Muscular Tissue

Muscular tissue consists of elongated cells that are specialized to respond to stimulation by contracting; thus, its primary job is to exert physical force on other tissues and organs—for example, when a skeletal muscle pulls on a bone, the heart contracts and expels blood, or the bladder contracts and expels urine. Not only do movements of the body and its limbs depend on muscle, but so do such

Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

176 Part One Organization of the Body

Table 5.9 Nervous Tissue

Nervous Tissue And Glial Cells
(a)

Nuclei of glial cells-i Axon-i Soma of neuron-i Dendrites-

Nuclei of glial cells-i Axon-i Soma of neuron-i Dendrites-

Cells Spinal Cord Smear

Figure 5.25 Spinal Cord Smear.

Figure 5.25 Spinal Cord Smear.

Microscopic appearance: Most sections show a few large neurons, usually with rounded or stellate cell bodies (somas) and fibrous processes (axon and dendrites) extending from the somas; neurons are surrounded by a greater number of much smaller glial cells, which lack dendrites and axons Representative locations: Brain, spinal cord, nerves, ganglia Function: Internal communication processes as digestion, waste elimination, breathing, speech, and blood circulation. The muscles are also an important source of body heat. The word muscle means "little mouse," apparently referring to the appearance of rippling muscles under the skin.

There are three histological types of muscle—skeletal, cardiac, and smooth—which differ in appearance, physiology, and function (table 5.10). Skeletal muscle consists of long, cylindrical cells called muscle fibers. Most of it is attached to bones, but there are exceptions in the tongue, upper esophagus, some facial muscles, and some sphincter24 (SFINK-tur) muscles (ringlike or cufflike muscles that open and close body passages). Each cell contains multiple nuclei adjacent to the plasma membrane. Skeletal muscle is described as striated and voluntary. The first term refers to alternating light and dark bands, or striations (stry-AY-shuns), created by the overlapping pattern of cytoplasmic protein filaments that cause muscle contraction. The second term, voluntary, refers to the fact that we usually have conscious control over skeletal muscle.

Cardiac muscle is essentially limited to the heart, though it extends slightly into the nearby blood vessels. It, too, is striated, but it differs from skeletal muscle in its other features. Its cells are much shorter, so they are commonly called myocytes25 rather than fibers. The myocytes are branched and contain only one nucleus, which is located near the center. A light-staining region of glycogen often surrounds the nucleus. Cardiac myocytes are joined end to end by junctions called intercalated26 (in-TUR-kuh-LAY-ted) discs. Electrical connections at these junctions enable a wave of excitation to travel rapidly from cell to cell, and mechanical connections keep the myocytes from pulling apart when the heart contracts. The electrical junctions allow all the myocytes of a heart chamber to be stimulated, and contract, almost simultaneously. Intercalated discs appear as dark transverse lines separating each myocyte from the next. They may be only faintly visible, however, unless the tissue has been specially stained for them. Cardiac muscle is considered involuntary because it is not usually under conscious control; it contracts even if all nerve connections to it are severed.

Smooth muscle lacks striations and is involuntary. Smooth muscle cells are fusiform (thick in the middle and tapered at the ends) and relatively short. They have only one, centrally placed nucleus. Small amounts of smooth muscle are found in the iris of the eye and in the skin, but most of it, called visceral muscle, forms layers in the walls of the digestive, respiratory, and urinary tracts, blood vessels, the uterus, and other viscera. In locations such as the esophagus and small intestine, smooth muscle forms adjacent layers, with the cells of one layer encircling the organ and the cells of the other layer running longitudinally. When the circular smooth muscle contracts, it may propel contents such as food through the organ. When the longitudinal layer contracts, it makes the organ shorter and

24sphinc = squeeze, bind tightly 25myo = muscle + cyte = cell 26inter = between + calated = inserted

Saladin: Anatomy & I 5. Histology I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 5 Histology 177

Table 5.10 Muscular Tissue

Skeletal Muscle

Cardiac Muscle

Smooth Muscle

Skeletal Muscle

Cardiac Muscle

Smooth Muscle

(a)

Intercalated disc- Glycogen-.

i—Muscle cells

Intercalated disc- Glycogen-.

i—Muscle cells

(b)

Figure 5.28 Smooth Muscle, Wall of Small Intestine.

Figure 5.26 Skeletal Muscle.

Microscopic appearance: Long, cylindrical, unbranched cells (fibers), relatively parallel in longitudinal tissue sections; striations; multiple nuclei per cell, near plasma membrane Representative locations: Skeletal muscles, mostly attached to bones but also in the tongue, esophagus, and voluntary sphincters of the lips, eyelids, urethra, and anus Functions: Body movements, facial expression, posture, breathing, speech, swallowing, control of urination and defecation, and assistance in childbirth; under voluntary control

Figure 5.27 Cardiac Muscle.

Microscopic appearance: Short branched cells (myocytes); less parallel appearance in tissue sections; striations; intercalated discs; one nucleus per cell, centrally located and often surrounded by a light zone Representative locations: Heart Functions: Pumping of blood; under involuntary control

Figure 5.28 Smooth Muscle, Wall of Small Intestine.

Microscopic appearance: Short fusiform cells overlapping each other; nonstriated; one nucleus per cell, centrally located Representative locations: Usually found as sheets of tissue in walls of viscera; also in iris and associated with hair follicles; involuntary sphincters of urethra and anus Functions: Swallowing; contractions of stomach and intestines; expulsion of feces and urine; labor contractions; control of blood pressure and flow; control of respiratory airflow; control of pupillary diameter; erection of hairs; under involuntary control thicker. By regulating the diameter of blood vessels, smooth muscle is very important in controlling blood pressure and flow. Both smooth and skeletal muscle form sphincters that control the emptying of the bladder and rectum.

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