Structure and Functions Body Tissues - Muscular and Nervous Tissues

Muscular Tissue and Nervous Tissue
Muscular Tissue 
Muscular tissue is composed of muscle fibers that contain actin and myosin filaments, whose interaction accounts for the muscle contraction. Muscle contraction, in turn, ac­counts for movement. Three types of muscular tissue are found in the body: skeletal, smooth, and cardiac muscle.
Skeletal muscle is attached to the bones of the skeleton and functions to move body parts. Skeletal muscle fibers are cylindrical and run the length of a muscle. They are multinucleated, with the nuclei appearing just in­side the plasma membrane. The fibers also have character­istic light and dark bands perpendicular to the length of the cell. These bands give the muscle a striated appearance. Skeletal muscle is under conscious or voluntary control and contracts faster than any other muscle type.
Smooth muscle is so named because it lacks striations. The spindle-shaped cells that make up smooth muscle are not under voluntary control and are said to be involuntary. Smooth muscle, which is found in the viscera (intestine, stomach, and so on) and in blood vessels, con­tracts more slowly than skeletal muscle, but can remain contracted for a longer time. The cells tend to form layers in which the thick middle portion of one cell is opposite the thin ends of adjacent cells. Consequently, the nuclei form an irregular pattern in the tissue.
Cardiac muscle, which is found only in the heart, is responsible for the heartbeat. Cardiac muscle seems to combine features of both smooth and skeletal muscle. Cardiac muscle has striations like those of skeletal muscle, but the contraction of the heart is involuntary for the most part. Cardiac muscle fibers also differ from skele­tal muscle fibers in that they are branched and seemingly fused, one with the other, so that the heart appears to be composed of one large, interconnecting mass of muscle cells. Actually, however, cardiac muscle fibers are separate and individual but they are bound, end-to-end, at interca­lated disks, areas of folded plasma membrane between the cells.
Muscular tissue contains actin and myosin filaments. These form a striated pattern in skeletal and cardiac muscle. but not in emooth muscle. Cardiac and smooth muscle are under Involuntary control. Skeletal muscle is under voluntary control.
Nervous Tissue 
Nervous tissue, found in the brain and spinal cord, con­tains specialized cells called neurons that conduct nerve impulses. A neuron (nu'ron) has three parts: (1) a dendrite conducts signals to the cell body; (2) the cell body contains the nucleus and most of the cytoplasm of the neu­ron; and (3) the axon generally conducts nerve impulses away from the cell body.
Long axons are called fibers. Outside the brain and spinal cord, fibers are bound together by connective tissue to form nerves. Nerves conduct impulses from sense or­gans to the spinal cord and brain, where the phenomenon called sensation occurs. They also conduct nerve impulses away from the spinal cord and brain to the muscles, caus­ing the muscles to contract.
In addition to neurons, nervous tissue contains neu­roglial (nu-rog'le-al) cells. These cells maintain the tissue by supporting and protecting the neurons. Schwann cells are neuroglial cells that encircle all long nerve fibers that are out­side of the brain or spinal cord. Each Schwann cell encircles only a small section (1 mm) of a nerve fiber. The gaps be­tween Schwann cells are called the nodes of Ranvier. A nerve impulse moves from node to node. Collectively, the Schwann cells give nerve fibers a protective layer of fatty in­sulation called a myelin sheath. Because the myelin sheath is white, all nerve fibers appear to be white.
Nervous tissue contains conducting cells called neurons. Neurons have processes called axons and dendrites. Outside the brain and spinal cord. these long axons (fibers) are found in nerves.

Posted in Human Anatomy and Physiology