Skeletal muscle is one of three major muscle types, the others being cardiac muscle and smooth muscle. It is a form of striated muscle tissue, under the voluntary control of the somatic nervous system. Most skeletal muscles are attached to bones by bundles of collagen fibers known as tendons. A skeletal muscle refers to multiple bundles of cells joined together called muscle fibers; the fibers and muscles are surrounded by connective tissue layers called fasciae. Muscle fibers, or muscle cells, are formed from the fusion of developmental myoblasts in a process known as myogenesis. Muscle fibers have more than one nucleus, they have multiple mitochondria to meet energy needs. Muscle fibers are in turn composed of myofibrils; the myofibrils are composed of actin and myosin filaments, repeated in units called sarcomeres, which are the basic functional units of the muscle fiber. The sarcomere is responsible for the striated appearance of skeletal muscle and forms the basic machinery necessary for muscle contraction.
Connective tissue is present in all muscles as fascia. Enclosing each muscle is a layer of connective tissue known as the epimysium. Muscle fibers are the individual contractile units within a muscle. A single muscle such as the biceps brachii contains many muscle fibers. Another group of cells, the myosatellite cells are found between the basement membrane and the sarcolemma of muscle fibers; these cells are quiescent but can be activated by exercise or pathology to provide additional myonuclei for muscle growth or repair. DevelopmentIndividual muscle fibers are formed during development from the fusion of several undifferentiated immature cells known as myoblasts into long, multi-nucleated cells. Differentiation into this state is completed before birth with the cells continuing to grow in size thereafter. MicroanatomySkeletal muscle exhibits a distinctive banding pattern when viewed under the microscope due to the arrangement of cytoskeletal elements in the cytoplasm of the muscle fibers; the principal cytoplasmic proteins are myosin and actin which are arranged in a repeating unit called a sarcomere.
The interaction of myosin and actin is responsible for muscle contraction. Every single organelle and macromolecule of a muscle fiber is arranged to ensure form meets function; the cell membrane is called the sarcolemma with the cytoplasm known as the sarcoplasm. In the sarcoplasm are the myofibrils; the myofibrils are long protein bundles about 1 micrometer in diameter each containing myofilaments. Pressed against the inside of the sarcolemma are the unusual flattened myonuclei. Between the myofibrils are the mitochondria. While the muscle fiber does not have smooth endoplasmic cisternae, it contains a sarcoplasmic reticulum; the sarcoplasmic reticulum surrounds the myofibrils and holds a reserve of the calcium ions needed to cause a muscle contraction. Periodically, it has dilated end sacs known as terminal cisternae; these cross the muscle fiber from one side to the other. In between two terminal cisternae is a tubular infolding called a transverse tubule. T tubules are the pathways for action potentials to signal the sarcoplasmic reticulum to release calcium, causing a muscle contraction.
Together, two terminal cisternae and a transverse tubule form a triad. Muscle architecture refers to the arrangement of muscle fibers relative to the axis of force generation of the muscle; this axis is a hypothetical line from the muscle's origin to insertion. For some longitudinal muscles, such as the biceps brachii, this is a simple concept. For others, such as the rectus femoris or deltoid muscle, it becomes more complicated. While the muscle fibers of a fascicle lie parallel to one another, the fascicles themselves can vary in their relationship to one another and to their tendons; the different fiber arrangements produce broad categories of skeletal muscle architectures including longitudinal, unipennate and multipennate. Because of these different architectures, the tension a muscle can create between its tendons varies by more than its size and fiber-type makeup. Longitudinal architectureThe fascicles of longitudinally arranged, parallel, or fusiform muscles run parallel to the axis of force generation, thus these muscles on a whole function to a single, large muscle fiber.
Variations exist, the different terms are used more specifically. For instance, fusiform refers to a longitudinal architecture with a widened muscle belly, while parallel may refer to a more ribbon-shaped longitudinal architecture. A less common example would be a circular muscle such as the orbicularis oculi, in which the fibers are longitudinally arranged, but create a circle from origin to insertion. Unipennate architectureThe fibers in unipennate muscles are all oriented at the same angle relative to the axis of force generation; this angle reduces the effective force of any individual fiber, as it is pulling off-axis. However, because of this angle, more fibers can be packed into the same muscle volume, increasing the Physiological cross-sectional area; this effect is known as fiber packing, and—in terms of force generation—it more than overcomes the efficiency loss of the off-axis orientation. The trade-off comes in the total excursion. Overall muscle shortening speed is reduced compared to fiber shortening speed, as is the total distance of shortening.
All of these effects scale with pennation angle.
