Human Anatomy & Physiology: Muscles

That movement is achieved through the coordinated action of the pectoralis major, latissimus dorsi, deltoid, teres major, and subscapularis muscles. The subscapularis is a deep muscle situated on the anterior, or front-facing, surface of the scapula. The teres minor, subscapularis, supraspinatus, and infraspinatus muscles together form the rotator cuff, which stabilizes the humeral head the ball portion of the ball-and-socket shoulder joint.

The muscles of the rotator cuff are common sites of injury in adults, particularly among people who perform overhead motions repeatedly e. Several of the rotator cuff muscles have tendons that run under the acromion, a bony prominence at the distal end of the scapula. The term distal describes a relative position away from the centre of the body; it often is contrasted with the term proximal , which describes a relative position near to the centre of the body.

The position of the tendons and of the subacromial bursae fluid-filled sacs located beneath the acromion leaves them vulnerable to compression and pinching, which can result in an injury known as shoulder impingement syndrome. In addition to aiding the movement of the shoulder, the muscles of the upper arm produce various movements of the forearm.

For example, the primary muscles involved in forearm flexion, in which the angle formed at the elbow becomes smaller i. Minor contributions to forearm flexion are provided by the coracobrachialis and by flexor muscles situated in the anterior compartment of the forearm the palm side of the forearm; also known as the flexor compartment , including the pronator teres, the flexor carpi radialis, the flexor digitorum superficialis, the palmaris longus, and the flexor carpi ulnaris.

Extension of the forearm increases the angle at the elbow, moving the hand away from the shoulder. That action is accomplished primarily by the triceps brachii. Other muscles that make minor contributions to forearm extension include the extensor muscles of the posterior compartment of the forearm the side of the forearm that is contiguous with the back of the hand; also known as the extensor compartment , including the extensor carpi radialis longus, the extensor carpi radialis brevis, the extensor digitorum, the extensor carpi ulnaris, and the anconeus.

Wrist flexion refers to movement of the wrist that draws the palm of the hand downward. That action is carried out by the flexor carpi radialis, the flexor carpi ulnaris, the flexor digitorum superficialis, the flexor digitorum profundus, and the flexor pollicis longus. Wrist extension, by contrast, shortens the angle at the back of the wrist. The muscles responsible for that action are the extensor carpi radialis longus and the extensor carpi radialis brevis, which also abduct the hand at the wrist move the hand in the direction of the thumb, or first digit ; the extensor digitorum, which also extends the index to little finger the second to fifth digits ; the extensor digiti minimi, which also extends the little finger and adducts the hand moves the hand in the direction of the little finger ; and the extensor carpi ulnaris, which also adducts the hand.

Other small muscles that cross the wrist joint may add to wrist extension, but they do so to only a small degree. Wrist supination is the rotation of the wrist that brings the palm facing up. The supinator muscle in the posterior compartment acts to supinate the forearm. The biceps brachii also adds to supination. Pronation is the opposing action, in which the wrist is rotated so that the palm is facing down.

The pronator quadratus, a deep muscle in the anterior compartment, along with the pronator teres, pronates the forearm. The hand is a complex structure that is involved in fine motor coordination and complex task performance.

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Its muscles generally are small and extensively innervated. Even simple actions, such as typing on a keyboard, require a multitude of precise movements to be carried out by the hand muscles. Because of that complexity, the following paragraphs cover only the primary action of each hand muscle. Several muscles that originate at the posterior surface of the ulna or the radius the other bone in the forearm have their actions in the hand.

Human Physiology/The Muscular System

Those include the abductor pollicis longus, which abducts and extends the thumb; the extensor pollicis brevis, which extends the metacarpophalangeal MCP joint of the thumb; the extensor pollicis, which extends the distal phalanx finger bone of the thumb; and the extensor indicis, which extends the index finger at the MCP joint. MCP joints are located between the metacarpal bones, which are situated in the hand, and the phalanges, which are the small bones of the fingers.

Although several of the muscles that move the hand have their origins in the forearm, there are many small muscles of the hand that have both their origin and their insertion within the hand. Those are referred to as the intrinsic muscles of the hand. They include the palmaris brevis, which assists with grip; the umbricals, which flex the MCP joints and extend the interphalangeal joints IPs; the joints between the phalanges of the fingers; the palmar interossei, which adduct the fingers toward the middle finger the third digit ; and the dorsal interossei, which abduct the fingers away from the middle finger.

Human Physiology/The Muscular System - Wikibooks, open books for an open world

The thenar eminence is located on the palm side of the base of the thumb and is composed of three muscles, the abductor pollicis brevis, the flexor pollicis brevis, and the opponens pollicis, all of which are innervated by the median nerve. The abductor pollicis brevis abducts the thumb; the flexor pollicis brevis flexes the MCP joint of the thumb; and the opponens pollicis acts to oppose the thumb to the other fingers. The adductor pollicis, which is not part of the thenar eminence, acts to adduct the thumb. The hypothenar enimence is located on the palm side of the hand below the little finger.

