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Histology/Cell Biology > Muscle > Flashcards

Muscle Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

voluntary muscle

A
  • conscious control (somatic nervous system)

- i.e. skeletal muscle

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2
Q

involuntary muscle

A
  • unconscious control (autonomic nervous system)

- i.e. smooth and cardiac muscle

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3
Q

What are skeletal muscle fibers and what are they formed from?

A
  • large multinucleate syncytia

- form from fusion of myoblasts during embryonic development

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4
Q

plasticity of muscle tissue

A
  • how muscle responds to dif things (i.e. workload)
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5
Q

myofibrils

A
  • present in skeletal muscle fiber by the hundreds, all in register
  • constitute the cross-striations at LM and EM level with repeats of 2-3 microns
  • includes: peripheral nuclei, basal lamina surrounding a sarcolemma, sarcoplasmic reticulum which envelops each myofibril, and T-system
  • also contain: glycogen, free ribosomes, mitochondria, sometimes lysosomes
  • PM has specializations at neuromuscular junction
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6
Q

hierarchy of muscle bundles

A
  • fibers enclosed by basal lamina and CT layer called endomysium
  • fascicles (group of fibers) enclosed by perimysium
  • entire muscle (collection of fascicles) enclosed by epimysium
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7
Q

myofibrils

A
  • present in skeletal muscle fiber by the hundreds, all in register
  • contractile unit of cell –> when triggered to shorten (contract), causes entire fiber to shorten)
  • also contain lots of mitochondria and sarcoplasmic reticulum
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8
Q

How can pathological conditions affect muscles?

A
  • can cause difference in size and composition of muscles

- nuclei at center of cell (usually at periphery of cell)

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9
Q

sarcomere

A

repeating unit of myofibril

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10
Q

thin myofilaments

A
  • double stranded helix of polymerized actin monomers
  • tropomyosin and troponin entwined with two actin strands
  • unipolar
  • attach to Z line and extend into A band to edge of H band
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11
Q

tropomyosin

A
  • protein that forms filaments that run I grove of F-actin molecules of thin filament
  • regulated by troponin
  • at rest, tropomyosin masks the myosin-binding site on actin molecule
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12
Q

troponin

A
  • complex of three globular subunits
  • each tropomyosin molecule contains one troponin complex
  • binds Ca2+, essential step in initiation of contraction
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13
Q

thick myofilaments

A
  • aggregates of myosin molecules tail-to-tail (bipolar)
  • tail segments overlap so globular heads project
  • bare zone does not have globular projections
  • restricted to central portion of sarcomere (A-band)
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14
Q

cross-bridges

A
  • extend from thick filaments and pull thin filaments in by “rowing motion” via attachment/detachment cycles
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15
Q

sliding filament model/theory

A
  • during contraction, sarcomere and I band shorten, while A band remains same length
  • to maintain myofilaments at constant length, shortening of sarcomere caused by increase in overlap of thick and thin filaments
  • H band narrows and thin filaments penetrate H band during contraction
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16
Q

Titan filaments

A
  • scaffolding protein which can sense tension and regulate protein synthesis and protein degradation by binding (depending on how protein is stretched)
  • stretches along whole sarcomere
  • functions as molecular spring, acting as force transducer
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17
Q

desmin

A
  • intermediate filament protein
  • forms lattice that surrounds sarcomere at level of Z lines, attaching them to one another and to plasma membrane via linkage protein –> creates stabilizing cross-links between neighboring myofibrils
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18
Q

alpha actin

A
  • short, bipolar, rod-shaped actin-binding protein
  • bundles thin filaments into parallel arrays and anchors them at Z line
  • also cross-links titan’s N terminus embedded in Z line
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19
Q

What are the two functions of ATP in relation to myosin within sarcomere?

A
  • dissociates myosin from thin (actin) filaments when it binds
  • moves myosin head (bending) via energy from hydrolysis
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20
Q

What function does Ca++ serve within sarcomere of skeletal muscle fibers?

A
  • Ca++ controls contraction/relaxation by binding to site on troponin which, in turn, alters tropomyosin’s position exposing myosin binding site on actin
  • usually 10^-7 M or less but activates contraction at 10^-6 M
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21
Q

Where is Ca++ stored and how is it released?

