muscles
TRANSCRIPT
The human muscular system is made up of over 600 muscles, which act in groups. Muscles, in turn, are made up of fibers and cells. Muscles are what enable you to do just about everything – from walking to lifting heavy objects to helping to pump blood throughout the body. Muscles are distinguished as either involuntary or voluntary. Involuntary muscles function within the body automatically, without you being able to control them. Voluntary muscles are the ones that are under your control.
All muscles are made up of the same type of material – a kind of an elastic tissue, akin to what rubber bands are made of. Each muscle is made up of thousands of tiny fibers. There are three kinds of muscles in the human muscular system: the skeletal muscle; the cardiac muscle; and the smooth muscle. Plus, the facial muscles and the tongue are a unique kind by themselves.
Muscle (from Latin musculus, diminutive of mus "mouse"[1]) is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to produce force and cause motion. Muscles can cause either locomotion of the organism itself or movement of internal organs. Cardiac and smooth muscle contraction occurs without conscious thought and is necessary for survival. Examples are the contraction of the heart and peristalsis which pushes food through the digestive system. Voluntary contraction of the skeletal muscles is used to move the body and can be finely controlled. Examples are movements of the eye, or gross movements like the quadriceps muscle of the thigh. There are two broad types of voluntary muscle fibers: slow twitch and fast twitch. Slow twitch fibers contract for long periods of time but with little force while fast twitch fibers contract quickly and powerfully but fatigue very rapidly.
Types of muscleAll muscles are made up of the same type of material – a kind of an elastic tissue, akin to what rubber bands are made of. Each muscle is made up of thousands of tiny fibers. There are three kinds of muscles in the human muscular system: the skeletal muscle; the cardiac muscle; and the smooth muscle. Plus, the facial muscles and the tongue are a unique kind by themselves.
The Skeletal Muscles: These are the voluntary type of muscles in the human muscular system. This means that they can be controlled by you. For example, you cannot pick up that mug of coffee with your hand unless you want your hand to do so. They are referred sometimes as striated muscles, because the dark and light fibrous material make them seem striped. These are also known as the musculoskeletal system, or the combination of the muscles and the bones that make up the skeleton.
Generally, skeletal muscles are attached to the ends of bones, stretching all across the joint and then attached
once more to another bone. Tendons, which are cords or bands of inelastic tissue, are what attach the muscles to the bones. Skeletal muscles are of different shapes and sizes, which enable them to perform a variety of tasks. The gluteus maximus, or the muscle that occurs in the buttocks, is the largest skeletal muscle in the human muscular system. Some of the other major skeletal muscles are the deltoid muscle in the shoulders, the biceps and triceps in the arm, the pectoralis in the chest, the rectus abdominus in the abdomen, the quadriceps and the hamstring muscles in the legs.
The Cardiac Muscle: The heart is made up of the cardiac muscle, which is also referred to as the myocardium. These muscles are thick and contract in order to pump out the blood and then relax in order to allow more blood in. The cardiac muscle is an involuntary muscle, or the type that works without your volition. Special type of cells in the cardiac muscle, called the pacemaker, help in controlling the heartbeat.
The Smooth Muscles: These are the involuntary muscles of the human muscular system, and they generally occur in layers or sheets, with one muscle layer behind another. These muscles are not under your control. The brain and the body control these muscles in performing their functions without any conscious volition from your part.
Some of the examples of smooth muscles are the stomach and the digestive system, which contract and relax in order to pass food through the alimentary canal of the body. The bladder is another example of smooth muscle, and so is the uterus in women. Smooth muscles also occur in the eyes, which help to keep the eyes focused. According to scientists, the eyes can move over 100,000 times in a day, making them the busiest muscles in the human muscular system.
The Facial Muscles: There are more than 30 muscles in the face. Not all of the facial muscles are attached to bones, as is the case in the other parts of the body. Many of the facial muscles are attached to the underside of facial skin. The contractions of these muscles are what give the face its various expressions, such as frowning, laughter, surprise, sadness and so on.
The Tongue: And another unique muscle is the tongue, which is free at one end and only attached on the other end. The tongue actually comprises of a group of muscles, which work in unison, enabling you to chew and swallow food, and talk.
Cardiac and skeletal muscles are "striated" in that they contain sarcomeres and are packed into highly-regular arrangements of bundles; smooth muscle has neither. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles (called intercalated discs). Striated muscle contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.
Skeletal muscle is further divided into several subtypes:
Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity.
Type II, fast twitch muscle, has three major kinds that are, in order of increasing contractile speed:[2]
o Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.
o Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often incorrectly attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB.[3]
o Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents this is the major fast muscle type, explaining the pale color of their flesh.
Anatomy
The anatomy of muscles includes both gross anatomy, comprising all the muscles of an organism, and, on the other hand, microanatomy, which comprises the structures of a single muscle.
Gross anatomy
The gross anatomy of a muscle is the most important indicator of its role in the body. The action a muscle generates is determined by the origin and insertion locations. The cross-sectional area of a muscle (rather than volume or length) determines the amount of force it can generate by defining the number of sarcomeres which can operate in parallel. The amount of force applied to the external environment is determined by lever mechanics, specifically the ratio of in-lever to out-lever. For example, moving the insertion point of the biceps more distally on the radius (farther from the joint of rotation) would increase the force generated during flexion (and, as a result, the maximum weight lifted in this movement), but decrease the maximum speed of flexion. Moving the insertion point proximally (closer to the joint of rotation) would result in decreased force but increased velocity. This can be most easily seen by comparing the limb of a mole to a horse - in the former, the insertion point is positioned to maximize force (for
digging), while in the latter, the insertion point is positioned to maximize speed (for running).
One particularly important aspect of gross anatomy of muscles is pennation or lack thereof. In most muscles, all the fibers are oriented in the same direction, running in a line from the origin to the insertion. In pennate muscles, the individual fibers are oriented at an angle relative to the line of action, attaching to the origin and insertion tendons at each end. Because the contracting fibers are pulling at an angle to the overall action of the muscle, the change in length is smaller, but this same orientation allows for more fibers (thus more force) in a muscle of a given size. Pennate muscles are usually found where their length change is less important than maximum force, such as the rectus femoris.
