Muscle Anatomy PDF

# Muscle By Dr/ Dalia El Marakby Prof. of Histology Kasr Al Ainy ## Muscular Tissue ### General Characteristics of Muscular Tissue: * The structural & functional unit is a specialized elongated cell called **muscle fiber**, containing contractile filaments (thin actin &thick myosin). * Cell membr...

# Muscle By Dr/ Dalia El Marakby Prof. of Histology Kasr Al Ainy ## Muscular Tissue ### General Characteristics of Muscular Tissue: * The structural & functional unit is a specialized elongated cell called **muscle fiber**, containing contractile filaments (thin actin &thick myosin). * Cell membrane of muscle fibers is called **sarcolemma**, the cytoplasm is called **sarcoplasm** (Lat: sarcose = flesh) and sER is called **sarcoplasmic reticulum**. * The sarcoplasm is acidophilic, contains organelles (mitochondria, sER &myofibrils) &cell inclusions (glycogen, myoglobin pigments & fat). ### Origin of the Muscular Tissue: * It arises from **Mesoderm**. * UMC differentiates into myoblasts (embryonic progenitor cells) differentiate into muscle cells (myocytes). * The muscle cells (myocyte) can form all types of muscle tissue. ### Types of the Muscular Tissues: (classified according to the shape and function of their cells) into: * 1-Skeletal muscle * 2-cardiac muscle * 3-smooth muscle. ## Skeletal Muscle ### Sites: Attached to the skeleton, except: Face, tongue, pharynx & upper 1/3 of esophagus, diaphragm and cremasteric muscles. ### Organization: Skeletal muscle consists of muscle fibers held and supported by C.T. that is organized into: * **Epimysium**: * dense C.T * surround the whole muscle * penetrated by the major vessels and nerve supply of the muscle * **Perimysium**: * less dense C.T * surrounds each bundle (fascicle) * It descends from the epimysium * large blood vessels and nerves pass in it. * **Endomysium**: * a layer of reticular fibers * It surrounds each muscle fiber * Small blood vessels and fine nerves pass in it. ### C.T. is essential for: * force transmission * connects the muscle fibers together * Rich in nerves and B.V. that nourish the muscle fibers through diffusion. ## Skeletal Muscle Fiber ### Histological structure: Each skeletal muscle fiber is a log cell with the following features: * **Shape**: cylindrical, non-branched (except in face & tongue) * **Size**: 10-100 µm in diameter & markedly variable in length * **Sarcolemma**: thick due to its fusion with the surrounding basal lamina and endomysium * **Nuclei**: multiple, oval nuclei, just under the sarcolemma. Their number in each cell correspond to the number of fused of mononucleated myoblasts. * **Sarcoplasm**: by LM is an acidophilic with uniform transverse striations that appear in longitudinal section. ### Sarcoplasm contains the following: #### A. Organelles: * **Myofibrils**: each muscle cell (fiber) is filled with numerous cylindrical parallel longitudinal fibrils. Extend muscle fiber * **Sarcoplasmic reticulum**: * Well-developed smooth endoplasmic reticulum * Network around the myofibrils * For Ca++ storage & pump * **Mitochondria**: numerous, arranged in rows between myofibrils * **Other organelles**: are few, found mainly in the perinuclear cytoplasm #### B. Inclusions: * **Myoglobin**: 02 binding pigmented protein, provide 02 for oxidative reactions * **Glycogen granules**: provide energy. They are found between myofibrils. * **lipid** ### EM Picture of Myofibrils: * Each myofibril shows alternating dark & light bands. This explains the appearance of transverse striations (hence the name striated muscle) * The dark bands in adjacent myofibrils are arranged beside each other at the same level and the light bands are also arranged beside each other (causing transverse striation) ## Parts of a Skeletal Muscle Fiber: * Illustrates the different parts of a skeletal muscle fiber including the I-band, A-band, Z-disc, M-line, H-zone and the Sarcolemma. * Also shows the Sarcoplasmic reticulum, T-tubule and the Triad. ## Appearance of Skeletal Muscle Bands under the Polarized Microscope: * **The light band (I band)** * isotropic (so called I-bands) as it does not alter the plane of polarized light * is divided by dense membrane called Z-line * **The dark band (A band)** * anisotropic (so called A-bands) as it alters the polarized light in two planes * has a pale area in the middle-called H-zone, which is bisected by thin M-line. * **The higher magnification of the electron microscope**: can explain A & I bands, Z line H zone and M line. **The myofibrils are composed of myofilaments:** * **Thin (actin filaments)** * **Thick (myosin filaments)**. * **The thick myosin filaments**: are present in the **A-band**. * **The thin actin filaments**: