Muscle Tissue classification, features, structure, function

Order of the zones and lines

• A Bands H-Zone M Line AHM?!

• Type IIb - Fast, White glycolytic fibers - Sprint

. Fewer mitochondria, less myoglobin, but abundant glycogen, make them pale in color. They depend largely on glycolysis for energy and are adapted for rapid contractions, but fatigue quickly.

Regeneration of muscle tissue:skeletal muscle

In skeletal muscle, although the nuclei are incapable of undergoing mitosis, the tissue can undergo limited regeneration by mesenchymal satellite cells that lie within the external lamina of mature muscle fiber. They are inactive unless an injury or certain other stimuli activate them to proliferate and form new skeletal muscle fibers. A similar activity of satellite cells has been implicated in muscle growth after extensive exercise.

Actin

composed of F-actin associated with tropomyosin and troponin. F-actin consists of long filamentous polymers, containing two strands of globular monomers - G-actin. Each G-actin monomer contains a binding site for myosin. Each Actin filaments anchored to the Z-line by actin binding protein - α-actinin

Myofibrils

composed of repetitive arrangement of sarcomeres

Muscle fiber (=muscle cell)

found in Fascicle, consists of many myofibrils, surrounded by sarcolemma and found in sarcoplasm environment including the nuclei. Blood vessels penetrate the muscle within the connective tissue septa, and form a rich capillary network in the endomysium. Connective tissue of the epimysium shows continuity with tendons, through myotendinous junctions. The collagen fibers of tendons are continuous with those in the connective tissue layers in the muscle, forming a strong unit that allows muscle contraction to move the skeleton.

• Type IIa - Fast, intermediate fibers

have many mitochondria and much myoglobin, but also have considerable glycogen. They intermediate between the two others muscle fibers, and adapted for rapid contractions as well to short bursts of activity

Hypertrophy

increased of cell volume which occur either by growth in the diameter of individual muscle fibers, or by increase in number of cells which called hyperplasia (most readily in smooth muscle, whose cells have not lost the capacity to divide by mitosis)

Myasthenia Gravis

is an autoimmune disorder, characterized by progressive muscular weakness. It is caused by a reduction in the number of functionally active acetylcholine receptors in the sarcolemma, affected by circulating antibodies that bind to the acetylcholine receptors and inhibit normal nerve muscle communication.

Z-Line

is the border between two repetitive functional-subunits of the contractile apparatus, called the Sarcomere. Sarcomere length is about 2.5µm.

A myocyte, Myoblast, Myogensis

A myocyte (also known as a muscle cell) is the type of cell found in muscle tissue. Myocytes are long, tubular cells that develop from myoblasts to form muscles in a process known as myogenesis.

Innervation of Skeletal Muscle:

A neuromuscular junction (sometimes called a myoneural junction or motor end plate) is a junction between nerve and muscle; it is a chemical synapse formed by the contact between the presynaptic terminal of a motor neuron and the postsynaptic membrane of a muscle fiber. At the neuromuscular junction a motor neuron is able to transmit a signal to the muscle fiber, causing muscle contraction.

Regeneration of muscle tissue Cardiac muscle

Cardiac muscle lacks satellite cells and has virtually no regenerative capacity beyond early childhood. Defects or damage in heart muscle are generally replaced by fibroblast proliferation, and growth of connective tissue, forming myocardial scars

Cardiac Muscle

Also has cross-striations and is composed of elongated, branched individual cells that lie parallel to each other. At sites of end to end contact are the intercalated disks, which found only in cardiac muscles. Contraction of cardiac muscles is involuntary and rhythmic.

Skeletal Muscle

Bundles of very long, cylindrical, multinucleated cells that show cross-striations. Nuclei found in the periphery of the cells. Their contraction is quick, forceful, and usually voluntary, which caused by the interaction of thin actin and thick myosin filaments whose molecular configuration allows them to slide upon one another.

Process (Part 1)

Calcium ions bind to sensor proteins on synaptic vesicles, triggering vesicle fusion with the cell membrane and subsequent neurotransmitter (ACh - Acetyl Choline) release from the motor neuron into the synaptic cleft ACh diffuses across the synaptic cleft and binds to receptors found on the sarcolemma. Sarcoplasmic reticulum release Calcium when there is enough Calcium in the muscle cell ("Calcium-induce-Calcium" release). Each muscle cell attaches to adjacent muscle cell by gap junctions, so these muscle cells practically aren't disconnected- they form a "syncytium" - which means that there is no separation between the cells, and when one cell contracts, the other one contract at the same time.

