1. The current world champion weightlifter has an annoying problem. She has always had difficulty with very fine movements. For example, she would over-power arm movements. She does quite well when applying her full strength, such as when bench pressing 550 pounds. However, when she tries to lift something gently, the result can be a disaster. Last week she went into a china shop and was checking the prices on small tea cups - after flipping a couple in the air she paid for the damage and left the shop in tears. Since she "bulked up" the problem has really become acute. Although she feels fine and is quite normal otherwise (well, as normal as possible under the circumstances) it seems that she has little fine control over strength. What is the problem related to?
Excess release of acetylcholine at the synaptic cleft.
Abnormal height of the muscle action potential.
A biochemical abnormality at the level of the sarcomere.
Unusual neuromuscular anatomical organization.
This is incorrect. Excess release of Ach, if it resulted in persistance of Ach in the synaptic cleft, might affect the state of polarization of the muscle membrane, which in turn affects its ability to be restimulated. Excess release of neurotransmitter should have no bearing on strength, however.
Sorry, this is incorrect. The "all-or-none" principle of muscle contraction holds that the signal coming to the mucle either triggers a depolarization followed by contraction, or it doesn't. The actual height of the action potential is not relevant, unless it is insufficient to stimuate release of neurotransmitter in the first place.
Incorrect. It is hard to think of what sort of abnormality would lead to excess strength of contraction of a sarcomere. Anyway, the problem was with fine control of muscle strength, and contraction in a muscle cell is all-or-none. Control does not lie primarily with regulation of strength of contraction or individual cells, except by repeated stimulation leading to tetanus.
Correct! They key word here is organization. It is the organization of motor neurons and muscle cells that allows fine control in the first place. The basic unit of contraction is the motor unit, consisting of a number of muscles cells and a single motor neuron which brances to innervate each of the muscle cells. The more motor units that are activated, the greater the strength of contraction. The fewer the number of individual motor units, the less fine control one has. Muscles that require extremely fine control, such as in the fingers or larynx, have a very low ratio of muscle cells to motor neurons, as low as 1:1. In bulky muscles, such as the gluteus, the ratio might be more like 1:1000. The weightlifter likely has very few motor units in her biceps relative to the average person, or she has somehow "bulked up" some motor units but not others. In any event, she is unable to recruit muscle strength in small increments. An attempt to slightly increase strength recruits too much, so her power increase in large instead of small increments.
Return to Question #1 or Return to Case/Problem menu or Return to Home Page