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Flashcards in Muscle Tissue Deck (116):

What causes muscle contractility?



What does conductivity allow muscle cells to do?

-allows muscle cells to transmit electrical impulse to other cells and to receive impulses from nerve cells.


Skeletal muscle location

-attached to the skeleton and are also present in some visceral organs.


Skeletal muscle innervation?



What causes skeletal muscle striation?

-due to arrangement of filaments in the cytoplasm


Skeletal muscle that is attached to bone function

primarily for locomotion, respiration, and other functions


Visceral skeletal muscle location and function

-tongue, pharynx, and the proximal 1/3 of esophagus.
-aid with swallowing and speech


Smooth muscle fiber location? Striated? Innervation?

-found in the walls of the internal organs (GI tract and middle walls of blood vessels)
-do not show striations in the cytoplasm


Most smooth muscle is slow, what smooth muscle is fast?

-ciliary muscle of the eye is fast

-erector pili of the skin is also smooth muscle


Cardiac muscle fiber location? Innervation?

-found in the heart

FYI: Evolved from smooth muscle but structurally like skeletal muscle


Specialized plasmalemma (plasma membrane) term?



Cytoplasm of muscle cells term?



Specialized smooth endoplasmic reticulum term?

-sarcoplasmic reticulum


Characteristics of skeletal muscle fibers?

-numerous cross striations
-are quick, but usually get tired fast: use up energy quick


Skeletal muscle origin?

-originate from myoblasts (myocytes line up) that fuse with each other to form long multinucleated postmitotic myotubes


How is a skeletal muscle fiber (skeletal muscle cell) formed?

-"cells" of the skeletal muscle is actually a multinucleated syncytium.

Syncytium is cytoplasm that is shared

-takes a long time and a lot of effort to repair


Sarcolemma of skeletal muscle fibers forms long process called ______? What is _____ important for?

T-tubules. (Only found in skeletal muscle)

T-tubules that extend into the cytoplasm are important for carrying the wave of depolarization deep into the sarcoplasm.

FYI: action potential in a neuron is along the surface. In muscle cells the action potential runs inside the cell through T-tubules


Nuclei of skeletal muscle cells?

-are peripheral in location and are found immediately beneath the sarcolemma.

-are peripheral because they are pushed near sarcolemma by myofibrils. This is unique to skeletal muscle


What 3 things does sarcoplasm of skeletal muscle cells contain?

-numerous myofibrils (found inside of fibers)
-numerous filamentous mitochondria


Arrangement of contractile filaments within the myofibrils?

-arrangement is regular and gives the myofibrils "striated" appearance.

-all the myofibrils are in register within the skeletal muscle fiber, so the whole fiber exhibits characteristic transverse striations.


Filamentous mitochondria location and function?

-lie between myofibrils and close to the sarcolemma.
-represent the source of ATP for the myofibrils



-an oxygen-binding protein

-gives muscle it's red color. May store glycogen. DM pt's can lower sugar by working out


Sarcoplasmic reticulum

-highly speacialized form of smooth endoplasmic reticulum and is used as a depot of Ca++
-when excited, the sarcoplasmic reticulum releases Ca++ into cytoplasm initiating the muscular contraction


Organization of skeletal muscles

-consist of fascicles or bundles of skeletal muscle fibers surrounded by a connective tissue sheath

Ex. Biceps->fascicles->skeletal muscle fiber (cell)->myofibril->myofilaments (thin/thick)



-formed by connective tissue sheath


Skeletal muscle fiber

-multinucleated cell or syncytium



-make up skeletal muscle fiber.
-extend the whole length of the muscle fiber.
-individual myofibril is striated


2 principle components of myofibril

-thick and thin filaments


Gower's sign

Trendelenburg (waddling) gait

-use forelimbs to push off hindlimbs to erect pelvis and upper body.
-variation of muscle fibers
-CT taking up place of fibers (endomysial)
-nuclei in abundance means inflammation
-numerous macrophages

Signs of Duchenne muscular dystrophy


Microstructues of a muscular fiber

-composed of hundreds of myofibrils that span the entire length of the muscle cell


Arrangement of microfibrils

-composed of thin and thick filaments
-thin (actin) filaments are arranged in a hexagonal array with 6 thin filaments surrounding 1 thick (myosin) filament


Proteins that make up thin filaments

-Troponin complex (Troponin T, I, and C)



-double-stranded helical filament (composed of G-actin)



-forms filaments that lie in the grooves between the two actin monomers.
-in the resting muscle tropomyosin masks the myosin-binding sites on the actin filament (sits over the actin binding site)


Troponin complex, what does it attach to?
What are the three globular subunits?

--attached to tropomysin
-Troponin T, Troponin I, Troponin C

FYI: tested for for possible MI


Troponin T binds to?

