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Muscle Questions
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1. An Action Potential arrives at a synaptic terminal of a motor neuron. Please describe the
sequence of major events that ultimately result in the contraction of a sarcomere: Tell me
all the relevant ions, integral proteins, protein subunits and protein filaments and describe
how they interact (12).
Arrival of AP causes voltage-gated Ca2+ channels in the axon terminal to open; Ca2+streams
in, causing secretory vesicles filled with ACh to fuse with plasma membrane of neuron;
Ach spills into synaptic cleft, diffuses across to sarcolemma and binds to Ach receptors
(which are gated channel proteins) on sarcolemma; Gated channels open, allowing Na+ to
rush in, along its concentration gradient; Na+ influx causes neighboring, voltage-gated Na+
channels to open, which in turn cause their neighboring channels to open; Influx of Na+
ions generates AP, which travels down t-tubules and causes the SR to eject its Ca2+ ions;
Ca2+ binds to troponin; troponin/ Ca2+ complex moves tropomyosin, and myosin binds to
active sites on actin subunits.
2. In a skeletal muscle cell, what molecule (we’ll call it molecule X from now on) provides
the energy necessary to do work? I mentioned two metabolic pathways that skeletal
muscle cells can use to produce molecule X. Please name those two pathways, then tell
me: where in the cell each one occurs, how much of molecule X each pathway produces,
and whether or not O2 is required. (7)
Molecule X = ATP.
Anaerobic Glycolysis (without O2): Occurs in cytoplasm. Pyruvate products quickly
converted to lactic acid. 1 Glucose = 2 ATP
Aerobic glycolysis (with O2): Occurs in cytoplasm. Pyruvate products quickly move into
mitochondria, where further electron stripping takes place. 1 Glucose = 38 total ATP
Creatine Phoshate cycle (without O2): Occurs in cytoplasm and in sarcomeres…Occurs
anywhere ATP is needed and CP and ADP are present. 1CP = 1 ATP
3. Working muscles fatigue either because they lack something (like a car running out of
gas), or because they’ve got too much of something. Tell me what two things they might
lack and what one thing they might have too much of. (3)
Lack O2 or ATP; Have too much lactic acid
4. The muscle I use to do a pull-up and the muscles I use to sneer at you are very different
in size and shape. Please tell me: what muscle I would mainly use to perform each
activity, how many motor units you expect each muscle to have, and how many muscle
fibers you expect each motor neuron to control. Tell me why for each case? (6)
Pull-up = Latissimus dorsi; Many motor units; many fibers innervated by each motor
neuron (large motor units). Only gross control is required for this large movement, and it’s
a big muscle.
Sneer = leveator labii superioris; few motor units; few fibers innervated by each motor
neuron (small motor units). Precise control of small, delicate movements is required & the
muscle itself is very small.
5. What ridiculously important ion is used in both the pre-synaptic motor neuron and the
post-synaptic muscle cell when an action potential stimulates a muscle contraction? What
does the ion do in each case? (3)
Ca2+ enters presynaptic terminal and causes vesicles full of Ach to fuse with PM. It also binds
to troponin to allow tropomyosin to expose active site on actin filaments