Author: Benjamin Aghoghovwia * Reviewer:Dimitrios Mytilinaios MD, PhD
Last review: March 16, 2022
Reading time: 16 minutes
Muscles are the largest of the soft tissues in the musculoskeletal structure. The Latin word “musculus”, which means “little mouse”, is what gives rise to muscle. Muscle fibre is a protein filament made of myosin and actin. They slide past each other, creating contractions that move internal organs as well as body parts.
Associated connective tissue connects muscle fibers into fascicles, or bundles. These connective tissues also carry nerve fibres (capillaries), and blood vessels (capillaries), to the muscles cells.
A macro- and microscopic view on a muscle
* Exitability- Ability to respond to stimuli
* Contractibility-ability to contract
* Flexibility -ability to stretch a muscle without tearing;
* Elasticity-ability to return to its original form.
The following functions are performed by the muscular system: support of the body, movement of force, change of posture, stability and joints, production heat to maintain normal body temperatures, as well as provision of form.
While muscles can produce heat energy, they still require energy to function properly. Although oxidation of fats, carbohydrates and other chemical reactions is the main source of energy for muscles, anaerobic chemical reaction are also utilized. These chemical reactions create adenosine Triphosphate (ATP), molecules which are used up during muscle contractions by myosin filaments.
There are three types. These are:
* Skeletal muscles that move bones and other structures (e.g. The eyes
* The heart muscles and the adjacent great vessels such as the aorta form the majority of the heart’s walls.
* Visceral or smooth muscles are part of most vessels’ walls and hollow organs. They move substances through viscera like the intestine and control blood vessel movement.
Histologically, muscles are divided into striated and non-striated. This is based on the structure characteristic known as “striations”, which refers to the arrangement of muscle fiber’s myosin and actin filaments. This microscopic classification allows for the grouping of skeletal and cardiac muscles as striated muscles while the visceral muscles are non-striated.
Development
The mesoderm layer is where the muscular system begins (except for the muscles in the iris, which develop from neuroectoderm and the muscles in the esophagus, which are thought to develop from transdifferentiation of smooth muscle). It consists of smooth, skeletal, and cardiac muscles. Mesenchyme (embryonic connective tissues) is the source of myoblasts, which are embryonic muscle cells. Actin and myosin are responsible for muscle contraction. They facilitate movement and drive physiologic processes such as respiration, circulation, and digestion. Smooth and cardiac muscle tissues are formed from mesenchymal cells in the local area (splanchnic mysoderm). Skeletal muscles are created from mesoderm within each somite.
Biceps brachii muscles (histology slide from fetal elbow). Myogenesis occurs within a somite, when cells respond to growth factor and activate myogenic basic loop-helix transcription factors (MyoD). These cells are called myoblasts or committed muscle precursor cells. They fuse to form multinucleated myotubes made up of terminally differentiated muscles cells. Adult muscle regeneration is thought to activate many of the molecular mechanisms responsible for embryonic muscle cell differentiation and proliferation. Wnt signaling, for example, can induc satellite cell proliferation and myoblast-to-myoblast fusion.
Skeletal muscle
Multinucleated skeletal muscle cells have thousands of nuclei. The more nuclei in a skeletal muscle cell, then the bigger it is. Non-branching fibers make up the skeletal muscles, which are not like cardiac. They are bound together by loose areolar tissue that contains the usual number of cells, such as macrophages and fibroblasts. Fluids such as pus cannot be spread through the epimysium or membranous envelope.
Skeletal muscle tissue (histological slides) Skeletal muscles can be found in many sizes and shapes. While the small muscles of an eye might only contain a few hundred cells while the vastus medialis of the thigh could contain hundreds of thousands of cells, the vastus lateralis may have many thousands of cells. The general architecture of a muscle determines its shape. This in turn influences the muscle’s function. Some muscles, like the gluteal, are very thick, while others, like the sartorius, are relatively short and slender. Others, such the extensors and fingers, have long tendons. These differences in the muscle structure and shape allow skeletal muscles to perform a wide variety of tasks.
Fascicles (or bundles of muscle fibers) can be organized into four basic structural patterns: parallel, convergent and pennate. This is why different skeletal muscles have different functional abilities and shapes.
Circular
This pattern is also known as sphincter. When the fascicles are placed in concentric rings, the fascicular pattern becomes circular. This arrangement of muscles surrounds the external body openings and they contract by contracting. These types of muscles are called “sphincter”, and “orbicular”. The orbicularis muscles around the eyes and mouth are examples.