-binds to tropomyosin, anchoring the Troponin complex


Troponin I binds to?

-binds to actin inhibiting the interaction with myosin


Troponin C binds to?

-binds to Ca++, which is an essential step in the initiation of the muscular contraction.

-smallest subunit


Thick filament are formed by?

-formed by hundreds of myosin molecules.
-Myosin II molecules is the muscle myosin


What does Myosin II consist of?

-2 heavy chains
-2 light chains


Two heavy chains of Myosin II characteristics?

-form coiled alpha-helices and two globular heads that exhibit ATPase and motor activity.
-globular heads contain binding sites for actin and ATP
-is a dimer


Two pairs of Myosin II light chains?

-attach to the heads.
-they are essential and regulatory (structural support)


Microstructure of a myofibril

-myofibril exhibits striations formed by alternating dark (A-bands) and light (I-bands) regions

A=anisotropic (same value when measured in different directions)
I=isotropic (different value when measured in different directions)



-Contains both thick and thin filaments.
-central part is called the H-zone (only thick filaments)
-overlap b/t thick and thin filaments cause darker part of A-band



-pale area in the middle of the A-band that contains thick filaments
-has no thin filaments



-line formed by accessory proteins, such as myomesin that hold the thick filaments in register



-pale area formed primarily by thin filaments
-proteins such as Titian and nebulin are also found in the I-band
-is bisected by the Z-line (Z-disk)


Z-line (Z-disk)

-composed of accessory proteins and its major function is to provide anchoring points for thin filaments and to support the architecture of the myofibril
-bisects I-band


Sarcomere characteristics

-portion of a myofibril between two adjacent Z-disks
-basic contractile unit of skeletal muscle
-measures 2-3 microns
-sarcomere of individual myofibrils are in register in one muscle fiber, so the entire muscle cell exhibits cross-striations.


Skeletal muscle contraction
-What happens with the I and A band? H-zone?

-due to the sliding of filaments which results in the shortening of the sarcomere, while the length of individual filaments does not change
-length of A-band does not change during contraction
-I-band and H-zone shrink during contraction due to the increase of overlap between thick and thin filaments


Where are accessory proteins found?

-primarily in the Z-disk and in the M-line


What do accessory proteins do?

-provide attachment of filaments to the Z-disk and attachment of Z-disks to each other and to the sarcolemma.


6 types of accessory proteins found in the skeletal muscle

-Titin (thick)
-Myomesin (thick)



-largest protein that anchors the thick filaments to the Z-disk
-extends to the M-line
-function is to keep the thick filaments in their position in the central part of the sarcomere



-holds the thick filaments in register at the M-line

Note: M-line runs through the middle of the H-zone



-anchors thin filaments into the Z-disk



-attached to the Z-disk and runs parallel to thin filaments.
-anchor the thin filaments and to regulate the length of thin filaments during muscular fiber development



-an intermediate filament that forms a lattice that surrounds Z-disks and attaches them to one another and also attaches Z-disks to the plasma membrane



-membrane-associated protein complex that links actin cytoskeleton to the extracellular matrix stabilizing the thin filaments


Electric capacitor of Sarcolemma

-voltage inside cell is negative
-outside cell is 0
-is a negative membrane potential in a resting cell, achieved by actively pumping Na+ ions out of the cell


Depolarized membrane of sarcolemma

-develops a positive membrane potential when it becomes more permeable for Na+ ions
-depolarization of membrane of muscle cell starts a cascade of reactions that cause muscular contraction


Action potential of muscle cell

-brief positive going changes in the membrane potential that are propagated along the length of the membrane at a speed up to 120 m/sec


Membrane triads

-formed by sarcolemma and the sarcoplasmic reticulum.
-on triad consists of one T-tubule and two cisternae of sarcoplasmic reticulum running parallel to it


T-tubular system

-formed by deep invaginations of the sarcolemma
-allow the impulse to travel down into the cell and excite the terminal cisternae of the sarcoplasmic reticulum


Terminal cisternae (formed by long chambers of sarcoplasmic reticulum)

-run parallel to the T-tubules on both sides
-contain high concentration of Ca++ ions
-running near the boundary of A-band and I-band


Action potential

-travels along the membrane
-descends down into the cell along the T-tubules, which causes excitation of the SR and release of Ca++ into sarcoplasm


Skeletal muscle contraction

1. resting muscle cell, myosin-binding site on the actin filament is concealed by tropomyosin filament
2. Presence of high concentration of Ca++ ions, Ca++ binds to Troponin C
3. Changes the spatial configuration of the Troponin molecule and causes the tropomyosin filament to shift->opens myosin-binding site on the actin filament
4. Myosin starts "walking" along the actin fibers
5. Myosin uses ATP to slide the actin along, myosin is an ATPase


Skeletal muscle relaxation

1. Following depolarization, Ca++activated ATPase membrane pumps transport Ca++ back into the sarcoplasmic reticulum. Ca++ disassociates from the Troponin C
2. Troponin complex returns to its original configuration and pulls the tropomyosin filament over the myosin binding site blocking the actin-myosin interaction. This stops the contraction unless there are new waves of depolarization coming with the nerve impulses
3. Rapid action that only takes 30 msec


Satellite cells

-after injury they become activated, proliferate and give rise to new myoblasts, which fuse to from a new fiber
-extensive damage results in the formation of a CT scar


How does muscle respond to:

-increasing in diameter


Connective tissue role in muscles?

