Involuntary and Voluntary Muscles
Dr.Kut | Feb 09, 2010 | Comments 0
Our human body composed of 35-40% of muscles that we happen to use every day from smiling, frowning, walking, chewing, and doing daily chores. There are two major types of muscles in our body and these are involuntary and voluntary muscles.
Involuntary muscles, otherwise known as smooth muscles appears as nucleated, nonstriated, non-branching muscle cells under a microscope. The definition of involuntary muscle is any muscle in our body which we cannot consciously control at will as they functionally move automatically. Our central nervous system and in some cases, our hormones influences or controls these involuntary muscles. There are also some of this muscles who function directly upon control of the spinal nerves without the participation of the brain. They act upon the specific external and internal stimuli depending on a determined threshold.
The types of involuntary muscles are smooth muscles and cardiac muscles. Examples of smooth muscle types are the iris (pupil contraction), skin muscles that causes goose bumps, vessel walls and any hollow internal organs; such as bronchi, esophagus, stomach, gallbladder, intestines, genital and urogenital organs. The muscles of the digestive tract that move food from the esophagus down to the gut are involved in mixing food and gastric juices in the process known as peristalsis.
Involuntary skeletal muscle is a skeletal muscle which is not under conscious control while involuntary striated muscle exclusively pertains to cardiac muscle. Although involuntary muscles are nonstriated, an exception was made for cardiac muscles. Cardiac muscle is an involuntary striated muscle which is classified differently due to its unique properties apart from the other smooth muscles. Our heart is composed of cardiac muscles which increases our heart rate when we exercise and also relaxes our heart automatically. These muscles are controlled by several nerve impulses from our natural pacemaker, sinoatrial node which is influenced by the sympathetic and parasympathetic systems. A hormone called adrenaline acts on our cardiac muscles to increase the cardiac output (how fast blood is pumped out of from our heart and throughout our body).
The rationale of involuntary muscle movement remains the same with a simple muscle contraction. It all started from the sliding interaction of actin and myosin filaments, but the difference is smooth muscles do not contain troponin and instead contains the thin filament protein tropomysin. The calcium activates phosphorylation which triggered the start of a contraction.
The second general muscle type is skeletal or voluntary muscles which are also known as striped muscles because of their histologic appearance. They are called voluntary because we have direct control over them when we want to initiate any movements. They are attached to bones and that is why they are also called skeletal muscle.
Voluntary muscles can be found in your face, jaws, arms and legs. These striped muscles work in pairs of antagonistic muscles. An example is when your biceps contract it forces to flex the elbow at the same time.
Skeletal or voluntary muscles contract according to the sliding filament model. It is initiated by an action potential in the muscle cell membrane which depolarizes the T tubules that further opens the calcium releasing channels. This further increases the intracellular calcium levels and this calcium binds to troponin C on the thin filaments resulting a conformational change in troponin leading to the cross-bridge cycle event. On this cycle, actin and myosin proteins bind, the heads of the cross-bridges pivot and these thin and thick filaments slide towards each other as ATP is being hydrolyzed. This cycle continues as long as calcium is bound to troponin C.
Voluntary and Involuntary muscles play an essential role in the normal functioning of our body systems. These muscles control our conscious movements and the automatic functions of the different organs in our body. With the differences in histologic appearances, voluntary and involuntary muscles also vary on their actions. Voluntary action involves conscious thought and free will and allows us to control our skeletal muscles. This action is controlled by our cerebrum with or without sensory stimulation. Voluntary actions are learned and allow us to respond with the benefit of experience. Involuntary actions do not involve thought and is not under control of will in which we have no control over its activities. Example of this is the pumping of our hearts and intestinal peristalsis. This is controlled by spinal cord (spinal cord reflex action) and hind-brain (cranial reflex action). Involuntary actions are not learned and therefore are inborn. Its role is to respond quickly to avoid danger and it usually involves a response to stimuli, example is a knee jerk. Other examples of involuntary actions are involuntary movements of smooth muscles, cardiac muscles, and glands.
Condition such as involuntary muscle spasms occurs when a group of muscles, suddenly contracts involuntarily. This sudden involuntary muscle contraction may be associated with pain that disappears after a while. In most cases this commonly occurs over a hollow organ or a contraction of an orifice. The spasmodic muscle contraction may be brought about by many medical conditions and this may include dystonias. On the other hand, involuntary muscle twitches is usually a mild uncontrollable twitching of a muscle group caused by diet deficiency, caffeine overdose and drug side effects. Common occurrence of twitches is on the eyelids, calf and thumb which are often triggered by stress or anxiety. Twitches are usually mild and disappear in a few days.
REFERENCES:
http://en.wikipedia.org/wiki/Involuntary_muscle
http://www.nlm.nih.gov/medlineplus/ency/article/003296.htm
http://www.purchon.com/biology/muscles.htm
http://www.informatics.jax.org/searches/GO.cgi?id=GO:0003011
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Filed Under: Bones, Joints and Muscles
