Histology of Muscle Tissue

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Muscle Contractility:

This is a fundamental property of cells and is exhibited by all cells in varying degrees. In muscle cells, this ability is highly developed.

Functions of Muscles:

• Voluntary locomotion (skeletal).

• Involuntary GI motility (smooth).

• Vasodilation/vasoconstriction (smooth).

• Secretion assistance via myoepithelial cells (smooth).

• Heart function (cardiac).

Muscle Tissue Composition:

• Muscle cells.

• Connective tissue which surrounds and supports muscles.

• Provides oxygen and nutrients.

• Provides framework against which contraction can act.

3 types of muscle are distinguished by their morphology.

Definitions of Muscle Cell Components:

Sarcolemma = plasmalemma.

Sarcoplasm = cytoplasm.

Sarcoplasmic reticulum = SER.

Sarcosome = mitochondria.

Sarcomere = linear morphological and functional unit.

Myofiber = muscle cell.

Myofibrils = bundles of cytoplasmic myofilaments Myofilaments = large linear molecular aggregates of either myosin or actin.

An entire skeletal muscle is enveloped by:

• Epimysium surrounds entire skeletal muscle.

• Perimysium surrounds bundles of muscle fasicles composed of groups of muscle fibers.

• Endomysium surrounds individual muscle cells or fibers and contains vessels and nerves.

• Lamina externa also surrounds muscle cells.

Skeletal muscle cells are very long (up to 30 cm). Increase by addition of new microfibrils and growth in diameter of individual cells called hypertrophy. Tissue growth by addition of new cells is hyperplasia which occurs in smooth muscle.

Cohnheim's fields:

These are small polygonal groups of bundled myofibrils separated by myofibril-free sarcoplasm, visible with the LM in transversely sectioned skeletal and cardiac muscle. In longitudinal section, they correspond to Leydig-Kolliker columns.

Hyperplasia does not occur in skeletal or cardiac muscle, but when young kids lift weights intensely, splitting can occur in skeletal muscle. An enlarged heart may result from overload (hypertrophy) or functional enlargement. Fibers become thinner with disuse and the number of myofilaments decrease. The change is termed atrophy of disuse.

Satellite cells:

These are a small population of undifferentiated cells that are positioned in shallow depressions of skeletal muscle fibers and are enclosed within a common lamina externa. They become activated by muscle injury and multiply to provide nuclei to new muscle cells, provided CT has not filled the void.

Skeletal muscle fiber types:

Red, slow twitch:

High content of myoglobin (an iron-containing protein similar to hemoglobin that holds O2 inside the cell until it is needed by mitochondria). Smaller than white fibers. More extensive blood supply. Great number of mitochondria. Capable of continuous, vigorous activity, ex. erector spinae (long back muscles), soleus (for standing) and breast muscles of migrating birds. Stamina.

White, fast twitch:

Low myoglobin content. Larger diameter than red. Contracts rapidly but can't sustain continuous heavy work, ex. extraocular muscles of human, gastrocnemius for sprinting and breast muscles of chickens and turkeys. Intermediate: Intermediate between red and white. Most mammalian muscle is intermediate. Human skeletal muscle is often a mixture of all 3 types.

Muscle contraction:

Most fibers do not contract individually but in motor units. Groups of muscle fibers of the same type innervated by branches of the same axon. A single motor neuron can innervate 100-300 fibers in the small muscles of the hand. 600-1,700 in large muscles of the arm and leg.Muscle fibers in one motor unit are not grouped together but are scattered over a considerable area which may be shared by 20 or more other motor units.

Via LM: #1 I band is actin attached to z discs that delineate the sarcomere. Called I because it is light or isotropic in polarizing light. #2 A band is is dark or anisotropic in polarizing light and includes both actin and myosin. Each myosin is surrounded by 6 actin molecules. M line is formed by myomesin or M-protein binding adjacent myosin molecules in a hexagonal array.

