Skeletal Muscle Physiology 1 (Week 8) Flashcards
Our skeletal muscles are composed of bundles of ___________
fascicles
Each fascicle is composed of linearly aligned ______________
muscle fibers (myofibres)
a single, multi-nucleated, elongated cell, composed of many sarcomeres that are arranged linearly
muscle fibre/myofiber
What the cell membrane of a muscle fibre called?
sarcolemma
Sarcomeres are composed of _______________
myofibrils
Note: myofibrils are organelles
the connective tissue sheath that surrounds the whole muscle, that extends from the tendons
epimysium
the sheath surrounding each fascicle
perimysium
the sheath surrounding each individual muscle fibre
endomysium
Note: below the endomysium is the sarcolemma (cell membrane of a muscle cell)
Each muscle fibre contains how many myofibrils?
1000-2000
Each myofibril is composed of many myofilaments. What is a myofilament composed of?
- contractile proteins (e.g., actin, myosin)
- regulatory proteins (e.g., tropomyosin, troponin)
- accessory proteins (e.g., actinin, dystrophin, titin, nebulin)
A sarcomere runs from __________________
Z disc to Z disc
the contractile unit of skeletal muscle
sarcomere
Thin filaments are made of __________
actin
Thick filaments are made of _____________
myosin
What gives the striated appearance of skeletal muscle?
the overlapping of thick and thin filaments
the center of the sarcomere
M-line
True or False: thin filament (actin) attaches to the Z discs and thick filament (myosin) attaches to the M-line
True
The A band (darker band) is composed of ___________
actin and myosin (they overlap)
Note: However, within the A band, there is a portion that is only myosin = H zone
The I band (lighter band) is composed of ____________
actin only
centre of I bands (light bands)
Z disc
What part of the sarcomere shortens during contraction?
I band
monomer of actin
G actin
polymer of actin
F actin
Thin filaments are composed of two strands of _____________ wound together
F actin
Each G actin (monomer of actin) has a binding site for ____________
myosin
Each unit of ___________ is composed of a head and tail region
myosin
Head region of myosin forms _____________ that interact with adjacent actin filaments
cross-bridges
What are the three important chemical features at the myosin head region?
1) ATP activity
2) actin-binding region
3) ATP binding region
Note: ATP is required to change the conformation of the hinge
___________ and __________ are regulatory proteins that associates with actin
tropomyosin and troponin
When a skeletal muscle is in a ______________ (relaxed/contracted) state, tropomyosin molecules cover the myosin-binding site on G-actin monomers
relaxed
Note: this prevents cross-bridge formation between actin and myosin
Troponin forms a complex of 3 subunits. What are they?
1) Troponin C
2) Troponin T
3) Troponin I
Troponin C binds to ?
calcium
Troponin T binds to?
tropomyosin
Troponin I binds to?
actin
True or False: When calcium binds to troponin C, the troponin complex undergoes a conformational change and troponin T “pulls” tropomyosin and troponin I off of the myosin-binding site of G actin subunits. This allows cross-bridge formation to occur.
True
invaginations within the sarcolemma that allows the action potential to be carried deep into the muscle fibre
transverse tubules (T tubules)
Note: T tubules are continuous with the extracellular fluid
a specialized endoplasmic reticulum containing high concentrations of calcium
sarcoplasmic reticulum
___________________ are specialized regions of the sarcoplasmic reticulum that associated with the T-tubules
terminal cisternae
The junction between the T-tubules and the terminal cisternae is called what?
muscle triad
(think of T tubule “sandwiched” between terminal cisternae on either side of it)
True or False: The concentration of calcium in the ECF (extracellular fluid) and the sarcoplasmic reticulum (SR) is generally low
False
The calcium concentration is high in both the ECF and SR (compared to the cytosol). This means that if you open up a calcium channel, it will go right into the cytosol.
a motor nerve axon contacts each muscle fiber near the middle of the fiber, forming a synapse called what the ________________
neuromuscular junction
the region of the sarcolemma in closest contact with the presynaptic nerve terminal is called the _____________
motor end plate
What neurotransmitter does the motor nerve synapse release onto the muscle fibre’s nicotinic receptors?
acetylcholine (Ach)
When Ach binds to the nicotinic receptors on the sarcolemma, this activates voltage-gated sodium channels which triggers an ______________, which propagates along the sarcolemma
action potential
Note: a skeletal muscle action potential looks very similar to a neuronal action potential but is slightly longer in duration
skeletal muscle AP = 5-10 msec
neuronal AP = 1-2 msec
The action potential travels into the T-tubule, bringing depolarization deep into the muscle fiber, signalling/exciting _____________ release from the sarcoplasmic reticulum
calcium
When the action potential propagates along the T-tubules, it activates _____________ channels in the sarcolemma
L-type Ca2+
Note: activation of these channels also triggers activation of Ryanodine receptors on the terminal cisternae… let’s out a ton of calcium out of the sarcoplasmic reticulum (SR)
After calcium flows down its concentration gradient (out of the SR and into the cytosol), it then binds to ______________, to expose the binding sites for myosin
troponin C
True or False: Most of the calcium that activates skeletal muscle comes from the ECF
False
Most comes from the sarcoplasmic reticulum (SR)
What is the average number of alpha motor neurons that innervate a typical muscle?
~100
The number of muscle fibers that a motor neuron reaches out to is referred to as what?
a motor unit
the result of a single action potential from a motor neuron that barely causes any force development in the muscle fibers that it innervates
a twitch
How do you get smooth, forceful movements?
occur when the alpha-motor neuron sends MANY action potentials to the motor unit, at a relatively high frequency
summation of the APs prevent calcium concentrations from dropping (calcium keeps getting released), creating a peak of force
True or False: fused tetanus/maximal force generation in skeletal muscle requires a lot of ATP
True
True or False: Individual “twitch times” vary depending on the role of the muscle
True
Example: Your soleus, meant to keep you standing, is a slow muscle. Therefore, one action potential results in a very long twitch (~200msec). Whereas your gastrocnemius helps you more when you’re running to help you push off the ground. This has a faster twitch time (~70msec), because it generates more force. Ocular muscle twitch times are very fast (~25msec)