are attached to the Z-line, pass through the I-band, and extend to the A-band, till the beginning of H zone. * **The I - band**: pale as it is formed of actin only. * **The Z-line**: condensed actin filaments with matrix material and other proteins (accessory proteins maintaining alignment of thin and thick filaments) as: * 1-Titin, a large protein, connects thick filaments to the Z line. * 2-Dystrophin, a large protein, is thought to link the external lamina of the muscle cell, to actin filaments. * **The A-band**: appears dark because it is formed of myosin & actin filaments. * **The H-zone**: pale as it is formed of myosin only. * **The M-line**: is produced by interconnections between adjacent myosin filaments. ## Sarcomere: * **The segment of a myofibril between 2 adjacent Z-lines.** * It is considered **the functional unit in contraction (the basic contractile unit)**. * It includes one dark band & 2 halves of light bands on both sides. * I-bands are very short in contracted muscle and longer in relaxed muscle, The lengths of A- bands remain constant during muscle contraction. ## The Triad Tubular System (T-system): * Formed of three tubules: * **one T-tubules (transverse tubules)** in the middle. The T-tubules are invaginations of the sarcolemma into the sarcoplasm to encircle the myofibrils. * T-tubule located at the level of the junction between the A & I band. The lumen of the T-tubule is continuous with the extracellular space. * **Two wide terminal cisternae of sarcoplasmic reticulum** from both sides. ## Function of the skeletal muscle and the role of the tubular system: * **Skeletal muscle**: * voluntary (controlled by will) * **pharynx, esophagus & cremasteric muscles**: * involuntary * **Arrival of nerve impulse to the neuromuscular junction**: depolarization of the sarcolemma →The depolarization is transmitted to the T-tubules → then to the depth of muscle fiber and stimulate the sarcoplasmic reticulum to release Ca (Ca pump)→ Ca facilitates the sliding of actin over myosin (till the middle of the A band). ### During contraction: * 1. H zone disappeared. * 2. I band decrease in size, Sarcomeres, myofibrils and muscle fibers are shortened. * 3. A band remains constant: length of actin and myosin filaments is not changed. * **When depolarization stop**: Ca is actively transported into the sarcoplasmic reticulum → ↓ Ca in cytoplasm → stop contraction (muscle relaxation). ## Types of Skeletal Muscle Fibers: * According to the required function, skeletal muscles are formed of three types of fibers but in different proportions. ### Differences between red and white skeletal muscle fibers: | Color | Type | Myoglobin | Vascularization | Size | Source of energy | Mitochondria | Glycogen | Contraction | Sites | |---------|---------|--------------|-------------------|---------|---------------------|-------------------|-----------|----------------|---------| | Red | Red (slow twitch) | Dark color - myoglobin, mitochondria & vascularity <br> Rich in myoglobin | Rich | Small | Aerobic respiration (many mitochondria & O2 binding myoglobin) so their contraction is prolonged with great resistance to fatigue | Larger & more numerous | Less | Slow, prolonged &no fatigue | 1- long back muscles in human to erect posture. <br> 2- Muscles of great endurance athletes (marathon runners). <br> 3- Chest muscle of flying birds.| | White | White (fast twitch) | Pale color - due to abundant glycogen & less myoglobin <br> Poor in myoglobin | Poor | Large | Depend on an anaerobic rapid & short generation of ATP by glycolysis so their contraction is rapid, short with easy fatigue | Less numerous | More | Fast, short & easy fatigue | 1- in muscles that move the eye and digit. <br> 2-in the muscles of short distance sprinter. | ### III Intermediate fibers: Their characters intermediate between red & white fibers. ### NB: The classification of fiber types in muscle biopsies has clinical significance for the diagnosis of certain diseases such as diseases due to mitochondrial disorders. ### Regeneration of Skeletal Muscles: * Regeneration occurs by satellite cells which are inactive reserve myoblasts present in adult muscle fibers between sarcolemma and basal lamina. * After injury→ satellite cells become activated, proliferate→ fuse to give new muscle fibers. ### Musculotendinous Junction: * The junction of the muscle with the tendon. ### NB: * **the muscle fibers stop (taper off)** * **-the C.T. components of the muscle continue with the tendon** * **Collagen fibers of the tendon insert among the muscle fibers.