Troponin-Tropomyosin System:

Thick myosin filaments "float" in the middle Thin filaments are made of F actin, tropomyosin and the troponin complex C, I, T. Tropomyosin lies in the groove between 2 actin filaments. The globular protein troponin attaches to tropomyosin at regular intervals and has 3 subunits: Troponin C binds to Ca++ ions Troponin I inhibits actin/myosin interaction Troponin T is attached to tropomyosin.

Steps in Muscle Contraction

• Ca+ binds troponin C which causes a conformational change in the protein.

• Tropomyosin is pulled away from the myosin binding site on actin.

• Myosin head binds ATP and attaches to binding site on actin.

• When myosin binds to the site, ATP is hydrolyzed.

• Hydrolysis causes myosin head to flex at junction between heavy and light chains.

• Flexion of many myosin heads ratchets the actin along the myosin.

• The sarcomere shortens, H and I bands disappear.

• ATP is again hydrolyzed and the myosin head is released.

• Cycle repeats 10-50 strokes/sec as long as ATP and Ca+ are available.

Axons terminate at myoneural junctions or motor end plates and release of synaptic vesicle contents causes sarcolemmal depolarization.

Acetylcholine is the neurotransmitter of skeletal muscle.

Sarcoplasmic reticulum (SR) and transverse tubules (TT) regulate Ca+ levels. At the A-I junction TT invaginate to bring in a wave of sarcolemmal depolarization. A network of SR extends between adjacent TT and the ends of each network expand to form terminal cisternae (TC).

Triad is one TT and two adjacent TC

Structure of Sarcoplasmic reticulum (SR) and transverse tubules (TT):

• TT and TC are joined by junctional feet, spanning proteins or junctional channel complexes. These are SR Ca++ channels and connect the 2 membranes.

• TT have voltage-sensing Ca+ channels.

• Ca+ is bound in the lumen of the SR by calsequestrin.

• Depolarization of the sarcolemma passes down the TT.

• Causes TT channel and SR channel to open and release Ca+ to the sarcoplasm where it participates in muscle contraction.

• Ca+ in the sarcoplasm is returned to the SR by a Ca+-ATPase pump in the SR membrane.

Accessory proteins maintain the architecture of the myofibril and provide elasticity. Titin- forms an elastic lattice that anchors thick filaments to z discs. It is the main structural basis for elasticity. It acts like springs to keep the myosin centered in the sarcomere. Alpha actinin bundles actin and anchors thin filaments to Z discs. Nebulin serves to act as a "molecular ruler" restricting the assembly of actin and therefore the total length of the actin filament. It also may assist alpha actinin to anchor thin filaments at the Z line.

Myomesin or M protein cross links adjacent thick filaments at the M line. Desmin is an IF protein also known as skeletin present at z discs. Cap Z is present at Z discs, caps the plus end of actin thin filaments and prevents actin from depolymerizing. Tropomodulin caps the minus end of actin thin filaments.

Heart Muscle Composition:

• Cardiac muscle cells - contractile.

• Purkinje cells - conductive (modified muscle cells not nerve cells).

• 1-2 pale staining nuclei intercalated discs.

• Mitochondria 40% cell volume (skeletal is 2%).

• Myofibrils more diffusely organized.

• Lipochrome pigment common.

• Cells branch.

Sarcotubules lack terminal cisternae but small expansions at the TT form diads TT occur at Z lines (not A-I junction as in skeletal).

What are Intercalated Discs?

• Intercalated discs are steplike junctions between cardiac cells that always occur at the Z line.

• Transverse components: desmosomes and fascia adherens, actin anchors to both.

• Longitudinal component: nexus and some desmosomes.

Actin is bound to intercalated discs by alpha actinin and vinculin and plakoglobulin between the membranes.

What is a Nexus?

Nexus provides ionic continuity so cells behave as a syncytium, allowing the signal to pass in a wave from cell to cell. Stimulation of any single cell is sufficient to excite the entire mass, ex. atria or ventricle. This is the all-or-nothing principle.

What hormones does the heart produce?

