The muscles of most animals consist of bundles of muscle fibers held together and wrapped in connective tissue, which provides structural support and allows communication and interaction between muscle fibers, nerves, and blood vessels. The connective tissue that surrounds each muscle, called the aponeurosis, is an elastic membrane that holds the muscle in place even during contraction. Nerves innervate the muscle fibers, providing electrical stimuli for muscle activation, while blood vessels provide the nutrients and oxygen necessary for muscle metabolism.
Types of muscles
Muscles are divided into two main categories:
- Voluntary muscles: these are consciously controlled by the central nervous system and are composed of striated muscle fibers. These fibers, organized into elongated, multinucleated structures, are attached to bones or skin by tendons or other connective structures, allowing precise and rapid movements. Voluntary muscles are highly vascularized, giving them a bright red color due to the abundant presence of hemoglobin.
- Involuntary muscles: these muscles are not under conscious control and are made up of smooth muscle fibers that are regulated by the autonomic nervous system. This type of muscle is found in the walls of internal organs and blood vessels and contributes to essential vegetative functions such as digestion, respiration, and circulation. Smooth muscles contract slowly and with less intensity than voluntary muscles, allowing for continuous, long-term processes without conscious intervention.
Structure of voluntary muscles
Voluntary muscles, often considered the primary organs of the musculoskeletal system, consist of a contractile portion, known as the muscle belly, and an inextensible portion, represented by the tendon. The muscle belly consists of bundles of striated muscle fibers that are bright red in color and have a consistency that varies depending on the degree of contraction.
Each muscle bundle is wrapped in a connective tissue sheath called the endomysium, while the entire group of bundles is surrounded by an outer layer of connective tissue called the perimysium, which helps distribute blood vessels and nerves within the muscle.
Internal structure and innervation
On a microscopic level, voluntary muscles contain a complex network of blood vessels, lymphatic vessels, and nerves that regulate their function. Motor nerves, which originate in the central nervous system, branch into the muscle fibers and terminate in a structure called the motor end plate, a neuromuscular junction that transmits the electrical impulse to contract the fiber. Sensory nerves, on the other hand, branch and terminate freely within the muscle, sending information about muscle position and tension to the brain, which is essential for movement coordination.
Physiology of contraction
Voluntary muscle contraction is based on a process called isotonic or isometric contraction, depending on whether the muscle is changing length or simply increasing tension. Contraction is regulated by the release of calcium ions which, after binding to troponin in the muscle fibers, allow the interaction between actin and myosin, two filamentous proteins responsible for the sliding mechanism that generates movement.
Functional differentiation
The main difference between voluntary and involuntary muscles lies in both structure and function: while voluntary muscles allow a wide range of movements and are under conscious control, involuntary muscles ensure the constant functioning of internal organs without requiring conscious intervention, operating under the control of the autonomic nervous system.
Revolutionary Magnetic Artificial Muscles Can Lift 1,000 Times Their Own Weight
More information: Minho Seong et al, Multifunctional Magnetic Muscles for Soft Robotics, Nature Communications (2024). DOI: https://dx.doi.org/10.1038/s41467-024-52347-w
A team of researchers led by Professor Hoon Eui Jeong from the Department of Mechanical Engineering at UNIST has unveiled a breakthrough magnetic composite artificial muscle that can support loads comparable to those of automobiles. This new material boasts a more than 2,700-fold increase in stiffness compared to conventional systems, marking a significant leap in the field. The study was published in the journal Nature Communications.
Soft artificial muscles, designed to mimic the smooth, flexible movements of human muscles, are becoming essential in fields such as robotics, wearable technology and biomedicine. While the flexibility of these materials allows for natural, fluid motion, traditional soft materials often lack the stiffness needed to lift heavier loads and achieve precise control, often resulting in unwanted vibrations.
In an effort to overcome these limitations, researchers have explored materials that can transition between soft and rigid states. However, existing materials have been limited in both stiffness modulation range and mechanical strength.
Professor Jeong’s team has pioneered a new approach by combining ferromagnetic particles with shape memory polymers to create a soft magnetic composite artificial muscle with greatly enhanced load capacity and elasticity. This advanced material combines ferromagnetic particles, which generate significant magnetic forces, with shape-memory polymers, which are known for their adaptable stiffness.
Through special surface treatments, the ferromagnetic particles form a strong physical bond with the shape memory polymer, creating a synergistic effect that increases the material’s mechanical strength and allows it to respond quickly and efficiently to external magnetic fields.
The result is an artificial muscle that can vary its stiffness up to 2,700 times and become eight times softer on demand. Under stiff conditions, it can withstand tensile forces up to 1,000 times its weight and compressive forces up to 3,690 times its weight.
In terms of actuation efficiency, these artificial muscles demonstrate exceptional performance, achieving up to 90.9% energy efficiency.
To further improve control, the team implemented a two-layer architecture and added a hydrogel layer to dampen excess vibrations. This structural innovation enables precise control of the artificial muscle’s movements, even during rapid actions.
“This research paves the way for revolutionary applications in various fields,” said Professor Jeong, “by providing mechanical properties and performance that go beyond the capabilities of existing artificial muscles.”
“With multi-stimulation methods such as laser heating and magnetic field control, we can remotely induce a range of movements, from basic stretching, contraction, bending and twisting to more sophisticated actions such as precise object manipulation,” he added.
This enhanced version highlights the technological advances, problem-solving approach, and potential future applications in a more engaging and streamlined way.