** * **Skeletal muscle hypertrophy**: takes place by the enlargement of the muscle fibers by exercise. * **Muscular dystrophy**: is a progressive degeneration of skeletal muscle fibers, in which lack of dystrophin in satellite cells→ fails to replace degenerated fibers resulting in decrease in muscle function. * **Muscle Cramps**: are sudden painful contractions of muscles due to lowered blood flow to them, lowered levels of potassium, r vigorous exercise without proper warming up (stretching). They usually involve the muscles of the lower leg. * **Myasthenia Gravis**: is an autoimmune that is characterized by weakening of skeletal muscles due to formation of antibodies against the skeletal muscle's acetylcholine receptors. The gradual weakening affects the most active muscles first (muscles of the face, eyes and tongue), muscles of respiration become compromised and the individual dies of respiratory insufficiency. ## Cardiac Muscle ### Site: *form the middle layer of the heart (Myocardium) *Thicker in the ventricles than in the atria. *Myocardium is surrounded by: *Epicardium (visceral layer of the pericardium) from outside. *Endocardium from inside. *The cardiac muscle is attached to the fibrous skeleton of the heart which is dense fibrous C.T. at the junction between atria and ventricles. ### Organization: *cardiac muscle is formed of branched and interconnected muscle fibers. *each fiber is surrounded by delicate C.T. rich in capillaries (endomysium). ### Histological structure: #### Cardiac muscle fiber *Each cardiac muscle fiber is made up of many separate cells (cardiac myocytes) linked to each other. *cardiac muscle cell is characterized by: * **Shape**: Cylindrical, may branch & join adjacent cells. * **Size**: diameter (15 µm) & length (80 µm). * **Sarcolemma**: thin cell membrane. * **Nucleus**: single (may be two), oval & central in each cardiac myocyte. * **Sarcoplasm**: by LM is acidophilic, showed non clear transverse striations and intercalated discs. ### Sarcoplasm contains the followings: #### A. Organelles: * **Myofibrils**: less than in skeletal muscles. * **Mitochondria**: numerous, large with closely packed cristae. * **Sarcoplasmic reticulum**: less in number, development and organization than skeletal muscle. Accompanies the T- tubule at the level of the Z-line forming diad tubular system (not triad). * **Other organelles**: few. #### B. Inclusions: * **glycogen granules** * **Lipofuscin granules**: which increase with age, forming brown atrophy of the heart. * **Atrial granules**: Present in atrial muscle cells & contain natriuretic hormone (endocrine function) which affect urinary excretion of sodium and water. ### The Intercalated Disc: * It is sites of junctions between sarcolemma of adjacent cardiac myocytes in cardiac muscle fiber. * **LM**: clear lines that cross the cardiac muscle fibres transversely. * **EM**: It is formed of transverse & lateral regions, give the discs stairway shape. #### Transverse components: * cross the cardiac muscle fibers transversely * contain: * Desmosomes & Adherent junctions (fascia adherens): bind cardiac cells together & prevent their separation during contraction. #### Lateral components: * run parallel to the muscle fibers * contain: * Gap junctions: allow contraction signals to pass from cell to another their position in the lateral part of the disc protects them from contraction forces. ## The Valves of the Heart: *(folds of the endocardium)* * It is covered with simple squamous epithelium and has middle supporting layer of dense fibrous C.T., rich in collagen, elastic fibers & macrophages. ## Function of Cardiac Muscle: * Cardiac muscle fibers involuntary rhythmic contraction. * Contraction modulated but not initiated by autonomic innervations and hormones. ## Purkinje Muscle Fibers: * Purkinje muscle fibers are highly specialized cardiac muscle fibers * present in Moderator band (located in the right ventricle) & atrioventricular (A -V) bundle & its branches. * They transmit impulses to ventricular muscles (via gap junction) faster than ordinary myocardium. * They are grouped into bundles which are surrounded with C.T. sheath. ## Histological characteristics of Purkinje fibers: * They are larger in diameter than ordinary cardiac muscle fibers. * Sarcoplasm: pale, vacuolated (due excess glycogen). * Nucleus: usually eccentric. * No-striation due to few myofibrils which peripherally situated parallel to the sarcolemma. * No intercalated discs. ## Repair of injured Cardiac Muscle cells: * Cannot regenerate as it does not contain progenitor cells. * If the cardiac myocytes are injured, as in cases of myocardial infarction, healing occurs by fibrous tissue. ### NB. * **Angina**: slowly progressive reduction of coronary artery lumen by atheroma impairs oxygenation of ventricular muscle leading to chest pain increases by exertion. * **Myocardial infarction, (complete obstruction of one of the coronary arteries leads to cardiac muscle fibers death), heart muscle cells release proteins such as troponin T and creatine kinase into the bloodstream. So it is advisable to measure these proteins (called cardiac enzymes) immediately in all patients who present with chest pain for early diagnoses and rapid treatment.