Muscle fibers contain filaments of contractile proteins actin and myosin, arranged side by side. These filaments are repeated along the muscle fiber, forming the sarcomere.
The sarcomere is the functional unit of muscle contraction.
For muscle contraction to occur, three elements are necessary:
- Stimulation of the nervous system;
- The contractile proteins, actin and myosin;
- Energy for contraction, provided by ATP.
How does muscle contraction occur?
Understand the step-by-step mechanism of muscle contraction in a skeletal muscle fiber:
The brain sends signals, through the nervous system, to the motor neuron that is in contact with the muscle fibers.
When close to the surface of the muscle fiber, the axon loses its myelin sheath and dilates, forming the motor end plate. Motor nerves connect to muscles through the motor end plates.
With the arrival of the nerve impulse, the axon endings of the motor nerve release acetylcholine , a neurotransmitter substance, onto its muscle fibers.
Acetylcholine binds to receptors on the muscle fiber membrane, triggering an action potential.
At this point, the actin and myosin filaments contract, leading to a decrease in the sarcomere and consequently causing muscle contraction.
Muscle contraction follows the “all or nothing rule”. In other words: the muscle fiber either contracts completely or does not contract at all. If the stimulus is not sufficient, nothing happens.
Types of Muscle Contraction
Muscle contraction can be of two types:
- Isometric contraction : when the muscle contracts without shortening its size. Example: maintaining posture involves isometric contraction.
- Isotonic contraction : when the contraction promotes the shortening of the muscle. Example: movement of the lower limbs;
- Isokinetic contraction: occurs when the muscle exerts a force against a constant resistance, such as in water, at a controlled speed. In this type of contraction, the tension is maximum at all angles of the joint involved. It is generally used to test muscle strength. Example: hydrotherapy.
