Muscular System

Plate 19 from Bernardino Genga's Anatomia per uso et intelligenza del disegno ricercata non solo su gl'ossi, e muscoli del corpo humano. Source: Wikimedia Commons.

The muscular system consists of the more than 600 muscles of the body and their associated fascia, tendons, tendon sheaths, and bursae. Muscles largely consist of skeletal muscle fibers, and are innervated by motor nerve fibers of peripheral nerves. Conscious and subconscious contractions of muscles affect body posture, and generate reflex, spontaneous (or automatic), and voluntary movements.

Other Names

Synonyms

• Musculature
• Muscles
• Skeletal muscle system
• Voluntary muscle system

Related terms

• Musculoskeletal system (also includes the Skeletal system and Articular system)
• Muscle
• Skeletal muscle tissue
• Muscular tissue

Description

Skeletal muscles

Structure of a skeletal muscle. Source: SEER training site.

Each skeletal muscle is considered an organ of the muscular system. Each organ or muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue.

Skeletal muscles vary considerably in size, shape, and arrangement of fibers. They range from extremely tiny strands such as the stapedium muscle of the middle ear to large masses such as the muscles of the thigh. Some skeletal muscles are broad in shape and some narrow. In some muscles the fibers are parallel to the long axis of the muscle, in some they converge to a narrow attachment, and in some they are oblique.

Each skeletal muscle fiber is a single cylindrical muscle cell. An individual skeletal muscle may be made up of hundreds, or even thousands, of muscle fibers bundled together and wrapped in a connective tissue covering. Each muscle is surrounded by a connective tissue sheath called the epimysium. Fascia, connective tissue outside the epimysium, surrounds and separates the muscles. Portions of the epimysium project inward to divide the muscle into compartments. Each compartment contains a bundle of muscle fibers. Each bundle of muscle fiber is called a fasciculus and is surrounded by a layer of connective tissue called the perimysium. Within the fasciculus, each individual muscle cell, called a muscle fiber, is surrounded by connective tissue called the endomysium.

Skeletal muscle cells (fibers), like other body cells, are soft and fragile. The connective tissue covering furnish support and protection for the delicate cells and allow them to withstand the forces of contraction. The coverings also provide pathways for the passage of blood vessels and nerves.

Commonly, the epimysium, perimysium, and endomysium extend beyond the fleshy part of the muscle, the belly or gaster, to form a thick ropelike tendon (which attaches muscle to bone) or a broad, flat sheet-like aponeurosis (which is connective tissue that serves as a connection between parts of a muscle). Typically a muscle spans a joint and is attached to bones by tendons at both ends. One of the bones remains relatively fixed or stable while the other end moves as a result of muscle contraction.

Muscle groups

There are more than 600 muscles in the body, which together account for about 40% of a person's weight.

Most skeletal muscles have names that describe some feature of the muscle. Often several criteria are combined into one name. The following are some terms relating to muscle features that are used in naming muscles:

• Size: vastus (huge); maximus (large); longus (long); minimus (small); brevis (short).
• Shape: deltoid (triangular); rhomboid (like a rhombus with equal and parallel sides); latissimus (wide); teres (round); trapezius (like a trapezoid, a four-sided figure with two sides parallel).
• Direction of fibers: rectus (straight); transverse (across); oblique (diagonally); orbicularis (circular).
• Location: pectoralis (chest); gluteus (buttock or rump); brachii (arm); supra- (above); infra- (below); sub- (under or beneath); lateralis (lateral).
• Number of origins: biceps (two heads); triceps (three heads); quadriceps (four heads).
• Origin and insertion: sternocleidomastoideus (origin on the sternum and clavicle, insertion on the mastoid process); brachioradialis (origin on the brachium or arm, insertion on the radius).
• Action: abductor (to abduct a structure); adductor (to adduct a structure); flexor (to flex a structure); extensor (to extend a structure); levator (to lift or elevate a structure); masseter (a chewer).

Head and neck

Humans have well-developed muscles in the face that permit a large variety of facial expressions. Because the muscles are used to show surprise, disgust, anger, fear, and other emotions, they are an important means of nonverbal communication. Muscles of facial expression include frontalis, orbicularis oris, laris oculi, buccinator, and zygomaticus.

There are four pairs of muscles that are responsible for chewing movements (mastication). All of these muscles connect to the mandible (jaw bone) and they are some of the strongest muscles in the body. Two of the muscles, temporalis and masseter are identified in the illustration above.

There are numerous muscles associated with the throat, the hyoid bone, and the vertebral column. Two of the more obvious and superficial neck muscles are identified in the illustration, the sternocleidomastoid and trapezius.

Muscles of the head and neck. Source: SEER training site.

Trunk

The muscles of the trunk include those that move the vertebral column, the muscles that form the thoracic and abdominal walls, and those that cover the pelvic outlet.

The erector spinae group of muscles on each side of the vertebral column is a large muscle mass that extends from the sacrum to the skull. These muscles are primarily responsible for extending the vertebral column to maintain erect posture. The deep back muscles occupy the space between the spinous and transverse processes of adjacent vertebrae.

The muscles of the thoracic wall are involved primarily in the process of breathing. The intercostal muscles are located in spaces between the ribs. They contract during forced expiration. External intercostal muscles contract to elevate the ribs during the inspiration phase of breathing. The diaphragm is a dome-shaped muscle that forms a partition between the chest cavity and the abdomen. It has three openings in it for structures that have to pass from the chest cavity to the abdomen.