Skeletal Muscle (Intro)

Consists of muscle fibers which are long, cylindrical multinucleated cells with diameters of 10-100µm. The multinucleation results from the fusion of embryonic mesenchymal cells called myoblasts. Oval nuclei found in periphery of the cell, under the sarcolemma

During contraction, neither the thick nor thin filaments change their length

Contraction is the result of an increase in the amount of overlap between those two. The contraction is induced by an action potential produced at a synapse, the neuromuscular junction. The attachment allows the contraction of the muscle, which releases only after the myosin binds a new ATP molecule. If no ATP is available, the actin-myosin complex becomes stable, which accounts for the extreme muscular rigidity, Rigor Mortis, that occurs after death.

Myosin

Dissociated into two identical heavy chains and two pairs of light chains. At the end of each heavy chain there are two heads, which have ATP binding sites and the ability to bind actin.

The bands durning contraction?

During contraction, the I-Band (lighter area with only actin presence) decreases in size as thin filaments penetrate the A-Band. The H-band (part of the A-Band with only myosin presence) diminishes in width, as the thin filaments completely overlap the myosin. The net result is the shortening of the sarcomere, and consequently the whole muscle fiber (cell).

Cardiac Muscle:(1)

During embryonic development, the mesoderm cells of the primitive heart tube align into chainlike arrays. Rather than fusing into multinucleated cells like in skeletal muscle fibers, cardiac muscle cells form complex junctions between extended processes. Cells within a fiber often branch and bind to cells in adjacent fibers. Consequently, the heart consists of tightly knit bundles of cells.

Skeletal Muscle Organization Endomysium

Each muscle fiber separately, that found in a fascicle, surrounded by even thinner connective tissue called the Endomysium, composed of a basal lamina synthesized by the multinucleated fibers themselves, as well as reticular fibers and fibroblasts. The endomysium main role is to transmit the mechanical forces generated by the contraction of muscle cells\fibers.

Skeletal Muscle Organization•Fascicle

In the inner portion of the Epimysium, found many bundles of muscle fibers, each one called Fascicle. The connective tissue surrounds each fascicle with its bundles of muscle fibers called Perimysium.

Regarding innervation of Smooth muscles

Regarding innervation of Smooth muscles, the cells occur either as multi-unit smooth muscle, in which each cell is innervated and can contract independently. More commonly are unitary (מאוחד) smooth muscles, in which only a few cells are innervated but all cells are interconnected by gap junctions which allow the stimulus for contraction to spread as a synchronized wave among adjacent cells. Because Smooth muscle is usually spontaneously active without nervous stimuli, its nerve supply serves primarily to modify activity rather than initiate it.

• Type I - Slow, Red Fiber - Marathon

Many mitochondria and abundant myoglobin, a protein with iron groups that bind oxygen and produce a dark red color. They adapted for continuous contractions over prolonged periods

Cardiac Muscle:(2)

Mature cardiac muscle cells are approximately 15µm in diameter and from 85-100 µm in length. They exhibit a cross striated banding pattern, similar to skeletal muscle. Unlike multinucleated skeletal muscle, each cardiac muscle cell possesses only one or two centrally located pale staining nuclei, surrounded by rich capillary network. A unique distinguishing characteristic of cardiac muscle is the presence of dark staining transverse lines that cross the chains of cardiac cells at irregular intervals. These intercalated discs represent the interface between adjacent muscle cells where many junctional complexes are present: desmosomes, fascia adherents - together these serve to bind cardiac cells firmly together. The more longitudinal portions of each disc have multiple gap junctions, which provide ionic continuity between cells for uniform contraction.

Intro: Muscle Tissue Topic

Muscle tissue is composed of cells differentiated for optimal use of the universal cell property termed contractility. Microfilaments and associated proteins generate the force which drives movement within certain organs and the body. Nearly all muscle cells are of mesodermal origin, and they differentiate mainly by a gradual process of cell lengthening with simultaneous synthesis of myofibrillar proteins

Muscle Fibers:

Skeletal muscle fibers of humans are classified into three types based on their physiological, biochemical and histochemical characteristics. All of them are normally found throughout most muscles. The classification has clinical significance for the diagnosis of muscle diseases.

Smooth Muscles (2-Size, appratus, organelles etc)

Smooth muscle cells may range in length from 20 µm in small blood vessels to 500 µm in the pregnant uterus. Diameter 5-15µm. Each cell has a single nucleus located in the center of the cell's broadest part. Concentrated near the nucleus are mitochondria, polyribosomes, cisternae of RER, and the Golgi.

