Question

A researcher obtained three cultures (A, B, and C) of human skeletal muscle cells. Each culture was known to represent one of the skeletal muscle fiber subtypes. The researcher made observations on two of the three cultures (Culture A and Culture B) and was able to infer the identity of Culture C from these observations.

Culture A appeared white when visualized with a microscope. When an electrical current was applied to the culture medium, the concentrations of lactate and H+ ions in that medium increased. The pH of the medium changed from 7 to 5 after 2 minutes of continuous, electrically-stimulated contractions.
Microscopic visualization showed that Culture B was red in appearance and had a high mitochondrial density, numerous intact capillaries, and abundant intracellular triglyceride deposits. Upon electrical stimulation, these cells exhibited a slow contraction phase of approximately 10 contractions per second and a very slow rate of ATP utilization.
In both cultures, the addition of acetylcholine to the culture media altered the intracellular electrical charge of the cells. However, only cells from Culture B were able to maintain contractile activity after 30 minutes of continuous acetylcholine infusion.
The researcher combined the results of the observations with the information in Table 1 to identify the muscle subtype represented by each cell culture.
Table 1
Biochemical and Contractile Properties of the Skeletal Muscle Fiber Subtypes
Property/ Muscle fiber subtype
I /IIa /IIX
Twitch duration long moderately fast fast
Size of motor neuron small medium large
Activity used for:aerobic/long-term anaerobic/short-term anaerobic
Maximum duration of use:hours/ <30 minutes <5 minutes
Power produced: low medium high
Oxidative capacity high high intermediate
Glycolytic capacity low high high

The muscle subtype represented by Culture C is LEAST likely to be characterized by:
A) a fast rate of muscle contraction.
B) the ability to engage in oxidative and anaerobic respiration.
C) the presence of medium-sized motor units.
D) low densities of mitochondria and capillaries.

Answer 1

Culture C is inferred to be a fast oxidative (FO) fiber type that possesses characteristics between those of the fast glycolytic (FG) fibers in Culture A and slow oxidative (SO) fibers in Culture B. Therefore, Culture C is LEAST likely to be characterized by low densities of mitochondria and capillaries, which is characteristic of FG fibers. B) the ability to engage in oxidative and anaerobic respiration.

Step-by-step explanation:

The identification of muscle fiber subtypes in the three cultures (A, B, and C) is based on specific characteristics observed in Culture A and Culture B, as well as the known properties of muscle fibers. Culture A exhibited increased lactate and H+ concentrations and a pH change from 7 to 5 after electrical stimulation, indicating that it is likely a fast glycolytic (FG) fiber type, which includes anaerobic glycolytic properties and a fast rate of muscle contraction. Culture B, showing high mitochondrial density, red appearance, slow contraction rate, and the ability to maintain contractility with acetylcholine for a prolonged time, suggests it is a slow oxidative (SO) fiber type, which is characterized by a slow rate of muscle contraction and high oxidative capacity.

Given that Culture C must differ from the observed characteristics of Cultures A and B, it is inferred to be a fiber type sitting between FG and SO, likely a fast oxidative (FO) fiber type. Therefore, the muscle subtype represented by Culture C is LEAST likely to be characterized by (D) low densities of mitochondria and capillaries, as FO fibers still have high oxidative capacities but are faster and possess both oxidative and glycolytic properties to a degree.