Your nervous system automatically controls involuntary muscles in your body. These muscles can stretch and maintain tension over long periods. They save you from the time-consuming regulation of your breathing, digestion, blood circulation and your heart. There are two kinds of involuntary muscles: visceral and cardiac. Smooth muscle tissue makes up these muscles and the cells have a spindle shape, no striations and a central nucleus.
Cardiac muscle, found only in the heart, can stretch like visceral muscle and contract like skeletal muscle. Short single contractions give it its name: twitch muscle. The autonomic nervous system controls the contractions through electrical impulses. The heart pumps blood which delivers the proteins, enzymes, nutrients and other needed substances throughout the body, insuring life.
You find visceral involuntary muscles in the digestive tract, respiratory passages, urinary and genital ducts, bladder, gallbladder and walls of the blood vessels. They contract more slowly than skeletal muscle with smooth, wavelike movements called peristalsis. For example, when you swallow food, the visceral muscles in the esophageal wall slide the food down to the stomach where muscles work with enzymes and stomach acids to break food down and gradually transfer it into the small intestine. The small intestine absorbs nutrients and peristalsis keeps the food moving through to the large intestine. There, the muscles in the walls move it to the rectum as the body reabsorbs water from it.
This dome-shaped muscle has the striated appearance of skeletal muscle but acts like both voluntary (skeletal) and involuntary muscle. It separates the abdomen from the chest cavity. When the doctor says to take a deep breath, you inhale and the diaphragm contracts downward to allow air to fill the lungs. When you exhale, it relaxes, compressing the lungs. You can control your breathing. However, you don't need to think about it. The autonomic nervous system also controls it and the diaphragm.
In women, the uterus or womb is a small, hollow, pear-shaped organ located in the lower central abdomen. Visceral involuntary muscle forms one layer of its wall. Hormones play a role in controlling uterine wall muscle. At the end of a woman's menstrual cycle, if an egg has not been fertilised, the uterus needs to shed its thickened inner lining. Decreasing levels of the hormones progesterone and estradiol signal the uterine wall muscles to contract. The contractions expel the uterine wall's inner lining. A pituitary hormone called oxytocin may give the signal to the uterus to begin labour to push a baby into the vaginal canal during birth.
- "Anatomy & Physiology for Dummies," Donna Rae Siegfried, 2004