There are approximately 639 skeletal muscles in the human body. However, the exact number is difficult to define because different sources group muscles differently.
Microanatomy
Muscle is mainly composed of muscle cells. Within the cells are myofibrils; myofibrils contain sarcomeres, which are composed of actin and myosin. Individual muscle fibres are surrounded by endomysium. Muscle fibers are bound together by perimysium into bundles called fascicles; the bundles are then grouped together to form muscle, which is enclosed in a sheath of epimysium. Muscle spindles are distributed throughout the muscles and provide sensory feedback information to the central nervous system.
Skeletal muscle is arranged in discrete muscles, an example of which is the biceps brachii. It is connected by tendons to processes of the skeleton. Cardiac muscle is similar to skeletal muscle in both composition and action, being comprised of myofibrils of sarcomeres, but anatomically different in that the muscle fibers are typically branched like a tree and connect to other cardiac muscle fibers through intercalcated discs, and form the appearance of a syncytium.
muscle contraction
The three (skeletal, cardiac and smooth) types of muscle have significant differences. However, all three use the movement of actin against myosin to create contraction. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by the nerves, the motor nerves and motoneurons in particular. Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells which regularly contract, and propagate contractions to other muscle cells they are in contact with. All skeletal muscle and many smooth muscle contractions are facilitated by the neurotransmitter acetylcholine.
Muscular activity accounts for much of the body's energy consumption. All muscle cells produce adenosine triphosphate (ATP) molecules which are used to power the movement of the myosin heads. Muscles conserve energy in the form of creatine phosphate which is generated from ATP and can regenerate ATP when needed with creatine kinase. Muscles also keep a storage form of glucose in the form of glycogen. Glycogen can be rapidly converted to glucose when energy is required for sustained, powerful contractions. Within the voluntary skeletal muscles, the glucose molecule can be metabolized anaerobically in a process called glycolysis which produces two ATP and two lactic acid molecules in the process (note that in aerobic conditions, lactate is not formed; instead pyruvate is formed and transmitted through the citric acid cycle). Muscle cells also contain globules of fat, which are used for energy during aerobic exercise. The aerobic energy systems take longer to produce the ATP and reach peak efficiency, and requires many more biochemical steps, but produces significantly more ATP than anaerobic glycolysis. Cardiac muscle on the other hand, can readily consume any of the three macronutrients (protein, glucose and fat) aerobically without a 'warm up' period and always extracts the maximum ATP yield from any molecule involved. The heart, liver and red blood cells will also consume lactic acid produced and excreted by skeletal muscles during exercise.
Efferent leg
The efferent leg of the peripheral nervous system is responsible for conveying commands to the muscles and glands, and is ultimately responsible for voluntary movement. Nerves move muscles in response to voluntary and autonomic (involuntary) signals from the brain. Deep muscles, superficial muscles, muscles of the face and internal muscles all correspond with dedicated regions in the primary motor cortex of the brain, directly anterior to the central sulcus that divides the frontal and parietal lobes.
In addition, muscles react to reflexive nerve stimuli that do not always send signals all the way to the brain. In this case, the signal from the afferent fiber does not reach the brain, but produces the reflexive movement by direct connections with the efferent nerves in the spine. However, the majority of muscle activity is volitional, and the result of complex interactions between various areas of the brain.
Nerves that control skeletal muscles in mammals correspond with neuron groups along the primary motor cortex of the brain's cerebral cortex. Commands are routed though the basal ganglia and are modified by input from the cerebellum before being relayed through the pyramidal tract to the spinal cord and from there to the motor end plate at the muscles. Along the way, feedback, such as that of the extrapyramidal system contribute signals to influence muscle tone and response.
Deeper muscles such as those involved in posture often are controlled from nuclei in the brain stem and basal ganglia.
Afferent leg
The afferent leg of the peripheral nervous system is responsible for conveying sensory information to the brain, primarily from the sense organs like the skin. In the muscles, the muscle spindles convey information about the degree of muscle length and stretch to the central nervous system to assist in maintaining posture and joint position. The sense of where our bodies are in space is called proprioception, the perception of body awareness. More easily demonstrated than explained, proprioception is the "unconscious" awareness of where the various regions of the body are located at any one time. This can be demonstrated by anyone closing their eyes and waving their hand around. Assuming proper proprioceptive function, at no time will the person lose awareness of where the hand actually is, even though it is not being detected by any of the other senses.
Several areas in the brain coordinate movement and position with the feedback information gained from proprioception. The cerebellum and red nucleus in particular continuously sample position against movement and make minor corrections to assure smooth motion.
Exercise
Exercise is often recommended as a means of improving motor skills, fitness, muscle and bone strength, and joint function. Exercise has several effects upon muscles, connective tissue, bone, and the nerves that stimulate the muscles.
Various exercises require a predominance of certain muscle fiber utilization over another. Aerobic exercise involves long, low levels of exertion in which the muscles are used at well below their maximal contraction strength for long periods of time (the most classic example being the marathon). Aerobic events, which rely primarily on the aerobic (with oxygen) system, use a higher percentage of Type I (or slow-twitch) muscle fibers, consume a mixture of fat, protein and carbohydrates for energy, consume large amounts of oxygen and produce little lactic acid. Anaerobic exercise involves short bursts of higher intensity contractions at a much greater percentage of their maximum contraction strength. Examples of anaerobic exercise include sprinting and weight lifting. The anaerobic energy delivery system uses predominantly Type II or fast-twitch muscle fibers, relies mainly on ATP or glucose for fuel, consumes relatively little oxygen, protein and fat, produces large amounts of lactic acid and can not be sustained for as long a period as aerobic exercise. The presence of lactic acid has an inhibitory effect on ATP generation within the muscle; though not producing fatigue, it can inhibit or even stop performance if the intracellular concentration
becomes too high. However, long-term training causes neovascularization within the muscle, increasing the ability to move waste products out of the muscles and maintain contraction. Once moved out of muscles with high concentrations within the sarcomere, lactic acid can be used by other muscles or body tissues as a source of energy, or transported to the liver where it is converted back to pyruvate. The ability of the body to export lactic acid and use it as a source of energy depends on training level.
