Show your muscles

Muscle tissue is one of the four basic tissue types in humans, and responsible for around 42% of body mass in men and around 36% of body mass in women. It has the simple function of contracting and relaxing. However, despite its simple appearance and function, there is a great level of diversity and complexity between various types of muscles.

Muscle structure

The smallest unit of a muscle is called a sarcomere, appearing as alternating dark and light bands under the microscope. The dark bands are a thick filament formed from a protein called myosin, and the light bands are a thinner filament formed from actin. Bands of actin are vertically attached to a Z line, and the sarcomere is the unit between two Z lines. Muscles contract via a series of events that let the myosin and actin chains slide along each other, shortening and lengthening the sarcomere.

Figure 1: Sarcomere under a microscope

Many sarcomeres form a regular, parallel bundle called myofibril, or a muscle fiber. Muscle fibers are bundled into fascicles, and several fascicles make a skeletal muscle.

The whole muscle structure is wrapped in thin layers of fibrous tissue called either epimysium, perimysium or endomysium, depending on its placement, which protect the muscle from friction against bones, other muscles and other parts of the same muscle, while also allowing the muscle to contract and move while maintaining its structure.

Figure 2: Schematic model of a skeletal muscle fibers.

Figure 3: Schematic model of a skeletal muscle.

Muscle types

The three types of muscle tissues are the skeletal, cardiac and smooth muscles.

Skeletal muscles are what we think of when somebody mentions muscles. They are the most abundant type, responsible for voluntary contractions. Examples are the muscles in an arm or leg, and every other „surface“ muscle beneath the skin. If you want to get in shape, this is the muscle type you train. This type of muscles is responsible for body movement and is made by voluntary contractions. It is also the muscle type that makes up blood vessels, nerve fibres and connective tissue, and is most often connected to the skeleton via tendons and ligaments. Its muscle cells, called myocytes, have multiple nuclei for faster energy expenditure and protein assembly.

Skeletal muscles are further divided into two major groups, the slow-twitch (also called Type 1) and the fast-twitch (called Type 2) muscles. The slow-twitch, slow oxidative, or red muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity longer, meaning it can be active for a prolonged period of time. Fast-twitch muscles can contract more quickly and with a greater amount of force than slow-twitch muscles, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful.

To help them contract in the right manner, each muscle is connected with up to 250 motor neurons' branches coming from a single or multiple motor neurons, with each branch forming a neuromuscular junction. To move a muscle, the brain first sends out a signal to the motor neuron. The motor neuron immediately responds by releasing acetylcholine into the muscle via the neuromuscular junction, which starts the cascading effect of contracting the muscle.

Smooth and cardiac muscles contract involuntarily, meaning a person has no autonomy or conscious will over their contractions. They are activated by the endocrine (hormonal) activation or by the intervention of the autonomic nervous system.

Smooth muscles are found in hollow structures such as intestines and some organs such as the stomach, uterus, and bladder. They are not striated, meaning they lack sarcomeres, and their contractions are usually slow and gradual. Both smooth and cardiac muscle myocytes predominantly have one nucleus per cell.

Cardiac muscle is striated, and found as the central band in the heart walls and contracts similarly to skeletal muscles, but is guided by the cardiac action potential. This is an impulse of positively charged calcium released from the cells' internal calcium storage. Coordinated contractions of cardiac muscle cells propel blood out of the atria and ventricles to the blood vessels of the left/body/systemic and right/lungs/pulmonary circulatory systems. Cardiac muscle cells, unlike most other tissues in the body, rely on an available blood and electrical supply to deliver oxygen and nutrients and to remove waste products such as carbon dioxide. The coronary arteries help fulfill this function.

Muscle conditions and diseases

Just as muscle function is essential to locomotion, so are metabolic processes that provide energy to drive muscle contraction. Our bodies convert the food we consume into ATP (the energy of the body), which provides energy and allows contraction in all muscle types. In contrast to all other otgans except skin, muscles can grow (hypertrophy) and decay (atrophy) depending on the availability of energy and other nutrients, combined with the amount and length of pressure the muscle is under. All types of muscles have this, mostly positive, feature. However, both hypertrophy and atrophy can also be the source of unwanted conditions. Cardiac hypertrophy and sarcopenia are common conditions affecting the muscle mass.

Another type of condition can be the lack of innervation and feedback loop from the brain to the skeletal muscles. A radical example of such condition is ALS, Amyotrophic Lateral Sclerosis, where the neurons connecting the muscles to the central nervous system die out. This leaves the muscles unable to get any input as to what to do, and they become progressively atrophic.

Other types of conditions include injury and overuse, which leads to inflammation, bruising, tendonitis, and incomplete healing; genetic problems that manifest as muscle atrophy; metabolic or endocrine disorders such as thyroid or adrenal disease and steroid abuse; or various toxicities, from alcoholism to pesticide poisoning.

Ongoing research

Despite the growing field of the fitness industry, some muscle diseases are still poorly understood. Additionally, the complexity of interconnected pathways needed for healthy and stable muscle contractions and their sustained growth has been put into practice just recently. The biggest research impact so far has been the understanding of the mind-muscle connection and the vastly beneficial effect of exercise on mental health. Ongoing research will certainly bring more revelations about our biggest organ in the near future!

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