16 Differences Between Voluntary and Involuntary Muscles

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Last Updated on September 16, 2020 by Sagar Aryal

Differences Between Voluntary and Involuntary Muscles
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Voluntary muscles definition

Voluntary muscles are the muscles that can be moved by the free will of the person and are almost always associated with the skeleton system.

  • These muscles are attached to the bones by means of tendons, and they are responsible for all kinds of movements in vertebrates.
  • Voluntary muscles account for about 40% of the total weight of the body and are usually long and present near the bones.
  • Voluntary muscles are striated muscles as the muscles are made up of long, thin, and multinucleated muscle fibers that are crossed with a regular pattern of red and white red and white lines, resulting in a striated appearance.
  • Each muscle cell is nucleated, where the nucleus remains at the periphery of the cell.
  • The muscle fibers are further covered with a specialized cell membrane called myolemma or sarcolemma.
  • The sarcolemma is thick in the voluntary muscles, and it connects the muscle fibers to each other and the connective tissues.
  • Besides, the muscle fibers have contractile units called sarcomeres which shorten, causing the muscle to contract and relax. The sarcomeres contain actin and myosin proteins, which together bring about the contraction of muscle by sliding against each other.
  • Each muscle fibers are connected to each other via connective tissue and interact with each other through nerves and blood vessels.
  • The voluntary control of these muscles is regulated by a part of the peripheral nervous system, termed the somatic nervous system.
  • The somatic nervous system consists of afferent and efferent nerves where the afferent nerves relay information to the central nervous system and the efferent nerves relay information from the CNS to the voluntary muscles for their contraction.
  • These muscles are not myogenic and require an outside nerve stimulus for contraction.
  • The voluntary muscles require a large amount of energy for their contraction and relaxation. Thus they possess multiple mitochondria to meet the energy needs.
  • Voluntary muscles exhibit rapid contraction and relaxation compared to the involuntary muscles. However, they also tire quickly and require regular intervals of rest.
  • These muscles are essential as they are involved in the movement of the body parts and locomotion of the body.
  • Some examples of voluntary muscles include the biceps, the triceps, the quadriceps, diaphragm, pectoral muscles, abdominals, hamstrings, etc.

Involuntary muscles definition

Involuntary muscles are the muscles that cannot be controlled by will or conscious and are often associated with organs that exhibit slow and regular contractions and relaxation.

  • Involuntary muscles are also termed as smooth muscles or non-striated muscles as there are no striations when viewed under the microscope.
  • These muscles are mostly found lining the walls of internal organs like the stomach, intestine, urinary bladder, and blood capillaries.
  • Individual muscle cells in smooth muscles are long, thin, and spindle-shaped with a centrally located nucleus.
  • The myolemma or sarcolemma is present as the cell membrane of the muscle fibers that functions to connect the muscle fibers to each other. The sarcolemma present is comparatively thinner and less in concentration.
  • One of the examples of involuntary muscles is the cardiac muscle which differs in structure and function to other involuntary muscles.
  • The cardiac muscle is composed of individual heart muscle cells called cardiomyocytes which are joined together by intercalated discs. These muscle cells are enclosed by collagen fibers and other substances that form the extracellular matrix.
  • The contraction of cardiac muscle is different from that of both skeletal and smooth muscles. The action potential is generated within the muscles in the form of electrical stimulation.
  • This potential causes the release of calcium ions from the cells into the sarcoplasm reticulum. The rise of calcium ions causes the myofilaments to slide past each other, causing excitation-contraction.
  • Cardiac muscle is myogenic, where the nerve stimulus is generated within the muscles.
  • Most of the muscle cells within the muscle fibers of the involuntary muscles function as a single unit where the contraction and relaxation occur together.
  • Involuntary muscles are controlled by the autonomous nervous system of the peripheral nervous system.
  • The motor nerves of the autonomous nervous system consist of a type of neurotransmitter-filled bulges termed varicosities.
  • Since gap junctions connect the cells in involuntary muscles, the neuron signals can be passed from one cell to another via neurotransmitters.
  • The contraction and relaxation of involuntary muscles are slow and occur at regular intervals of time.
  • As a result, these muscles do not tire quickly and can work continuously.
  • They also have less energy requirement when compared to voluntary muscles and thus have fewer mitochondria.
  • Involuntary muscles are involved in the movements of the internal organs, and also aid in the passage of fluids and food in the digestive system.
  • Some examples of involuntary muscles include the cardiac muscle and smooth muscle lining the intestinal tracts, blood vessels, urogenital tracts, respiratory tract, etc.

