Striated muscle tissue
Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. The presence of sarcomeres manifests as a series of bands visible along the muscle fibers, responsible for the striated appearance observed in microscopic images of this tissue. There are two types of striated muscles: Cardiac muscle Skeletal muscle Striated muscle tissue contains T-tubules which enables the release of calcium ions from the sarcoplasmic reticulum. Skeletal muscle includes skeletal muscle fibers, blood vessels, nerve fibers, connective tissue. Skeletal muscle is wrapped in epimysium, allowing structural integrity of the muscle despite contractions; the perimysium organizes the muscle fibers, which are encased in collagen and endomysium, into fascicles. Each muscle fiber contains sarcolemma and sarcoplasmic reticulum; the functional unit of a muscle fiber is called a sarcomere. Each myofiber is composed of myosin myofibrils repeated as a sarcomere. Based on their contractile and metabolic phenotypes, skeletal muscle can be classified as slow-oxidative or fast-oxidative.
Cardiac muscle lies the endocardium in the heart. Cardiac muscle fibers only contain one nucleus, located in the central region, they contain many myoglobin. Unlike skeletal muscle, cardiac muscle cells are unicellular; these cells are connected to each other by intercalated disks, which contain gap junctions and desmosomes. The main difference between striated muscle tissue and smooth muscle tissue is that striated muscle tissue features sarcomeres while smooth muscle tissue does not. All striated muscles are attached to some component of the skeleton, unlike smooth muscle, which composes hollow organs such as the intestines or blood vessels; the fibers of striated muscle have a cylindrical shape with blunt ends, whereas those in smooth muscle can be described as being spindle-like with tapered ends. Two other characteristics that differentiate striated muscle from smooth muscle are that the former has more mitochondria and contains cells that are multinucleated; the main function of striated muscle tissue is to create contract.
These contractions will either pump blood throughout the body or powers breathing, movement or posture. Contractions in cardiac muscle tissue are due to pacemaker cells; these cells respond to signals from the autonomic nervous system to either increase or decrease the heart rate. Pacemaker cells have autorhythmicity; the set intervals at which they depolarize to threshold and fire action potentials is what determines the heart rate. Because of the gap junctions, the pacemaker cells transfer the depolarization to other cardiac muscle fibers, in order to contract in unison. Signals from motor neurons cause myofibers to depolarize and therefore release calcium ions from the sarcoplasmic reticulum; the calcium drives the movement of actin filaments. The sarcomere shortend which causes the muscle to contract. In the skeletal muscles connected to tendons that pull on bones, the mysia fuses to the periosteum that coats the bone. Contraction of the muscle will transfer to the mysia the tendon and the periosteum before causing the bone to move.
The mysia may bind to an aponeurosis or to fascia. Adult humans cannot regenerate cardiac muscle tissue after an injury, which can lead to scarring and thus heart failure. Mammals have the ability to complete small amounts of cardiac regeneration during development. Vertebrates can regenerate cardiac muscle tissue throughout their entire life span. Skeletal muscle is able to regenerate far better than cardiac muscle due to satellite cells, which are dormant in all healthy skeletal muscle tissue. There are three phases to the regeneration process; these phases include the inflammatory response, the activation and fusion of satellite cells, the maturation and remodeling of newly formed myofibrils. This process begins with the necrosis of damaged muscle fibers, which in turn induces the inflammatory response. Macrophages induce phagocytosis of the cell debris, they will secrete anti-inflammatory cytokines, which results in the termination of inflammation. These macrophages can facilitate the proliferation and differentiation of satellite cells.
The satellite cells re-enter the cell cycle to multiply. They leave the cell cycle to self-renew or differentiate as myoblasts. Sarcopenia Polymyositis Dermatomyositis Inclusion body myositis Coronary Artery Disease Arrhythmia Cardiomyopathy Smooth muscle tissue Skeletal Muscle Cardiac Muscle
Perimysium is a sheath of connective tissue that groups muscle fibers into bundles or fascicles. Recent advances in muscle physiology suggest that the perimysium plays a role in transmitting lateral contractile movements; this hypothesis is supported in one exhibition of the existence of "perimysial junctional plates" in ungulate flexor carpi radialis muscles constructed by E. Passerieux; the overall comprehensive organization of the perimysium collagen network, as well as its continuity and disparateness, have still not been observed and described everywhere within the muscle. Endomysium Epimysium Connective tissue in skeletal muscle Histology at cytochemistry.net
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.