Humans are genetically predisposed with a larger percentage of one type of muscle group over another. An individual born with a greater percentage of Type I muscle fibers would theoretically be more suited to endurance events, such as triathlons, distance running, and long cycling events, whereas a human born with a greater percentage of Type II muscle fibers would be more likely to excel at anaerobic events such as a 200 meter dash, or weightlifting. People with high overall musculation and balanced muscle type percentage engage in sports such as rugby or boxing and often engage in other sports to increase their performance in the former.[citations needed]
Delayed onset muscle soreness is pain or discomfort that may be felt one to three days after exercising and subsides generally within two to three days later. Once thought to be caused by lactic acid buildup, a more recent theory is that it is caused by tiny tears in the muscle fibers caused by eccentric contraction, or unaccustomed training levels. Since lactic acid disperses fairly rapidly, it could not explain pain experienced days after exercise.[4]
Disease
Main article: Neuromuscular disease
Symptoms of muscle diseases may include weakness, spasticity, myoclonus and myalgia. Diagnostic procedures that may reveal muscular disorders include testing creatine kinase levels in the blood and electromyography (measuring electrical activity in muscles). In some cases, muscle biopsy may be done to identify a myopathy, as well as genetic testing to identify DNA abnormalities associated with specific myopathies and dystrophies.
Neuromuscular diseases are those that affect the muscles and/or their nervous control. In general, problems with nervous control can cause spasticity or paralysis, depending on the location and nature of the problem. A large proportion of neurological disorders leads to problems with movement, ranging from cerebrovascular accident (stroke) and Parkinson's disease to Creutzfeldt-Jakob disease.
A non-invasive elastography technique that measures muscle noise is undergoing experimentation to provide a way of monitoring neuromuscular disease. The sound
produced by a muscle comes from the shortening of actomyosin filaments along the axis of the muscle. During contraction, the muscle shortens along its longitudinal axis and expands across the transverse axis, producing vibrations at the surface.[5]
Atrophy
There are many diseases and conditions which cause a decrease in muscle mass, known as muscle atrophy. Example include cancer and AIDS, which induce a body wasting syndrome called cachexia. Other syndromes or conditions which can induce skeletal muscle atrophy are congestive heart disease and some diseases of the liver.
During aging, there is a gradual decrease in the ability to maintain skeletal muscle function and mass, known as sarcopenia. The exact cause of sarcopenia is unknown, but it may be due to a combination of the gradual failure in the "satellite cells" which help to regenerate skeletal muscle fibers, and a decrease in sensitivity to or the availability of critical secreted growth factors which are necessary to maintain muscle mass and satellite cell survival. Sarcopenia is a normal aspect of aging, and is not actually a disease state.
Physical inactivity and atrophy
Inactivity and starvation in mammals lead to atrophy of skeletal muscle, accompanied by a smaller number and size of the muscle cells as well as lower protein content.[6] In humans, prolonged periods of immobilization, as in the cases of bed rest or astronauts flying in space, are known to result in muscle weakening and atrophy. Such consequences are also noted in small hibernating mammals like the golden-mantled ground squirrels and brown bats.[7] Representatives of the Ursid species make for an interesting exception to this expected norm.
Bears are famous for their ability to survive unfavorable environmental conditions of low temperatures and limited nutrition availability during winter by means of hibernation. During that time Ursids go through a series of physiological, morphological and behavioral changes.[8] Their ability to maintain skeletal muscle number and size at time of disuse is of a significant importance. During hibernation bears spend four to seven months of inactivity and anorexia without undergoing muscle atrophy and protein loss.[7] There are a few known factors that contribute to the sustaining of muscle tissue. During the summer period, Ursids take advantage of the nutrition availability and accumulate muscle protein. The protein balance of bears at time of dormancy is also maintained by lower levels of protein breakdown during the winter time.[7] At times of immobility, muscle wasting in Ursids is also suppressed by a proteolytic inhibitor that is released in circulation.[6] Another factor that contributes to the sustaining of muscle strength in hibernating bears is the occurrence of periodic voluntary contractions and involuntary contractions from shivering during torpor.[9] The three to four daily episodes of
muscle activity are responsible for the maintenance of muscle strength and responsiveness in bears during hibernation.[9]
Strength
A display of "strength" (e.g. lifting a weight) is a result of three factors that overlap: physiological strength (muscle size, cross sectional area, available crossbridging, responses to training), neurological strength (how strong or weak is the signal that tells the muscle to contract), and mechanical strength (muscle's force angle on the lever, moment arm length, joint capabilities). Contrary to popular belief, the number of muscle fibres cannot be increased through exercise; instead the muscle cells simply get bigger. Muscle fibres have a limited capacity for growth through hypertrophy and some believe they split through hyperplasia if subject to increased demand.[citation needed]
The "strongest" human muscle
Since three factors affect muscular strength simultaneously and muscles never work individually, it is misleading to compare strength in individual muscles, and state that one is the "strongest". But below are several muscles whose strength is noteworthy for different reasons.
In ordinary parlance, muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of 4,337 N (975 lbf) for 2 seconds. What distinguishes the masseter is not anything special about the muscle itself, but its advantage in working against a much shorter lever arm than other muscles.
If "strength" refers to the force exerted by the muscle itself, e.g., on the place where it inserts into a bone, then the strongest muscles are those with the largest cross-sectional area. This is because the tension exerted by an individual skeletal muscle fiber does not vary much. Each fiber can exert a force on the order of 0.3 micronewton. By this definition, the strongest muscle of the body is usually said to be the quadriceps femoris or the gluteus maximus.