The abdomen, unlike the chest cavity and pelvis, has no bony reinforcements or protection. The wall consists entirely of four muscle pairs, arranged in layers, and the fascia that envelops them.

The pelvic outlet is formed by two muscular sheets and their associated fascia.

Muscles of the trunk. Source: SEER training site.

Upper extremity

The muscles of the upper extremity include those that attach the scapula (shoulder blade) to the chest cavity, those that attach the humerus (upper arm bone) to the scapula, and those that are located in the arm or forearm.

Muscles that move the shoulder and arm include the trapezius and serratus anterior. The pectoralis major, latissimus dorsi, deltoid, and rotator cuff muscles connect to the humerus and move the arm.

The muscles that move the forearm are located along the humerus. Some of these are the triceps brachii, biceps brachii, brachialis, and brachioradialis. The 20 or more muscles that cause most wrist, hand, and finger movements are located along the forearm.

Muscles of the upper extremity. Source: SEER training site.

Lower extremity

The muscles that move the thigh have their origins on some part of the pelvic girdle and their insertions on the femur. The largest muscle mass belongs to the posterior group, the gluteal muscles, which, as a group, move the thigh away from the body. The iliopsoas flexes the thigh. The muscles in the compartment nearest the center of the body move the thigh inwards.

Muscles that move the leg are located in the thigh region. The quadriceps femoris muscle group straightens the leg at the knee. The hamstrings are antagonists to the quadriceps femoris muscle group and are used to flex the leg at the knee.

The muscles located in the leg that move the ankle and foot are divided into anterior, posterior, and lateral compartments. The tibialis anterior, which flexes the foot so that the toes move towards the head, acts in the opposite direction as the gastrocnemius and soleus muscles, which flex the foot so the toes point towards the ground.

Muscles of the lower extremity. Source: SEER training site.

Role of the Muscular System in the Body

The integrated action of joints, bones, and skeletal muscles produces obvious movements such as walking and running. Skeletal muscles also produce more subtle movements that result in various facial expressions, eye movements, and respiration.

Nearly all movement in the body is the result of muscle contraction. Exceptions to this are the action of cilia, the flagellum on sperm cells, and amoeboid movement of some white blood cells.

In addition to movement, muscle contraction also fulfills some other important functions in the body, such as posture, joint stability, and heat production. Posture, such as sitting and standing, is maintained as a result of muscle contraction. The skeletal muscles are continually making fine adjustments that hold the body in stationary positions. The tendons of many muscles extend over joints and in this way contribute to joint stability. This is particularly evident in the knee and shoulder joints, where muscle tendons are a major factor in stabilizing the joint. Heat production, to maintain body temperature, is an important by-product of muscle metabolism. Nearly 85% of the heat produced in the body is the result of muscle contraction.

How It Works

Skeletal muscles have an abundant supply of blood vessels and nerves. This is directly related to the primary function of skeletal muscle, contraction. Before a skeletal muscle fiber can contract, it has to receive an impulse from a nerve cell. Generally, an artery and at least one vein accompany each nerve that penetrates the epimysium of a skeletal muscle. Branches of the nerve and blood vessels follow the connective tissue components of the muscle of a nerve cell and with one or more minute blood vessels called capillaries. The blood vessels bring nutrients that provide the energy the muscle needs to contract.

Diseases of the Muscular System

Conditions

Diseases that affect the muscles include:

• Amyotrophic lateral sclerosis (Lou Gehrig disease)
• Botulism

Procedures

• EMG (electromyography)

Related Professions

• A chiropractor is a health care professinal trained to provide medical care for disorders of the musculoskeletal system and the nervous system.
• A physical therapist is a health care professional trained to diagnose and treat disorders that affect an individual's ability to move and perform functional activities in daily life.
• An orthopedic surgeon is a medical doctor who is trained in the diagnosis and management of disorders of the musculoskeletal system.
• A neurologist is a medical doctor trained in treating disease of the nervous system, and muscular dystrophies and atrophies.
• A rheumatologist is a medical doctor trained in treating diseases of joints, bones, and soft tissues, including muscles.

History

Sir Bernard Katz was co-awarded the Nobel Prize in Medicine or Physiology in 1970. He and his co-awardees received the prize for their discoveries concerning "the humoral transmitters in the nerve terminals and the mechanisms for their storage, release, and inactivation." Katz, in particular, made important discoveries in how the neurotransmitter acetylcholine is released and acts at the nerve-muscle junction.^[1]

Research

A review of data obtained in the 2001 National Health Interview Survey showed that only a small percentage of Americans over the age of 50 engage in regular exercise that meets the goals set forth by the Healthy People 2010 program. Almost half did not exercise at all.^[2]

References

1. ↑ Nobel Prize Foundation Web site. The Nobel Prize in Physiology or Medicine 1970.
2. ↑ Kruger J, Carlson SA, Buchner D. How active are older Americans? Prev Chronic Dis. 2007 Jul;4(3):A53. Epub 2007 Jun 15. Abstract | Full Text

External Links

American Physical Therapy Association

American Academy of Orthopaedic Surgeons

American Orthopaedic Association