Skeletal muscle is composed of fascicles, bundled units of multiple muscle fibers or muscle cells. Each muscle fiber consists of a cross-banded structure that is further divided into myofibrils that contain thick myosin and thin actin myofilaments, which give muscle its stripe appearance. This structure gives skeletal muscle a characteristic striated structure. Muscle contraction occurs when these myofilaments move closer together when stimulated by the release of the neurotransmitter acetylcholine from a nerve cell that communicates with the muscle fiber.
Common examples of skeletal muscles include major muscle groups that control movement of the arms, legs, and body, such as the biceps, triceps, glutes, quadriceps, hamstrings, and abdominals.
Involuntary muscles are muscles 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 heartbeat regulation, circulation, breathing, digestion, and urination. Most involuntary muscles are made up of smooth muscle. Smooth muscle lacks the striated structure of skeletal muscle and instead consists of sheets or layers of smooth muscle cells.
When stimulated by the autonomic nervous system to contract from the release of hormones or other chemical signals, smooth muscle cells shorten via the movement of actin and myosin myofilaments.
Involuntary smooth muscles include the diaphragm , intestines, bladder, and walls of blood vessels. The one exception of an involuntary muscle is the myocardium, or heart muscle. The myocardium is composed of a specialized type of muscle cell called cardiac muscle that is only found 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. Neuromuscular disorders , also called skeletal muscle disorders, are conditions that affect the nerves that send electrical signals to voluntary skeletal muscles to control movement.
When the nerves are damaged, communication between the nerves and muscles becomes disrupted. This results in significant muscle weakness, atrophy, and loss of function. The majority of neuromuscular disorders are genetic or caused by problems with the immune system. Nerves communicate with muscles through the release of neurotransmitters at the neuromuscular junction, the space between a nerve cell and muscle fiber.
Neuromuscular disorders can damage the nerve itself or the neuromuscular junction, where the signal is transmitted from a nerve to a muscle. Symptoms of neuromuscular disorders include:. Common neuromuscular disorders include:. Muscle weakness can also result from electrolyte imbalances , such as altered levels of sodium, potassium, calcium, or magnesium. Skeletal muscles that attach to bones such as the muscles of your arms, legs, neck, back, and trunk are voluntary muscles that you can consciously control.
Weakness or inability to control voluntary skeletal muscles can signal a health issue like a neuromuscular disorder or electrolyte imbalance. Involuntary muscles include those involved in automatic internal processes needed for survival that control your blood vessels and organs like your heart, lungs, and digestive system.
Skeletal muscles are muscles under voluntary control that cause parts of your body like your arms, legs, and body to move when contracted. If you experience skeletal muscle weakness, make sure to discuss your type and duration of symptoms with your doctor, as this might be a sign of a medical condition such as a neuromuscular disorder. Structurally, smooth muscles are fusiform in shape, i. Smooth muscles are made up of thick and thin filaments that are not arranged into sarcomeres resulting in a nonstriated pattern.
Microscopically they appear to be homogenous and hence, named smooth muscles. The cytoplasm of the smooth muscles contains actin and myosin in large amounts. Smooth muscles also have calcium-containing sarcoplasmic reticulum.
This calcium-containing sarcoplasmic reticulum is responsible for prolonged contraction. Smooth muscles can be single-unit or multi-unit muscles. Single unit smooth muscles contract and relax as a single unit i.
The contractions in the digestive tract are an example of the slow and steady involuntary contractions in the single-unit smooth muscles. Vesicles of a nerve fiber or buttons surround the smooth muscle fibers and carry neurotransmitters. Smooth muscles have a greater elastic property in comparison to striated muscles. The elasticity of smooth muscles is a critical property as it helps to maintain contractile tone in organs like the urinary bladder.
Due to the absence of the sarcomeres, the organization and stretchability of the smooth muscles are not limited. Instead, smooth muscles exhibit a stress-relaxation response wherein the muscles of a hollow organ are stretched when the organ fills up.
This mechanical stress due to the stretching of the organ triggers the contraction. However, muscle relaxation immediately after contraction ensures that the organ does not empty its content prematurely. This phenomenon is significant for the urinary bladder, wherein the smooth muscle tone ensures the efficient functioning of the excretory system. Do you have a question on smooth muscle tissues? Ask our community. Join our Forum: Smooth muscle vs dense regular connective tissue.
Though both cardiac muscles and smooth muscles are involuntary, they differ from each other. The differences are:. The movement of the involuntary muscles, i. Almost all the hollow organs like the stomach, bladder, and tubular structures like blood vessels, bile ducts, sphincters, uterus, eye, etc. Some of the body movements can be controlled at will, others cannot. The body has a motor program, which is the pattern of neural activities required to perform a movement. Learn more about this topic in this tutorial.
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Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum. Table of Contents. Involuntary muscle biology definition : A muscle that contracts without conscious control. Examples: 1 the smooth muscles , which are muscles lacking striations when viewed under a microscope.
This is why involuntary muscles are sometimes called non-striated or un-striped muscles. The smooth muscles are found lining the internal organs such as the esophagus, stomach , intestines, etc. The cardiac muscle has striations when viewed under the microscope but its contractions are not under the control of the will.
Organization at the Tissue Level. Search for:. Muscular Tissue. Characteristics of Muscle Tissue The three types of muscle tissue are skeletal, smooth, and cardiac. Learning Objectives Describe the types of muscle tissue. Key Takeaways Key Points Muscle tissue can be divided functionally, is it under voluntary or involuntary control; and morphologically, striated or non-striated. By applying these classifications three muscle types can be described; skeletal, cardiac and smooth.
Skeletal muscle is voluntary and striated, cardiac muscle is involuntary and straited and smooth muscle is involuntary and non-striated.
Key Terms skeletal muscle : The voluntary muscle of vertebrates, which is striated and anchored by tendons to bone, is used to effect skeletal movement such as locomotion. Licenses and Attributions. CC licensed content, Shared previously.
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