Involuntary muscles are either nonstriated or striated. They are found in the walls of hollow organs, in your eyes and around your hair follicles. Cardiac or heart muscle cells appear striated under a microscope because of the large number of contractile proteins in the cells. Smooth muscles are involuntary because they contract through stimulation from your nervous system, without you actively controlling their movement.
Heart Muscle
Your heart muscle is a collection of smooth, involuntary muscle cells running in a parallel fashion and connecting from end to end. This anatomical set up enables your heart to effectively contract and push blood out to your body. The size and shape of your heart takes on the characteristic shape of your chest cavity. If you are long and thin, your heart will be elongated. If you are short and stocky, your heart will be wider than it is longer. Improve the structure and function of your heart through regular, moderately- to vigorously-intense aerobic exercise.
Precapillary Sphincter
The precapillary sphincter is a ring of smooth muscle at the beginning of capillaries. It controls the flow of blood at specific tissues. For example, when you go out for a run, the precapillary sphincters in your leg muscles open up to let more blood into your working muscles. The precapillary sphincters in your digestive organs constrict those capillaries to reduce blood flow to your gut.
Muscularis
The muscularis muscle is located in your digestive tube or GI tract. It runs from your esophagus all the way to your large intestines. The muscularis forms two layers in your esophagus, three layers in your stomach, two in your small intestine and one layer in your large intestine. The primary role of this muscle is to move food through your GI tract so it can be broken down, absorbed and then eliminated from your body.
Eye Muscles
The iris and ciliary muscles are in your eye. The iris controls the size of your pupil while the ciliary muscle regulates the shape of the lens in your eyes. Your iris contracts and relaxes to let in more or less light through your pupils, helping to form clear images. The ciliary muscle changes the shape of your lens depending on whether the object you are focusing on is close to you or far away from you.
The Myometrium
The middle layer of a woman's uterus is made of three layers of smooth muscle fibers going in all directions--diagonally, longitudinally and transversely. The myometrium is thicker on the upper part of the uterus, but thinner on the lower end, the cervix. This arrangement of muscle fibers makes it possible for the uterus to contract forcefully on the upper end and expand wider on the lower end.
References
- "Anatomy & Physiology"; Gary Thibodeau, Ph.D. and Kevin Patton, Ph.D.; 2007
- "Exercise Physiology, Energy, Nutrition & Human Performance"; William McArdle, Frank Katch and Victor Katch; 2007
