Motor Unit Activation in Skeletal Muscles

How is motor unit activation achieved in skeletal muscles?

A. Through the recruitment of additional motor units

B. Through the stimulation of muscle fibers by motor neurons

C. Through the interaction of actin and myosin filaments

Answer:

B. Through the stimulation of muscle fibers by motor neurons

Motor unit activation in skeletal muscles is achieved through the stimulation of muscle fibers by motor neurons. A motor unit consists of a single motor neuron and all the muscle fibers it innervates. When the motor neuron fires an action potential, it causes the muscle fibers in the motor unit to contract.

The process of motor unit activation begins with the release of neurotransmitters from the motor neuron into the neuromuscular junction. These neurotransmitters bind to receptors on the muscle fiber membrane, leading to depolarization and the generation of an action potential along the muscle fiber.

Once the action potential reaches the sarcoplasmic reticulum, it triggers the release of calcium ions, which then bind to troponin and initiate the contraction process. This results in the shortening of sarcomeres and ultimately the generation of force by the muscle.

Motor unit activation can be graded by the recruitment of additional motor units. When greater force is required, more motor units are recruited to increase the overall muscle tension. This phenomenon is known as recruitment of motor units and is essential for generating varying levels of force in skeletal muscles.