Muscle Flashcards
1
Q
voluntary muscle
A
- conscious control (somatic nervous system)
- i.e. skeletal muscle
2
Q
involuntary muscle
A
- unconscious control (autonomic nervous system)
- i.e. smooth and cardiac muscle
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
4
Q
plasticity of muscle tissue
A
- how muscle responds to dif things (i.e. workload)
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
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
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
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)
9
Q
sarcomere
A
repeating unit of myofibril
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
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
12
Q
troponin
A
- complex of three globular subunits
- each tropomyosin molecule contains one troponin complex
- binds Ca2+, essential step in initiation of contraction
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)
14
Q
cross-bridges
A
- extend from thick filaments and pull thin filaments in by “rowing motion” via attachment/detachment cycles
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
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
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
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
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
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
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
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
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
calsequestrin
- 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
dihydropyridine receptors (DHPR)
- depolarization-sensitive transmembrane channels that are activated when the plasma membrane depolarizes
27
muscle spindle
- 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
Where do skeletal muscle cells get their energy supply?
- energy supply of skeletal muscles comes from: capillaries surrounding all cells, lipid droplets, glycogen granules, and mito
29
skeletal muscle fiber types
- determined by color (red, intermediate, white) and by speed (fast oxidative/type IIa, fast glycolytic/type IIb, or slow oxidative/type I)
30
fast muscles
- have higher myosin ATPase conc
31
red muscles
- more oxidative and have more mito, myoglobin, and blood
32
satellite cell
- 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
myoblasts
- 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
hypertrophy
- process by which muscle fibers grow
| - fiber types respond differently to stimuli for growth and atrophy
35
Muscular Dystrophy (Duchenne)
- results from absence of dystrophin
| - destabilizes glycoprotein complex and leads to break down of linkage between ECM and cytoskeleton
36
dystrophin
- prevents membrane damage and keeps Ca++ from rising in cell at sarcolemma and T-tubules
37
intercalated discs
- 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
cardiac myocyte
- 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
atrial granules
- found in atrial cells
| - release (by exocytosis) important regulatory agents like atrial natriuretic factor (ANF) --> diuretic hormone
40
differences in primary sequences of contractile protein of cardiac muscle relative to that of skeletal muscle
- similar to skeletal muscle contractile system except that myofibrils appear highly branched
- thin (actin) filaments insert into intercalated disc
41
differences in mechanism of contraction of cardiac muscle relative to that of skeletal muscle
- 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
How do cardiac cells depolarize?
- 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
What is the preferred substrate of cardiac muscle?
- fatty acids
| - cardiac muscle has rich capillary bed and high oxidative metabolism
44
purkinje fibers
- 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
What induces distinct isoforms of cardiac muscle?
- work-load and thyroxin
| - atrial vs. ventricular vs. purkinje cells
46
By what process does cardiac muscle grow?
- 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
fibers of smooth muscle
- consist of groups of spindle shaped cells which are coupled electrically by gap junctions
48
characteristics of smooth muscle cells
- 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
dense bodies
- have thin filaments of smooth muscle cell attachment to them
- distributed throughout sarcoplasm in network of intermediate filaments containing desmin
50
What CT proteins does vascular smooth muscle contain?
- vimentin filaments
| - along with desmin filaments, interconnects dense bodies and prevents excess stretching
51
contractile system (myofibrils) of smooth muscle
- 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
caldesmon
- actin-binding protein that blocks myosin-binding site
| - action is Ca++ dependent and controlled by phosphorylation of myosin heads
53
control of contraction in smooth muscle (Myosin-linked Ca++ regulation)
- 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
activation of smooth muscle to contract
- 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
energy supply of smooth muscle
- not highly oxidative (few mito)
| - not highly vascular
56
growth of smooth muscle
- 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