A
  • Ca stored in sacs of sarcoplasmic reticulum
  • membrane of T-tubule/SR contains voltage sensitive protein
  • when cytosol receives influx of Na+ (caused by ACh at NM junction), T tubule membrane depolarized, causing conformation change allowing Ca++ to exit
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22
Q

neuromuscular (NM) junction

A
  • motor neuron synapses on muscle
  • release of ACh via vesicles, bind to specific receptors in post-synaptic cell (muscle cell) membrane
  • binding of ACh causes depolarization of sarcolemma and T-tubules which triggers Ca++ release from SR
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23
Q

t-tubules

A
  • tubular invaginations of plasma membrane (continuous with PM)
  • penetrate to all levels of muscle fiber
  • contain dihydropyridine receptors (DHPR)
24
Q

sarcoplasmic reticulum

A
  • stores Ca++ prior to activation by motor neuron
  • Ca++ release channels have receptors for drug ryanodine which blocks Ca++ release
  • has several proteins including a Ca++ activated ATPase involved in pumping Ca++ through membrane and high affinity Ca++ binding proteins
  • i.e. calsequestrin
25
Q

calsequestrin

A
  • highly acidic calcium-binding protein found on luminal surface of SR
  • allows Ca++ required for initiation of muscle contraction to be stored at high conc
  • free Ca++ conc within lumen of SR remains very low
26
Q

dihydropyridine receptors (DHPR)

A
  • depolarization-sensitive transmembrane channels that are activated when the plasma membrane depolarizes
27
Q

muscle spindle

A
  • specialized stretch receptor found in all skeletal muscles
  • sits in perimysium
  • consists of two types of modified muscle fibers called spindle cells and neuron fibers
  • nerve fibers contained within internal capsule, muscle spindle contained within external capsule
28
Q

Where do skeletal muscle cells get their energy supply?

A
  • energy supply of skeletal muscles comes from: capillaries surrounding all cells, lipid droplets, glycogen granules, and mito
29
Q

skeletal muscle fiber types

A
  • determined by color (red, intermediate, white) and by speed (fast oxidative/type IIa, fast glycolytic/type IIb, or slow oxidative/type I)
30
Q

fast muscles

A
  • have higher myosin ATPase conc
31
Q

red muscles

A
  • more oxidative and have more mito, myoglobin, and blood
32
Q

satellite cell

A
  • pool of undifferentiated cells that have potential to undergo myogenic differentiation
  • interposed between plasma membrane of muscle fiber and its external lamina
  • limited regenerative capacity
  • fuses with fiber during hypertrophy (fiber growth)
33
Q

myoblasts

A
  • early and late
  • after muscle tissue injury, satellite cells can be activated and become myogenic precursors of muscle cells –> gives rise to new myoblasts
  • myoblasts fuse to form myotube, which matures to form muscle fiber
34
Q

hypertrophy

A
  • process by which muscle fibers grow

- fiber types respond differently to stimuli for growth and atrophy

35
Q

Muscular Dystrophy (Duchenne)

A
  • results from absence of dystrophin

- destabilizes glycoprotein complex and leads to break down of linkage between ECM and cytoskeleton

36
Q

dystrophin

A
  • prevents membrane damage and keeps Ca++ from rising in cell at sarcolemma and T-tubules
37
Q

intercalated discs

A
  • join branched cells of cardiac muscle fibers
  • consists of gap and adherent junctions (fascia adherent in transverse component, gap junctions in lateral component)
  • forms functional syncytium as a result of electrical coupling
38
Q

cardiac myocyte

A
  • cardiac cell
  • each has one or sometimes two centrally located nuclei
  • display cross-striations at LM and EM level
  • contain abundant mito, free ribosomes and glycogen, large T-tubules in the ventricle but small in atria and poorly developed SR, abundant lipid droplets
39
Q

atrial granules

A
  • found in atrial cells

- release (by exocytosis) important regulatory agents like atrial natriuretic factor (ANF) –> diuretic hormone

