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Questions and Answers
What does motor unit recruitment primarily depend on?
What does motor unit recruitment primarily depend on?
- The intensity of a task (correct)
- The speed of contraction
- The type of exercise performed
- The size of the muscle fibers
Which statement best describes the size principle?
Which statement best describes the size principle?
- Motor units are recruited from smallest to largest during tasks. (correct)
- Larger motor units are always recruited first.
- Smaller motor units are recruited at higher exercise intensities.
- Motor units fire randomly regardless of size.
What characterizes the all or none principle in muscle contraction?
What characterizes the all or none principle in muscle contraction?
- All motor units pause before contracting.
- Once a motor unit fires, all its muscle fibers contract simultaneously. (correct)
- The contraction strength can be varied across motor units.
- Some muscle fibers contract while others do not.
Which of the following is NOT a mechanism of fatigue?
Which of the following is NOT a mechanism of fatigue?
What determines the gradation of force during muscle actions?
What determines the gradation of force during muscle actions?
What effect does an increase in the rate of motor unit firing have on muscle force generation?
What effect does an increase in the rate of motor unit firing have on muscle force generation?
What defines a muscle twitch?
What defines a muscle twitch?
What is the term for the phenomenon when a second action potential increases tension before the muscle has relaxed?
What is the term for the phenomenon when a second action potential increases tension before the muscle has relaxed?
What does 'neural load' refer to during physical activity?
What does 'neural load' refer to during physical activity?
What type of fatigue occurs within the central nervous system?
What type of fatigue occurs within the central nervous system?
What effect does prolonged exercise have on the central nervous system's (CNS) workload?
What effect does prolonged exercise have on the central nervous system's (CNS) workload?
Which statement best describes a low stimulus to fatigue ratio in training?
Which statement best describes a low stimulus to fatigue ratio in training?
What role do muscle spindles play in the stretch reflex?
What role do muscle spindles play in the stretch reflex?
What type of receptor do muscles and tendons contain that generate reflex actions?
What type of receptor do muscles and tendons contain that generate reflex actions?
What is the process of reciprocal inhibition during muscle contractions?
What is the process of reciprocal inhibition during muscle contractions?
What is the primary consequence of impaired calcium handling during peripheral fatigue?
What is the primary consequence of impaired calcium handling during peripheral fatigue?
Which neurotransmitter's accumulation is most likely associated with feelings of sleepiness and lethargy during central fatigue?
Which neurotransmitter's accumulation is most likely associated with feelings of sleepiness and lethargy during central fatigue?
What effect does afferent inhibitory feedback have on motor neuron excitability during fatigue?
What effect does afferent inhibitory feedback have on motor neuron excitability during fatigue?
What primarily leads to the accumulation of hydrogen ions during prolonged exercise?
What primarily leads to the accumulation of hydrogen ions during prolonged exercise?
Which factor is NOT a contributing element to neural load during high-intensity exercise?
Which factor is NOT a contributing element to neural load during high-intensity exercise?
What is a possible sign of central fatigue in an athlete?
What is a possible sign of central fatigue in an athlete?
How does training affect neural load in athletes?
How does training affect neural load in athletes?
What role does the postsynaptic nicotinic receptor play during peripheral fatigue?
What role does the postsynaptic nicotinic receptor play during peripheral fatigue?
What is the primary function of muscle spindles?
What is the primary function of muscle spindles?
Which statement about extrafusal and intrafusal fibers is correct?
Which statement about extrafusal and intrafusal fibers is correct?
What happens during the coactivation of alpha and gamma motor neurons?
What happens during the coactivation of alpha and gamma motor neurons?
Golgi tendon organs (GTOs) primarily function to:
Golgi tendon organs (GTOs) primarily function to:
In which scenario will Golgi tendon organs inhibit the motor neuron?
In which scenario will Golgi tendon organs inhibit the motor neuron?
Which statement about alpha motor neurons is true?
Which statement about alpha motor neurons is true?
What occurs if the stretch is held for approximately 30-60 seconds?
What occurs if the stretch is held for approximately 30-60 seconds?
Which type of muscle would likely contain more muscle spindles per gram?
