Neuroscience Chapter on Muscle Force

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Questions and Answers

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?

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?

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?

Reflex fatigue Signup and view all the answers

What determines the gradation of force during muscle actions?

Motor unit recruitment and rate code Signup and view all the answers

What effect does an increase in the rate of motor unit firing have on muscle force generation?

It increases muscle force generation. Signup and view all the answers

What defines a muscle twitch?

A single muscle contraction in response to one nerve impulse. Signup and view all the answers

What is the term for the phenomenon when a second action potential increases tension before the muscle has relaxed?

Twitch summation. Signup and view all the answers

What does 'neural load' refer to during physical activity?

The amount of energy expenditure required by the central nervous system. Signup and view all the answers

What type of fatigue occurs within the central nervous system?

Central fatigue. Signup and view all the answers

What effect does prolonged exercise have on the central nervous system's (CNS) workload?

The CNS must increase its effort to preserve motor output. Signup and view all the answers

Which statement best describes a low stimulus to fatigue ratio in training?

It implies insufficient training stimulus relative to high fatigue. Signup and view all the answers

What role do muscle spindles play in the stretch reflex?

They cause the stretched muscle to contract and the opposing muscle to relax. Signup and view all the answers

What type of receptor do muscles and tendons contain that generate reflex actions?

Proprioceptors Signup and view all the answers

What is the process of reciprocal inhibition during muscle contractions?

It allows the agonist muscle to relax while the antagonist contracts. Signup and view all the answers

What is the primary consequence of impaired calcium handling during peripheral fatigue?

Reduced calcium release from the sarcoplasmic reticulum Signup and view all the answers

Which neurotransmitter's accumulation is most likely associated with feelings of sleepiness and lethargy during central fatigue?

Serotonin Signup and view all the answers

What effect does afferent inhibitory feedback have on motor neuron excitability during fatigue?

Decreases lower motor neuron excitability Signup and view all the answers

What primarily leads to the accumulation of hydrogen ions during prolonged exercise?

Inability to remove lactate efficiently Signup and view all the answers

Which factor is NOT a contributing element to neural load during high-intensity exercise?

Nutrition level Signup and view all the answers

What is a possible sign of central fatigue in an athlete?

Decreased muscular strength Signup and view all the answers

How does training affect neural load in athletes?

Decreases neural load as movements become more coordinated Signup and view all the answers

What role does the postsynaptic nicotinic receptor play during peripheral fatigue?

Decreases sensitivity, impairing excitation Signup and view all the answers

What is the primary function of muscle spindles?

Detect changes in muscle length Signup and view all the answers

Which statement about extrafusal and intrafusal fibers is correct?

Intrafusal fibers are aligned parallel to extrafusal fibers. Signup and view all the answers

What happens during the coactivation of alpha and gamma motor neurons?

Muscle spindles maintain their sensitivity. Signup and view all the answers

Golgi tendon organs (GTOs) primarily function to:

Trigger muscle relaxation in response to excessive tension. Signup and view all the answers

In which scenario will Golgi tendon organs inhibit the motor neuron?

When muscle tension is too high. Signup and view all the answers

Which statement about alpha motor neurons is true?

They innervate force-generating extrafusal fibers. Signup and view all the answers

What occurs if the stretch is held for approximately 30-60 seconds?

Autogenic inhibition will trigger muscle relaxation. Signup and view all the answers

Which type of muscle would likely contain more muscle spindles per gram?

Muscles involved in fine motor skills. Signup and view all the answers

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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Description

Explore the concepts of neural drive, gradation of force, and motor unit recruitment in this comprehensive quiz. Delve into principles such as the Size Principle and the All or None Principle, alongside the importance of rate coding in muscle contractions. Test your understanding of how muscles adapt to varying tasks and intensities.