-protection and force transduction
-provides passage for nerves and blood vessels that supply the muscle



-most internal layer of reticular fibers that surround individual muscle fibers



-thicker layer of collagenous connective tissue that surrounds groups of muscle fibers, known as fascicles



-most external thick layer of connective tissue that surrounds a group of fascicles that constitutes a named muscle
*dense CT collagen type 1


Smooth muscle (simplest type of muscular tissue) characteristics?

-does not exhibit cross striation in the cytoplasm, contractile filaments are not well organized inside the cell
-specialized for slow/rhythmic prolonged contractions of visceral organs, can provide rapid and precise contraction (eye)


Fusiform cells

-smooth muscle cells are elongated spindle shaped (fusiform cells) and are 20-200 micrometers in length (20 in walls of small blood vessels and 500 in the pregnant uterus)


Nuclei of smooth muscle

-centrally placed and long (cigar-shaped) with tapered ends

*similar looking to fibroblasts


Sarcoplasm of smooth muscle

-contains myofilaments and organelles (numerous mitochondria)
-myofilaments of smooth muscle cells are more randomly distributed throughout the cytoplasm than in skeletal muscle and are poorly organized


Dense bodies of smooth muscle

-anchoring points of thin filaments, formed by a-actinin.
-are anchored into the network of intermediate filaments by desmin
*dense bodies are somewhat analogous to the Z-disks of skeletal muscle fibers
*thick filaments are scattered throughout the sarcoplasm


Caveolae-like invaginations of sarcolemma

-act as sarcolemmal vesicles and deliver the depolarization to the chambers of sarcoplasmic reticulum, located beneath the caveolae
-may act similar to T-tubules of skeletal muscle


Communicating junctions of smooth muscle

-found interconnecting individual muscle cells
-small molecules or ions can pass through junctions from cell to cell and regulate contraction of the entire bundle of muscle cells
-can initiate contraction of neighboring muscle cells by allowing ions to pass through gap junctions and depolarize plasma membranes of other cells


Reticular fibers of smooth

-important role in the force transduction in the smooth muscle tissue


Thin filaments of smooth muscle

-similar to skeletal but w/out Troponin complex


F-actin and tropomyosin (smooth)

F-actin: principal component of thin filaments in the smooth muscle
Tropomyosin: wraps around actin in a fashion similar to skeletal muscle
-no troponin is present in the thin filaments of smooth muscle



-smooth muscle-specific actin-binding protein that masks the myosin-binding site on the actin fibers
-opens Ca++ binding site when Ca++ is near


Thick filament (smooth)

-formed by Myosin II


Myosin II of smooth muscle

-composed of two heavy polypeptide chains and four light chains
-only binds to actin when phosphorylated
-Myosin molecule is folded (inactive!) when dephosphorylated


Excitation of smooth muscle

-neural stimulation occurs through the postganglionic fibers of the autonomic nervous system
-neurotransmitter is released in the close proximity of a muscle cell and has to diffuse to the muscle sells through the CT that surrounds the muscle cells
-impulse transmission from cell to cell occurs through gap junctions


Chemical stimulation of smooth muscle contraction

-elicited by various hormones, angiotensin II (kidney), vasopressin, etc


Mechanical stimulation of smooth

-passive stretching of the organ, can lead to the initiation of a muscular contraction


Smooth muscle contraction

-driven by the high Ca++ concentration in the sarcoplasm
-relaxed muscle cells the highest concentration of Ca++ is in the sarcoplasmic reticulum


Excitation of smooth muscle steps

1. Ca++ is releases into sarcoplasm
2. Ca++ binds calmodulin (specific to smooth muscle)
3. Ca++/calmodulin complex binds to caldesmon releasing it from actin and opening the myosin-binding site on F-actin
4. Ca++/calmodulin complex activates myosin light-chain kinase (has to unfold)


What happens in phosphorylation in smooth muscle contraction?

