Muscle Tissue

Muscle contraction

-nerve impulse transmitted down nerve cell axon, reaches pre-syn. membrane, Ca2+ enter pre-syn. terminal, synaptic vesicles attach inner surface of pre-syn. membrane and liberate Ach into synaptic cleft. -Ach binds receptors on post-syn. membr. (sarcolemma), Na+ enters, K+ leave causing depolarization, which spreads over entire sarcolemma and into T-tubules. -depolarization of T-tubules triggers change in permeability of sarcoplasmic reticulum, calsequestrin which stores Ca ions releases them into cytoplasm. -Ca binds TnC site of troponin, change in comformation and causes TnT to push tropomyosin deeper into actin groove, exposing actin active sites and allowing binding of actin to myosin. ATP input required for dissociation of myosin head and actin. -myosin head pulls actin filament over myosin filament, sliding actin toward midline of sarcomere. -sarcomere shortens, Z discs closer, I-band narrows, H-band shortens-almost disappears and A-band remains the same.

Dystrophin

Accessory protein of sarcomere. Cytoplasmic protein that binds actin (thin) filaments to laminin (a component of the external lamina surrounding the muscle fiber).

Rigor Mortis

After death, actin-myosin complexes cannot dissociate from one another since ATP is not available. Muscles stiffen.

Myasthenia gravis

Autoimmune disease causing muscle weakness. Individual produce antibodies that bind Ach receptor proteins on sarcolemma of skeletal muscle cells, forming antibody-receptor complexes. The complexes block Ach from binding sarcolemma and prevent muscle contraction. The complexes are continuously destroyed, decreasing number of receptor sites on sarcolemma and further weakening the muscle cells.

Dyad at Z disc of cardiac muscle

Cardiac muscle with one T-tubule per sarcomere. T-tubule + 1 SR forms dyad seen at Z-disc of these muscle cells. -T-tubules carry calcium from EC space into cardiac muscle cells. T-tubules of ventricular muscle are larger and more abundant than those of skeletal muscle.

Z-line (disk)

Component of sarcomere. Attachment for thin filaments. Contains alpha actinin, which anchors actin filaments to the Z-disk and bisects the I-band. -contains accessory proteins alpha actinin to which each thin filament is anchored to the disc as well as nebulin, which is a nonelastic protein, wrapped around each thin filament and also anchors thin filaments to the Z-disk (2 molecules) -contains accessory protein titin (4 molecules) anchor thick filaments to Z-disc -alpha-actinin, nebulin and titan create an even, precise and orderly arrangement of thin and thick filaments, exhibited as the characteristic light and dark banding of skeletal muscle.

A-band

Component of sarcomere. Consists of dark band of thin and thick filaments and forms the widest band of the sarcomere.

M-line

Component of sarcomere. Consists of protein structures lying between thick filaments, holding them in register. Creatine kinase is major protein here. Bisects the H-band. -accessory proteins myomesin and C-protein secure thick filaments in register at M-line

H-band

Component of sarcomere. Consists of thick filaments only and bisects to A-band.

I-band

Component of sarcomere. Light band made up of thin filaments only. Is made up of portions of adjacent sarcomeres

Fascia adherens

Component of transverse portion of cardiac intercalated discs. Functions to anchor actin filaments of terminal sarcomere to plasma membrane.

Epimysum

Connective tissue wrapping that supports and protects underlying structures. "Deep fascia" of muscle, dense irregular collagenous connective tissue. Acts as an envelope enclosing gross muscle (many fascicles). Sends septa within the muscle so that vessels and nerve fibers may reach deeper aspects of the muscle.

Perimysium

Connective tissue wrapping that supports and protects underlying structures. Surround fascicles (bundles) of muscle fibers/cells. Thin collagenous connective tissue derived from epimysium.

Endomysium

Connective tissue wrapping that supports and protects underlying structures. Surrounds each muscle fiber and consists of delicate layer of reticular fibers.

Thin filaments of muscle (thin myofilaments)

Consist of protein, primarily actin and include: -actin -troponin -tropomyosin (-actin + troponin complexes + tropomyosin alpha helixes assemble to form the composite "thin filaments" of muscle)

Sarcomere

Contractile unit of skeletal and cardiac muscle (Z to Z). Cylindrical structure and in relaxed state, exhibits: -A-band -H-band -I-band -Z-line/disk -M-line

Triad at A-I junction

Formed by 1 T-tubule and 2 terminal cisternae of SR will appear at A-I junction of sarcomere

Multiunit smooth muscle

Found in iris of the eye-sphincter and dilator pupillae muscles. Each muscle fibers has its own nerve supply, this can contract independently of each other (rich innervation)

Unitary (visceral) smooth muscle

Found in walls of intestine, ureters and uterus. Have abundant gap junctions. One nerve fibers innervates few smooth muscle cells (poor innervation)

Duchenne muscular dystrophy (DMD)

Genetic, degenerative disease, affecting primarily the voluntary skeletal muscle. Xp21 region on X chromosome mutated and results in inability to produce dystrophin protein. Dystrophin is an accessory protein of sarcomere. Cytoplasmic protein that binds actin (thin) filaments to laminin (a component of the external lamina surrounding the muscle fiber). Characterized by weakness and atrophy of skeletal muscle, early death.

