Contractile cells Flashcards
4 groups of contractile cells
muscle cells
myoepithelial cells
myofibroblasts
pericytes
muscle cells
striated (voluntary)
cardiac and smooth (involuntary)
myofibroblasts
contractile role and collagen secretion
pericytes
smooth, muscle-like cells surrounding blood vessels
myoepithelial cells
component of certain secretory glands
skeletal muscle
voluntary movement
influenced by nervous system
maintenance of posture
development of skeletal muscle
embryogenesis
fusion of precursor cells (myoblasts)
syncytium with hundreds of nuclei beneath cell membrane
structure of skeletal muscle
long thin cylindrical structure 50-60 um diameter up to 10cm long hexagonal profile (cross section) fibrocollagenous septa numerous nuclei at side mitochondria and glycogen external lamina
satellite cells
adult muscle
muscle precursor cells dividing to form new muscle cells
sarcolemma
cell membrane
sarcoplasm
cell cytoplasm
sarcoplasmic reticulum
ER
contractile elements of skeletal muscle cells
myofibrils
thin cylindrical structures 1-2 um in diameter
overlapping, repeating assemblies of thick (myosin) and thin (actin) filaments
sarcomere
functional unit of myofibrils
plates of accessory proteins hold filaments in place and divide myofibrils
arrangement of contractile proteins in sarcomeres
thick filament surrounded by 6 thin filaments
bands in contraction
thick and thin filaments slide past eachother
decrease in width of light bands
width of dark bands is unchanged
A band
dark band
thick filament band
includes zone where thin filaments overlap with thick filaments
H zone
pale staining area in centre of A band
no thin filaments overlap with thick filaments
I band
zone of thin filaments not overlapping with thick filaments
Z line
dark band in centre of I band
M line
runs down centre of H band
myosin molecules
2 tadpole shaped heavy chains
tails coil around eachother
4 small light chains attached to head portions
tail portions aggregate to form filament
head portions project out in regular helical pattern
thin filaments
8nm diameter
thin filament (F actin) formed by polymerisation of single molecules of globular actin (G actin)
polar
G-actin molecules point in same direction
2 actin filaments attach by tail ends to alpha actinin in Z line, opposite direction
thick filaments
myosin polar 2 myosin filaments attach by tail ends face opposite direction, away from M line isoforms present
accessory proteins of molecules
actinin nebulin myomesin titin desmin C protein
actinin
holds actinin filaments in lattice arrangement in Z disk
Z disk proteins
actinin
filamin
amorphin
Z protein
nebulin
associated with actin containing thin filaents
myomesin
holds myosin filaments in lattice arrangement
region of M line
associated with creatine kinase and M protein
titin
long elastic protein
runs parallel to filament array
links ends of thick filaments to Z disk
desmin
link adjacent myofibrils to eachother
link myofibrils to membrane
C protein
myosin binding protein
localised in 7 stripes running parallel to M band in first half of A band
use of ATP by myosin binding to actin
ATP bound to myosin head hydrolysed to ADP and phosphate
myosin binds loosely to actin
phosphate released and myosin binds tightly to actin
binding of myosin to actin
folding of myosin to cause movement of it relative to actin filament
ADP released, fresh ATP binds and myosin returns to non attached state
tropomyosin
long rod like protein winding around actin to stabilise and stiffen it
troponin complex
regulates actin and myosin binding
attached to tropomyosin and contains 3 polypeptides: troponins T, I and C
troponin T
binds complex to tropomyosin
positions complex at site where actin binds to myosin
troponin I
physically prevents myosin binding to actin
troponin C
binds Ca2+ ions, causing a conformational change in troponin complex allowing myosin access to actin
how are nerve signals conveyed?
