Motor Units, Recruitment, and Muscle Tension - Anatomy and Physiology
- What is the size principle of motor unit recruitment?
- Motor units are recruited from smallest to largest as the force needed increases, so small, fatigue-resistant units are used first.
- What is recruitment in skeletal muscle?
- Increasing muscle force by activating more motor units.
- A muscle lengthens while producing tension, as when slowly lowering a weight. What type of contraction is this?
- An eccentric contraction: the muscle generates tension while it is lengthening.
- Why do muscles that control the eyes have very small motor units?
- Few fibers per motor neuron allow fine, precisely graded control of movement.
- What is a motor unit?
- One motor neuron and all the skeletal muscle fibers it innervates.
- What fiber-type composition does a single motor unit have?
- All fibers in one motor unit are of the same type, either slow or fast, because one neuron controls them all.
- A muscle produces tension but its length does not change, as when holding a weight still. What type of contraction is this?
- An isometric contraction: tension rises but never exceeds the load, so the muscle does not shorten.
- What are the three phases of a muscle twitch, in order?
- Latent period, period of contraction and period of relaxation.
- What is muscle tone?
- The slight, continuous contraction of a resting muscle produced by spinal reflexes that activate different motor units in turn.
- What is the difference between unfused and fused tetanus?
- In unfused tetanus the fiber partly relaxes between stimuli; in fused tetanus stimuli are so rapid it never relaxes, giving smooth maximal tension.
- What is the length-tension relationship in skeletal muscle?
- A fiber produces the most tension at an intermediate length with optimal thick-thin filament overlap, and less when overstretched or overshortened.
- How does increasing stimulus frequency increase the tension of a muscle?
- Twitches occur before the fiber fully relaxes, so they summate (wave or temporal summation) and produce greater tension.