40
Q

differences in primary sequences of contractile protein of cardiac muscle relative to that of skeletal muscle

A
  • similar to skeletal muscle contractile system except that myofibrils appear highly branched
  • thin (actin) filaments insert into intercalated disc
41
Q

differences in mechanism of contraction of cardiac muscle relative to that of skeletal muscle

A
  • similar to skeletal muscle contraction mech except phosphorylation of myosin light chains and troponin can modulate strength of contraction at a given Ca++ level
  • Ca++ release can be regulated (graded) by phosphorylation –> useful since all cells fire in heart
  • Ca++ enters through cell membrane channels and mito as well as SR
42
Q

How do cardiac cells depolarize?

A
  • no NM junction
  • cells depolarize spontaneously but contraction coordinated through gap junctions
  • nerves modulate rate and force through nodes and conducting fibers (purkinje cells)
43
Q

What is the preferred substrate of cardiac muscle?

A
  • fatty acids

- cardiac muscle has rich capillary bed and high oxidative metabolism

44
Q

purkinje fibers

A
  • highly specialized conducting fibers containing cardiac conducting cells
  • generate and rapidly transmit contractile impulse to various parts of myocardium in precise sequence
  • large amount of glycogen present around nucleus
45
Q

What induces distinct isoforms of cardiac muscle?

A
  • work-load and thyroxin

- atrial vs. ventricular vs. purkinje cells

46
Q

By what process does cardiac muscle grow?

A
  • grows by hypertrophy
  • generally no cell division in vivo and no analog of satellite cell
  • some evidence of mitosis in advanced stage human hypertrophy exists which may originate from stem cells
47
Q

fibers of smooth muscle

A
  • consist of groups of spindle shaped cells which are coupled electrically by gap junctions
48
Q

characteristics of smooth muscle cells

A
  • small, surrounded by basal lamina, containing single central nucleus and lacking striations
  • enclosed by variable amounts of CT
  • found in walls of hollow organs like uterus, intestines, stomach and blood vessels
  • contain rough ER, sparse SR, many micropinocytotic vesicles, and dense bodies
49
Q

dense bodies

A
  • have thin filaments of smooth muscle cell attachment to them
  • distributed throughout sarcoplasm in network of intermediate filaments containing desmin
50
Q

What CT proteins does vascular smooth muscle contain?

A
  • vimentin filaments

- along with desmin filaments, interconnects dense bodies and prevents excess stretching

51
Q

contractile system (myofibrils) of smooth muscle

A
  • does not display clear sarcomere structure
  • thick myosin filaments similar to those in striated muscles, but vary in length
  • thick filaments depolymerize during relaxation in manner regulated by dephosphorylation of myosin
  • thin actin filaments lack troponin but have protein caldesmon and insert into dense bodies
  • sliding filament mechanism required for shortening
52
Q

caldesmon

A
  • actin-binding protein that blocks myosin-binding site

- action is Ca++ dependent and controlled by phosphorylation of myosin heads

53
Q

control of contraction in smooth muscle (Myosin-linked Ca++ regulation)

A
  • involves Ca++ but dif mech than striated muscles
  • similar actin-myosin interaction
  • Ca++ binds calmodulin (in cytosol) which activates a myosin light-chain kinase
  • results in phosphorylated myosin which can only then bind to actin and generate tension and shorten
  • Ca++ induced dissociation of caldesmon from thin filament, exposing myosin binding site on actin
54
Q

activation of smooth muscle to contract

A
  • activated by hormones, stretch, spontaneous depolarization and nerves (no traditional NM junctions)
  • Ca++ comes from outside cell through membrane channels/vesicles –> channels regulated via phosphorylation
  • can result in graded response in individual cells via phosphorylation of contractile proteins and calcium channels
55
Q

energy supply of smooth muscle

A
  • not highly oxidative (few mito)

- not highly vascular

56
Q

growth of smooth muscle

A
  • hypertrophy and hyperplasia (cell division)
  • cells appear to dedifferentiate (loose contractile protein) before dividing, then redifferentiate
  • proliferation of smooth muscle cells important in thickening of uterus and formation of lesions in atherosclerosis

Histology/Cell Biology (12 decks)

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