E-C coupling and skeletal muscle contraction Flashcards
what is the primary function of muscle?
to generate force or movement in response to a physiological stimulus
all muscles transduce a chemical or electrical signal into a (blank) response
mechanical
what are the three types of muscle
cardiac, smooth and skeletal
what is the trigger for all 3 types of muscle
an increase in calcium ions
how does skeletal muscle contract
in response to neuromuscular synaptic transmission
a skeletal muscle cell has a (blank) where ACh receptors are concentrated
single NMJ
are ACh receptors selective or non-selective cation channels
non-selective
when the ACh cation channels open in response to ACh binding what is the result
depolarization of EM known as an end plate potential (epp)
what happens if the EPP exceeds the threshold for activating the V-gated sodium ion channels
an action potential is generated
what happens when there is a generation of an action potential
there is a sequence of events leading to contraction
what inactivated ACh rapidly
ACh-esterase
name properties of muscle fibers
excitability
contractability
extensibility
elasticity
they are elongated and cylindrical
muscle fibres have multiple nuclei and have many mitochondria
what makes up a muscle
muscle fiber
what makes up muscle fibers
myofibrils
what makes up myofibrils
thick and thin myofilaments
what makes up the A band
thick and thin filaments with the protein myosin. (appears dark)
what makes up the I band
there are only thin filaments in the I band and this appears lighter
what is the z line
it is a boundary for each sarcomere and it is disk-shaped, there is also actin found here
what is the m line
the m line is in the center of the A band and contains a protein called myomesin.
what is the H zone
the h zone is in the middle of the A band, and the m line runs down the middle of it. in the h zone thick and thin filaments do not overlap and it only contains thick filaments.
what are the levels of organization in a skeletal muscle
whole skeletal muscle (an organ)
muscle fiber (single cell)
myofibril (a specialised intracellular structure)
thick and thin filaments (cytoskeletal elements)
myosin and actin (protein molecules)
how large is a single muscle fiber
10-100 micrometers and up to approx. 75cm in length
what are myofibrils
specialized contractile elements that extend the entire length of the muscle fiber
muscle fibers can contain 100s-1000s of myofibrils
what makes up thick and thin filaments
in thick there is myosin
in there there is actin
what makes up a thin filament
there are the actin molecules with the binding site for attachment with myosin.
these molecules form an actin helix of 2 chains.
tropomyosin acts as a ribbon and covers the myosin-binding site stopping troponin from binding and the troponin binds to tropomyosin
Heterotrimers
what do troponin T, C and I bind to
T binds to tropomyosin
C binds to calcium ions
I binds to actin and inhibits contraction
what does each heterotrimer of troponin interact with
a single molecule of tropomyosin which in turn interacts directly with 7 actin monomers
what is the structure of myosin forming the thick filaments
2 identical subunits- tails intertwined with the globular head sticking out. the tail is 100nm long
what is on the globular head of the myosin filaments
the actin-binding site and myosin ATPase site
each myosin II molecule is a double trimer composed of what
2x intertwined heavy chains
2x regulatory light chains
2x alkali (or essential) light chains
the two heavy chains have 3 regions a tail, a hinge, and a head region. go into detail about all of these
the tail portions are alpha helices that intertwine
at the hinge region the molecules opens to form 2 globular heads
The head regions (aka S1 fragments) are the cross-bridges between the thick and thin filaments of the sarcomere
the head portions of each myosin form a complex with 2 light chains, 1 alkali and 1 regulatory
the alkali light chain stabilizes the myosin head region
the regulatory light chain regulates the ATPase activity of myosin. The activity of this chain is regulated via phosphorylation by kinases
explain the muscle contraction cycle
myosin-II heads bind to actin,
these cross bridges become distorted
myosin heads detach from actin
where does the energy come from for the muscle contraction
hydrolysis of ATP
in all 3 muscle types, what does the increase of free calcium ions trigger
contraction by removing the inhibition (tropomyosin) of cross bridges cycling
what happens to the calcium ions upon stimulation
rises from resting levels of less than 10-7M to greater than 10-5M
what happens when there is a decrease in calcium ions
the decrease is the signal to cease cross-bridge cycling and relax
how do calcium ions modulate contraction
via regulatory proteins rather than interacting directly with the contractile proteins
what happens in the absence of Ca2+
the regulatory proteins act together to inhibit actin-myosin interactions, thus inhibiting the contractil process
what happens when Ca2+ binds to one or more of the regulatory proteins
a conformational change takes place in regulatory complex that releases the inhibition of the contraction