-myosin light-chain kinase phosphorylares the regulatory (light) chain of the myosin molecule
-when light regulatory chain is phosphorylated, myosin molecule unfolds and actin-binding site on the myosin head becomes open and myosin binds to actin
A. Filaments slide at an angle
B. Cell shortens, nucleus folds and becomes corkscrew-shaped
C. Slow action (1 second) ant it often takes over a second to achieve the contraction


Smooth muscle relaxation

-Ca++ is pumped back into sarcoplasmic reticulum
-Ca++ levels in the sarcoplasm drop
-Calmodulin disassociates from the light-chain kinase, which deactivates the latter
-myosin is dephosphorylated and becomes inactive (folds to cover actin binding site)
-Caldesmon binds to the myosin-binding site on the actin filament
-slow action, has a prolonged effect on the tissue and requires very little energy spent compared to the skeletal muscle (10% of ATP that skeletal muscle uses)


Long-term contraction of smooth muscle

-secondary mechanism that requires minimum energy


Latch state of smooth muscle

-caused by the decrease of ATP activity while the myosin head is attached to actin
-causes prolonged contraction used to sustain the tone of blood vessels
-condition resembles the rigor Morris of the skeletal muscle


Location of smooth muscles (broad)

-walls of larger hollow organs
-iris and ciliary body
-present in dermis of skin


Smooth muscle in walls of larger hollow organs:

-walls of veins, arteries and lymphatic vessels
-extramural part of large ducts of glands
-GI tract, from distal esophagus to the anal canal
-M/F reproductive tract (ductus deferents, uterus, vagina)
-urinary system (bladder, ureter)


Smooth muscle of iris and ciliary body

-responsible for the constriction and dilation of the pupil


Smooth muscle fibers in the dermis of the skin

-attached to the hair follicles and form so called arrectores pilorum muscles


Smooth muscle regeneration and blood supply

-moderate blood supply through capillaries in the CT immediately surrounding the muscle cells (uses less energy than skeletal and cardiac)
-capable of active regenerative response. Can go through mitosis and replace the damaged or lost cells


Cardiac muscle characteristics

-exhibit cross-striations (same arrangement of contractile filaments as skeletal)
-involuntary (rhythmic contractions to pump blood through the cardiovascular system)


Cardiac muscle blood supply

-receives extensive blood supply through a network of capillaries


Cardiac muscle regeneration

-no ability of regeneration
-localized injury result in the replacement of muscle cells with fibrous CT that forms a scar
-function is lost in area of scar
-injury and repair is seen in the myocardial infarction


Cardiac muscle fibers arrangement

-fibers are formed by individual cells that are about 100 micrometers long
-different from the skeletal muscle fibers, which are formed by a syncytium


Cardiac muscle cell organelles

-centrally placed rounded nuclei
-myofibrils pass around the nucleus outlining a nuclear region where most organelles, including numerous mitochondria are concentrated, as well as inclusions, such as glycogen granules
-large flattened mitochondria are densely packed b/t the myofibrils


Muscle cells of cardiac
Shape and arrangement?

-rectangular in outline
-formed by long chains of muscle cells that attach to each other like cars in a train
-connections b/t individual cells are marked with intercalated disks
-when two cells attach there is branching of muscle fibers


Intercalated disks of cardiac

-represent junctions b/t individual muscle cells
-step-like Junction that consists of transverse and lateral portions


Transverse portion of intercalated disks

-runs across the fiber at a right angle, contains anchoring junctions that provide mechanical stability for the tissues and prevent cells from being pulled apart during contraction
-two parts: adherens junctions and desmosomes


Adherens junctions (fasciae adherentes) of transverse portion of intercalated disks

-connect the microfilaments of the two neighboring cardiac muscle cells.
-similar to the zonula adherentes found in the epithelial cells


Desmosomes (maculae adherentes) of the transverse portion of intercalated disks

-connect intermediate filaments (formed by desmin) of two adjacent cells


Lateral portion of intercalated disks

-runs parallel to the myofilaments
-contains communication, of gap junctions that provide ionic continuity b/t adjacent cardiac muscle cells and allow the signal to contract to pass from cell to cell and generate a wave of contraction


T-tubules of cardiac muscles

-large and found at the Z-disks and not at the junction of A and I bands like skeletal muscle fibers
-contraction is Ca++ dependent and similar to contraction of skeletal muscle


Sarcoplasmic reticulum of cardiac muscle

-not as well-developed as in the skeletal muscle
-no large terminal cisternae; instead there are small chambers that run parallel to the T-tubules only on one side of the tubule forming diads


Cardiac conducting system

-heartbeat is initiated, coordinated, and regulated by modified cardiac muscle cells that form the cardia conducting system


Purkinje fibers

-cardiac conduction cells
-modified for the conduction of electric impulses in a way similar to nerve cells
-from nodes and bundles including the sinoatrial and atrioventricular nodes and the bundle of His