Anatomical terms of muscle
Muscles are described using unique anatomical terminology according to their actions and structure. There are three types of muscle tissue in the human body: skeletal and cardiac. Skeletal striated muscle, or "voluntary muscle" joins to bone with tendons. Skeletal muscle maintains posture. Smooth muscle tissue is found in parts of the body; the majority of this type of muscle tissue is found in the digestive and urinary systems where it acts by propelling forward food and feces in the former and urine in the latter. Other places smooth muscle can be found are within the uterus, where it helps facilitate birth, the eye, where the pupillary sphincter controls pupil size. Cardiac muscle is specific to the heart, it is involuntary in its movement, is additionally self-excitatory, contracting without outside stimuli. As well as anatomical terms of motion, which describe the motion made by a muscle, unique terminology is used to describe the action of a set of muscles. Agonist muscles and antagonist muscles refer to muscles that inhibit a movement.
Agonist muscles cause a movement to occur through their own activation. For example, the triceps brachii contracts, producing a shortening contraction, during the up phase of a push-up. During the down phase of a push-up, the same triceps brachii controls elbow flexion while producing a lengthening contraction, it is still the agonist, because while resisting gravity during relaxing, the triceps brachii continues to be the prime mover, or controller, of the joint action. Agonists are interchangeably referred to as "prime movers," since they are the muscles considered responsible for generating or controlling a specific movement. Another example is the dumbbell curl at the elbow; the "elbow flexor" group is the agonist. During the lowering phase the "elbow flexor" muscles lengthen, remaining the agonists because they are controlling the load and the movement. For both the lifting and lowering phase, the "elbow extensor" muscles are the antagonists, they shorten during the dumbbell lowering phase.
Here it is important to understand that it is common practice to give a name to a muscle group based on the joint action they produce during a shortening contraction. However, this naming convention does not mean; this term describes the function of skeletal muscles. Antagonist muscles are the muscles that produce an opposing joint torque to the agonist muscles; this torque can aid in controlling a motion. The opposing torque can slow movement down - in the case of a ballistic movement. For example, during a rapid discrete movement of the elbow, such as throwing a dart, the triceps muscles will be activated briefly and to accelerate the extension movement at the elbow, followed immediately by a "burst" of activation to the elbow flexor muscles that decelerates the elbow movement to arrive at a quick stop. To use an automotive analogy, this would be similar to pressing your gas pedal and immediately pressing the brake. Antagonism is not an intrinsic property of a particular muscle group. During slower joint actions that involve gravity, just as with the agonist muscle, the antagonist muscle can shorten and lengthen.
Using the example above of the triceps brachii during a push-up, the elbow flexor muscles are the antagonists at the elbow during both the up phase and down phase of the movement. During the dumbbell curl, the elbow extensors are the antagonists for both the lifting and lowering phases. Antagonist and agonist muscles occur in pairs, called antagonistic pairs; as one muscle contracts, the other relaxes. An example of an antagonistic pair is the triceps. "Reverse motions" need antagonistic pairs located in opposite sides of a joint or bone, including abductor-adductor pairs and flexor-extensor pairs. These consist of an extensor muscle, which "opens" the joint and a flexor muscle, which does the opposite by decreasing the angle between two bones. However, muscles don't always work this way. Sometimes during a joint action controlled by an agonist muscle, the antagonist will be activated, naturally; this occurs and is not considered to be a problem unless it is excessive or uncontrolled and disturbs the control of the joint action.
This serves to mechanically stiffen the joint. Not all muscles are paired in this way. An example of an exception is the deltoid. Synergist muscles help perform, the same set of joint motion as the agonists. Synergists muscles act on movable joints. Synergists are sometimes referred to as "neutralizers" because they help cancel out, or neutralize, extra motion from the agonists to make sure that the force generated works within the desired plane of motion. Muscle fibers can only contract up to 40% of their stretched length, thus the short fibers of pennate muscles are more suitable where power rather than range of contraction is required. This limitation in the range of contraction affects all muscles, those that act over several joints may be unable to shorten sufficiently to produce
Inferior pharyngeal constrictor muscle
The Inferior pharyngeal constrictor, the thickest of the three constrictors, arises from the sides of the cricoid and thyroid cartilage. To the superior and middle pharyngeal constrictor muscles, it is innervated by the vagus nerve by branches from the pharyngeal plexus and by neuronal branches from the recurrent laryngeal nerve; the muscle is composed of two parts. The first arising from the thyroid cartilage and the second arising from the cricoid cartilage. On the thyroid cartilage it arises from the oblique line on the side of the lamina, from the surface behind this nearly as far as the posterior border and from the inferior cornu. From the cricoid cartilage it arises in the interval between the Cricothyreoideus in front, the articular facet for the inferior cornu of the thyroid cartilage behind. From these origins the fibers spread backward and medialward to be inserted with the muscle of the opposite side into the fibrous pharyngeal raphe in the posterior median line of the pharynx; the inferior fibers are continuous with the circular fibers of the esophagus.