General characteristics of cardiac muscle

Heart, striated, involuntary muscle. Contraction is rigorous and rhythmic.

Heart's intrinsic impulse-generating and conducting system

SA node, AV node, bundle of His and Purkinje fibers collectively form this system and relay electrical impulses to cardiac muscle fibers of the atria and ventricles, to coordinate the contraction of these chambers. -can be superimposed by ANS (vagus-parasympathetic, T1-4 spinal nerves-sympathetic)

Sarcolemma

Structure of muscle fiber/cell. Made up of cell membrane, external lamina and reticular lamina. -acetylcholine binds receptor on sarcolemma (post-synaptic membrane), Na+ ions enter, K+ ions exit and sarcolemma becomes depolarized, which spread over entire sarcolemma and into the T-tubules.

Myosin

Thick filament of muscle tissue. Consists of 4 light chains, 2 globular heads and a short twisted tail in alpha-helical configuration. ***

Arrangement of muscle fibers/cells

Thin and thick filaments (actin and myosin) form a sarcomere. Long cylindrical series of end-to-end sarcomeres form a myofibril and may parallel myofibrils are enclosed within a muscle fiber or cell

Troponin

Thin filament of muscle tissue. Complex of 3 subunits: 1. TnT subunit attaches to tropomyosin 2. TnC subunit binds Ca2+ 3. TnI subunit binds troponin complex to actin molecules, inhibiting actin-myosin interactions

Tropomyosin

Thin filament of muscle tissue. Consists of two polypeptide chains forming alpha helix. Each alpha helix rests on 7 G-actin molecules and covers their active sites (for myosin). Alpha helices line up head-to-tail to form long filaments.

Actin filament

Thin filament of muscle tissue. Monomers of globular action assemble to form polymers. 2 G-actin polymers become twisted to form a double-stranded helix (spiral), called filamentous action or F-actin.

Troponin test

Blood test helps diagnose heart attack, specific to cardiac muscle damage and remains elevated 1-2 weeks following damage.

Dense bodies

Membrane-associated attach to inner surface of the cell membrane while cytoplasmic dense bodies are suspended in the cytoplasm. Both made up of myofilament bundles, criss-crossing. -both contains alpha-actinin (similar to Z-discs) and serve as attachment for thin and intermediate filaments.

Innervation of skeletal muscle

Motor unit: lower motor neuron (a nerve cell) and all of the muscle fibers it innervates. Motor end plate (neuromuscular junction/myoneuronal junction): one more end plate per muscle fiber, consists of: -axon terminal (with presynaptic membrane) containing synaptic vesicles with acetylcholine -synaptic cleft (narrow space) -muscle cell sarcolemma (postsynaptic membrane)

General characteristics of smooth muscle

Non-striated (no sarcomeres), involuntary muscle, forms sheet. Pupillary constriction and dilation, accommodation of eye lens, piloerection, peristalsis, blood vessel constriction and dilation. Contraction is slow. -contain actin and myosin as well as tropomyosin but NO troponin. -Mononuclear cells with central nucleus -each fiber coated with external lamina and reticular fibers. -dense bodies formed via myofilament bundles -no T-tubules, caveolae release Ca2+ for cell contraction -unitary (visceral) and multiunit -contracts spontaneously. ANS and hormones modify rate of contraction but do not initiate it.

Caveolae

Pinocytotic vesicles of smooth muscle. Form bubble-like structures on inner surface of cell membrane and release Ca2+ into the cell for contraction

Satellite cells of skeletal muscle

Regenerative cells that lie between skeletal muscle cell and external lamina, will undergo proliferation following injury. New myoblasts derived from satellite cells will assemble to form myotubes which form muscle tissue. External lamina must be present and intact for this to occur. If not, fibroblasts will form collagen and scar tissue.

Purkinje fibers

Specialized (modified) cardiac muscle cells located in the endocardium. Large, pale staining cells, cytoplasm contains glycogen deposits and a few myofibrils. -act as "batteries" which relay electrical impulses to cardiac muscle fibers.

Intercalated discs

Specialized intercellular junctions (connections) of cardiac muscle cells. -transverse portion is perpendicular to long axis of muscle fiber, includes fascia adherens and maculae adherents (spot desmosomes) -transmits contractile forces from cell to cell

Maculae adherents

Spot desmosomes of transverse portion of cardiac intercalated discs. Functions to bind cells together.

General characteristics if skeletal muscle

Striated, striped, voluntary movement. Contraction is quick and forceful. -contain abundance of contractile (thin and thick) filament -multinucleated, nuclei are oval fusiform shaped and euchromatic -cells covered by external lamina and reticular fibers -myofibrils extend entire length of muscle fiber and appear as "dots" on c/s if muscle fiber -transverse T-tubules invagination of sarcolemma-continuous with extracellular space and function carry nerve impulses deep into cell. -sarcoplasmic reticulum forms "lacy" sleeve around each myofibril. -terminal cisternae of SR stores calcium in the lumen -satellite cells for regeneration -connective tissue appear between muscle fibers