Problem:
Muscular Dysgeny
A case study nervous and muscular physiology
You are a research scientist interested in finding the cause of the cellular defect associated with the genetic disease Muscular dysgeny. You already know it is a lethal recessive, genetic disease of mice that is caused by a mutation in the mdg gene. Animals with this mutation die shortly after birth. You are interested in finding the exact defect associated with this mutation.
You surgically remove a single motor unit from a normal mouse fetus and a dysgenic mouse fetus. You place the neuron/muscle fiber unit in an experimental chamber in order to study abnormalities.
Question 1: Define each of the following:
motor neuron
synapse
synaptic knob (endbulb)
sarcolemma
t-tubules
sarcoplasmic reticulum
motor end plate.
Question 2: Place the following steps for an action potential in the correct order:
1. Voltage gated Na+ channels open
2. Na+ rushes in
3. Potential moves toward and above zero to + 30 mV
4. Voltage gated K+ channels open
5. K+ rushes out
6. Na+/K+ pump restores RP
7. Graded potential reaches TP
8. Potential moves toward RP and past to -90 mV
9. Voltage gated K+ channels close
10. Voltage gated Na+ close
a. 7,1,3,4,2,6,5,8,10,9
b. 7,1,2,3,10,9,4,5,8,6
c. 7,1,2,3,10,4,5,8,9,6
d. 7,1,2,8,9,6,4,5,10,3
e. 7,1,4,5,3,1,2,8,10,6
Question 3: Place the following steps in synaptic transmission in the correct order:
1. Ca++ enters
2. Exocytosis of ACH occurs
3. ACH binds to chemical gated channel
4. Voltage gated Ca++ channels opened
5. EPSP or IPSP occurs
6. AP arrives at telodendrion
7. Vesicles migrate and fuse to membrane
8. Acetylcholinesterase breaks down ACH
a. 4,1,3,2,5,7,6,8
b. 6,7,1,2,4,3,5,8
c. 6,1,3,4,2,5,7,8
d. 6,1,7,2,3,5,4,8
e. 6,4,1,7,2,3,5,8
Question 4: You briefly apply acetylcholine (ACH) directly to the motor end plate and observe the results. After ACH is applied to the muscle fiber, there is an AP generated on the sarcolemma of both the normal and dysgenic muscle fibers.
Question 5: Which of the following is NOT PROVEN by the above result?
ACH receptors are present in the membrane of the motor end plate
ACH binds to receptors
Sodium permeability of the end plate membrane is increased
Acetylcholinesterase breaks down ACH.
The normal mouse fiber contracts after the ACH application but the dysgenic mouse fiber does not.
Question 6: Do you suspect that the motor neurons are working normally? Why or Why not? Does this test whether the defect seen in dysgeny is in the motor neuron or the muscle fiber. Briefly discuss how you came to your conclusion.
Question 7: Draw and label a sarcomere: Include all bands and zones and indicate where the thick and thin filaments are located. List the proteins that make up the thick filament and the thin filament. List the binding sites found on each.
Question 8: Place the following steps in a muscle contraction in the correct order:
1. AP spreads down sarcolemma and t-tubules
2. AP on t-tubules opens voltage gated Ca++ channels on the sarcoplasmic reticulum
3. Ca++ leaves sarcoplasmic reticulum and binds to troponin
4. End-plate potential triggers an action potential on the sarcolemma
5. ATP is split and energy released is used to re-cock the myosin heads
6. Action is repeated over and over until AP ends and Ca++ is returned to sarcoplasmic reticulum
7. Troponin changes shape and pulls tropomyosin off the myosin binding sites on actin
8. Myosin heads bind to actin
9. ACH binds to chemical gated Na+/K+ channels
10. Na+ enters and K+ leaves resulting in an end-plate potential
11. Myosin heads rotate toward center of sarcomere pulling thin filament
12. Sarcomere shortens
13. ATP binds to myosin heads and myosin-actin bond is broken
a. 9,10,4,1,8,7,11,13,5,6,2,3,12
b. 9,10,4,1,8,7,11,13,2,3,5,6,12
c. 9,10,4,1,8,7,2,3,11,13,5,6,12
d. 9,10,4,1,2,3,7,8,11,12,13,5,6
e. 9,10,4,1,2,3,7,8,11,5,6,12,13
Question 9: Draw and label the expected graph of a muscle twitch of a normal fiber (time = 60 ms, force = 3 gms).
Label and briefly describe the activities in the muscle during the:
latent period
contraction period
relaxation period
Question 10: Draw a muscle twitch (max force muscle can produce is 3 gms) with a 2.5 gm weight attached and with a 5gm weight attached. Include the length of the muscle as well as the tension produced. Label as to whether the graph shows an isotonic or isometric contraction
Question 11: You artificially raise the calcium concentration inside the myofibrils and observe that both the normal and dysgenic fibers contract. From this observation, where would you conclude that the defect occurs in the steps of muscle contraction? Briefly explain how you came to this conclusion.
Exposure of the active sites on actin
Binding of the calcium to troponin
Release of the calcium ions from the sarcoplasmic reticulum into the myofibril
The repeated cycles of cross-bridge formation, myosin power strokes, and detachment
Question 12: Where in the muscle fiber do you suspect that the normal protein made by the mdg gene functions in normal mice?
Question 13: What causes the mice to die shortly after birth (i.e. What do they die from?)
Question 14: Is this spastic paralysis or flaccid paralysis? Which of the following would have the same effect (same type of paralysis)--on the muscles of the mouse? Briefly discuss your reasoning.
a. Botulism toxin
b. Poisoning with atropine
c. Poisoning with military nerve gas
d. Tetanus toxin
Please read all the questions carefully and answer them.