Myoendocrine cell granules occur in the atria (predominantly) and ventricles and contain a hormone precursor called atrial naturetic peptide (ANP) or factor, cardiodilatins (CDD), auriculin or atriopeptin. ANP causes vasodilation, lowering of blood pressure and drop in blood volume. It also acts on the kidneys to cause sodium and water loss. Choroid plexus and arachnoid villi also have receptors for ANP, so ANP may have a role in maintenance of fluid balance in the brain.

Smooth muscle from middle esophagus to internal anal sphincter ducts of GI glands walls of respiratory tract from trachea to alveolar ducts walls of blood vessels myometrium of uterus arrector pili muscles annular band of constrictor pupillae muscle of iris precapillary arterioles myoepithelium of glands smaller than skeletal.

Types of Smooth Muscle:

Visceral:

• In circular and longitudinal sheets around hollow organs of viscera, ex. intestine, uterus, ureter, bile ducts.

• Gap junctions couple cells in the sheet.

• Each cell does not have its own neuromuscular junction.

• When one cell is stimulated, action potential is conducted to surrounding fibers by direct electrical coupling.

• Sustained slow contraction, ex. peristalsis neural impulses regulate rather than initiate contraction.

Multiunit:

• Each fiber operates independently.

• Relative rapid contraction.

• Parallel bundles of cells found in places where lumenal diameter or sphincter changes are important, ex. iris, piloerector muscles of hair, large blood vessels.

• Require precise and highly controlled degree of contraction.

Smooth Muscle Innervation and Control:

Nerve fibers generally branch diffusely on a sheet of fibers and don't make contact, instead form diffuse junctions and secrete their neurotransmitter into the interstitial fluid. It then diffuses. ~50% of contractions are initiated by stimulatory factors such as: local conditions in interstitial fluid low O2 causes vasodilation excess CO2 causes vasodilation decreased body temp causes vasodilation hormones estrogen promotes contraction progesterone has the opposite effect.

Structure of Smooth Muscle:

• Have lamina externa (so do skeletal and cardiac) and network of reticular fibers and collagen.

• No striations.

• Single nucleus.

• Fusiform.

• Cells joined by gap junctions.

• Only 1/20 to 1/400 as much energy needed to sustain smooth muscle contraction as skeletal.

No TT or TC, instead there are a series of vesicles termed caveolae. Ca++ is regulated by these and they are analogous to the T-system.

Bundles of actin and myosin crisscross obliquely in the cell forming a lattice network.

Types and Function of Dense Bodies:

Two types of dense bodies occur: cytoplasmic dense bodies and plasmalemmal dense bodies. Are sites of attachment of actin, hence are analogous to Z line. Are interconnected to IF vimentin in vascular smooth muscle and to desmin in non-vascular smooth muscle.

Membrane excitation sweeps from cell to cell and causes release of Ca++ from sarcoplasmic reticulum in regions under caveolae. Ca++ associates with calmodulin to form a calcium-calmodulin complex. The complex allows myosin and actin to interact Interaction pulls on IF. This is transmitted to dense bodies

Can contract 1/4 to 1/2 its stretched length (skeletal 25-35%) Synthesizes: collagen elastin proteoglycans laminin lamina externa components.

Additional Reading:

Basic Histology

1. Introduction to Histology
2. Basic Cell Physiology
3. Actin, Microtubules, and Intermediate Filaments
4. Mitochondria, Nucleus, Endoplasmic Reticulum, Golgi
5. Epithelium (Epithelial Tissue)
6. Connective and Adipose Tissue
7. Types of Cartilage
8. Osteogenesis
9. Nervous Tissue
10. Muscle Tissue
11. Cardiovascular System
12. Blood and Hematopoiesis
13. Lymphoid Tissue
14. Digestive Tract I: Oral Cavity
15. Digestive Tract II: Esophagus through Intestines
16. Liver, Pancreas, and Gall Bladder
17. Respiratory System
18. Integument
19. Urinary System
20. Endocrine System
21. Male Reproductive System
22. Female Reproductive System
23. Eye and Ear

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