Muscles

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The primary job of muscle is to move the bones of the skeleton, but muscles also enable the heart to beat and constitute the walls of other important hollow organs.

Muscle is a large, active endocrine organ, affecting physical function, quality of life, metabolism, and inflammation.

Approximately 40% of body mass consists of muscle tissue.

There are three types of muscle, skeletal or striated, cardiac, and smooth. 

Muscle action can be categorized  as being either voluntary or involuntary.

 Cardiac and smooth muscles contract without conscious thought and are termed involuntary.

Skeletal muscles contract upon command.

Skeletal muscle is the largest and most labile reserve of protein in the body, and it constantly adapts its mass and function  to both physiologic and pathological stimuli by altering flux through signaling pathways that influence protein turnover

 Skeletal muscles in turn can be divided into fast and slow twitch fibers.

This type of muscle creates movement in the body.

Muscle cells are multinuclear cells, which means they  contain many nuclei.

 

New nuclei are formed whenever muscles are subjected to overload from any muscle building activity. 

 

These new nuclei may never be lost, which means that permanent physiological changes have taken place in the muscle fibers: muscles grow faster in individuals resuming muscle building activity in comparison to those who have never performed any muscle building activity in the past. 

 

This referred to as muscle memory.

Muscle tissue is made up of bundles of muscle fibers. 

Muscle fibers are long, skinny cells that can be up to several inches long and, in the case of skeletal muscle, may contain several nuclei. 

The cytoplasm of muscle fibers contains long, thread-like structures called myofibrils, which are made up of bundles of thick, myosin filaments and thin  actin filaments. 

Surrounding the actin and myosin filaments is a structure called the sarcoplasmic reticulum (SR); which is a network of tubules that store calcium ions. 

The SR also plays an important role in transmitting electrical signals.

These electrical signals are delivered to the muscle cells by neurons.

Muscle tissue is made of bundles of muscle fibers.

Each myofibril is made of a chain of repeating contractile units called sarcomeres. 

At the end of each sarcomere is a Z disc.

There are more than 600 skeletal muscles, and they makes up about 40 percent of a person’s body weight.

When the nervous system signals the muscle to contract, groups of muscles work together to move the skeleton.

These signals and movements are nearly involuntary, yet they do require conscious effort.

In normal muscle contraction, calcium is released from the sarcoplasmic reticulum through the ryanodine receptor channel, which causes the tension-generating interaction of actin and myosin.

 

When muscle contracts, calcium re-uptake by the calcium-activated ATPase of the sarcoplasmic reticulum is brought about by magnesium.

Contracting muscles release multiple substances known as myokines which promote the growth of new tissue, tissue repair, and various anti-inflammatory functions, which in turn reduce the risk of developing various inflammatory diseases.

In response to muscle contraction, muscles release cytokines and myokines  into the circulation.

Myokines may be protective against the risk for depression.

Muscle strength has a role in preventing and managing the development of chronic disease.

Muscle strength is associated with lower risk of cardiovascular disease risk factors and cardiovascular disease morbidity and mortality.

Muscle strength is associated inversely with all cause mortality in community residing populations.

Low muscle mass is repeatedly and significantly associated with worse survival and increased dose limiting toxicity in advanced cancers.

In younger men, a diminished grip strength predisposes to incident coronary heart disease, and enhanced isometric muscle strength is extended by less risk of mortality from cardiovascular disease.

 

Hand grip strength and chair-rise test performance, individually, are associated with the risk of all-cause and

 

cardiovascular disease mortality, irrespective of the frequency of physical exercise.

 

In healthy older women arm strength, hand grip strength, and leg strength serve as valid  predictors of all-cause mortality, cardiovascular disease, and stroke.

Genetic factors influence muscular strength, however the primary determinant of muscular strength is resistant exercise, which is recommended in addition to aerobic exercise in order to reduce disease incidence and attenuate disease progression.

Muscular strength is inversely associated with the incidence of metabolic syndrome, and obesity, which are diagnoses that often precede type two diabetes.

Cardiac muscle:

Cardiac muscle is involuntary muscle, that makes up the walls of the heart and creates the steady, rhythmic pulsing that pumps blood through the body from signals from the brain.

This muscle type also creates the electrical impulses that produce the heart’s contractions.

Smooth muscle:

Smooth muscle makes up the walls of hollow organs, respiratory passageways, and blood vessels.

Smooth muscle wavelike movements propel things through the bodily system, such as food through your stomach or urine

Smooth muscle is involuntary and also contracts in response to stimuli and nerve impulses.

Muscle movement happens when neurological signals produce electrical changes in muscle cells.

During muscle movement, calcium is released into the cells and brings about a short muscle twitch.

Some muscular disorders and conditions that affect muscles include:

Muscle pain

Sprains and strains

Bruising

Cramping

Myopathy

Muscular dystrophy

Parkinson’s disease

Fibromyalgia

Multiple sclerosis

Almost all muscles develop from the mesoderm.

Cardiac muscles and smooth muscle develop from the splanchnic mesoderm.

Skeletal muscles develop from myotomes.

Skeletal muscles mesencyhmal cell in the myotomes differentiate into myotubes that elongate and fuse into tubular structures, the myoblasts.

Fibroblasts and external laminae encase muscles in a fibrous sheath.

 

Muscle strength is considered the most reliable measure of muscle function in older adults, superseding the role of muscle mass. 

Muscle mass begins to decline about age 40 years.

Low muscle mass is associated with several negative outcomes across the healthcare continuum.

Mean muscle loss about 8% per decade after age 40 per decade.

For adults 60 years and older there is a decline in muscle strength (between 1.5 and 3.5%) and power.

Above 70 years muscle mass atrophy rate accelerates to 25-40% per decade.

Muscle protein synthesis decreases with aging partly due to endocrine changes with reduction in sex hormones and growth factors.

Muscle strength in elderly has a higher association with mortality than muscle mass.

Muscle strength deteriorates more rapidly than muscle mass.

Lean muscle uses more calories than fat.

In geriatric patients muscle mass measurement is mainly performed by dual energt x-ray and bioelectrical impedance analysis.

MRI, ultrasound, CT imaging, are used for muscle mass measurement.

CT scans are the best to evaluate muscle mass in cancer patients.

Abnormal, skeletal muscle measurements at baseline, such as low muscle mass and resultant fat infiltration is associated with adverse cancer related outcomes, and all cause mortality in patients with solid and hematologic malignanciess, including those undergoing hematopoetic stem cell transplant.

Skeletal muscle cells can modify immune response through muscle cell derive cytokines (myokines), cell surface molecules, and cell to cell interactions, which alter T sell homeostasis.

 

There are  2 skeletal muscle fiber types – slow-twitch and fast-twitch muscle fibers. 

There is a genetic predisposition to have more fast-twitch type muscle fibers, or  more slow-twitch type muscle fibers.

Men build muscle more efficiently than women because they have larger reserves of testosterone with a greater capability of producing growth hormone. 

 

Women have shorter muscle fibers, accounting for a reduction in strength, and differences in strength could also be a result of lean tissue distribution.

 

Women generally build muscle about 2-3 times slower in comparison to men.

 

During exercise, muscle proteins get broken down into free amino acids and then undergo the process of oxidizing in mitochondria to produce energy. 

 

Eating whey protein supplements enhance the body’s protein recovery and synthesis after exercise because it increases the free amino acids in the body’s free amino acid pool.

 

The main effect of exercise on glucose uptake in the working skeletal muscle is insulin‐independent and is partly mediated via AMPK.