A shorter muscle will be stronger "pound for pound" (i.e., by weight) than a longer muscle. The myometrial layer of the uterus may be the strongest muscle by weight in the human body. At the time when an infant is delivered, the entire human uterus weighs about 1.1 kg (40 oz). During childbirth, the uterus exerts 100 to 400 N (25 to 100 lbf) of downward force with each contraction.
The external muscles of the eye are conspicuously large and strong in relation to the
small size and weight of the eyeball. It is frequently said that they are "the strongest muscles for the job they have to do" and are sometimes claimed to be "100 times stronger than they need to be." However, eye movements (particularly saccades used on facial scanning and reading) do require high speed movements, and eye muscles are exercised nightly during rapid eye movement sleep.
The statement that "the tongue is the strongest muscle in the body" appears frequently in lists of surprising facts, but it is difficult to find any definition of "strength" that would make this statement true. Note that the tongue consists of sixteen muscles, not one.
The heart has a claim to being the muscle that performs the largest quantity of physical work in the course of a lifetime. Estimates of the power output of the human heart range from 1 to 5 watts. This is much less than the maximum power output of other muscles; for example, the quadriceps can produce over 100 watts, but only for a few minutes. The heart does its work continuously over an entire lifetime without pause, and thus does "outwork" other muscles. An output of one watt continuously for eighty years yields a total work output of two and a half gigajoules.
The Skeletal Muscles: These are the voluntary type of muscles in the human muscular system. This means that they can be controlled by you. For example, you cannot pick up that mug of coffee with your hand unless you want your hand to do so. They are referred sometimes as striated muscles, because the dark and light fibrous material make them seem striped. These are also known as the musculoskeletal system, or the combination of the muscles and the bones that make up the skeleton.
Generally, skeletal muscles are attached to the ends of bones, stretching all across the joint and then attached once more to another bone. Tendons, which are cords or bands of inelastic tissue, are what attach the muscles to the bones. Skeletal muscles are of different shapes and sizes, which enable them to perform a variety of tasks. The gluteus maximus, or the muscle that occurs in the buttocks, is the largest skeletal muscle in the human muscular system. Some of the other major skeletal muscles are the deltoid muscle in the shoulders, the biceps and triceps in the arm, the pectoralis in the chest, the rectus abdominus in the abdomen, the quadriceps and the hamstring muscles in the legs.
The Cardiac Muscle: The heart is made up of the cardiac muscle, which is also referred to as the myocardium. These muscles are thick and contract in order to pump out the blood and then relax in order to allow more blood in. The cardiac muscle is
an involuntary muscle, or the type that works without your volition. Special type of cells in the cardiac muscle, called the pacemaker, help in controlling the heartbeat.
The Smooth Muscles: These are the involuntary muscles of the human muscular system, and they generally occur in layers or sheets, with one muscle layer behind another. These muscles are not under your control. The brain and the body control these muscles in performing their functions without any conscious volition from your part.
Some of the examples of smooth muscles are the stomach and the digestive system, which contract and relax in order to pass food through the alimentary canal of the body. The bladder is another example of smooth muscle, and so is the uterus in women. Smooth muscles also occur in the eyes, which help to keep the eyes focused. According to scientists, the eyes can move over 100,000 times in a day, making them the busiest muscles in the human muscular system.
The Facial Muscles: There are more than 30 muscles in the face. Not all of the facial muscles are attached to bones, as is the case in the other parts of the body. Many of the facial muscles are attached to the underside of facial skin. The contractions of these muscles are what give the face its various expressions, such as frowning, laughter, surprise, sadness and so on.
The Tongue: And another unique muscle is the tongue, which is free at one end and only attached on the other end. The tongue actually comprises of a group of muscles, which work in unison, enabling you to chew and swallow food, and talk.
Muscle cells (also called muscle fibers) are cylindrical, and are multinucleated (in vertebrates and insects). The nuclei of these muscles are located in the peripheral aspect of the cell, just under the plasma membrane, which vacates the central part of the muscle fiber for myofibrils. (Conversely, when the nucleus is located in the center it is considered a pathologic condition known as centronuclear myopathy.)
Skeletal muscles have one end (the "origin") attached to a bone closer to the centre of the body's axis and the other end (the "insertion") is attached across a joint to another bone further from the body's axis. The bones rotate about the joint and move relative to one another by contraction of the muscle (lifting of the upper arm in the case of the origin and insertion described here).
There are several different ways to categorize the type of skeletal muscle fibers (see below). One method uses the type of protein contained in myosin (one of the important proteins that is responsible for the ability of muscle to contract). Using this classification scheme,
there are two major types of fibers for skeletal muscles: Type I and Type II. Type I fibers appear reddish. They are good for endurance and are slow to tire because they use oxidative metabolism. Type II fibers are whitish; they are used for short bursts of speed and power, and use both oxidative metabolism and anaerobic metabolism depending on the particular sub-type, and are therefore quicker to fatigue.
Muscle force is proportional to physiologic cross-sectional area (PCSA), and muscle velocity is proportional to muscle fiber length[1]. The strength of a joint, however, is determined by a number of biomechanical principles, including the distance between muscle insertions and pivot points and muscle size. Muscles are normally arranged in opposition so that as one group of muscles contract, another group 'relaxes' (in fact simply stretched) or lengthens. Antagonism in the transmission of nerve impulses (epsp and ipsp lateral balance) to the muscles means that it is impossible to stimulate the contraction of two antagonistic muscles at any one time. During ballistic motions such as throwing, the antagonist muscles act to 'brake' the agonist muscles throughout the contraction, particularly at the end of the motion. In the example of throwing, the chest and front of the shoulder (anterior Deltoid) contract to pull the arm forward, while the muscles in the back and rear of the shoulder (posterior Deltoid) also contract and undergo eccentric contraction to slow the motion down to avoid injury. Part of the training process is learning to relax the antagonist muscles to increase the force input of the chest and anterior shoulder.
