Chapter 7: Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling Flashcards
are innervated by large, myelinated nerve fibers that originate from large motoneurons in the anterior horns of the spinal cord
skeletal muscle fibers
each nerve ending makes a junction (with the muscle fiber near its midpoint) called the?
neuromuscular junction
it initiated in the muscle fiber by the nerve signal travels in both directions toward the muscle fiber ends.
action potential
with the exception of about 2% of the muscle fibers how many junction per muscle fiber are there?
only one such junction
the entire structure, where it is covered by one or more schwann cells that insulate it from the surrounding fluids
motor end plate
invaginated membrane
synaptic gutter or synaptic trough
space between the terminal and the fiber membrane is called?
synaptic space or synaptic cleft
at the bottom of the gutter are numerous smaller folds of the muscle membrane called?
sub neural clefts
the energy source that is used for synthesis of an excitatory transmitter
acetylcholine
Acetylcholine is synthesized in the ___________ of the terminal
cytoplasm
in the synaptic space are large quantities of the enzyme _______________, which destroys acetylcholine a few milliseconds after it has been released from the synaptic vesicles
acetylcholinesterase
when an action potential spreads over the terminal, these channels open and allow calcium ions to diffuse from the synaptic space to the interior of the nerve terminal
voltage-gated calcium channels
a process when the vesicles then fuse with the neural membrane and empty their acetylcholine into the synaptic space
exocytosis
they are located almost entirely near the mouths of the sub neural clefts lying immediately below the dense bar areas, where the acetylcholine is emptied into the synaptic space.
acetylcholine-gated ion channels
the complex is composed of five subunit proteins, what are these?
two alpha proteins, beta, delta, and gamma proteins
__________ has a diameter of about 0.65 nanometer, which is large enough to allow the important positive ions __________, __________, __________ to move easily through the opening
acetylcholine-gated channel
sodium (Na), potassium (K) calcium (Ca)
more ________ flow through the acetylcholine-gated channels than any other ions.
sodium ions
this creates a local positive potential change inside the muscle fiber membrane called?
end plate potential
it initiates an action potential that spreads along the muscle membrane and thus causes muscle contraction
end plate potential
creating a local potential
end plate potential
stimulation of the nerve fiber at rates greater than 100 times per second for several minutes often diminishes the number of acetylcholine vesicles so much that impulses fail to pass into the muscle fiber
fatigue of the neuromuscular junction
compound that has the same effect on the muscle fiber as does acetylcholine
methacholine, carbachol, and nicotine
these particularly well-known drugs, inactivate the acetylcholinesterase in the synapses so that it no longer hydrolyzes acetylcholine.
neostigmine, physostigmine, diisopropyl fluorophosphate
it causes when even a few nerve impulses reach the muscle
muscle spasm
combine with acetylcholinesterase to inactivate the acetylcholinesterase for up to several hours, after which these drugs are displaced from the acetylcholinesterase so that the esterase once becomes active.
neostigmine and physostigmine
a group of drugs that can prevent passage of impulses from the nerve ending into the muscle.
curariform drugs
blocks the action of acetylcholine on the muscle fiber acetylcholine receptors, thus preventing sufficient increase in permeability of the muscle membrane channels to initiate an action potential
D-tubocurarine
causes muscle paralysis because of inability of the neuromuscular junctions to transmit enough signals from the nerve fibers to the muscle fibers
myasthenia gravis
Some of quantitative aspects of muscle potentials: (3)
- resting membrane potential
- duration of action potential
- Velocity of conduction
about -80 to -90 millivolts in skeletal fibers, the same as in large myelinated nerve fibers
resting membrane potential
1 to 5 milliseconds in skeletal muscle about five times as long as in large myelinated nerves
duration of action potential
penetrate all the way through the muscle fiber from one side of the fiber to the other.
Transverse tubules (T tubules)
T-tubule cause release of calcium ions inside the muscle fiber in the immediate vicinity of the myofibrils and these calcium ions then cause contraction. this overall process called?
Excitation-contraction coupling
are small and run transverse to the myofibrils. they begin at the cell membrane and penetrate all the way from one side of the muscle fiber to the opposite side.
transverse tubules
sarcoplasmic reticulum composed of two major parts;
- large chambers called terminal cisternae
- long longitudinal tubules that surround all surfaces of the actual contracting myofibrils
as the action potential reaches the t tubule, the voltage change is sensed by __________ receptors that are linked to calcium release channels also called?
dihydropyridine
ryanodine receptor
activation of ____________ triggers the opening of the calcium release channels in the cisternae as well as in their attached longitudinal tubules
dihydropyridine receptors
the normal resting state concentration of calcium ions in the cytosol that bathes the myofibrils is too little to elicit contraction
<10-⁷ molar
keeps the actin filaments inhibited and maintains a relaxed state of the muscle
troponin-tropomyosin
full excitation of the _________ and ___________ causes enough release of calcium ions to increase the concentration in the myofibrillar fluid
T-tubule and sarcoplasmic reticulum system
the total duration of the calcium pulse in the skeletal muscle fiber last about?
1/20 of a second
in heart muscle, the calcium pulse last about?
1/3 of a second
during the calcium pulse, MUSCLE CONTRACTIONS OCCUR.
