(mŭs′ĕl )
[Pronunciation]
[L. musculus]

A type of tissue composed of contractile cells or fibers that effects movement of an organ or part of the body.
The outstanding characteristic of muscular tissue is its ability to shorten or contract. It also possesses the properties of irritability, conductivity, and elasticity. Muscle tissue possesses little intercellular material; hence, its cells or fibers lie close together.
SYN: muscular tissue; ; musculus

Figure: MUSCLES OF THE TRUNK
(A) anterior, (B) posterior



See: leg; arm; cell; face

TYPES
Three types of muscle occur in the body, differentiated on the basis of histologic structure. These muscle types are smooth, skeletal (striated), and cardiac.

Figure: MUSCLE TISSUES
(Orig. mag. ×430)




Comparison of Properties of Three Types of Muscle

	Smooth	Cardiac	Striated
Synonyms	Involuntary	Myocardial	Voluntary
	Nonstriated		Skeletal
	Visceral
Fibers
Length (in/m)	50–200		25,000
Thickness (in/m)	4–8		75
Shape	Spindles		Cylinders
Markings	No striation	Striation	Marked striation
Nuclei	Single	Single	Multiple
Effects of cutting related nerve	Slight	Regulation of heart rate is lost	Complete paralysis

Smooth (Involuntary): Smooth muscle is found principally in the internal organs, esp. the digestive tract, respiratory passages, urinary and genital ducts, urinary bladder, gallbladder, and walls of blood vessels. It lacks the cross striations characteristic of other types of muscle. This type of muscle tissue is called involuntary because it is not under conscious control. Smooth muscle cells are fusiform or spindle-shaped, each containing a central nucleus. The cells are usually arranged in sheets or layers, but may occur as isolated units in connective tissue.

Striated, Skeletal (Voluntary): Striated muscle is found in all skeletal muscles. It also occurs in the tongue, pharynx, and upper portion of esophagus. Since movement is under conscious control, this type of muscle tissue is called voluntary. The cytoplasm (sarcoplasm) contains bundles of myofibrils made of sarcomeres, the unit of contraction. The cell membrane is called the sarcolemma. Muscle fibers are grouped into bundles called fasciculi, each of which is surrounded by a sheath or connective tissue called perimysium. The fibers within a fasciculus are surrounded by and held together by delicate reticular fibrils forming the endomysium.

Cardiac: Cardiac muscle fibers are often branched. Intercalated disks at end-to-end junctions increase the efficiency of electrical impulse transmission throughout the myocardium. Purkinje fibers are cardiac cells that have a faster natural rate of depolarization and thus form the conducting pathways leading from the atrioventricular node to the ventricles.

ANATOMY
Muscle is a contractile tissue that shortens and moves a body part. A typical skeletal muscle is spindle-shaped and consists of a central portion called the belly and two or more attaching sites (with tendons) at the ends. The more stationary attachment to bone is called the origin; the more movable attachment is the insertion. Most tendons are cylindrical but a few are flat (an aponeurosis); they are made of fibrous connective tissue that merges with the deep fascia surrounding the muscle and with the periosteum covering the bone.

Figure: SKELETAL MUSCLE





BLOOD SUPPLY
It is obtained from small blood vessels that enter the muscular tissue and subdivide into capillaries that permeate throughout.

NERVE SUPPLY
Voluntary: These muscles are innervated by somatic branches of cranial or spinal nerves; it is because of this that the skeletal muscles are under conscious control. Involuntary: Smooth and cardiac muscles receive their nerve supply from the autonomic nervous system and function involuntarily without conscious control.

Subentries:
abdominal muscles
abducens muscle
abductor muscle
abductor digiti minimi muscle
abductor pollicis brevis muscle
adductor muscle
adductor brevis muscle
adductor longus muscle
adductor magnus muscle
adductor pollicis muscle
agonist muscle
anconeus muscle
antagonist(ic) muscle
antigravity muscles
appendicular muscle
arrector pili muscle
articular muscle
auditory muscles
axial muscle
bipennate muscle
ciliary muscle
constrictor muscle of pharynx
deep neck m.
deltoid muscle
digastric muscle
erector spinae m.'s
external intercostal muscles
external oblique muscle
extraocular eye muscle
extrinsic muscle
fibularis longus muscle
fixation muscle
flexor pollicis brevis muscle
fusiform muscle
gastrocnemius muscle
gemellus muscle
genioglossus muscle
gluteus maximus muscle
gluteus medius muscle
hyoglossus muscle
iliacus muscle
infraspinatus muscle
internal intercostal muscles
intrinsic muscle
involuntary muscle
laryngeal muscle (intrinsic)
latissimus dorsi muscle
lumbrical muscle
mastication muscle
mimetic muscles
multipennate muscle
nasalis muscle.
nonstriated muscle
obturator muscle
opponens pollicis muscle
orbicular muscle
orbicularis oculi muscle
orbicularis oris muscle
papillary muscle
pectinate muscle
peroneus longus muscle
postaxial muscle
postural muscles
preaxial muscle
procerus muscle
rectus abdominis muscle
rectus femoris muscle
muscles of respiration
semimembranosus muscle
serratus muscle
serratus anterior muscle
skeletal muscle
smooth muscle
somatic muscle
sphincter muscle
sphincter muscle of urinary bladder
stabilizer muscle
striated muscle
supraspinatus muscle
synergistic muscles
teres major muscle
thenar muscle
thyroepiglottic muscle
tibialis anterior muscle
tonic muscle.
triangular muscle
tricipital muscle
twitch muscle.
unipennate muscle
unstriated muscle
vastus intermedius muscle
vastus lateralis muscle
vastus medialis muscle
voluntary muscle