Skeletal muscle cells are stimulated by acetylcholine, which is released at neuromuscular junctions by motor neurons [2] . Once the cells are "excited", their sarcoplasmic reticulum will release ionic calcium (Ca2+) which interacts with the myofibrils to induce muscular contraction (via the sliding filament mechanism). This process also requires adenosine triphosphate (ATP). The ATP is produced by metabolizing creatine phosphate and glucose (stored as glycogen or absorbed from blood) within the muscle cells by mitochondria, as well as by metabolizing fatty acids obtained from the blood and within the cell. Each motor neuron activates a group of muscle cells, and collectively the neurons and muscle cells are known as motor units. When more strength is required than can be obtained from a single motor unit, more units will be stimulated; this is known as motor unit recruitment. This is spatial summation. If more strength is required than can be obtained from the current number of motor units, the motor neurons continue to recruit more motor units. When all the motor units are recruited, there will be no further increase in contraction strength. To increase the force of contraction, it is necessary to increase the frequency of neuronalT firing. This results in tetanic contraction, which is a smooth contraction. This is temporal summation...
TENDON
A tendon (or sinew) is a tough band of fibrous connective tissue that usually connects muscle to bone [1]
and is capable of withstanding tension. Tendons are similar to ligaments and fascia as they are both made of collagen except that ligaments join one bone to another bone, and fascia connect muscles to other muscles. Tendons and muscles work together and can only exert a pulling force.
Structure
Normal healthy tendons are composed of parallel arrays of collagen fibers closely packed together. The fibers are mostly collagen type I, however there are also collagen type III and V present. These collagens are held together with other proteins, particularly the proteoglycan, decorin and, in compressed regions of tendon, aggrecan. The tenocytes produce the collagen molecules which aggregate end-to-end and side-to-side to produce collagen fibrils. Fibril bundles are organized to form fibers with the elongated tenocytes closely packed between them. Collagen fibers coalesce into macroaggregates. Groups of macroaggregates are bounded by connective tissue endotendon and are termed fascicles. Groups of fascicles are bounded by the epitendon and peritendon to form the tendon organ.
Blood vessels may be visualized within the endotendon running parallel to collagen fibers, with occasional branching transverse anastomoses.
The internal tendon bulk is thought to contain no nerve fibers, but the epi- and peritendon contain nerve endings, while Golgi tendon organs are present at the junction between tendon and muscle.
Tendon length varies in all major groups and from person to person. Tendon length is practically the discerning factor where muscle size and potential muscle size is concerned. For example, should all other relevant biological factors be equal, a man with a shorter tendons and a longer biceps muscle will have greater potential for muscle mass than a man with a longer tendon and a shorter muscle. Successful bodybuilders will generally have shorter tendons. Conversely, in sports requiring athletes to excel in actions such as running or jumping, it is beneficial to have longer than average Achilles tendon and a shorter calf muscle.[2]
Tendon length is determined by genetic predisposition, and has not been shown to either increase or decrease in response to environment, unlike muscles which can be shortened by trauma, use imbalances and a lack of recovery and stretching.
Function
Tendons have been traditionally considered to simply be a mechanism by which muscles connect to bone, functioning simply to transmit forces. However, over the past two decades, much research focused on the elastic properties of tendons and their ability to function as springs. This allows tendons to passively modulate forces during locomotion, providing additional stability with no active work. It also allows tendons to store and recover energy at high efficiency. For example, during a human stride, the Achilles tendon stretches as the ankle joint dorsiflexes. During the last portion of the stride, as the foot plantar-flexes (pointing the toes down), the stored elastic energy is released. Furthermore, because the tendon stretches, the muscle is able to function with less or even no change in length, allowing the muscle to generate greater force.
Pathology
Tendinitis refers to inflammation of a tendon.
Tendinosis refers to non-inflammatory injury to the tendon at the cellular level. The tendons in the foot are highly complex and intricate. If any tendons break it is a long painful healing process, not to mention the intricacy of the repairing (if fully severed) process. Most people that do not receive medical attention within the first 48 hours of the injury will suffer from severe swelling, pain, and an on-fire feeling where the injury occurred. They are very painful when they are inflamed or not in use.
Muscular disorders
Muscular dystrophy (MD) is a group of more than 30 inherited diseases. It causes weakening and breaking down of muscle fibers. The muscles become weak and susceptible to damage. This disease affects the voluntary or skeletal muscles, which control the movements of legs, arms and trunk. It can also affect the heart muscles and other involuntary muscles, such as muscles in the gut. Some forms of MD are found in infancy or childhood, while some may not appear until middle age. This progressive disease is more common in boys than girls.
Arthrogryposis, also known as Arthrogryposis Multiplex Congenita, is a rare congenital disorder that is characterized by multiple joint contractures and can include muscle weakness and fibrosis. It is a non-progressive disease. The disease derives its name from Greek, literally meaning 'curved or hooked joints'.
Centronuclear myopathies (CNM) are a group of congenital myopathies where cell nuclei are abnormally located in skeletal muscle cells. In CNM the nuclei are located at a position in the center of the cell, instead of their normal location at the periphery. Although all forms of centronuclear myopathy are considered rare, the most commonly known form of CNM is Myotubular Myopathy (MTM). Symptoms of CNM include severe hypotonia, hypoxia-requiring breathing assistance, and
scaphocephaly. Among centronuclear myopathies, the X-linked myotubular myopathy form typically presents at birth, and is thus considered a congenital myopathy. However, some centronuclear myopathies may present later in life
Diastasis recti (also known as abdominal separation) is a disorder defined as a separation of the rectus abdominis muscle into right and left halves. [1] Normally, the two sides of the muscle are joined at the linea alba at the body midline. Diastasis of this muscle occurs principally in two populations: newborns and pregnant women. In the newborn, the rectus abdominis is not fully developed and may not be sealed together at midline. Diastasis recti is more common in premature and African American newborns. In pregnant or postpartum women, the defect is caused by the stretching of the rectus abdominis by the growing uterus. It is more common in multiparous women due to repeated episodes of stretching. When the defect occurs during pregnancy, the uterus can sometimes be seen bulging through the abdominal wall beneath the skin.