Convergent
A convergent muscular has a broad origin and its fascicles connect to a single tendon for insertion. This type of muscle can be triangular, fan-shaped, or both. An example of such a muscle is the pectoralis minor muscle in the anterior thorax.
Parallel view of the Pectoralis Major Muscle (anterior view).
Parallel arrangement: The length of the fascicles runs parallel to the long axis. There are three types:
* Strap muscles are those with a narrow belt-like, strap-like stomach; such as the sartorius Muscle of the thigh.
* Fusiform muscles with a spindle-shaped, extended belly similar to the biceps brachii muscles of the arm.
* Fan-shaped muscles: The fibers of fan-shaped muscles are characterized by a divergence from a narrow attachment and eventually end with a noticeably larger one. The pectoralis major is an example.
Biceps brachii muscle (lateral-right view)Pennate
Pennate patterns have fascicles that are shorter and attach obliquely with a central tendon running the length of the muscle. There are three types of pennate muscles:
Unipennate: The fascicles are inserted into one side of the tendon as in the extensor digitalorum longus muscle in the leg.
* Bipennate is when the fascicles insert in the tendon from the opposite side. The tendon is located in the center, giving the muscle the appearance of a feather. Bipennate is the rectus foemoris on the thigh.
* Multipennate is a combination of many feathers placed side-by side with all the quills in one tendon. Multipennate is the deltoid muscles, which create the roundness in the shoulder.
The posterior view of the Deltoid Muscle. Skeletal muscles attach directly or indirectly to bones, cartilages and ligaments. Some attach to organs, such as the eyeball, skin (such facial muscles), or to the mucous membrane (e.g. The intrinsic tongue muscles. The skeleton and other body components are moved by the skeletal muscles.
Because many major skeletal muscles can be controlled at will, they are sometimes called voluntary muscles. However, not all of their actions are automatically controlled. The diaphragm contracts automatically. However, a person can control it by taking deep breaths. Skeletal muscles have striated. Skeletal muscles attach with their tendons to bones. Some tendons form flat sheets known as aponeuroses, which anchor one muscle to another.
Cardiac muscle
Each cell of a cardiac muscles also has one nucleus. The cardiac muscle is composed of a larger number of cells, which are shorter and more branch-like. Under microscopic inspection, a portion of the border membranes between adjacent cardiac muscle cells makes intricate interdigitations (branchings). This wideness and interdigitations in the cardiac muscle increase the surface area available for impulse conduction. The cells of the muscle are organized in spirals and whorls. Each chamber of the heart is emptied by mass contraction, rather than peristalsis. Contrary to smooth muscle fibers, the cardiac muscle fibres have a striated appearance and are joined end to end by cell junctions made by intercalated disks.
Cardiac muscle tissue (histological slides)The muscular wall of your heart is made up of cardiac muscle (i.e. The myocardium of the heart. The walls of the aorta and pulmonary veins contain some cardiac muscles.
Like the smooth muscle innervation, innervation is controlled by the autonomic nerve system (ANS). A pacemaker (the SA Node) regulates heart rate intrinsically. It is made of special cardiac muscle fibres, which are also influenced by and innervated the ANS.
Smooth muscle
A smooth muscle cell (fibre), has one nucleus, like other body cells. Smooth muscle is composed of narrow, spindle-shaped cells that are usually arranged in parallel. Peristalsis is an anti-directional movement in hollow organs. They are arranged in a longitudinal or circular fashion, such as in the alimentary and ureter. Contractile impulses are transmitted between muscle cells at specialized sites called “nexuses” (or gap junctions). These are places where adjacent cell membranes are unusually close together.
Smooth muscle (histological slides) Smooth muscles can be found in the middle layer (tunica median) of most blood vessels and the muscular portion of the digestive tract. They can also be found in the eyeball which controls the thickness of the lens and the size of your pupil.
The smooth muscle innervation is done by ANS. Many cells don’t receive nerve fibers because they are separated by the gaps between smooth muscle cells.
Clinical correlation
Dermatomyositis, polymyositis
Inflammation of the muscles can be caused by polymyositis or dermatologomyositis. These rare conditions affect only one percent of the population each year. These disorders affect more women than men. The peak age at which the disorder is most common is 50, but it can happen at any age.
These disorders cause muscle weakness, which usually worsens over time. However, in some cases, symptoms appear suddenly. The affected muscles are located close to the trunk, rather than in the wrists and feet. They can also affect the hips, shoulders, neck, and neck muscles. Both sides of the body can be affected. Sometimes, the muscles may become tender or sore. Sometimes, problems with swallowing can be caused by the muscles of the pharynx, which is the tube that connects the throat and stomach. This can lead to severe pneumonia if food is not directed from the esophagus into the lungs.