An axon, or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, that conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons and glands. In certain sensory neurons, such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body, from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders which can affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, group C nerve fibers. Groups A and B are myelinated, group C are unmyelinated; these groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, Type IV. An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron.
Axons are distinguished from dendrites by several features, including shape and function. Some types of neurons have no transmit signals from their dendrites. In some species, axons can emanate from dendrites and these are known as axon-carrying dendrites. No neuron has more than one axon. Axons are covered by a membrane known as an axolemma. Most axons branch, in some cases profusely; the end branches of an axon are called telodendria. The swollen end of a telodendron is known as the axon terminal which joins the dendron or cell body of another neuron forming a synaptic connection. Axons make contact with other cells—usually other neurons but sometimes muscle or gland cells—at junctions called synapses. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse. At a synapse, the membrane of the axon adjoins the membrane of the target cell, special molecular structures serve to transmit electrical or electrochemical signals across the gap.
Some synaptic junctions appear along the length of an axon as it extends—these are called en passant synapses and can be in the hundreds or the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches. A single axon, with all its branches taken together, can innervate multiple parts of the brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system, a fascicle in the peripheral nervous system. In placental mammals the largest white matter tract in the brain is the corpus callosum, formed of some 20 million axons in the human brain. Axons are the primary transmission lines of the nervous system, as bundles they form nerves; some axons can extend up to more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot; the diameter of axons is variable. Most individual axons are microscopic in diameter.
The largest mammalian axons can reach a diameter of up to 20 µm. The squid giant axon, specialized to conduct signals rapidly, is close to 1 millimetre in diameter, the size of a small pencil lead; the numbers of axonal telodendria can differ from one nerve fiber to the next. Axons in the central nervous system show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain. There are two types of axons in the nervous system: unmyelinated axons. Myelin is a layer of a fatty insulating substance, formed by two types of glial cells Schwann cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of a myelinated axon. In the central nervous system oligodendrocytes form the insulating myelin.
Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an rapid mode of electrical impulse propagation called saltatory conduction; the myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in the brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum. Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures; the largest of these is the corpus callosum that connects the two cerebral hemispheres, this has around 20 million axons. The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, the axonal region as the other.
The axonal region or compart
The Purkinje fibres are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibres are specialised conducting fibres composed of electrically excitable cells that are larger than cardiomyocytes with fewer myofibrils and a large number of mitochondria and which conduct cardiac action potentials more and efficiently than any other cells in the heart. Purkinje fibres allow the heart's conduction system to create synchronized contractions of its ventricles, are, essential for maintaining a consistent heart rhythm. Purkinje fibers are a unique cardiac end-organ. Further histologic examination reveals; the electrical origin of atrial Purkinje fibers arrives from the sinoatrial node. Given no aberrant channels, the Purkinje fibers are distinctly shielded from each other by collagen or the cardiac skeleton; the Purkinje fibers are further specialized to conduct impulses. Purkinje fibers take up stain differently from the surrounding muscle cells because of fewer myofibrils than other cardiac cells and the presence of glycogen around the nucleus causes Purkinje fibers to appear, on a slide and larger than their neighbors, arranged along the longitudinal direction.
They are binucleated cells. Heart rate is governed by many influences from the autonomic nervous system; the Purkinje fibres do not have any known role in setting heart rate unless the SA node is compromised. They are influenced by electrical discharge from the sinoatrial node. During the ventricular contraction portion of the cardiac cycle, the Purkinje fibres carry the contraction impulse from both the left and right bundle branch to the myocardium of the ventricles; this causes the muscle tissue of the ventricles to contract and generate force to eject blood out of the heart, either to the pulmonary circulation from the right ventricle or to the systemic circulation from the left ventricle. Purkinje fibres have the ability of firing at a rate of 15-40 beats per minute if upstream conduction or pacemaking ability is compromised. In contrast, the SA node in normal state can fire at 60-100 beats per minute. In short, they at a slower rate than sinoatrial node; this capability is suppressed. Thus, they serve as the last resort.
