Question

Can individual myocytes change their force of contraction?

Answer 1

Individual myocytes can change their force of contraction based on the number of myofibers receiving action potentials, the frequency of these potentials, and the cellular environment, such as substrate stiffness and conformational changes of the myosin heads.

Step-by-step explanation:

Yes, individual myocytes can change their force of contraction. This is largely influenced by the number of myofibers within the muscle that receive an action potential, which in turn is controlled by the neuron associated with that fiber. When the motor cortex sends out fewer signals, as when lifting a light object like a pencil, fewer myofibers contract. In contrast, lifting a heavy object like a piano requires more extensive neuronal signaling, causing all myofibers to participate, generating nearly maximum force. Moreover, the frequency of action potentials can also increase the force by increasing Ca2+ concentration around the tropomyosin. Electron microscopic examinations provide evidence that myosin head conformation is flexible and supports different stable conformations, essential for the contraction cycle. Additionally, cellular mechanics suggest that myosin II mini filaments within the cytoskeleton are individual molecular force generators that influence the contraction force. Lastly, experiments demonstrate that factors like substrate stiffness can regulate cytoskeletal organization and myofibril assembly, influencing the contraction strength, and showing that myocytes adapt and change force according to various internal and external cues.

Answer 2

Individual myocytes can change their force of contraction by varying the number of myofibers that receive a neural action potential, the frequency of these potentials, and the myosin head conformation, responding dynamically to different stimuli.

Step-by-step explanation:

Yes, individual myocytes (muscle cells) can change their force of contraction. The force of contraction within a myocyte is primarily determined by the number of myofibers within the muscle that receive an action potential from the neuron that controls that fiber. For instance, picking up a pencil only requires a few myofibers to contract, whereas lifting a heavy object like a piano involves the contraction of all available myofibers in the muscle, producing maximum force. Additionally, increasing the frequency of action potentials can boost the force slightly, due to elevated calcium influx making tropomyosin more receptive to these signals. Electron microscopic analysis reveals that the myosin heads are flexible and can adopt different conformations during the contraction cycle, suggesting that this molecular flexibility facilitates adjustment in force generation.

The way myocytes respond to changes, such as being stretched, is also indicative of their ability to modify contraction force. Cells can dynamically adjust their contractile force through mechanisms like actin filament depolymerization and re-polymerization in a calcium-dependent manner. In turn, these cellular adjustments influence force production at a tissue level, reflecting how individual myocytes can fine-tune their force of contraction in response to varying stimuli.