Voluntary Vs Involuntary Muscles Overview

Voluntary muscles’ definition

Voluntary muscles that can be moved by a person’s free will are known as “voluntary muscles,” and they are virtually always connected to the skeletal system.

• In vertebrates, these muscles, which are connected to the bones via tendons, control a variety of actions.
• About 40% of the body’s weight is made up of voluntary muscles, which are typically lengthy and located close to the bones.
• The lengthy, narrow, multinucleated muscular fibres that comprise voluntary muscles are striated because they are constantly crossed by a pattern of red and white lines, which gives the muscles a striated look.
• The nucleus of each muscle cell is positioned at the cell’s periphery.
• The myolemma or sarcolemma, a specialised cell membrane, is used to further protect the muscle fibres.
• The sarcolemma, which binds the muscle fibres to one another and the connective tissues, is thick in the voluntary muscles.
• In addition, the sarcomeres, which are contractile units in the muscle fibres, shrink when the muscle contracts and relaxes. Actin and myosin, two proteins found in sarcomeres, work together to contract muscles by gliding against one another.
• Connective tissue links each muscle fibre to the next, while nerves and blood vessels mediate interactions between them.
• A section of the peripheral nervous system known as the somatic nervous system controls the voluntary regulation of these muscles.
• The somatic nervous system is composed of afferent and efferent nerves that transport signals from the central nervous system to the voluntary muscles for contraction via the afferent nerves.
• Since they lack myogenic characteristics, these muscles need external nerve input to contract.
• The contraction and relaxation of the voluntary muscles consume a lot of energy. They therefore have several mitochondria to suit their energy requirements.
• Compared to involuntary muscles, voluntary muscles exhibit fast contraction and relaxation. However, they also become tired easily and need rest breaks frequently.
• Due to their involvement in both the movement of the body’s organs and its motility, these muscles are crucial.
• The biceps, triceps, quadriceps, diaphragm, chest muscles, abdominals, hamstrings, etc. are instances of voluntary muscles.

Involuntary muscles definition

Involuntary muscles are those that cannot be regulated by willpower or consciousness, and they are frequently linked to organs that exhibit steady, consistent contractions and relaxation.

• Since they lack striations when observed under a microscope, these muscles are also referred to as smooth muscles or non-striated muscles.
• The walls of internal organs like the stomach, colon, urine bladder, and blood capillaries typically have these muscles lining them.
• The smooth muscle’s individual muscle cells have a central nucleus and are long, thin, and spindle-shaped.
• The cell membrane of the muscle fibres, known as the myolemma or sarcolemma, serves as a link between the muscle fibres. The sarcolemma is nevertheless there. It is comparatively thinner and less concentrated than normal.
• The cardiac muscle, which differs from other involuntary muscles in structure and function, is one example of an involuntary muscle.
• Cardiomyocytes, which are the individual heart muscle cells that make up the cardiac muscle, are connected by intercalated discs. Collagen fibres and other components that make up the extracellular matrix surround these muscle cells.
• Cardiovascular muscles contract differently from skeletal and smooth muscles. Electrical stimulation causes the muscles to produce an action potential.
• This potential results in the liberation of calcium ions into the sarcoplasm reticulum from the cells. Myofilaments move past one another as calcium ions rise, resulting in excitation-contraction.
• Myogenic heart muscle generates the nerve stimulus inside the muscle itself.
• The bulk of muscle cells within the fibres of the involuntary muscles function as a unit, contracting and relaxing at the same time.
• The autonomous nervous system of the peripheral nervous system regulates involuntary muscles.
• The motor nerves of the autonomous nervous system are made up of varicosities, which are neurotransmitter-filled lumps.
• Gap junctions link the cells that make up involuntary muscles, allowing neurotransmitters to transmit signals through one cell to the next.
• Involuntary muscle contractions and relaxation take place slowly and at predictable intervals.
• As a result, these muscles can function continuously and do not get tired easily.
• Additionally, they demand less energy than voluntary muscles, which results in fewer mitochondria.
• Internal organ movements are aided by involuntary muscles, which also help food and liquids flow through the digestive system.
• The heart muscle, as well as the smooth muscles lining the gastrointestinal tract, blood arteries, urogenital tracts, respiratory tract, etc., are several instances of involuntary muscles.

Examples of voluntary muscles

Diaphragm

• The diaphragm, the major respiratory muscle, aids in breathing by altering the volume of the thoracic wall.
• This is a dome-shaped skeletal muscle situated behind the diaphragm, the heart, and the lungs. It divides the chest from the abdomen.
• The diaphragm is a voluntary muscle controlled by the phrenic nerve, which extends from the neck to the diaphragm.
• The esophageal aperture for the vagus nerve, the aortic aperture for the aorta, as well as the caval aperture for the inferior vena cava, are only a few examples of the enormous openings in the diaphragm for the passage of various components.
• The diaphragm also plays a role in non-respiratory processes in addition to respiratory ones. In order to aid in the elimination of vomit, pee, and faeces, the diaphragm increases abdominal pressure. In order to avoid acid reflux, it also exerts pressure on the oesophagus.
• The hiccupping sound is a result of the diaphragm’s spasmodic movements during inspiration.

Biceps

• bicep brachii as well as the bicep femoris are the two heads or origins of the bicep muscle in humans.
• The bicep brachii is the muscle situated above the front side of the upper arm. Tendons connect this muscle to the inner projection near the apex of the radius, one of the two bones of the forearm.
• As it flexes the forearm in the direction of the upper arm, the biceps brachii is used in lifting and pulling movements.
• The size of the bicep brachii is regarded as a physical strength indicator.
• The bicep femoris is a muscle found in the rear of the thighs.
• It is joined to the heads of the fibula and tibia and develops from the back of the isthmus and the back of the femur.
• The flexing of the knee and thigh movements are both controlled by it.

Examples of involuntary muscles

Cardiac muscle

• The heart’s wall is covered with cardiac muscle, an involuntary striated muscle that contracts and relaxes at regular intervals.
• Cardiomyocytes, which are the individual heart muscle cells that make up the cardiac muscle, are connected by intercalated discs. Collagen fibres and other components that make up the extracellular matrix surround these muscle cells.
• Cardiovascular muscles contract differently from skeletal and smooth muscles.
• The nerve stimulus is produced within the muscles of the myogenic cardiac muscle.
• Electrical stimulation causes the muscles to produce an action potential.
• This potential results in the release of calcium ions into the sarcoplasm reticulum from the cells. Myofilaments move past one another as calcium ions rise, resulting in excitation-contraction.
• The sympathetic and vagal nerves that innervate the cardiac muscles regulate the contraction of the muscle fibres.

Smooth muscle

• One-unit or unitary muscles and multiunit muscles make up the involuntary, nonstriated muscle known as smooth muscle.
• The walls of several internal organs, including the gastrointestinal system, urinary tract, and blood arteries, are lined with smooth muscle.
• The ciliary muscle in the eye is a smooth muscle that also controls iris dilation, which modifies lens shape.
• Single-unit smooth muscles are those in which the entire muscle contracts and relaxes simultaneously. The multiunit muscles have the ability to contract and relax in distinct units.
• Nerve fibres are made up of vesicles known as varicosities or boutons that transport neurotransmitters around the muscle fibres.

References and Sources

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• Waugh A and Grant A. (2004) Anatomy and Physiology. Ninth Edition. Churchill Livingstone.
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