Muscular dystrophy
Muscular dystrophy refers to a variety of muscle diseases that affect the musculoskeletal system. It can also hinder locomotion. Muscular dystrophies include progressive weakness of the skeletal muscles, deficiency in muscle proteins, death of muscle cells and tissue, as well as a loss or destruction of muscle fibres.
This is a grouping of inherited diseases that causes the muscles responsible for controlling movement to gradually weaken. Dys- is a prefix that means abnormal. The root, -trophy refers to normal nutrition, structure, and function. Duchenne muscular dystrophy is the most common type in children and only affects males. The symptoms usually appear between the ages 2 and 6, and sufferers can live into their teens or early 20s.
Muscle atrophy
Muscle atrophy can also be called Muscle wasting or Atrophy of the muscles. Muscle atrophy in the majority of people is caused by ‘disuse’. Sedentary workers and seniors with reduced activity may experience significant muscle atrophy. With vigorous exercise, this type of atrophy can be reversed. People who are bedridden can experience significant muscle loss. After a few days of weightlessness, astronauts can experience decreased muscle tone, loss of calcium, and other symptoms.
Muscle atrophy is usually caused by disease and not disuse. It can be one of two types: damage to the nerves that supply muscles or disease of the muscle.
The following are examples of diseases that affect the nerves controlling muscles:
* poliomyelitis;
* Amyotrophic lateral sclerosis (ALS) or Lou Gehrig’s disease;
* Guillain-Barre syndrome.
Some examples of diseases that affect primarily the muscles include:
* muscular dystrophy;
* myotonia congenita;
* Myotonic dystrophy
* As well as congenital, inflammatory and metabolic myopathies.
You wake up with a twitching sound
A hypnagogic massive or simply a “hypnic jerk” is a phenomenon where the twitching sensation occurs in the first stages of sleep. It is also known as a “sleep start”. Although there has not been much research, there are some theories.
The body experiences physiological changes when it falls asleep, such as changes in body temperature, breathing rate, and muscle tone. Muscle changes may cause hypnic jerks. Another theory is that relaxation occurs when the body transitions from the sleeping to the waking state. The brain might interpret relaxation as a signal to the body to relax and then signal the legs and arms to get up. Studies using electroencephalograms have shown that sleep starts can affect nearly 10 percent of the population, with 80 percent experiencing it occasionally and 10 percent only rarely.
During the Rapid Eye Movement (REM) phase of sleep, muscle movement and twitching may occur. Dreams also occur during this phase. During the REM phase, all muscle activity ceases with a decrease in muscle tone. However, some people may feel a slight eyelid or ear twitching, or slight jerks. RBD (REM behavioral disorder) may have more severe muscular twitching or full-fledged activity while sleeping. They do not experience muscle paralysis and, as such, can act out their dreams.
Sources
Kenhub’s content is reviewed by anatomy and medical experts. All information on Kenhub is based on peer-reviewed research and academic literature. Kenhub does NOT provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.References:
* K.M. Baldwin and F. Haddad, The muscular system: Muscleplasticity. History of Exercise Physiology (2014), p. 337
* K.L. Moore and T.V. N. Persaud, The developing human (Clinically-oriented embryology), 8th Edition, (2007) p.
* K.L. Moore and A.F. Dalley: Clinically Oriented Anatomy 4th Edition, (1999), pp.26-32.
* M.H. Stone, M. Stone, and W.A. Sands: Principles of Resistance Training, 1st Edition, (2007) p.
* R.M.H McMinn, Last’s anatomy (Regional & Applied), 9th Edition, Ana-Maria Dulea (2014). pp. 5-8.
Illustrators:
* A macro- and microscopic view a muscle by Paul Kim
* Orbicularis Oculi Muscle (anterior view). – Yousun KOH
* Pectoralis major Muscle (anterior view) by Yousun Koh
* Biceps brachii muscle (lateral-right view) – Yousun Koh
* Deltoid muscle (posterior view) – Yousun Koh
Want to know more about muscle tissue and muscles?
Engaging videos, interactive quizzes and HD atlas will help you get top results quicker.
Which do you prefer to learn from?
Kenhub has cut my study time in half,” -Read more. Kim Bengochea Regis University, Denver
(c) All content, including illustrations, are the exclusive property of Kenhub GmbH and are protected under German and international copyright laws. All rights reserved.
