lesson 7-8 gritti Flashcards
how are the nuclei of the cells positioned in the skeletal muscle?
in the periphery of the fibers because the fibers are filled of contractile elements (actin and myosin)
how are cardiac muscle cells connected?
by intercalated discs
contraction of skeletal muscle
quick, forceful and under voluntary control
contraction of cardiac muscle
rhytmic, involuntary and powerful
types of muscle tissue
skeletal muscle
cardiac muscle
smooth muscle
contraction of smooth muscle
slow and involuntary
which muscle tissue is not striated?
smooth muscle tissue
name of the cells that compose the skeletal muscle
muscle fibers, organized into bundles, supplied by blood vessels and innervated by motor neurons.
embryonic origin of skeletal muscle fibers
cells migrate from the mesoderm and become myoblasts that then differentiate into myocytes and, at the end, they fuse together forming multinucleate myotubes.
satellite cells
are remnants of primitive myoblasts.
are stem cells that reside in the muscle tissue.
these cells in the skeletal muscle don’t proliferate in physiological conditions but only in case of an injury–>muscle regeneration
NB: skeletal muscle is generally a quiescent tissue
PAX-7
master transcription factor and marker of satellite cells
when can we say that the number of skeletal muscle fibers is fixed?
right before or right after birth
the number of muscle fibers remain fixated throughout life. satellite cells proliferate ONLY in pathological conditions.
endocrine control of muscle fibers
by IGF1 (insulin growth factor)
the growth hormone acts on the liver
the liver produces IGF1
IGF1 stimulates muscle growth reducing, as a consequence, adipose tissue
No Change in Fiber Number through life
The number of muscle fibers (myofibers) remains constant after birth in humans under normal conditions. Increases in muscle mass due to GH and IGF-1 are attributed to:
Increased fiber size (hypertrophy)
Enhanced protein synthesis within fibers
Improved recruitment of satellite cells
myostatin (GDF8)
inhibitory of muscle fibers growth
no myostatin–>hyper development of the muscle system
what is the name of the layer that covers each muscle?
epimysium
what’s the name of the layer that covers each fascicle?
perimysium
what’s the name of the layer that covers each muscle fiber?
endomysium
each muscle is formed by bundles of muscle fibers, each bundle is called?
fascicle
each muscle fiber contains?
myofibrils
each myofibril is made of?
repeating units of sarcomeres
each sarcomere is made of
actin (thin filaments) and myosin (thick filaments)
the arrangement of actin and myosin give to the skeletal muscle the striated appearance
the sliding of these filaments allow the sarcomere to shorten and the muscle to contract
T tubules
invaginations of the sarcolemma that make contact with the sarcoplasmic reticulum
encircle myofibrils near the bundaries of each sarcomere
multiple sarcomeres form
a myofibril
multiple myofibrils form
a muscle fiber
what gives the striation to the skeletal muscle?
the disposition of actin and myosin filaments in the sarcomeres
structure of a sarcomere
I band: band made of parallel thin filaments (actin) that anchor to the Z line and move in the opposite direction from the Z line
A band:band made of parallel thick filaments that anchor to the M line and point in the opposite direction
H band: region in the middle of the A band not reached by thin filaments
M line: in the middle of the H band
Z line: dark region in the center of the I band
a sarcomere extends from one Z line to the next one
tropomyosin and troponin
associated with actin filaments
tropomyosin–>fibrous protein
troponin–>globular protein
these proteins are the ones in which we have the binding sites for calcium ions
alfa actinin
accessory protein bonded to the Z line that keeps the sarcomere in place
titin
accessory protein bonded to the Z line that has recoiling properties and is essential for when the contraction ends and the sarcomere relaxes
dystrophin
an actin binding protein, it keeps the sarcomere attached to the sarcolemma
triad
the T tubule + the two terminal cisternae of the sarcoplasmic reticulum in contact with it
in corrispondence of the Z line
what is the power stroke?
