Muscle

A muscle is an organ of the muscular system. Muscles are largely composed of skeletal muscle tissue and are innervated by motor nerve fibers in spinal and cranial nerves. Many muscle insert into bones and are involved in body posture and movement. Other muscles have other functions, for example, some muscles of the head and neck, the diaphragm, pelvic diaphragm and perineal muscles.

Other Names

Synonyms

• Somatic muscle
• Skeletal muscle

Related terms

• Skeletal muscle tissue
• Muscular tissue
• Muscular system

Types

The names of specific muscles often describe the location, size, shape, attachment, structure, or function of the muscle. In addition, their features can be described using a set of general terms, including those listed below:

Location descriptors

• Muscles of head
• Muscles of neck
• Muscles of back
• Muscles of thorax
• Muscles of abdomen
• Muscles of upper limb
• Muscles of lower limb

Shape descriptors

• Fusiform muscle
• Flat muscle
• Straight muscle (M. rectus)
• Triangular muscle
• Quadrate muscle (M. quadratus)

Structure descriptors

• Two-bellied muscle
• Two-headed muscle (M. biceps)
• Three-headed muscle (M. triceps)
• Four-headed muscle (M. quadriceps)
• Semipennate (or unipennate) muscle
• Pennate (or bipennate) muscle
• Multipennate muscle

Attachment descriptors

• Orbicular muscle
• Cutaneous muscle

Function descriptors

• Abductor muscle
• Adductor muscle
• Rotator muscle
• Flexor muscle
• Extensor muscle
• Pronator muscle
• Supinator muscle
• Opponens muscle
• Sphincter muscle
• Dilator muscle

Description

A muscle is largely composed of skeletal muscle tissue, but it also contains connective tissue, blood vessels and nerves. Muscles are covered by epimysium (a thin layer of dense fibrous connective tissue), and divided into fascicles by perimysium (thin dense fibrous connective tissue septa). Fascicles consist of bundles of skeletal muscle fibers, each fiber surrounded by endomysium (a very thin sheath of loose connective tissue). Vessels and nerves are located within the connective tissue components of the muscle.
Descriptions of specific muscles usually include its attachments (origin and insertion), innervation, and actions. More elaborate descriptions may include shape and structure descriptors, and blood supply.