Muscle Questions

Only need info. In BOLD to get full credit

  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.

  1. 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

  1. 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

  1. 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.

  1. 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