** ## Smooth Muscle ### Site: *Wall of hollow viscera e.g. digestive, urinary, genital & respiratory tracts. *Wall of blood vessels. * - in skin & eye ### Organization: * Smooth muscle fibers are connected together by gap junction. * Each fiber is surrounded by thin reticular fibers (endomysium). * Fibers are packed to form sheets, layers or bundles. * Each bundle is surrounded by thin collagenous C.T. perimysium, containing blood vessels and nerves. ### NB. *Smooth muscle cells may be affected by hormones such as oxytocin which is potent stimulator of smooth muscle contraction. It plays an essential role in uterine contraction during parturition, so used in induce or enhance labor. ### Histological structure: #### Smooth muscle fiber * **Size**: small diameter (8 µm). Length: varies from (20 µm) in blood vessels to (500 µm) in pregnant uterus. * **Shape**: Non-branched spindle shaped. Each fiber is a cell with wider central area & tapering ends. Cells are arranged so that their tapered parts lie adjacent to the broad parts of the neighboring cells, so they form sheaths. Therefore, in transverse section, smooth muscles appeared with variable diameter. * **Sarcolemma**: thin surrounded by basal lamina, no T-tubules instead, there are invaginations along the cell surface called Caveolae can control Ca release & muscle contraction. ***Nucleus**: -single, central & oval. - corkscrew appearance during contraction. * **Sarcoplasm**: acidophilic, contains numerous mitochondria, sarcoplasmic reticulum, small Golgi, free ribosomes & glycogen granules. They are present mainly at the perinuclear region. * **Myofibrils**: - thick myosin and thin actin filaments irregularly arranged. Therefore, striations do not appear. * Actin filaments insert into a dense body (on the inner aspect of sarcolemma) → interact with myosin filaments → interact with another actin filaments (inserted in the intracytoplasmic dense bodies). Dense bodies correspond to Z line in striated muscle). * Abundant intermediate desmin filaments also inserted into the dense bodies. * **The attachment of actin and intermediate filaments to dense bodies transmit contractile force to adjacent smooth muscle cells.** ### Functions of smooth muscles: * it produces slow, sustained involuntary contraction that is modulated by autonomic innervations and hormones. * It can secrete C.T. matrix & fibers as in the wall of blood vessels. ### Regeneration & growth of smooth muscle: * smooth muscles can * grow in size (hypertrophy) * Increase number (hyperplasia), mitotic division.. * e.g. in arteries in hypertension, in pregnant uterus. * Regeneration of smooth muscles * Pericytes can repair smooth muscles of blood vessels ### Relaxed smooth muscle cell: * Shows the structure of a relaxed smooth muscle cell. * Highlights key components such as the dense body and nucleus. ### Contracted smooth muscle cell: * Shows the structure of a contracted smooth muscle cell. * Highlights key components such as the dense body and nucleus. ## Differences between the three types of muscle | Site | Skeletal muscle | Cardiac muscle | Smooth muscle | |---|---|---|---| | | Attached to bone Face, tongue, pharynx & upper 1/3 of esophagus, diaphragm and cremasteric muscles. | The heart wall (myocardium) | Wall of the viscera digestive, urinary, genital and respiratory tracts. -Wall of blood vessels. - in skin and eye | | | | | | | Size | Largest | Medium sized | Smallest | | Single fiber | Single cell | Several cells | Single cell | | Shape | Cylindrical | Cylindrical | Spindle-shaped | | Branching | Rarely (branched in face & tongue) | Branched | Non-branched | | Sarcolemma | Thick | Very thin | Thin | | Striation | Striated | Non-clear striation | Non-striated | | Nuclei | Multiple & peripheral | One & central in each cardiac myocyte | One & central | | Sarcomere | regular | irregular | absent | | Tubular system | Triad system | Diad system | Absent | | Cell junctions | Non | Intercalated disc: fascia adherens, desmosome & gap junction | Gap junction | | Regeneration | Satellite cells | Cannot regenerate in normal conditions | Mitosis or from pericytes | | Action | Voluntary | Involuntary | Involuntary | | Innervations | Motor | Autonomic | Autonomic | | Modification | Muscle spindle | Purkinje fibers | -------- |