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Involuntary muscles are those that contract and relax without conscious control. They are regulated by the autonomic nervous system, which controls the activity of organs and blood vessels needed for essential daily functions, such as digestion and breathing. The heart is an example of an involuntary muscle, contracting and beating automatically to pump blood through the body. Involuntary muscles are also found in the digestive system, blood vessels, bronchi, uterus, and bladder. These muscles are typically made up of smooth muscle cells, which lack the striated structure of skeletal muscles.
| Characteristics | Values |
|---|---|
| Definition | Muscles that contract without conscious control |
| Controlled by | The autonomic nervous system |
| Types | Smooth muscles, cardiac muscles |
| Location | Heart, respiratory and digestive systems, blood vessels, bronchi, uterus, bladder |
| Function | Cause variations in shape or inner diameter of organs |
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What You'll Learn
The heart is an involuntary muscle
Cardiac muscle is striated like skeletal muscle, but it is under involuntary control. The heart is made up of three layers: the pericardium, myocardium, and endocardium. The myocardium is the cardiac muscle, composed of sarcomeres that allow for contractility. The endocardium is not cardiac muscle and forms the inner lining of the heart chambers and valves. The pericardium is a fibrous sac surrounding the heart. The cardiac muscle is responsible for the contractility of the heart and, therefore, the pumping action.
The heart begins as a single chamber, but four separate chambers are created through the growth of various septa. The muscular ventricular septum originates from the bottom of the ventricle, with a membranous septum forming shortly after. The contractile functions of the heart require ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones. The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force.
Cardiac muscle cells (cardiomyocytes) are striated, branched, and contain many mitochondria. Each myocyte contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma contains voltage-gated calcium channels, which skeletal muscle does not possess. The sarcolemma is the cardiomyocyte plasma membrane containing transverse tubules (t-tubules). These t-tubules are highly branched invaginations that function in excitation-contraction coupling (ECC), action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction.
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Smooth muscles are involuntary
In the gastrointestinal tract, smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and platelet-derived growth factor receptor alpha (PDGFRα) work together as an electrically coupled functional syncytium. Smooth muscle cells are spindle-shaped myocytes with a single nucleus. They can tense and relax, and in their relaxed state, each cell is 30 to 200 micrometers long, thousands of times shorter than skeletal muscle cells. Smooth muscle lacks the striated structure of skeletal muscle and instead consists of sheets or layers of smooth muscle cells.
The contraction of smooth muscle cells is stimulated by the autonomic nervous system through the release of hormones or other chemical signals. This causes the cells to shorten via the movement of actin and myosin myofilaments. Myosin is primarily class II in smooth muscle, containing two heavy chains (MHC) that form a head and tail domain. These heavy chains can also be involved in contraction and arise from multiple genes. Additionally, myosin II contains four light chains (MLC), with two per head, and these light chains bind the heavy chains in the "neck" region.
Smooth muscles differ from skeletal and cardiac muscles in terms of structure, function, and regulation of contraction. They demonstrate greater elasticity and function within a larger length-tension curve than striated muscle. This ability to stretch and maintain contractility is crucial in organs like the intestines and urinary bladder. Smooth muscles also play a role in the mechanical tension that assists in the movement of substances inside the body and the stabilization of organs.
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Involuntary muscles are controlled by the autonomic nervous system
Involuntary muscles are those that are not under conscious control. Their contraction and relaxation are regulated by the autonomic nervous system, which controls the activity of organs and blood vessels needed for essential daily functions, such as digestion and breathing. The autonomic nervous system is a subdivision of the peripheral nervous system and controls the body's involuntary motor responses. It operates independently of the central nervous system, which is responsible for voluntary actions.
The autonomic nervous system has two branches: the sympathetic nervous system, which prepares the body for a "fight or flight" response, and the parasympathetic nervous system, which controls the "rest and digest" functions. The sympathetic nervous system increases heart rate to supply more oxygen to muscles during exercise, while the parasympathetic nervous system regulates digestion and manages the contraction of smooth muscles in the digestive tract to move food along. The autonomic nervous system also controls the release of hormones and other chemical signals that stimulate smooth muscle cells to contract.
