Skeletal Muscle Flashcards
Where are skeletal muscles located?
between bones
What are the functions of skeletal muscles? (3)
- body movement (locomotion)
- maintenance of posture
- respiration (diaphragm and intercostal contractions)
Are skeletal muscle fibres striated or non-striated?
striated
Describe the organization of skeletal muscle fibres.
- muscle contains fascicles (bundles) of muscle fibres
- muscle fibres contain myofibrils
What are the 3 investments of skeletal muscle?
epimysium
perimysium
endomysium
What is epimysium?
dense collagenous connective tissue that surrounds entire muscle
What is perimysium?
collagenous connective tissue that surrounds fascicles
What is endomysium?
fine sheath of connective tissue composed of reticular fibres that surround individual muscle fibres
Is skeletal muscle contraction voluntary or non-voluntary?
voluntary
What is the spontaneity of skeletal muscle contraction controlled by?
motor neurons – cell body of motor neuron is localized in ventral horn of spinal cord
What is the basal lamina?
external lamina – layer of extracellular matrix secreted by epithelial cells
Where is the basal lamina?
under endomysium, between plasma membrane of muscle fibre and endomysium
What does the basal lamina contain?
glycoproteins and collagen
How is the basal lamina continuous with the inside of the cell
basal lamina binds myofiber via dystroglycan-containing complex
What is the dystroglycan-containing complex?
multi-protein complex
What is the sarcolemma?
plasma membrane of muscle fibre
What is the myotendinous junction?
interface between muscle and tendon, where force is transmitted between the two tissues
What does the myotendinous junction do?
attaches muscles to bones
- muscle fibres end at level of tendon
- finger-like extensions at ends of muscle fibres insert into connective tissue of tendon
Describe the structure of myofibers.
- shape
- striated or non-striated
- nuclei
- lond, cylindrical
- striated
- multinucleated
Where are nuclei of myofibers?
at periphery, just under plasma membrane
What are myofibrils composed of?
myofilaments
How are myofibrils organized?
- aligned in parallel
- separated by mitochondria and SR vesicles
What are myofibrils?
contractile muscle fibers
What is the sarcoplasmic reticulum?
smooth ER
Where is the sarcoplasmic reticulum and how is it organized?
- runs longitudinally to myofibril
- surrounds individual myofibrils
What does sarcoplasmic reticulum form? (2)
- interconnected network of tubules
- chambers (lateral sacs) called terminal cisternae on either side of T-tubules
What does the sarcoplasmic reticulum do in skeletal muscle?
- major role in storing Ca2+ when muscle is at rest
- releases Ca2+ into sarcoplasm when muscle is stimulated
What are transverse tubules (T-tubules)?
deep invaginations of sarcolemma, perpendicular to length of myofiber
What is a triad junction?
single T-tubule + 2 terminal cisternae
What receptors do T-tubules carry?
dihydropyridine receptors = voltage-gated Ca2+ channels
What receptors do terminal cisternae carry?
ryanodine receptors = Ca2+ release channel
What is the sarcomere?
contractile unit of myofibril made of 2 myofilaments
in skeletal and cardiac muscle
What are the 2 types of myofilaments?
- myosin (thick) myofilament
- actin (thin) myofilament
What is the myosin myofilament made of?
myosin molecule, composed of 2 heavy chains of myosin
What is the purpose of the myosin molecule head?
power stroke during contraction
What are the two sites on the myosin molecule head?
- actin binding site
- myosin ATPase site: where ATP binds and hydrolyzes into ADP and Pi
What are actin myofilaments made of?
- two actin filaments twisted to form helix
- proteins – tropomyosin, troponin complex
What are actin filaments made of?
many g-actin (monomeric form of actin)
What is a motor unit?
motor neuron and all muscle fibres it innervates
What is the motor endplate?
specialized domain of sarcolemma
What does the motor endplate do?
highly excitable, and is responsible for initiating AP that propagates across myofiber and causes contraction
What is the neuromuscular junction (NMJ)?
junction between axon terminal of motor neuron and motor endplate
(synaptic contact between presynaptic terminal and postsynaptic membrane of muscle fiber)
What are synpatic vesicles filled with?
ACh (neurotransmitter)
What are junctional folds?
folds in postsynaptic membrane, particular to NMJ only (otherwise, membrane is usually flat)
What is the synaptic cleft?
space between presynaptic membrane and postsynaptic membrane, where ACh in vesicles is released
Does the NMJ have a basal lamina?
has a prominent basal lamina
Does the NMJ have myelin?
no
What is the nerve terminal filled with?
organelles – mitochondria, synaptic vesicles
Where does endplate potential (EPP) occur?
in motor endplate, located underneath nerve terminal
Can myosin head bind to actin at low Ca2+ concentration?
no
- myosin binding sites on actin are masked by tropomyosin
- blocks myosin head access to myosin binding site
Can myosin head bind to actin at high Ca2+ concentration?
yes
- myosin binding sites on actin are not masked by tropomyosin
- myosin head can access myosin binding site and bind actin
Where are there delays in contraction? Why?
between AP, Ca2+ release, and degeneration of force
Ca2+ needs to be released from SR, travel through sarcoplasm, find troponin complex, etc. – process takes time
What is a single twitch?
muscle fiber is restimulated after it has completely relaxed – second twitch is of same magnitude as first twitch
What is summation?
muscle fiber is restimulated before it has completely relaxed – second twitch is added on to first twitch
What is tetanus?
muscle fiber is stimulated so rapidly that it does not have opportunity to relax at all between stimulations – maximal sustained contraction (tetanus)
What happens when tetanus ends?
muscle fatigue leads to relaxation of muscle
What happens if ACh is constantly present at synaptic cleft?
- muscle spasms
- ACh is degraded by AChE until another contraction needs to occur
In skeletal muscle, are dihydropyridine receptors in contact with ryanodine receptors?
yes
Skeletal Muscle Contraction Process
–
What is the sliding filament model of contraction?
actin and myosin filaments overlap slightly
actin filaments slide along myosin, resulting in greater overlap and shortening of sarcomere → contraction
Does the length of myofilaments (thick or thin) get shorter during contraction and/or relaxation?
no
- during contraction, actin and myosin filaments slide along each other and make the whole sarcomere shorter