The cricopharyngeal muscle is synonymous with the upper esophageal sphincter, which controls the opening of the cervical esophagus, is sometimes referred to as the cricopharyngeal inlet. As soon as the bolus of food is received in the pharynx, the elevator muscles relax, the pharynx descends, the constrictors contract upon the bolus, convey it downward into the esophagus. During deglutition, they cause peristaltic movement in the pharynx. Uncoordinated contraction, and/or Cricopharyngeal Spasm and/or impaired relaxation of this muscle are considered the main factors in development of a Zenker's diverticulum. Zenker's diverticulum develops between the two bellies of the inferior constrictor in a small gap called Killian's dehiscence. A diverticulum can form. Food or other materials may reside here. Motor incoordination of the cricopharyngeus can cause difficulty swallowing. Upper esophageal sphincter This article incorporates text in the public domain from page 1142 of the 20th edition of Gray's Anatomy lesson8 at The Anatomy Lesson by Wesley Norman
Obliquus capitis inferior muscle
The obliquus capitis inferior muscle is the larger of the two oblique muscles of the neck. It arises from the apex of the spinous process of the axis and passes laterally and upward, to be inserted into the lower and back part of the transverse process of the atlas, it lies deep to the semispinalis trapezius muscles. The muscle is responsible for rotation of first cervical vertebra, it forms the lower boundary of the suboccipital triangle of the neck. The naming of this muscle may be confusing, as it is the only capitis muscle that does NOT attach to the cranium; the obliquus capitis inferior muscle, like the other suboccipital muscles, has an important role in proprioception. This muscle has a high density of Golgi organs and muscle spindles which accounts for this, it is believed that proprioception may be the primary role of the inferior oblique allowing accurate positioning of the head on the neck. This article incorporates text in the public domain from page 402 of the 20th edition of Gray's Anatomy
Rectus capitis posterior minor muscle
The rectus capitis posterior minor arises by a narrow pointed tendon from the tubercle on the posterior arch of the atlas, widening as it ascends, is inserted into the medial part of the inferior nuchal line of the occipital bone and the surface between it and the foramen magnum, takes some attachment to the spinal dura mater. The synergists are the rectus. Connective tissue bridges were noted at the atlanto-occipital joint between the rectus capitis posterior minor muscle and the dorsal spinal dura. Similar connective tissue connections of the rectus capitis posterior major have been reported as well; the perpendicular arrangement of these fibers appears to restrict dural movement toward the spinal cord. The ligamentum nuchae was found to be continuous with the posterior cervical spinal dura and the lateral portion of the occipital bone. Anatomic structures innervated by cervical nerves C1-C3 have the potential to cause headache pain. Included are the joint complexes of the upper three cervical segments, the dura mater, spinal cord.
The dura-muscular, dura-ligamentous connections in the upper cervical spine and occipital areas may provide anatomic and physiologic answers to the cause of the cervicogenic headache. This proposal would further explain manipulation's efficacy in the treatment of cervicogenic headache. Atlanto-occipital joint Rectus capitis lateralis Rectus capitis posterior major muscle Rectus capitis anterior muscle This article incorporates text in the public domain from page 401 of the 20th edition of Gray's Anatomy Anatomy figure: 01:07-01 at Human Anatomy Online, SUNY Downstate Medical Center Anatomy photo:01:10-0101 at the SUNY Downstate Medical Center PTCentral
Longus colli muscle
The Longus colli muscle is a muscle of the human body. The Longus colli is situated on the anterior surface of the vertebral column, between the atlas and the third thoracic vertebra, it is broad in the middle and pointed at either end, consists of three portions, a superior oblique, an inferior oblique, a vertical. The superior oblique portion arises from the anterior tubercles of the transverse processes of the third and fifth cervical vertebræ and, ascending obliquely with a medial inclination, is inserted by a narrow tendon into the tubercle on the anterior arch of the atlas; the inferior oblique portion, the smallest part of the muscle, arises from the front of the bodies of the first two or three thoracic vertebræ. The vertical portion arises, from the front of the bodies of the upper three thoracic and lower three cervical vertebræ, is inserted into the front of the bodies of the second and fourth cervical vertebræ, it is injured in rear end whiplash injuries resulting from a car crash.
This muscle is in front of the spine and is thought by some scientists that it may cause some whiplash patients to have an unnatural lack of curvature in the patients' neck. Acute calcific tendinitis of the longus colli muscle can occur; this presents with acute onset of neck pain, stiffness and odynophagia, must be distinguished from retropharyngeal abscess and other sinister conditions. Imaging diagnosis is by CT or MRI, demonstrating calcification in the muscle in addition to retropharyngeal oedema. Treatment is supportive, with non-steroidal anti-inflammatory drugs; this article incorporates text in the public domain from page 394 of the 20th edition of Gray's Anatomy Specific PTCentral spinal injury database
Rectus capitis lateralis muscle
The Rectus capitis lateralis, a short, flat muscle, arises from the upper surface of the transverse process of the atlas, is inserted into the under surface of the jugular process of the occipital bone. Atlanto-occipital joint Rectus capitis posterior major muscle Rectus capitis posterior minor muscle Rectus capitis anterior muscle This article incorporates text in the public domain from page 395 of the 20th edition of Gray's Anatomy PTCentral