excitation of muscle cell membrane conveyed to interior of cell via membranous channels (transverse tubular system of T tubules) extending from muscle surface to surround each myofibril
terminal cisternae
2 portions of sarcoplasmic reticulum running alongside T tubule
high conc of Ca2+ ion channels in wall
membrane excitation of tubule system causes channels to open
membrane triad
association of T tubules and sarcoplasmic reticulum forms 3 tubules in cross section
surrounds every myofibril in AI junction - 2 triads to each sarcomere
T tubular system
internal membrane system
extends from the surface membrane as thin tubules
deep through muscle fibre along region of AI junction
2 extensions of sarcoplasmic reticulum with high Ca2+ conc
differences between cardiac and skeletal muscle
cardiac muscle cells mononuclear and shorter (nuclei)
long cardiac muscle fibres produced by lining cells via anchoring cell type junctions
population of stem cells, analogous to satellite cells of skeletal muscle, not present in cardiac cells - no regeneration
structure of cardiac muscle cells
15 um diameter
100 um long
central nucleus
intercalated disks
3 types of cell junction in intercalated disks
desmosomal junctions - intermediate filaments
adherent-type junctions anchor actin fibres of sarcomeres to end of cell
communicating gap junctions facilitate passage of membrane excitation and synchronisation of contraction
molecular basis of contraction of cardiac muscle
transverse T tubule have wider invaginations of cell surface
sarcoplasmic reticulum not as regular or as organised as in skeletal muscle
association of cardiac sarcoplasmic reticulum is form of dyads, not triads, and is in region of Z lines rather than AI junction
smooth muscle cells structure
less organised system of contractile proteins than skeletal and cardiac muscle cells
anchored together into functional units by BM material
spindle shaped
20um - 500 um
single, centrally located nucleus elongated in shape
external lamina
bundles by fine collagenous tissue containing blood vessels and nerves
where are smooth muscle cells located?
gut, urinary bladder and uterus
blood vessel walls and secretory gland ducts
sustained slow or rhythmic contractions not under voluntary control
cross sectional/longitudinal smooth muscle cells
polygonal profiles
linear bundles
contractile proteins in smooth muscle cells
bundles of contractile proteins crisscrossing the cell inserted into anchoring points (focal densities)
how does a mass of smooth muscle cells function as one unit?
tension generated by contraction transmitted through focal densities surrounding network of external lamina
how is energy supplied in smooth muscle cells?
numerous mitochondria
located with ER around nucleus, in area devoid of contractile filaments
nexus junctions
gap junctions in smooth muscle
invaginations in smooth muscle cells
similar to T tubules
contraction of smooth muscle cells
contraction of cell membrane results in shortening of the cell, which assumes a globular shape
elongated shape in relaxed state
filaments in smooth muscle
thin filaments of actin (isoform specific to smooth muscle) associated with tropomyosin - no troponin
thick filaments composed of myosin (different type than skeletal muscle) - only binds to actin if its light chain is phosphorylated
control of Ca2+ ion movements
relaxed - Ca2+ ions sequestered in sarcoplasmic reticulum
excitation - released into cytoplasm and bind to calmodulin (calcium binding protein)
calcium-calmodulin complex activates enzyme myosin light-chain kinase - phosphorylates myosin light chain and allows it to bind to actin
functional types of smooth muscle
unitary and multiunit
sheets with cells arranged circumfrentially or longitudinally
unitary smooth muscle
generate own low level of rhythmic contraction, stimulated by stretch, transmitted cell to cell via gap junctions
innervated by ANS
increases or decreases levels of spontaneous contraction, not initiate it
slow contraction, no action potentials, low fast myosin
phasic smooth muscle
iris of eye
ANS controls contraction
vas deferens and large arteries
rapid contractions, distinct action potentials, high fast myosin
myofibroblasts
spindle shaped cells
secrete collagen
well defined contractile properties
myofibroblasts structure
actin and desmin
lack external lamina
spindle shaped cells
myofibroblasts after damage
become active and proliferate
repair defects from tissue death
secrete collagen for firm scaffold to consolidate a damaged area
myofibrils contract and pulls extracellular matrix together to reduce physical size of damaged area
pericytes
spindle-shaped cells circumfrentially arranged around capillaries and venules external lamina little cytoplasmic differentiation actin and myosin
pericytes after tissue damage
proliferate
assume role of primitive mesenchymal cells
differentiate into myofibroblasts and mesenchymal tissue
myoepithelial cells structure
layer of flat cells around acini and ducts dark staining rounded nuclei clear cytoplasm contractile proteins desmosomes desmin
myoepithelial cells around acini
stellate, multi processes morphology in 3D
contractile meshwork enclosing secretory units of glands
myoepithelial cells around ducts
fusiform
surround periphery of ducts
myoepithelial cells function
controlled by ANS and contract and expel glandular secretions
