Peritaltismo

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BIOPAC Systems, Inc., 42 Aero Camino, Goleta, CA 93117 Ph (805) 685-0066, Fax (805) 685-0067, Web: www.biopac.com

BSL PRO Lesson A05
©2002 BIOPAC Systems, Inc. Updated 12-22-03

PHYSIOLOGICAL PROPERTIES OF VISCERAL SMOOTH MUSCLE

Overview
Dr. Katja Hoehn
Mt. Royal College Chemical Biological and Environmental Sciences Calgary, AB, CANADA There are three types of muscle tissue in thehuman body, each specialized for certain tasks in maintaining homeostasis.
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Skeletal muscle, generally under voluntary control, is so named because it is in most cases attached to the skeleton. Contraction of skeletal muscle moves parts of the body with respect to one another or moves the entire body. Cardiac muscle is the muscle found only in the heart, where it functions to circulatethe blood in order to deliver nutrients to cells and remove wastes from cells throughout the body. Smooth muscle is generally not under voluntary control. It is usually organized in sheets found in the walls of hollow organs of the digestive, urinary, reproductive and respiratory system and in the walls of all but the smallest of blood vessels. Contraction of smooth muscle controls movementthrough these organs and blood vessels as well as the pressure within them.

Smooth muscle is characterized histologically by small spindle-shaped cells with a central nucleus. They do not exhibit the characteristic striations (stripes) seen in skeletal muscle. They lack myofibrils and do not have the clearly defined bands of actin and myosin, which make up the A-bands and the I-bands of thesarcomeres of skeletal muscle. In spite of their structural differences, a number of similarities exist between skeletal muscle and smooth muscle.
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In both types of muscle, contraction is achieved through the interaction of actin and myosin via the sliding filament mechanism. Both types of muscle use ATP to energize the sliding process.

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In both types of muscle the final trigger forcontraction is a rise in the cytosolic Ca 2+ concentration.

However, there are a number of important differences between excitation-contraction coupling in smooth muscle versus excitation contraction-coupling in skeletal muscle.
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In smooth muscle the sarcoplasmic reticulum is poorly developed and there are no ttubules. On depolarization of the smooth muscle membrane, cytosolic Ca 2+rises due to both a release of some Ca 2+ from the sarcoplasmic reticulum as well as the entry of Ca 2+ into the cell from the extracellular fluid. The cytosolic Ca 2+ does not bind troponin as it does in skeletal muscle (smooth muscle does not contain troponin), but instead interacts with a regulatory molecule, calmodulin. The Ca 2+ calmodulin complex then binds to and activates a molecule calledmyosin kinase (or myosin light chain kinase), which catalyzes the transfer of a phosphate from ATP to myosin. This phosphorylated myosin can then interact with actin of the thin filaments and that initiates the cross-bridge cycling. Relaxation comes about when the cytosolic Ca 2+ concentration returns to precontraction levels and the myosin is dephosphorylated (the phosphate group is removed.) Ca2+ in the cytosol is actively pumped back into the sarcoplasmic reticulum and back across the plasma membrane.

Smooth muscle can be classified into two major categories: multi-unit smooth muscle and singleunit (or visceral) smooth muscle.
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Mulit-unit smooth muscle is made up of individual muscle fibers, which can operate independently, are not connected by gap junctions, and tend notto be spontaneously active. Multi-unit smooth muscle is found only in a few locations in the body. This includes the internal eye muscles that adjust the focus and the pupil size, the smooth muscle of the large airways and large arteries, and the erector pili muscles attached to the hair follicles of the skin (which, when they contract, give you “goose bumps”). Its properties tend to be...
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