When a Purkinje fibre does fire, it is called a premature ventricular contraction or PVC, or in other situations can be a ventricular escape. It plays a vital role in the circulatory system, they are named after Jan Evangelista Purkyně who discovered them in 1839. Anatomy photo: Circulatory/heart/purkinje/purkinje1 - Comparative Organology at University of California, Davis - "Mammal heart, purkinje fibers" Anatomy Atlases - Microscopic Anatomy, plate 05.78 MedEd at Loyola Histo/practical/cardio/hp8-21.html Human Cardiac Muscle, histology slides at UC San Diego Cardiac Muscle Tissue with Purkinje Fibers, Lonestar College North Harris Biology
Connective tissue is one of the four basic types of animal tissue, along with epithelial tissue, muscle tissue, nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in the body, including the nervous system. In the central nervous system, the three outer membranes that envelop the brain and spinal cord are composed of connective tissue, they protect the body. All connective tissue consists of three main components: ground substance and cells. Not all authorities include blood or lymph as connective tissue because they lack the fiber component. All are immersed in the body water; the cells of connective tissue include fibroblasts, macrophages, mast cells and leucocytes. The term "connective tissue" was introduced in 1830 by Johannes Peter Müller; the tissue was recognized as a distinct class in the 18th century. Connective tissue can be broadly subdivided into connective tissue proper, special connective tissue. Connective tissue proper consists of loose connective tissue and dense connective tissue Loose and dense connective tissue are distinguished by the ratio of ground substance to fibrous tissue.
Loose connective tissue has much more ground substance and a relative lack of fibrous tissue, while the reverse is true of dense connective tissue. Dense regular connective tissue, found in structures such as tendons and ligaments, is characterized by collagen fibers arranged in an orderly parallel fashion, giving it tensile strength in one direction. Dense irregular connective tissue provides strength in multiple directions by its dense bundles of fibers arranged in all directions. Special connective tissue consists of reticular connective tissue, adipose tissue, cartilage and blood. Other kinds of connective tissues include fibrous and lymphoid connective tissues. Fibroareolar tissue is a mix of fibrous and areolar tissue. New vascularised connective tissue that forms in the process of wound healing is termed granulation tissue. Fibroblasts are the cells responsible for the production of some CT. Type I collagen is present in many forms of connective tissue, makes up about 25% of the total protein content of the mammalian body.
Characteristics of CT: Cells are spread through an extracellular fluid. Ground substance - A clear and viscous fluid containing glycosaminoglycans and proteoglycans to fix the body water and the collagen fibers in the intercellular spaces. Ground substance slows the spread of pathogens. Fibers. Not all types of CT are fibrous. Examples of non-fibrous CT include adipose blood. Adipose tissue gives "mechanical cushioning" to the body, among other functions. Although there is no dense collagen network in adipose tissue, groups of adipose cells are kept together by collagen fibers and collagen sheets in order to keep fat tissue under compression in place; the matrix of blood is plasma. Both the ground substance and proteins create the matrix for CT. Connective tissues are derived from the mesenchyme. Types of fibers: Connective tissue has a wide variety of functions that depend on the types of cells and the different classes of fibers involved. Loose and dense irregular connective tissue, formed by fibroblasts and collagen fibers, have an important role in providing a medium for oxygen and nutrients to diffuse from capillaries to cells, carbon dioxide and waste substances to diffuse from cells back into circulation.
They allow organs to resist stretching and tearing forces. Dense regular connective tissue, which forms organized structures, is a major functional component of tendons and aponeuroses, is found in specialized organs such as the cornea. Elastic fibers, made from elastin and fibrillin provide resistance to stretch forces, they are found in the walls of large blood vessels and in certain ligaments in the ligamenta flava. In hematopoietic and lymphatic tissues, reticular fibers made by reticular cells provide the stroma—or structural support—for the parenchyma—or functional part—of the organ. Mesenchyme is a type of connective tissue found in developing organs of embryos, capable of differentiation into all types of mature connective tissue. Another type of undifferentiated connective tissue is mucous connective tissue, found inside the umbilical cord. Various types of specialized tissues and cells are classified under the spectrum of connective tissue, are as diverse as brown and white adipose tissue, blood and bone.
Cells of the immune system, such as macrophages, mast cells, plasma cells and eosinophils are found scattered in loose connective tissue, providing the ground for starting inflammatory and immune responses upon the detection of antigens. There are many types of connective tissue disorders, such as: Connective tissue neoplasms including sarcomas such as hemangiopericytoma and malignant peripheral nerve sheath tumor in nervous tissue. Congenital diseases include Ehlers-Danlos Syndrome. Myxomatous degeneration – a pathological weakening of connective tissue. Mixed connective tissue disease – a disease of the autoimmune system undifferentiated connective tissue disease. Systemic lupus erythematosus – a major autoimmune disease of connective tissue Scurvy, caused by a deficiency of vitamin C, necessary for the synthesis of collagen. For microscopic viewing, most of the connective tissue staining-techniques, colour tissue fibers in contrasting shades. Collagen may be differentially stained by any of the following: Van Gieson's stain Masson's trichrome stain Mallory's t