Skeletal Muscle Physiology- Allard Flashcards
What are the characteristics of smooth muscle?
- smooth
- involuntary
- stimulated by autonomic nervous system, hormones, stretch, and pacemaker cells
- slow, sustained contractions and low fatigability
What are the characteristics of skeletal muscle?
- striated
- voluntary
- requires somatic nervous system
- rapid contractibility and high fatigability
What are the characteristics of cardiac muscle?
- striated
- involuntary
- self-stimulated stimulation and ANS modification
- continuous contraction/ relaxation and no fatigability
What are the connective tissue layerings of a skeletal muscle fiber from the inner to outer most?
endomysium
perimysium
epimysium
What is the basic contractile unit of skeletal muscle?
the sacromere
What protein stabilizes the myosin to the Z line and is responsible for elasticity of our muscles?
titin
Which zones in the sacromere of skeletal muscle shorten?
H and I zone
“HI wave”
T-tubule is continuous with the sarcoplasm reticulum (SR). What is the function of SR?
regulates calcium ion concentration in the cytoplasm of skeletal muscle
Myosin, the thick filament, has what types of protein chains?
- heavy chains with ATPase activity and bind to actin
- light chains that are of two types: essential and regulatory
Actin, the thin filament, is composed of globular actin molecules that come together to form what? These G-actin also have binding site for what?
- filamentous (F) actin
- each G-actin has binding site for myosin
Troponin is a complex of 3 proteins that bind to what? What occurs when calcium binds?
tropomyosin
when calcium binds to troponin, troponin moves tropomyosin from myosin binding sites making them exposed
What is the function of titin and nebulin?
titin (largest known protein): extends from z disc and runs through the core of the myosin filament; connects myosin to the Z-lines and is responsible for muscle flexibility
nebulin: binds with and stabilizes F-actin and attaches it to the Z-disc
they are for stabilizing filaments
What is function of myomesin and dystrophin?
myomesin: binds myosin and titin to the M line
dystrophin: maintains structural integrity; anchors actin to the sacrolemma (deficiencies in this protein leads to muscular dystrophy)
What is the cause of Duchenne muscular dystrophy and what does it cause?
X-linked recessive disease caused by a mutation in the dystrophin gene; causes loss of structural stability of myocytes
During this process, the heads of the myosin filaments pull the actin filaments toward the M line, in part increasing the area of overlap between the thick myosin and thin actin filaments.
muscle shortening AKA contraction
What is a motor unit?
- a somatic motor neuron plus all the skeletal muscle fibers it stimulates
- all of the muscle fibers in one motor unit contract in unison
- the motor neurons and all of the muscle cells that it innervates
One muscle may have many motor units of different fiber types. True or false?
True!!
But a single motor unit has to have the same fibers
Fibers of a single motor unit can be of different types. True or false?
FALSE!!!
Fibers of a single motor unit are all of the same type!!!
Motor units are stimulated by what?
upper motor neurons
What are the acetylcholine receptors on skeletal muscles called?
nicotinic receptor
Where are the cell bodies of low motor neurons for skeletal muscle located and where do their axons exit and enter?
- cell bodies are located in the ventral horn of the spinal cord
- axons exit the spinal cord via the ventral root and enter in spinal nerves to innervate target muscles
SOMATIC motor neurons control skeletal muscle innervation
What are the steps of excitation at the neuromuscular junction?
- action potential is generated and travels down the axon of the lower motor neuron
- voltage-gated calcium channels open and release calcium into the axon terminal of the neuron binding to synaptotagmin
- calcium entry causes the exocytosis of ACh into the synaptic cleft
- ACh binds its nicotinic receptors on the skeletal muscle
- nicotinic channel allows flow of monovalent positive ions (sodium into the cell and potassium out of the cell)
What type of channels are nicotinic receptors?
ligand-gated channels
How is the endplate membrane potential generated?
endplate membrane potential is a local change in the membrane potential of muscle fiber that is triggered by the opening of the nicotinic receptor (channel) that allow the flow of Na+ inside the cell and K+ outside of the cell; MORE Na+ flows in than K+ out allowing for a depolarizing current to occur
IT IS essentially depolarization at the motor end plate AKA graded potential.
What breaks down ACh in the synaptic cleft to prevent continual stimulation of the nicotinic receptors?
acetylcholinesterase: limits the impact of ACh in the synaptic cleft
In the neuromuscular junction, each axon terminal innervates ________muscle fiber. Each muscle fiber has a specialized region of ___________with junctional folds containing ACh receptors.
ONE
sarcolemma
What macromolecules is the synaptic cleft rich in?
glycoprotein and collagen fibers
What is the difference between an end plate potential and an action potential?
- end plate potential occurs at the post-synaptic membrane (NMJ) with LIGAND-gated channels of Na+ AND K+; is a graded potential
- action potential occurs at the muscle membrane with VOLTAGE-gated channels of ONLY Na+
What is the mechanisms of botulinum toxin?
destroys the SNARE proteins responsible for the fusion and release of the synaptic vesicle content
This is an autoimmune disease of the NMJ where antibodies bind to nicotinic ACh receptors inhibiting contraction.
Myasthenia gravis
these binding antibodies destroy the ACh receptors thus decreasing the number of receptors that ACh can bind to
Chronic inflammation of myasthenia gravis causes structural changes of the NMJ in what ways?
- flattening out of the junctional folds of the sarcolemma
- spreading out of AChR and acetylcholinesterase
- 66% decrease in number of AChR
- an increased junctional gap
What are the initial symptoms of myasthenia gravis?
drooping of the eyelids, clumsiness
How is the AP in skeletal muscle different from AP in neurons?
- not much of a hyperpolarization due to potassium already reaching its equillbrium potential through the repolarization
- chloride channels play a much bigger role as to where the resting membrane potential is of the skeletal muscle
What is the relationship between the timing of the skeletal muscle AP and the timing of the force generation in the skeletal muscle?
the AP is done significantly before the force generation
-this means you can generate another AP (or more) and add to the contractile force of the skeletal muscle (increase in force generation)
What is consisted in a triad?
1 T-tubule and 2 terminal cisterns of the sacroplasmic reticulum
What is the function of calsequestrin within the terminal cisternae of sarcoplasmic reticulum?
it is a low-affinity, high capacity calcium binding protein that keeps the calcium concentration in the terminal regions of the SR high
-it creates an allusion that there is not as much calcium (hoards the calcium in the terminal regions of SR)
What is the voltage-activated protein embedded on the T-tubule that engages with the ryanodine receptor on the SR to allow for calcium release into the myoplasm?
dihydropyridine receptor
- AP changes the configuration of dihydropyridine receptor on T-tubule which in turn knocks open the ryanodine channel by changing its configuration.
- Calcium then flows into the cytosol (inside cell) to meet troponin whose been waiting for it.
What type of receptor is the ryanodine receptor?
calcium channel