Which type of muscle would likely contain more muscle spindles per gram?
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Study Notes
Neural Drive
- Neural drive is the magnitude of motor output that dictates muscle force and movement.
Gradation of Force
- How muscles produce different amounts of force for various tasks.
- Dictated by:
- Motor unit recruitment
- Rate code
Motor Unit Recruitment
- The number of motor units activated during muscle action.
- Higher intensity tasks recruit more motor units.
- Dependent on task intensity, force, and speed.
Size Principle
- Motor units are recruited from smallest to largest.
- Smaller motor neurons (type I fibers) have lower thresholds and are recruited first.
- Larger motor neurons (type IIa and IIx fibers) have higher thresholds and require greater neural drive.
The All or None Principle
- When a motor unit fires, all muscle fibers within that unit contract simultaneously.
- Either a stimulus elicits an action potential or it does not.
Rate Code
- The frequency of motor unit firing.
- Higher frequency leads to greater force generation.
- Muscle twitch is the smallest unit of muscle contraction.
- Twitch summation occurs when motor units fire before relaxation, leading to greater tension.
- Tetanus is a sustained muscle contraction from maximal motor unit firing.
Neural Load and Fatigue
- Neural load is the stress on the nervous system during activity.
- It relates to the amount of neural activity and energy required for muscle coordination.
- Fatigue is a reduction in force production due to:
- Central fatigue (processes within the CNS reducing neural drive)
- Peripheral fatigue (processes at or distal to the neuromuscular junction)
Mechanisms of Fatigue
- Central fatigue mechanisms:
- Decreased motor cortex activity
- Neurotransmitter imbalance
- Peripheral fatigue mechanisms:
- Substrate depletion (muscle glycogen, phosphocreatine)
- Metabolite accumulation (lactate, H+)
- Calcium handling (reduced release and uptake)
- Impaired excitation (receptor sensitivity, ion disruption)
- Motor unit recruitment, afferent inhibitory feedback, and duration/intensity of feedback all contribute to muscle fatigue.
Stimulus to Fatigue Ratio
- A high stimulus to fatigue ratio indicates effective training programs that promote adaptation.
- Low stimulus to fatigue ratios suggest inadequate training or disproportionate fatigue.
Reflexes
- Involuntary, near-instantaneous actions in response to stimuli.
- Occur via a reflex arc (sensory neuron, interneuron, motor neuron).
Proprioceptors
- Specialized receptors within muscles and tendons that detect movement and position:
- Muscle spindles
- Golgi tendon organs
Muscle Spindles
- Detect muscle stretch, specifically rate of lengthening.
- Cause the stretched muscle to contract and the opposing muscle to relax.
- Initiate the stretch reflex and reciprocal inhibition.
Stretch Reflex
- Regulates muscle length by increasing contractility in response to stretching.
- Example: knee jerk reflex.
Reciprocal Inhibition
- Relaxation of antagonist muscles to accommodate agonist muscle contraction.
Muscle Spindle Structure
- Intrafusal fibers: within the spindle, they do not generate force
- Extrafusal fibers: outside the spindle, they generate force
- More muscle spindles per gram of muscle are present in muscles involved in fine motor skills.
Alpha and Gamma Motor Neurons
- Alpha motor neurons innervate extrafusal fibers.
- Gamma motor neurons innervate intrafusal fibers and control their length.
Alpha-Gamma Coactivation
- Simultaneous activation of alpha and gamma motor neurons maintains sensitivity during contraction.
Golgi Tendon Organs
- Located at the myotendinous junction.
- Detect muscle tension/force.
- Trigger a relaxation response when tension is too high.
Autogenic Inhibition
- A protective mechanism that prevents excess force generation.
- Inhibits the motor neuron when excessive tension is detected.
- Can occur during prolonged stretching, leading to muscle relaxation.
Length-Tension Relationship
- The amount of force a muscle can produce varies with its length.
- Optimal length allows for maximum cross-bridge formation and force generation.
Force-Velocity Relationship
- As muscle contraction velocity increases, force production decreases.
- A slower contraction allows for greater force generation.
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