Smooth muscles of the human body are activated by your autonomic nervous system, or the part of your nervous system creating involuntary movement within your body. You cannot consciously control these muscles. These muscles contract to perform vital movements in the human body, such as digestion, blood flow and vision control. Smooth muscles are found in hollow organs, blood vessels, eyes and skin.
Location
Smooth muscles can be found in your stomach and intestines. These muscles help squeeze food through your digestive tract. You also have smooth muscles in your blood vessels that help distribute blood throughout your body. The smooth muscles in your eye help you focus, allowing more or less light into your eyes. Smooth muscles also are attached to your hair follicles making your hair stand up, creating goose bumps when you are cold.
Features
Unlike skeletal or heart muscle, each smooth muscle cell is small and spindle-shaped. These cells do not have transverse striations and have only one nucleus, giving them a smooth, unstriated appearance under the microscope. Though the smooth muscle cells themselves are small, they are arranged in long, flat layers to contract in all directions.
Method of Control
Smooth muscles are controlled by the sympathetic division of your autonomic nervous system. These contractions of your smooth muscles are slow, weak and rhythmic. In a fight-or-flight situation like running away from a dog, the smooth muscles of your blood vessels automatically contract and expand to direct blood toward your skeletal muscles and heart. When you eat, the muscles of your stomach automatically receive signals from your sympathetic nerves to contract and move the food through your digestive tract. The muscles controlling your eyes automatically adjust your iris when you go from a dark room into bright light.
Innervation
Sympathetic nerves controlling your smooth muscles originate from the middle third of your spinal cord. When these nerves fail to work properly, it can result in pain, loss of sensation, abnormally high or low blood pressure and digestive problems. Injuries to these nerves may be due to diseases like diabetes and cancer. Excessive alcohol use and exposure to poisonous substances can damage these nerves as well. Nerve damage may be hereditary and can occur with prolonged compression of the nerve.
Disease States of Smooth Muscle
Atherosclerosis occurs in part due to the build-up of smooth muscle cells in the walls of the arteries, thereby narrowing the arteries. Excessive smoking leads to a build-up of myofibroblasts, cells functioning as smooth muscle cells and structural cells. These cells lay down excessive connective tissue, decreasing the lung's ability to transport oxygen from the air you breath, into the blood stream.
References
- "Exercise Physiology, Energy, Nutrition &Human Performance"; Lippincott Williams & Wilkins; 2007
- Medline Plus: Peripheral Neuropathy
- CytoChemistry: Smooth Muscle