Smooth Muscles (1-Intro)

Smooth muscle fibers are elongated, tapering, and non-striated cells, each of which is enclosed by a thin basal lamina and a network of reticular fibers. The connective tissues serve to combine the forces generated by each smooth muscle fiber, into a concerted action. E.g. peristalsis movements in the intestine.

muscle cell organelles

Some of the muscle cell organelles have different names than usual; The cytoplasm of muscle cells is called sarcoplasm, the SER is called sarcoplasmic reticulum and the membrane of muscle cell is called sarcolemma or plasmalemma.

Process: (Intro)

Synaptic transmission at the neuromuscular junction begins when an action potential reaches the presynaptic terminal of a motor neuron, which activates voltage-dependent calcium channels to allow calcium ions to enter the neuron

Cardiac Muscle:(3)The Transverse Tubules

The Transverse Tubules of cardiac muscle are more numerous and large than in skeletal muscle, the sarcoplasmic reticulum is less developed. It contains numerous mitochondria, occupy 40% of the cytoplasmic volume (only 2% in skeletal muscle), reflecting the need for continuous aerobic metabolism in heart muscle. Fatty acids are the main fuel for the cardiac muscles, and storage in lipid droplets which seen in many of the cells.

Smooth muscle ( The characteristic contractile)

The characteristic contractile activity of smooth muscle is related to the structure and organization of its actin and myosin filaments, which do not exhibit the organization present in striated muscles; Rather, bundles of actin and myosin crisscross (שתי וערב) obliquely through the cell. The contraction of smooth muscle is similar in mechanism to that of the striated muscles. Contraction of smooth muscle is not under voluntary control, but is regulated by autonomic nerves, certain hormones, and local physiological conditions.

A-Bands

The darker bands The A and I banding pattern in sarcomeres is due to the regular arrangement of two types of myofilaments - thin Actin myofilament, and thick Myosin myofilament.

A-Bands - Actin Myosin POV

The darker central area of the sarcomere (A-band) filled mainly by thick Myosin myofilament, which is 1.6µm long and 15 nm wide The thin Actin myofilaments, 1µm long and 8nm wide, run between and parallel to the thick filaments, and have one end attached to the Z-Line (by α-actinin).

I-Bands

The lighter bands. Each I-band is seen to be bisected by a dark transverse line, the Z-Line. The A and I banding pattern in sarcomeres is due to the regular arrangement of two types of myofilaments - thin Actin myofilament, and thick Myosin myofilament. The I-band, lighter lateral area of the sarcomere, consists of the thin filaments that don't overlap to the Myosin.

Skeletal Muscle Organization:•Epimysium

The masses of fibers that make up the various types of muscle are arranged in regular bundles surrounded by the Epimysium, an external sheath of dense CT surrounding the entire muscle.

Sarcoplasmic Reticulum & Transverse Tubule System (Process part 2)

The smooth endoplasmic reticulum (sarcoplasmic reticulum in muscles) is specialized for Calcium ion storage and release. The release of calcium from the SER is initiated at the specialized myoneural junction on the surface of the muscle cell To provide a uniform contraction, a uniform release of Calcium from the SER is needed, thus the skeletal muscle fibers have a system of transverse tubules which encircle every myofibril. At the end of each tubule, terminal cisternae of the sarcoplasmic reticulum presence, and depolarization that leads to release of Calcium is transmitted to the sarcoplasmic reticulum membrane

Troponin

Troponin is a complex of three subunits: TnT - attached to tropomyosin, TnC - binds calcium ions, TnI - inhibits the actin-myosin interaction.

Muscle Fibers

Under LM or TEM, longitudinally sectioned skeletal muscle fibers show cross-striations of alternating light and dark bands.

• H-zone

a bit lighter zone - a region consisting only of Myosin with no Actin present,, Found in the middle of the A Band.

Smooth muscle

composed of simpler mononucleated cells, is capable of more active regenerative response. The smooth muscle cells may undergo mitosis and replace the damaged tissue. Contractile Pericytes (contractile cells that wrap around the endothelial cells of capillaries and venules throughout the body) participate in the repair of vascular smooth muscles.

• M-Line

connection point between two Myosin myofilaments in the middle of the H-zone, made by protein myomesin- binding protein

Smooth Muscle

consists of collection of fusiform (spindle shaped) cells that do not show striations. Their contraction process is slow, and involuntary

Muscle contraction depends on (Process part 3)

the availability of Calcium ions and muscle relaxation is related to an absence of Calcium. The sarcoplasmic reticulum specifically regulates calcium flow. After Calcium ion released by the SER, it binds to troponin and allow bridging between actin and myosin. The binding of Calcium to troponin makes it changes its shape, lead to moving of Tropomyosin, which expose the Myosin binding site in Actin filaments.