Muscle - Lecture 2

Question 1

First step of cross-bridge cycle + myosin head group state

Answer 1

Myosin dissociates from actins by releasing ADP + Pi and binding ATP. Myosin-relaxed state

Question 2

Second step of cross-bridge cycle + myosin head group state

Answer 2

Myosin hydrolizes ATP (ADP +Pi in actin), is still dissociated from actins but is in active state (direction where it reaches for actins)

Question 3

Third step of cross-bridge cycle + myosin head group state

Answer 3

After myosin is in the direction where it reaches for actins, it binds to actins (ADP + Pi in actin)

Question 4

Fourth step of cross-bridge cycle + myosin head group state

Answer 4

Binding of myosin to actins results in power stroke, thin filaments pushed to M-line. Myosin back to relaxed state but still binds actins

Question 5

How myosin binds to actins

Answer 5

Each actin (actins are globular and organized in 2 strands forming a helix) has a binding site for myosin head groups

Question 6

Why dead organism stays stiff for approx 24 hours after death (contracted muscles)

Answer 6

Step 1 can’t happen because ATP levels in cells drop after death so release of ADP + Pi and ATP binding (it’s ATP binding in fact) not possible for myosin dissociation

Question 7

Region name in brain responsible for movement

Answer 7

motor cortex

Question 8

2 steps of signal going from brain to muscle cells (muscle fibers)

Answer 8

1) Neuron from motor cortex goes in spinal cord and does synapse with efferent fiber (motor neuron) in spinal cord.
2) Motor neuron does synapse with multiple muscle fibers

Question 9

What is a motor unit (2)

Answer 9

Motor neuron and group of muscle fibers it innervates

Question 10

Range of how many muscle fibers can be innervated by a single neuron and what this influences

Answer 10

10 to thousands. influences movement precision. The more fibers it communicates to = less precise movement

Question 11

One motor neuron can innervate ___________ muscle fiber(s) but one muscle fiber can be innervated by ________ neuron(s)

Answer 11

multiple. one.

Question 12

Spinal cord physiology

Answer 12

White matter (myelinated axons) on outside and grey matter (cell bodies, dendrites, synapses) on inside (H form). Dorsal and ventral sides.

Question 13

Synapses between neurons from motor cortex to motor neurons in the spinal cord happen in which region of the spinal cord

Answer 13

Happen in grey matter (because synapses) in ventral region

Question 14

Motor neuron - Muscle fiber synapses size vs Brain neurons synapses size

Answer 14

Very large compared to synapses in the brain

Question 15

What is called the synapse between motor neurons and muscle fibers

Answer 15

Neuromuscular junction

Question 16

2 important structures found in presynaptic terminal of motor neuron

Answer 16

Active zone and ACh synaptic vesicles (Acetylcholine)

Question 17

Which neurotransmitter involved in neuromuscular junction

Answer 17

Acetylcholine

Question 18

What’s the neurotransmitter receptor found on muscle fiber membrane at the neuromuscular junction

Answer 18

nACh receptors - nicotinic acetylcholine receptors

Question 19

Name of region where nACh receptors are found and 2 characteristics

Answer 19

Is called the endplate (postsynaptic membrane of the neuromuscular junction). 1) Has multiple junctional folds (of the membrane) (folds are regions that go on the inside) 2) Has regions of the membrane rich in nACh receptors - specialized regions

Question 20

First step of neuromuscular junction

Answer 20

AP potential in motor neuron reaches presynaptic terminal and ACh is released at presynaptic terminal

Question 21

Second step of neuromuscular junction

Answer 21

ACh activates nACh receptors and Na+ flows inside the endplate

Question 22

Third step of neuromuscular junction

Answer 22

Endplate potential (special name for the EPSP) is created and one is obtained to reach AP threshold.

Question 23

Fourth step of neuromuscular junction (+ where neuromuscular junction is found on a muscle fiber)

Answer 23

Fiber (muscle cell) action potential propagates in both directions in the muscle fiber because neuromuscular junction is in the middle

Question 24

Why one endplate potential is sufficient to reach AP threshold and resting membrane potential + Variation in membrane potential caused by one endplate potential

Answer 24

Because synapse is very large, lots of ACh released. lots of nACh receptors released. Resting potential is -90 mV (-70 in neurons), and one endplate potential causes +20 to +30 variation in membrane potential

Question 25

How muscle fibers propagate APs

Answer 25

Have voltage-gated sodium channels along their membrane. Not same gene as those in neurons but are similar

Question 26

2 important structures in muscle fiber

Answer 26

T-tubules and sarcoplasmic reticulum

Question 27

T-tubules characteristics (2)

Answer 27

1) Irrigate cell inside and are continuous with external environment
2) They separate 2 sarcomeres

Question 28

Sarcoplasmic reticulum characteristics (2)

Answer 28

1) Covers and surrounds each myofibril (each myofibril surrounded by sarcoplasmic reticulum)
2) Rich in calcium (cytoplasm is not)

Question 29

Receptors on cell membrane name and Receptors on sarcoplasmic reticulum name

Answer 29

on cell membrane : DHP receptor (calcium channel but calcium does not cross it)
on sarcoplasmic reticulum : Ryanodine receptors

Question 30

How DHP receptors can be activated

Answer 30

Binding of ions.

Question 31

How calcium flows in the cytoplasm

Answer 31

When AP reaches DHP receptors, it activates them, they change their conformation and this changes ryanodine receptors’ conformation because DHP and ryanodine receptors are coupled. Activated ryanodine lets Ca 2+ flow out of SR into cytoplasm.