Question

Using the concept of motor units, explain how skeletal muscles in vivo produce graded and sustained contractions.

Short answer

In vivo, skeletal muscles produce graded and sustained contractions through the concept of motor units, which consist of a motor neuron and the muscle fibers it innervates. Graded contractions are achieved by motor unit recruitment, where smaller motor units are activated first for minimal force and larger units are recruited as more force is needed, and by frequency of stimulation, where the muscle can produce stronger contractions through wave summation or tetanic contractions if stimuli are provided before the muscle fully relaxes. Sustained contractions, or muscle tone, are maintained by the brain continuously activating a rotating pool of motor units to prevent fatigue. Overall, motor unit recruitment and varied frequency of stimuli allow muscles to finely control the force of contraction and maintain contraction for extended periods without fatigue.

Step-by-step solution

Define Motor Unit

A motor unit refers to a single motor neuron and all of the skeletal muscle fibers it innervates. When a motor neuron is activated, all the muscle fibers connected to it contract.

Explain the Process of Graded Muscle Contractions

Varying degrees of force output from a muscle, or graded contractions, are achieved through two main mechanisms: motor unit recruitment and frequency of stimulation.

Explaining the Principle of Motor Unit Recruitment

Motor unit recruitment refers to the process by which the brain increases the force of muscular contraction by activating more motor units. When minimal force is needed, only smaller motor units (containing fewer muscle fibers) are activated. As more force is needed, larger motor units (containing more muscle fibers) are activated. This is known as the 'size principle,' where smaller motor units are recruited first, followed by larger ones.

Explain the Frequency of Stimulation

The frequency of stimulation is the second method of controlling the force exerted by the muscle. If a second stimulus is given after the first but before the muscle has fully relaxed, the second contraction will be stronger than the first. This is known as wave summation. If the stimuli are provided often enough, the muscle does not relax fully between stimuli, resulting in a stronger contraction known as a tetanus.

Describe the Sustained Contractions

Sustained contractions, or muscle tone, are low-level, continuous contractions that occur even when the muscle is at rest. Muscle tone is important for maintaining posture, stabilizing joints and keeping muscles ready for action. During a sustained contraction the brain continuously activates a rotating pool of motor units to prevent fatigue.

Combine the Above Concepts

To sum up, in vivo, motor units play a critical role in allowing skeletal muscles to produce graded and sustained contractions. Through the principle of motor unit recruitment and varied frequency of stimuli, muscles can finely control the force of contraction. And, by rotating activations among motor units, muscles can maintain contraction for extended periods without fatigue.