Sporadic inclusion body myositis (sIBM) is an inflammatory muscle disease, characterized by slowly progressive weakness and wasting of both distal and proximal muscles, most apparent in the muscles of the arms and legs. In sporadic inclusion body myositis [MY-oh-sigh-tis] muscle, two processes, one autoimmune and the other degenerative, appear to occur in the muscle cells in parallel. The inflammation aspect is characterized by the cloning of T cells that appear to be driven by specific antigens to invade muscle fibers. The degeneration aspect is characterized by the appearance of holes in the muscle cell (vacuole)s, deposits of abnormal proteins within the cells and in filamentous inclusions (hence the name inclusion body myositis).
Macrophagic Myofasciitis, or MMF, is a rare muscle disease identified in 1993. The disease is characterized by microscopic lesions found in muscle biopsies that show infiltration of muscle tissue by PAS-positive macrophages. [1] Specific causes of MMF are unknown, but the disease is most often associated with the pathological persistence of aluminium hydroxide used in some vaccines. Clinical symptoms include muscle pain, joint pain, muscle weakness, fatigue, fever, and muscle tenderness. A diagnosis can only be identified with an open muscle biopsy of the vaccinated muscle. [2]
Nemaline myopathy (also called rod myopathy or nemaline rod myopathy) is a congenital, hereditary neuromuscular disorder that causes muscle weakness, generally nonprogressive, of varying severity."Myopathy" means "muscle disease," and a biopsy of muscle from a person with nemaline myopathy shows abnormal thread-like rods, called nemaline bodies, in the muscle cells. People with nemaline myopathy (or NM) usually experience delayed motor development and weakness in the arm, leg, trunk, throat, and face muscles.
Pelvic Floor Muscle Disorder the muscles of the pelvic floor remain tightened. Normally these muscles are under voluntary control, but for some excessive tension can develop. Reasons for this are not well known but can be resultant from a natural disposition, learned reaction to stress or pain, trauma, or any combination of these. Excessive pelvic floor tension can result in various problems including frequent urination (due to the bladder's inability to expand) or pain. Treatments involve relaxing the muscles, using medication (such as Tamsulosin), Biofeedback, or physical therapy.
Thyrotoxic Myopathy (TM) is a neuromuscular disorder that develops due to the overproduction of the thyroid hormone thyroxine. Also known as hyperthyroid myopathy, TM is one of many myopathies that lead to muscle weakness and muscle tissue breakdown. Evidence indicates the onset of TM may be caused by hyperthyroidism (Kazakov, 1992). There are currently two known causes of hyperthyroidism that lead to development of TM including a multinodular goiter and Graves disease. Physical symptoms of TM may include muscle weakness, the breakdown of muscle tissue, fatigue, and heat intolerance (Quin, 1951).� Physical acts such as lifting objects and climbing stairs may become increasingly difficult (Horak, 2000). If untreated TM can be an extremely debilitating disorder that can, in extreme rare cases, lead to death. If diagnosed and treated properly the effects of TM can be controlled and in most cases reversed leaving no lasting effects.
Writer's cramp, also called mogigraphia and scrivener's palsy, causes a cramp or spasm affecting certain muscles of the hand and/or fingers[1]. Writer's cramp is a task-specific focal dystonia of the hand [2]. 'Focal' refers to the symptoms being limited to one location (the hand in this case), and 'task-specific' means that symptoms first occur only when the individual engages in a particular activity. Writer's cramp first affects an individual by inhibiting their ability to write.[3]
Almost all skeletal muscles either originate or insert on the skeleton. When a muscle moves a portion of the skeleton, that movement may involve flexion, extension, adduction, abduction, protraction, retraction, elevation, depression, rotation, circumduction, pronation, supination, inversion, eversion, lateral flexion, or opposition.
Actions can be described by one of two methods. The first, a muscle such as the biceps brachii is said to perform "flexion of the forearm." The second method, of increasing use among specialists such as kinesiologists, identifies the joint involved. With this method, the action of the biceps brachii muscle would be "flexion at (or of) the elbow." Both methods are valid, and each has its advantages. We shall primarily use the latter method.
When complex movements occur, muscles commonly work in groups rather than individually. Their cooperation improves the efficiency of a particular movement. For example, large muscles of the limbs produce flexion or extension over an extended range of motion. Although these muscles cannot develop much tension at full extension, they are generally paired with one or more smaller muscles that provide assistance until the larger muscle can perform at maximum efficiency. At the start of the movement, the smaller muscle is producing maximum tension while tension production by the larger muscle is at a minimum. The importance of this smaller "assistant" decreases as the movement proceeds and the efficiency of the primary muscle increases.
On the basis of size and range of motion, muscles are described as follows:
An agonist, or prime mover, is a muscle whose contraction is chiefly responsible for producing a particular movement. The biceps brachii muscle is an agonist that produces flexion at the elbow.
Antagonists are muscles whose actions oppose that of the agonist under consideration. The triceps brachii muscle is an agonist that extends the elbow. It is therefore an antagonist of the biceps brachii muscle, and the biceps brachii is an antagonist of the triceps brachii. Agonists and antagonists are functional opposites; if one produces flexion, the other will produce extension. When an agonist contracts to produce a particular movement, the corresponding antagonist will be stretched, but it will usually not relax completely. Instead, it will contract eccentrically, with the tension adjusted to control the speed of the movement and ensure its smoothness. You may find it easiest to learn about muscles in agonist/antagonist pairs (flexors/extensors, abductors/adductors) that act at a specific joint. This method highlights the functions of the muscles involved, and it can help organize the information in a logical framework. The tables in this chapter are arranged to facilitate this approach.