when the myosin head pivot pulling the actin filaments closer to the center of the sarcomere
the skeletal muscle fiber is (in terms of nuclei)?
multinucleated, due to the fact that derives from the fusion of multiple myoblasts in a process called myogenesis
the nuclei are positioned in the periphery of the fibers
nuclei are more elongated
cardiomyocytes in terms of nuclei?
have one or two nuclei and are positioned in the center of the cell
nuclei are more rounded
smooth muscle cells in terms of nuclei
nuclei resemble a spindle
placed at the center
smooth muscle cells are uninucleated
how do smooth muscle cells run in an histological sample?
all in the same direction with elongated nuclei
smooth muscle in the skin
arrector pili muscle associated to hair follicles
alpha bungarotoxin
irreversible agonist for acetylcholine
leads to paralysis
if paired with fluorofore can be used to localize acetylcholine receptors
neuromuscular junction
motor unit
the motor neuron innervates one or more muscle fibers forming NMJs
motor unit–>branches of the axon of a single motor neuron and the muscle fibers in contact with them
Proprioceptors
transmit infos to the CNS from the musculoskeletal system about the stretching and the degree of tension
Golgi tendon organ–>a type of proprioreceptor present at the boarder of tendon and muscles
(nerve fibers surrounded by a connective tissue capsule )
muscle spindles
-Sorrounded by extrafusal muscle fibers
-made of flattened cells with interstitial fluid, a few muscle fibers (intrafusal) and are wrapped around by sensory nerve axons
-detect changes in length (distension) of the muscle fibers around (extrafusal) and transmit the info to the spinal cord
-help in mantaining posture and regulate the activity of opposite muscles
what’s the position of nuclei in newly formed muscle fibers?
in the middle of the fiber
this allows us to distinguish between resident old fibers and newly formed
Which type of tissue is the epicardium?
simple squamous mesothelium+connective tissue+nerves and vessels
which type of tissue is the myocardium?
muscle tissue made for the major part by cardiac muscle cells
contain also a layer of connective tissue
endocardium, which type of tissue it is?
simple squamous endothelium+ connective tissue+ smooth muscle
intercalated discs are made of?
desmosomes for a physical mechanical connection
gap junctions for an electrical connection
do we have satellite cells in cardiac muscles?
NO WE DON’T–>THIS IMPAIRS REGENERATION OF THE CARDIAC TISSUE
In which heart cells the T tubules are more developed? and in general the sarcomeric apparatus?
ventricular cardiac cells
have a more developed sarcomeric apparatus and t tubules.
in the atrial ones T tubules are smaller and often absent
diads
instead of triads in the cardiac muscle we have diads
one cisterna and one t tubule in cardiac cells
conduction cells, names and where they are situated
purkinje cells and pacemaker cells situated in the sinoatrial node
which is the muscle cell type that can change in number throughout life?
smooth muscle cells have proliferative capacity as opposed to cardiomyocites and myofibers
caveolae
invaginations of the plasma membrane of smooth muscle cells
dense bodies
points where actin filaments randomly disposed in the cytoplasm of smooth muscle cells attach
-also point of attachment of intermediate filaments (not contractile filaments)
contraction in smooth muscle cells
-calmodulin is activated by calcium
-then calmodulin activates MLCK (myosin light chain kinase)
-MLCK phosphorylates myosin head that grabs actin
-the result is an overall shortening and, as a cons, contraction of the smooth muscle cells
varicosities
branches of the nerve fibers that are close but NOT in contact with the smooth muscle cells.
they contain vesicles with neurotransmitters (such as acetylcholine or norepinephrine)
single unit smooth muscles
cells are electrically connected by gap junctions and the stimulus start from the layer closer to the neurotransmitter and then propagates as a wave
multi units smooth muscle
-cells are not connected by gap junctions, each smooth muscle cell contracts independently.
-more precise regulation of contraction, very important in small muscles where precision is needed (in the eyes for example)
in which type of muscle you can have both hyperplasia and hypertrophy?
smooth muscle