It contains three muscles that are innervated by the deep branch of the ulnar nerve. The abductor digiti minimi abducts the little finger. The flexor digiti minimi flexes the little finger. The opponens digiti minimi opposes the little finger with the thumb. There are three muscular layers of the abdominal wall, with a fourth layer in the middle anterior region. The fourth layer in the midregion is the rectus abdominis, which has vertically running muscle fibres that flex the trunk and stabilize the pelvis.

To either side of the rectus abdominis are the other three layers of abdominal muscles.

The deepest of those layers is the transversus abdominis, which has fibres that run perpendicular to the rectus abdominus; the transversus abdominis acts to compress and support the abdomen and provides static core stabilization. The internal oblique layers run upward and forward from the sides of the abdomen, and the external oblique layers, which form the outermost muscle layers of the abdomen, run downward and forward.

The hip joint is a complex weight-bearing ball-and-socket joint that can sustain considerable load. The socket of the joint is relatively deep, allowing for stability but sacrificing some degree in range of motion. The movements described in this section include flexion, extension, abduction, and adduction. Hip flexion is the hip motion that brings the knee toward the chest.

The major muscles of hip flexion include the iliopsoas, which is made up of the psoas major, psoas minor, and iliacus. Together, those muscles act mainly to flex the hip, but they also contribute to abdominal flexion and hip stabilization. Other hip flexors include the sartorius, the rectus femoris, the pectineus, and the gracilis. The sartorius also contributes to external hip rotation and knee extension and abduction, and the rectus femoris also acts in knee extension. The pectineus is also involved in hip adduction and internal rotation.

Hip extension is accomplished primarily by the muscles of the posterior thigh and buttocks, which when contracted serve to move the thigh from a flexed position toward the midline of the body or the trunk of the body from a bent position toward a more-erect posture. Hip extension is accomplished mostly by the gluteus maximus, the biceps femoris which is divided into two heads, the long head and the short head , the semitendinosus, and the semimembranosus.

A minor contribution is also provided by the adductor magnus and other small pelvic muscles. The movement of adduction is used to describe a direction of limb motion that serves to take the limb from a lateral position to its more-axial alignment.

Muscular System Physiology

During a jumping-jack exercise, for example, abduction of the leg occurs when it is moved away from the midline and adduction when it is moved back toward the midline. The main abductors of the hip are the gluteus medius, gluteus minimus, and tensor fascia lata. Those three muscles also serve to internally rotate the thigh in an extended position and externally rotate the thigh in the flexed position.

Another minor contributor is the piriformis. There are three classes of levers, but the vast majority of the levers in the body are third class levers. A third class lever is a system in which the fulcrum is at the end of the lever and the effort is between the fulcrum and the load at the other end of the lever. The third class levers in the body serve to increase the distance moved by the load compared to the distance that the muscle contracts. The tradeoff for this increase in distance is that the force required to move the load must be greater than the mass of the load.

For example, the biceps brachia of the arm pulls on the radius of the forearm, causing flexion at the elbow joint in a third class lever system. A very slight change in the length of the biceps causes a much larger movement of the forearm and hand, but the force applied by the biceps must be higher than the load moved by the muscle. Nerve cells called motor neurons control the skeletal muscles. Each motor neuron controls several muscle cells in a group known as a motor unit.

When a motor neuron receives a signal from the brain, it stimulates all of the muscles cells in its motor unit at the same time. The size of motor units varies throughout the body, depending on the function of a muscle. Muscles that need a lot of strength to perform their function—like leg or arm muscles—have many muscle cells in each motor unit. One of the ways that the body can control the strength of each muscle is by determining how many motor units to activate for a given function.

This explains why the same muscles that are used to pick up a pencil are also used to pick up a bowling ball. Muscles contract when stimulated by signals from their motor neurons. Motor neurons release neurotransmitter chemicals at the NMJ that bond to a special part of the sarcolemma known as the motor end plate. The motor end plate contains many ion channels that open in response to neurotransmitters and allow positive ions to enter the muscle fiber. The positive ions form an electrochemical gradient to form inside of the cell, which spreads throughout the sarcolemma and the T-tubules by opening even more ion channels.

Tropomyosin is moved away from myosin binding sites on actin molecules, allowing actin and myosin to bind together. ATP molecules power myosin proteins in the thick filaments to bend and pull on actin molecules in the thin filaments. Myosin proteins act like oars on a boat, pulling the thin filaments closer to the center of a sarcomere. As the thin filaments are pulled together, the sarcomere shortens and contracts.

Myofibrils of muscle fibers are made of many sarcomeres in a row, so that when all of the sarcomeres contract, the muscle cells shortens with a great force relative to its size. Muscles continue contraction as long as they are stimulated by a neurotransmitter. When a motor neuron stops the release of the neurotransmitter, the process of contraction reverses itself.