When a synergist contracts, it helps a larger agonist work efficiently. Synergists may provide additional pull near the insertion or may stabilize the point of origin. Their importance in assisting a particular movement may change as the movement progresses. In many cases, they are most useful at the start, when the agonist is stretched and unable to develop maximum tension. For example, the latissimus dorsi muscle is a large trunk muscle that extends, adducts, and medially rotates the arm at the shoulder joint. A much smaller muscle, the teres major muscle, assists in starting such movements when the shoulder joint is at full flexion. Synergists may also assist an agonist by preventing movement at another joint and thereby stabilizing the origin of the agonist. Such synergists are called fixators.
NAMES OF SKELETAL MUSCLES
Except for the platysma and the diaphragm, the complete names of all skeletal muscles include the term muscle. Although the full name, such as the biceps brachii muscle, will usually appear in the text, for simplicity only the descriptive name (biceps brachii) will be used in figures and tables.
You need not learn every one of the approximately 700 muscles in the human body, but you will have to become familiar with the most important ones. Fortunately, anatomists assigned names to the muscles that provide clues to their identification. If you can learn to recognize the clues, you will find it easier to remember the names and identify the muscles. The name of a muscle may include information about its fascicle organization, location, relative position, structure, size, shape, origin and insertion, or action.
Fascicle Organization
A muscle name may refer to the orientation of the muscle fibers within a particular skeletal muscle. Rectus means "straight," and rectus muscles are parallel muscles whose fibers generally run along the long axis of the body. Because we have several rectus muscles, the name typically includes a second term that refers to a precise region of the body. For example, the rectus abdominis muscle is located on the abdomen, and the rectus femoris muscle on the thigh. Other directional indicators include transversus and oblique for muscles whose fibers run across or at an oblique angle to the longitudinal axis of the body, respectively.
Location
They are common as modifiers that help identify individual muscles, as in the case of the rectus muscles. In a few cases, the muscle is such a prominent feature of the region that the regional name alone will identify it. Examples include the temporalis muscle of the head and the brachialis muscle of the arm.
Relative Position
Muscles visible at the body surface are often called externus or superficialis, whereas deeper muscles are termed internus or profundus. Superficial muscles that position or stabilize an organ are extrinsic; muscles located entirely within the extrinsic organ are intrinsic.
Structure, Size, and Shape
Some muscles are named after distinctive structural features. The biceps brachii muscle, for example, has two tendons of origin; the triceps brachii muscle has three; and the quadriceps group, four. Shape is sometimes an important clue to the name of a muscle. For example, the trapezius, deltoid, rhomboideus, and
orbicularis muscles look like a trapezoid, a triangle, a rhomboid, and a circle, respectively. Long muscles are called longus (long) or longissimus (longest), and teres muscles are both long and round. Short muscles are called brevis. Large ones are called magnus (big), major (bigger), or maximus (biggest); and small ones are called minor (smaller) or minimus (smallest).
Origin and Insertion
Many names tell you the specific origin and insertion of each muscle. In such cases, the first part of the name indicates the origin, and the second part the insertion. The genioglossus muscle, for example, originates at the chin (geneion) and inserts in the tongue (glossus).
Action
Many muscles are named flexor, extensor, retractor, abductor, and so on. These are such common actions that the names almost always include other clues as to the appearance or location of the muscle. For example, the extensor carpi radialis longus muscle is a long muscle along the radial (lateral) border of the forearm. When it contracts, its primary function is extension at the carpus (wrist).
A few muscles are named after the specific movements associated with special occupations or habits. The sartorius muscle is active when you cross your legs. Before sewing machines were invented, a tailor would sit on the floor cross-legged, and the name of this muscle was derived from sartor, the Latin word for "tailor." On the face, the buccinator muscle compresses the cheeks, as when you purse your lips and blow forcefully. Buccinator translates as "trumpet player." Another facial muscle, the risorius muscle, was supposedly named after the mood expressed. The Latin term risor, however, means "laugher"; a more appropriate description for the effect would be "grimace."
AXIAL AND APPENDICULAR MUSCLES
The separation of the skeletal system into axial and appendicular divisions provides a useful guideline for subdividing the muscular system as well:
1. The axial musculature arises on the axial skeleton. It positions the head and spinal column and also moves the rib cage, assisting in the movements that make breathing possible. It does not play a role in movement or support of either the pectoral or pelvic girdle or the limbs. This category encompasses roughly 60 percent of the skeletal muscles in the body.
2. The appendicular musculature stabilizes or moves components of the appendicular skeleton and includes the remaining 40 percent of all skeletal muscles.
The major axial and appendicular muscles of the human body. These are the superficial muscles, which tend to be relatively large. The superficial muscles cover deeper, smaller muscles that cannot be seen unless the overlying muscles are either removed or reflected--that is, cut and pulled out of the way. Later figures that show deep muscles in specific regions will indicate whether superficial muscles have been removed or reflected for the sake of clarity.
Paying attention to patterns of origin, insertion, and action, we will now examine representatives of both muscular divisions. This discussion assumes that you already understand skeletal anatomy. As you examine the figures in this chapter, you will find that some bony and cartilaginous landmarks are labeled for orientation purposes. These labels are shown in italics, to differentiate them from the muscles that are the primary focus of each figure.
Innervation is the distribution of nerves to a region or organ; the tables indicate the nerves that control each muscle. Many of the muscles of the head and neck are innervated by cranial nerves, such as the facial nerve, or seventh cranial nerve (N VII), which innervates the facial musculature. Cranial nerves originate at the brain and pass through the foramina of the skull. Spinal nerves are connected to the spinal cord and pass through the intervertebral foramina. For example, spinal nerve L1
passes between vertebrae L1 and L2 . Spinal nerves may form a complex network, or plexus; one branch, such as the sciatic nerve of the thigh, may contain axons from several spinal nerves. Thus, many tables include the spinal nerves as well as the names of the peripheral nerves.
