Peristalsis per ı stal sis a wavelike motion that

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Peristalsis (per ı˘-stal sis), a wavelike motion that keeps food moving through the small intestine, is typical of its activity. Nervous Tissue When we think of nervous tissue, we think of cells called neurons. All neurons receive and con- Nuclei of supporting cells Diagram: Nervous tissue Photomicrograph: Neurons (100x) Nuclei of supporting cells Cell processes Cell body of neuron Cell body of neuron Brain Cell processes Spinal cord FIGURE 3.21 Nervous tissue. Neurons and supporting cells form the brain, spinal cord, and nerves. duct electrochemical impulses from one part of the body to another; thus irritability and conductivity are their two major functional characteristics. The structure of neurons is unique (Figure 3.21). Their cytoplasm is drawn out into long processes (exten- sions), as much as 3 feet or more in the leg, which allows a single neuron to conduct an impulse over long distances in the body. Neurons, along with a special group of supporting cells that insulate, support, and protect the delicate neurons, make up the structures of the nervous system the brain, spinal cord, and nerves. Tissue Repair (Wound Healing) The body has many techniques for protecting itself from uninvited guests or injury. Intact physical bar- riers such as the skin and mucous membranes, cilia, and the strong acid produced by stomach glands are just three examples of body defenses exerted at the local tissue level. When tissue injury does occur, it stimulates the body’s inflammatory and immune responses, and the healing process
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begins almost immediately. Inflammation is a gen- eralized (nonspecific) body response that attempts to prevent further injury. The immune response, on the other hand, is extremely specific and mounts a vigorous attack against recognized in- vaders (bacteria, viruses, toxins). These protective responses are considered in detail in Chapter 12. Here we will concentrate on the process of tissue repair itself. Tissue repair, or wound healing, occurs in two major ways: by regeneration and by fibrosis. Regeneration is the replacement of destroyed tissue by the same kind of cells, whereas fibrosis involves repair by dense (fibrous) connective tis- sue, that is, by the formation of scar tissue . Which occurs depends on (1) the type of tissue damaged and (2) the severity of the injury. Generally speaking, clean cuts (incisions) heal much more successfully than ragged tears of the tissue. Tissue injury sets a series of events into motion. The capillaries become very permeable. This allows fluid rich in clotting proteins and other substances to seep into the injured area from the bloodstream. Then leaked clotting proteins construct a clot, which stops the loss of blood, holds the edges of the wound to- gether, and walls off the injured area, prevent- ing bacteria or other harmful substances from spreading to surrounding tissues. Where the clot is exposed to air, it quickly dries and hard- ens, forming a scab. Granulation tissue forms. Granulation tis- sue is a delicate pink tissue composed largely of new capillaries that grow into the damaged area from undamaged blood vessels nearby.
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