18 Questions
What is the purpose of the different pattern of motor unit activity in a muscle?
To delay muscle fatigue
What is the relationship between motor unit size and the precision of movements?
Smaller motor units produce more precise movements
How are motor units recruited as the force requirement increases?
Motor units are recruited in a specific order, with progressively stronger ones added
How does the recruitment of motor units contribute to smooth movements?
It allows for precise control of muscle force
How do the different recruitment patterns of motor units contribute to muscle function?
They allow for the production of both powerful and precise movements
What is the main difference between the role of large and small motor units?
Small motor units are used for precise movements, while large motor units are used for powerful contractions
What is a motor unit?
A group of muscle fibers dispersed throughout a muscle
Which type of muscles have small motor units with as few as two or three muscle fibers per unit?
Muscles controlling voice production
What determines the total strength of a muscle contraction?
The number of muscle fibers in a motor unit
How long do twitches of skeletal muscle fibers typically last?
20 to 200 msec
In response to a single action potential, what occurs in a motor unit?
All the muscle fibers in the motor unit contract briefly
Which muscles have large-scale motor units with as many as 2000 to 3000 muscle fibers?
Muscles responsible for powerful movements
What is a motor unit composed of?
A somatic motor neuron and all the skeletal muscle fibers it stimulates
What determines the total force a single muscle fiber can produce?
The rate at which nerve impulses arrive at the neuromuscular junction
How many skeletal muscle fibers does an average motor unit consist of?
150
What is the main factor that influences the maximum tension a muscle fiber can generate?
The frequency of stimulation by nerve impulses
What does the force of muscle fiber contraction depend on?
The size of the muscle fiber
How do all the muscle fibers in one motor unit contract?
In unison
Study Notes
Muscle Fibers and Motor Units
- Muscle fibers of a motor unit are dispersed throughout a muscle rather than clustered together.
- Whole muscles controlling precise movements, such as the larynx (voice box), consist of many small motor units, with as few as 2-3 muscle fibers per motor unit.
- Muscles controlling eye movements may have 10-20 muscle fibers per motor unit.
- In contrast, skeletal muscles responsible for large-scale and powerful movements, such as the biceps brachii muscle in the arm and the gastrocnemius muscle in the calf of the leg, have as many as 2000 to 3000 muscle fibers in some motor units.
Twitch Contraction
- A twitch contraction is the brief contraction of all the muscle fibers in a motor unit in response to a single action potential in its motor neuron.
- Twitches of skeletal muscle fibers last anywhere from 20 to 200 msec, much longer than the brief 1-2 msec of a muscle action potential.
- A single nerve impulse in a somatic motor neuron elicits a single muscle action potential in all the skeletal muscle fibers with which it forms synapses.
Motor Units
- A motor unit consists of a somatic motor neuron plus all the skeletal muscle fibers it stimulates.
- A single somatic motor neuron makes contact with an average of 150 skeletal muscle fibers, and all the muscle fibers in one motor unit contract in unison.
- Typically, the different motor units of an entire muscle are not stimulated to contract in unison, with some motor units contracting while others are relaxed.
Recruitment and Muscle Fatigue
- The weakest motor units are recruited first, with progressively stronger motor units added if the task requires more force.
- Recruitment is one factor responsible for producing smooth movements rather than a series of jerks.
- This pattern of motor unit activity delays muscle fatigue and allows contraction of a whole muscle to be sustained for long periods.
Clinical Connection
- Aerobic training, such as jogging or aerobic dancing, increases the supply of oxygen-rich blood available to skeletal muscles for aerobic cellular respiration.
- Strength training, such as weight lifting, relies more on anaerobic production of ATP through glycolysis.
Test your knowledge on the pattern of motor unit activity during muscle contraction and how motor units are recruited based on force requirements for sustained movements.
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