Week 4 - Muscle Structure and Function Flashcards
What % of total body mass is made up of skeletal muscle?
40-50%
over 600 voluntary skeletal muscles
5 functions of skeletal muscle
1) Force production for locomotion and breathing
2) Force production for postural support
3) Heat production during cold stress to maintain internal body temperature
4) Acts as an endocrine organ
5) Support and stabilization of joints
Explain the 2 muscle actions.
- Flexors (decrease angle joint)
- Extensors (increase angle joint)
They attach to bones by tendons
Define the key words that make up the structure of skeletal muscle.
a) Epimysium
b) Perimysium
c) Endomysium
d) Basement membrane
e) Sarcolemma
a) Surrounds entire muscle
b) Surrounds fascicles (bunch of muscle fibres)
c) Surrounds individual muscle fibers
d) Below endomysium
e) muscle cell membrane
What do myofibrils contain?
contain contractile proteins; actin (thin filament) and myosin (thick filament) in the sarcomere
Sarcomere
The main functional/ contractile unit of a muscle fiber.
Includes Z line, M line, H zone, A band and I band.
a) Z line
b) M line
c) A band
d) I band
a) define the boundaries of a sarcomere
b) middle of sarcomere
c) contains actin and myosin
d) contains actin
Sarcoplasmic reticulum
storage sites for calcium and the terminal cisternae lies between each sarcoplasmic reticulum
Transverse tubules
extend from sarcolemma to sarcoplasmic reticulum
Role of satellite cells
Undifferentiated cells reside above sarcolemma - they have a key role in muscle growth and repair. During muscle growth, they increase the number of nuclei in mature muscles fibers (muscle hypertrophy).
Motor unit
motor neuron and all the muscle fibers it innervates
Neuromuscular junction
chemical synapse between motor neuron and muscle fiber - made up of axon terminal, synapse and motor end plate of muscle fiber.
Acetylcholine
neurotransmitter released from the motor neuron into the synapse where it binds to nicotinic receptors on the motor end plate of the sarcolemma that causes an end-plate potential (EPP) (influx of sodium ions into muscle fiber) thus depolarization of the muscle fiber and signaling for muscle contraction.
Myonuclear domain - What is it and why is it important?
the volume of sarcoplasm surrounding an individual nucleus and its important as a single nucleus is responsible for the gene expression for its surrounding sarcoplasm.
What does more myonuclei allow for?
greater protein synthesis resulting in muscle hypertrophy
Training adaptations of the neuromuscular junction
- Increased size of NMJ
- Increased number of synaptic vesicles releasing ACh
- Increase number of ACh receptors on post-synaptic membrane
Sliding filament model of muscular contraction
refers to the formation of cross bridges which allows actin filaments to slide over myosin and the muscle shortens (power stroke). There is a reduction in the distance between the Z lines of the sarcomere.
ATP role in muscle contraction
ATP hydrolyzed via enzyme myosin ATPase (ATP –> ADP + P) provides energy required for cross-bridge formation and a power stroke.
Sources of ATP: PCr, glycolysis, and oxidative phosphorylation.
True or False - A band changes size during contraction
False - A band stays the same size
What % of resting length is a muscle shortened during a single contraction cycle? During a repeated contraction cycle, what % of resting length can muscle shorten?
1% + up to 60%
Brief outline of the 5 steps of crossbridge cycling.
1) ATP binds to myosin head and breaks link between actin and myosin.
2) ATP hydrolysis (via myosin ATPase) with ADP and Pi remaining attached to myosin head.
3) Pi dissociated from myosin which allows myosin to bind with actin binding sites and form a cross-bridge.
4) Power stroke occurs with the muscle shortening to generate force.
5) ADP dissociates from myosin and myosin awaits another ATP molecule.
What is excitation-contraction coupling?
The sequence of events where nerve impulses reaches the muscle membrane and leads to muscle shortening by cross-bridge activity.
What is released to provide the energy needed for the myosin head to pull on the actin filament and initiate a powerstroke?
the release of Pi from the myosin head
Sequence of events leading to muscle contraction
- Signal from motor nerve reaches axon terminal
- Stimulates synaptic vesicles to release ACh across synaptic cleft and to the ACh receptors on the sarcolemma of muscle fibre.
- The release of ACh causes excitation of muscle fiber (sodium ions cause depolarization).
- This causes the propagation of AP which travels along the sarcolemma and down the T tubule which causes depolarization of muscle fibre.
- This results in opening of calcium ion channels from sarcoplasmic reticulum and terminal cisterna of SR. This increase intracellular Ca2+ concentration in the muscle fiber (calcium released into sarcoplasm).
- Calcium ions binds to troponin (causing conformational change) which causes tropomyosin to move from the actin-binding sites. (ATP used)
- Energized myosin heads bind to the actin binding site to form crossbridges and pulls on the actin molecule to produce a back-and-forth movement (power stroke). This causes muscular contraction.
- When ACh release/nerve impulse to muscle stops (muscle fiber is repolarized) and Ca2+ is removed by reuptake into the SR, crossbridge cannot form and the muscle relaxes.
Fatigue
a decline in muscle power output