stimulus disappears).
When the sarcomere shortens, the muscle fibers, and therefore the corresponding muscle contracts (see Contraction and relaxation infography).
Isometric and isotonic contraction
Not all contractions in the body are produced in the same way or cause the same effect.
There are two types of muscle contractions that work together and complement each other in order to carry out their different activities.
Isometric contraction (or of the same length) tensions the muscle without altering its length, therefore, it does not cause movement and maintains a set position; an example of this is the tension of soldiers when standing. Isotonic contraction (or of the same tension) shortens the muscle and causes a steady, tensioning force generating movement; an example is the work done when writing.