The axial muscles fall into logical groups on the basis of location, function, or both. The groups do not always have distinct anatomical boundaries. For example, a function such as extension of the vertebral column involves muscles along its entire length and flexion at each of the intervertebral joints. We will discuss the axial muscles in four groups:
1. The muscles of the head and neck. This group includes muscles that move the face, tongue, and larynx. They are therefore responsible for verbal and nonverbal communication—laughing, talking, frowning, smiling, whistling, and so on. You also use this group of muscles when you eat—especially in sucking and chewing—and even when you look for something to eat, by controlling your eye movements. This group does not include muscles of the neck that are involved with movements of the vertebral column.
2. The muscles of the spine. This group includes numerous flexors, extensors, and rotators of the vertebral column.
3. The oblique and rectus muscles. This group forms the muscular walls of the thoracic and abdominopelvic cavities between the first thoracic vertebra and the pelvis. In the thoracic area these muscles are partitioned by the ribs, but over the abdominal surface they form broad muscular sheets. The neck also has oblique and rectus muscles. Although they do not form a complete muscular wall, they share a common developmental origin with the oblique and rectus muscles of the trunk.
4. The muscles of the pelvic floor. These muscles extend between the sacrum and pelvic girdle. This group forms the perineum, a muscular sheet that closes the pelvic outlet.
MUSCLES OF THE HEAD AND NECK
We can divide the muscles of the head and neck into several groups by function. The muscles of facial expression, the muscles of mastication (chewing), the muscles of the tongue, and the muscles of the pharynx originate on the skull or hyoid bone. Muscles involved with sight and hearing are also based on the skull. We will consider here the extrinsic eye muscles--those associated with movements of the eye. We shall discuss the intrinsic eye muscles, which control the diameter of the pupil and the shape of the lens, and the tiny skeletal muscles associated with the auditory ossicles. In the neck, the extrinsic muscles of the larynx adjust the position of the hyoid bone and larynx.
Muscles of Facial Expression
The muscles of facial expression originate on the surface of the skull. At their insertions, the fibers of the epimysium are woven into those of the superficial fascia and the dermis of the skin: When they contract, the skin moves. These muscles are innervated by the facial nerve.
The largest group of facial muscles is associated with the mouth. The orbicularis oris muscle constricts the opening, and other muscles move the lips or the corners of the mouth. The buccinator muscle has two functions related to eating (in addition to its importance to musicians). During chewing, it cooperates with the masticatory muscles by moving food back across the teeth from the space inside the cheeks. In infants, the buccinator provides suction for suckling at the breast.
Smaller groups of muscles control movements of the eyebrows and eyelids, the scalp, the nose, and the external ear. The epicranius, or scalp, contains two muscles, the frontalis and the occipitalis muscles. These muscles are separated by the galea aponeurotica, a collagenous sheet. The platysma covers the ventral surface of the neck, extending from the base of the neck to the periosteum of the mandible and the fascia at the corner of the mouth.
Extrinsic Eye Muscles
Six extrinsic eye muscles, or oculomotor muscles, originating on the surface of the orbit control the position of each eye. These muscles are the inferior rectus, medial rectus, superior rectus, lateral rectus, inferior oblique, and superior oblique muscles. The extrinsic eye muscles are innervated by the third (oculomotor), fourth (trochlear), and sixth (abducens) cranial nerves.
Muscles of Mastication
The muscles of mastication move the mandible at the temporomandibular joint. The large masseter muscle is the strongest jaw muscle. The temporalis muscle assists in elevation of the mandible. The pterygoid muscles, used in various combinations, can elevate, depress, or protract the mandible or slide it from side to side, a movement called lateral excursion. These movements are important in making efficient use of your teeth while you chew foods of various consistencies. The muscles of mastication are innervated by the fifth cranial nerve, the trigeminal nerve.
Muscles of the Tongue
The muscles of the tongue have names ending in glossus, the Greek word for "tongue." Once you can recall the structures referred to by palato-, stylo-, genio-, and hyo-, you will follow this group. The palatoglossus muscle originates at the palate, the styloglossus muscle at the styloid process of the temporal bone, the genioglossus muscle at the chin, and the hyoglossus muscle at the hyoid bone. These muscles, used in various combinations, move the tongue in the delicate and complex patterns necessary for speech, and they manipulate food within the mouth in preparation for swallowing. Most are innervated by the hypoglossal nerve (N XII), a cranial nerve whose name indicates its function and its location
Muscles of the Pharynx
The muscles of the pharynx are responsible for initiating the swallowing process. The pharyngeal constrictors (superior, middle, and inferior) move materials into the esophagus. The laryngeal elevators elevate the larynx. The palatal muscles, the tensor veli palatini and the levator veli palatini, raise the soft palate and adjacent portions of the pharyngeal wall and also pull open the entrance to the auditory tube. As a result, swallowing repeatedly can open the entrance to the auditory tube to help you adjust to pressure changes when you fly or dive.
Anterior Muscles of the Neck
The anterior muscles of the neck include (1) muscles that control the position of the larynx, (2) muscles that depress the mandible and tense the floor of the mouth,
and (3) muscles that provide a stable foundation for muscles of the tongue and pharynx. The digastric muscle has two bellies, as the name implies. One belly extends from the chin to the hyoid bone, and the other continues from the hyoid bone to the mastoid portion of the temporal bone. Depending on which belly contracts and whether fixator muscles are stabilizing the position of the hyoid bone, the digastric muscle can open the mouth by depressing the mandible, or it can elevate the larynx by raising the hyoid bone. The digastric muscle overlies the broad, flat mylohyoid muscle, which provides a muscular floor to the mouth. The stylohyoid muscle forms a muscular connection between the hyoid bone and the styloid process of the skull. The sternocleidomastoid muscle extends from the clavicle and the sternum to the mastoid region of the skull. The omohyoid muscle attaches to the scapula, the clavicle and first rib, and the hyoid bone. The other members of this group are straplike muscles that extend between the sternum and larynx (sternothyroid) or hyoid bone (sternohyoid), between the larynx and hyoid bone (thyrohyoid), and between the hyoid bone and chin (geniohyoid).