Involuntary muscles are primarily made up of smooth muscle, which lacks the striated structure of skeletal muscle. Instead, smooth muscle consists of sheets or layers of smooth muscle cells, which are crossed by thin filaments called myofibrils. Smooth muscles contract and relax more slowly than skeletal muscles. They are present in the walls of the digestive system, blood vessels, bronchi, uterus, and bladder.
In addition to smooth muscles, the autonomic nervous system also controls cardiac muscle, which is a specialized type of muscle cell found only in the heart. Cardiac muscle is striated like skeletal muscle but is controlled by both the autonomic nervous system and its own pacemaker cells, causing it to contract automatically and rhythmically. The autonomic nervous system regulates heart rate, ensuring blood circulation even when distracted.
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Involuntary muscles are found in the digestive system
Involuntary muscles are those whose contraction is controlled by the autonomic nervous system and are not under conscious control. They include muscles involved in automatic internal processes needed for survival, such as the heart, lungs, and digestive system. The digestive system contains involuntary muscles that help propel food through the gastrointestinal tract in a process called peristalsis.
Peristalsis is a series of wave-like muscle contractions that move food through the digestive tract. It begins in the oesophagus, where strong wave-like motions of the smooth muscle move swallowed food to the stomach. Peristalsis is mediated by the vagus nerve at the dorsomotor nucleus and myenteric plexus level. The myenteric plexus's afferent (sensory) nerves deliver information to interneurons, which then communicate with efferent nerves, stimulating an action potential within smooth muscle cells.
In the stomach, food is churned into a liquid mixture called chyme, which then moves into the small intestine. Peristalsis in the small intestine is frequent, allowing for optimal absorption and digestion. The parasympathetic nervous system stimulates peristalsis in the small intestine, aiding in the rapid evacuation of faeces. Peristalsis in the large intestine occurs approximately 2 to 4 times daily and is most substantial in the hour following a meal.
Another type of involuntary muscle movement in the digestive system is segmentation, which occurs mainly in the intestines. Segmentation activates circular muscles that contract to move food back and forth, allowing it to mix with gastric juices and be broken down further. Segmentation slows the progress of food through the gastrointestinal tract, but peristalsis continues to move it gradually along.
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Cardiac muscle is involuntary
The human body has three types of muscle tissue: skeletal, smooth, and cardiac. Cardiac muscle, also known as myocardium or heart muscle, is involuntary. This means that its movement is not under conscious control. Instead, cardiac muscle contracts and relaxes involuntarily, keeping the heart pumping and blood circulating throughout the body.
Cardiac muscle is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs and encased in collagen fibres and other substances that form the extracellular matrix. These cells appear striated or striped under a microscope due to the alternating filaments of myosin and actin proteins they contain. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments toward each other, causing the cell to shrink. This process is powered by ATP.
The cardiac muscle forms a thick middle layer of the heart wall, between the outer layer (the pericardium) and the inner layer (the endocardium). The heart wall is a three-layered structure, with the myocardium sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium). The sheets of muscle that wrap around the left ventricle closest to the endocardium are oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner, they allow the ventricle to squeeze in several directions simultaneously, maximising the amount of blood pumped out of the heart with each heartbeat.
The contraction of cardiac muscle cells is regulated by the autonomic nervous system and the heart's own "pacemaker" cells. Pacemaker cells are a specialised type of cardiomyocyte that generates electrical impulses, or action potentials, that tell cardiac muscle cells to contract and relax. They are distributed throughout the heart and are responsible for several functions, including spontaneously generating and sending out electrical impulses that control the heart rate and determine how fast the heart pumps blood.
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Frequently asked questions
Involuntary muscles are muscles that contract and relax without conscious control. They are regulated by the autonomic nervous system.
The heart is an example of an involuntary muscle. It beats without any conscious input and is composed of cardiac muscle cells. Other involuntary muscles include those in the digestive system, blood vessels, bronchi, uterus, and bladder.
Voluntary muscles are skeletal muscles that can be consciously controlled. They are attached to bones and make up 30-40% of human body weight. Involuntary muscles, on the other hand, are typically smooth muscles that line the internal organs and exhibit slow and rhythmic contractions.
Involuntary muscles are essential for automatic internal processes that control organs and blood vessels, ensuring the body's survival. They help with breathing, digestion, and maintaining normal body temperature.