Calcium returns to the sarcoplasmic reticulum; troponin and tropomyosin return to their resting positions; and actin and myosin are prevented from binding. Sarcomeres return to their elongated resting state once the force of myosin pulling on actin has stopped. Certain conditions or disorders, such as myoclonus, can affect the normal contraction of muscles. You can learn about musculoskeletal health problems in our section devoted to diseases and conditions. Also, learn more about advances in DNA health testing that help us understand genetic risk of developing early-onset primary dystonia.

A single nerve impulse of a motor neuron will cause a motor unit to contract briefly before relaxing. This small contraction is known as a twitch contraction. If the motor neuron provides several signals within a short period of time, the strength and duration of the muscle contraction increases. This phenomenon is known as temporal summation. If the motor neuron provides many nerve impulses in rapid succession, the muscle may enter the state of tetanus, or complete and lasting contraction.

A muscle will remain in tetanus until the nerve signal rate slows or until the muscle becomes too fatigued to maintain the tetanus. Not all muscle contractions produce movement. Isometric contractions are light contractions that increase the tension in the muscle without exerting enough force to move a body part. When people tense their bodies due to stress, they are performing an isometric contraction. Holding an object still and maintaining posture are also the result of isometric contractions.

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A contraction that does produce movement is an isotonic contraction. Isotonic contractions are required to develop muscle mass through weight lifting. Muscle tone is a natural condition in which a skeletal muscle stays partially contracted at all times. All muscles maintain some amount of muscle tone at all times, unless the muscle has been disconnected from the central nervous system due to nerve damage.

Skeletal muscle fibers can be divided into two types based on how they produce and use energy: Type I and Type II. Muscles get their energy from different sources depending on the situation that the muscle is working in. This calcium uncovers the actin binding sites. When the muscle no longer needs to contract, the calcium ions are pumped from the sarcomere and back into storage in the sarcoplasmic reticulum. Heart muscles are distinct from skeletal muscles because the muscle fibers are laterally connected to each other.

Furthermore, just as with smooth muscles, their movement is involuntary. Heart muscles are controlled by the sinus node influenced by the autonomic nervous system.

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Smooth muscles are controlled directly by the autonomic nervous system and are involuntary, meaning that they are incapable of being moved by conscious thought. Functions such as heartbeat and lungs which are capable of being willingly controlled, be it to a limited extent are involuntary muscles but are not smooth muscles. Neuromuscular junctions are the focal point where a motor neuron attaches to a muscle. Acetylcholine, a neurotransmitter used in skeletal muscle contraction is released from the axon terminal of the nerve cell when an action potential reaches the microscopic junction called a synapse.

A group of chemical messengers cross the synapse and stimulate the formation of electrical changes, which are produced in the muscle cell when the acetylcholine binds to receptors on its surface. Calcium is released from its storage area in the cell's sarcoplasmic reticulum. An impulse from a nerve cell causes calcium release and brings about a single, short muscle contraction called a muscle twitch. If there is a problem at the neuromuscular junction, a very prolonged contraction may occur, such as the muscle contractions that result from tetanus.

Also, a loss of function at the junction can produce paralysis. Skeletal muscles are organized into hundreds of motor units , each of which involves a motor neuron, attached by a series of thin finger-like structures called axon terminals. These attach to and control discrete bundles of muscle fibers. A coordinated and fine tuned response to a specific circumstance will involve controlling the precise number of motor units used.

While individual muscle units contract as a unit, the entire muscle can contract on a predetermined basis due to the structure of the motor unit. Motor unit coordination, balance, and control frequently come under the direction of the cerebellum of the brain. This allows for complex muscular coordination with little conscious effort, such as when one drives a car without thinking about the process. At rest, the body produces the majority of its ATP aerobically in the mitochondria [1] without producing lactic acid or other fatiguing byproducts. At lower activity levels, when exercise continues for a long duration several minutes or longer , energy is produced aerobically by combining oxygen with carbohydrates and fats stored in the body.

During activity that is higher in intensity, with possible duration decreasing as intensity increases, ATP production can switch to anaerobic pathways, such as the use of the creatine phosphate and the phosphagen system or anaerobic glycolysis. Aerobic ATP production is biochemically much slower and can only be used for long-duration, low-intensity exercise, but produces no fatiguing waste products that can not be removed immediately from the sarcomere and the body, and it results in a much greater number of ATP molecules per fat or carbohydrate molecule.

Aerobic training allows the oxygen delivery system to be more efficient, allowing aerobic metabolism to begin quicker. It allows for the highest levels of exercise intensity, but intramuscular stores of phosphocreatine are very limited and can only provide energy for exercises lasting up to ten seconds. Recovery is very quick, with full creatine stores regenerated within five minutes.