Key differences (Voluntary muscles vs Involuntary muscles)

Basis for Comparison

Voluntary muscles

Involuntary muscles

DefinitionVoluntary muscles are the muscles that can be moved by the free will of the person and are almost always associated with the skeleton system.Involuntary muscles are the muscles that cannot be controlled by will or conscious and are often associated with organs that exhibit slow and regular contractions and relaxation.
Also known asVoluntary muscles are also known as striated muscles or skeletal muscles.Involuntary muscles are also known as nonstriated muscles or smooth muscles.
FoundVoluntary muscles are found attached to bones by means of tendons.Involuntary muscles are found lining the walls of internal organs like the stomach, intestine, urinary bladder, and blood capillaries.
Shape of the cellThe muscle cells in the voluntary muscles are long cylindrical and unbranched with the nucleus located towards the periphery of the cell.The muscle cells in involuntary muscles are thin, long, and spindle-shaped with a centrally located nucleus.
Type of cellThe muscle cells are multinucleated with a large number of mitochondria.The muscle cells are uninucleated with fewer mitochondria.
SarcolemmaSarcolemma surrounding the voluntary muscle fibers is thicker.Sarcolemma surrounding the involuntary muscle fibers is thinner.
SarcomeresSarcomeres are present in the muscle fibers.Sarcomeres are absent in the muscle fibers.
Intercalated discsIntercalated discs are not found in the voluntary muscles.Some involuntary muscles like cardiac muscle consist of intercalated discs.
ControlVoluntary muscles can be controlled by will or conscious.Involuntary muscles are the muscles that cannot be controlled by the will.
Nervous systemThe somatic nervous system controls all voluntary muscles.The autonomic nervous system controls involuntary muscles.
Nerve stimuliThe nerve stimulus in the voluntary muscle is created from the outside by the nervous system.Some involuntary muscles are myogenic where the stimulus is generated within the muscle.
Type of contractionsThe contractions and relaxation of voluntary muscles are rapid and robust.The contractions and relaxations of involuntary muscles are rhythmic and slower.
Energy requirementMore energy is required for the contraction and relaxation of voluntary muscles.Less energy is required for the contraction and relaxation of involuntary muscles.
Fatigue and restVoluntary muscles tire quickly and require regular intervals of rest.Involuntary muscles do not fatigue and can work continuously.
Involved inThese muscles are important as they are involved in the movement of the body parts and locomotion of the body.Involuntary muscles are involved in the movements of the internal organs and also aid in the passage of fluids and food in the digestive system.
ExamplesSome examples of voluntary muscles include the biceps, the triceps, the quadriceps, diaphragm, pectoral muscles, abdominals, hamstrings, etc.Some examples of involuntary muscles include the cardiac muscle and smooth muscle lining the intestinal tracts, blood vessels, urogenital tracts, respiratory tract, etc.

Examples of voluntary muscles

Diaphragm

  • The diaphragm is a primary respiratory muscle that assists the process of breathing by increasing and decreasing the volume of the thoracic wall.
  • It is a dome-shaped skeletal muscle present underneath the lungs and the heart. It separates the abdominal area from the chest.
  • The diaphragm is a voluntary muscle that is controlled by the phrenic nerve which runs from the neck to the diaphragm.
  • There are large openings in the diaphragm for the passage of certain structures like the esophageal opening for the vagus nerve, aortic opening for the aorta, and the caval opening for the inferior vena cava.
  • Besides the respiratory functions, the diaphragm is also involved in other nonrespiratory functions. The diaphragm increases abdominal pressure to assist the removal of vomit, urine, and feces. It also generates pressure on the esophagus to prevent acid reflux.
  • Spasmodic inspiratory movement of the diaphragm produces the characteristic sound known as hiccupping.

Biceps

  • Bicep muscles are the muscle with two heads or two points of origin, which in humans are bicep brachii and bicep femoris.
  • The bicep brachii is the muscle present on the front side of the upper arm. Its tendons connect this muscle to the inner protrusion near the head of the radius, one of the two bones of the forearm.
  • The biceps brachii bends the forearm toward the upper arm and is thus used in lifting and pulling movements.
  • The size of the bicep brachii is considered to be a symbol of bodily strength.
  • The bicep femoris is one of the muscles present at the back of the thighs.
  • It originates from the back of the isthmus and back of the femur and is attached to the head of the fibula and tibia.
  • It is involved in the movement of the thighs and flexing of the leg at the knee.

Examples of involuntary muscles

Cardiac muscle

  • Cardiac muscle is an involuntary striated muscle that is present on the wall of the heart and undergoes contractions and relaxations at regular intervals.
  • The cardiac muscle is composed of individual heart muscle cells called cardiomyocytes which are joined together by intercalated discs. These muscle cells are encased by collagen fibers and other substances that form the extracellular matrix.
  • The contraction of cardiac muscle is different from that of both skeletal and smooth muscles.
  • Cardiac muscle is myogenic, where the nerve stimulus is generated within the muscles.
  • The action potential is generated within the muscles in the form of electrical stimulation.
  • This potential causes the release of calcium ions from the cells into the sarcoplasm reticulum. The rise of calcium ions causes the myofilaments to slide past each other, causing excitation-contraction.
  • Cardiac muscles are innervated with vagal and sympathetic nerves that control the contractions of the muscle fibers.

Smooth muscle

  • Smooth muscle is an involuntary nonstriated muscle that consists of single-unit or unitary muscle and multiunit muscles.
  • Smooth muscle lines the wall of various internal organs like the intestinal tract, urinary tract, and blood vessels.
  • The ciliary muscle of the eye is also a smooth muscle that dilates and controls the iris that alters the shape of the lens.
  • Single-unit smooth muscles are the muscles where the contraction and relaxation of the entire muscle occur as a whole. The multiunit muscles can contract and relax separately as different units.
  • The muscle fibers are surrounded by nerve fibers consisting of vesicles called varicosities or boutons that carry neurotransmitters.

References and Sources

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