week 7 central fatigue

Questions and Answers

What role do synergists play in motor activation?

They primarily increase muscle fatigue.

They assist in stabilizing adjacent joints. (correct)

They inhibit muscle contraction.

They solely activate antagonist muscles.

What is the impact of electrical stimulation on central and peripheral fatigue?

It worsens both types of fatigue.

It restores central fatigue but does not affect peripheral fatigue. (correct)

It completely alleviates both central and peripheral fatigue.

It only works on peripheral fatigue.

During maximum voluntary contraction (MVC), what happens to the firing rates of motor units?

Firing rates stabilize at a constant rate.

Firing rates become completely erratic.

Firing rates increase continuously throughout the contraction.

Firing rates decrease by up to 50%. (correct)

Which of the following factors contribute to the decrease in motor unit firing rates during prolonged maximal effort?

Adaptation of alpha-motoneurons. Signup and view all the answers

In a scenario focusing on endurance and low intensity, what happens to motor units?

Motor units are de-recruited and then later re-activated. Signup and view all the answers

What is the basic firing rate range observed for motor units?

5-10 Hz Signup and view all the answers

How does the firing rate of motor units correlate with force output?

Firing rates and motor unit recruitment regulate force output. Signup and view all the answers

What typically occurs in terms of neuronal firing patterns at the onset of fatigue?

Firing rates start to decrease. Signup and view all the answers

What happens to the superimposed twitch during central fatigue?

It grows, indicating more can be added. Signup and view all the answers

How is voluntary activation assessed using twitch interpolation?

By analyzing the difference between superimposed and rest twitch responses. Signup and view all the answers

What is the typical increase in maximum force from effective training interventions on MVC?

Over 30% Signup and view all the answers

What is a primary reason that electrical stimulation is less effective compared to voluntary contraction?

The brain and spinal cord can more effectively coordinate muscle output. Signup and view all the answers

During the phase after MVC, which term describes the twitch response?

Rest twitch Signup and view all the answers

What is the relationship between voluntary activation and central fatigue?

Voluntary activation decreases as central fatigue increases. Signup and view all the answers

What does the term 'superimposed twitch' refer to during muscle stimulation?

The additional contraction generated over voluntary output. Signup and view all the answers

What is a common practical application of twitch interpolation?

To measure central fatigue in muscles. Signup and view all the answers

What role do Golgi tendon organs (GTO) and Renshaw cells play in muscle fatigue?

They have a low firing rate and play no role in muscle fatigue. Signup and view all the answers

What does the superimposed twitch method suggest about supraspinal fatigue?

It contributes around 60% to force loss in low intensity, long-duration exercise. Signup and view all the answers

According to current understanding, how does motoneuron excitability change with repeated firing?

It decreases, although the underlying mechanisms are unclear. Signup and view all the answers

What is the effect of pharmacological interventions on central fatigue as per the discussed neurotransmitters?

Their effects on central fatigue are inconsistent and unpredictable. Signup and view all the answers

What implication does increased drive from the motor cortex have during physical exertion?

It results in transient facilitation and potential spinal inhibition compensation. Signup and view all the answers

What role does 5-HT (serotonin) play in muscle fatigue according to its effects?

Small amounts lead to excitation while large amounts lead to inhibition. Signup and view all the answers

What is the observed effect of methylphenidate on performance and perceived effort during exercise?

It raises core temperature without altering perceived effort. Signup and view all the answers

How does spinal inhibition alter the drive from the motor cortex during prolonged exercise?

It can contribute to a decrease in effectiveness of the motor cortex drive. Signup and view all the answers

What occurs to the firing rates of all motor units at low intensity (50% MVC)?

They show decreased firing rates. Signup and view all the answers

What happens to motoneuron excitability following repeated current injection?

Excitability decreases. Signup and view all the answers

What is the primary function of Group Ia spindle afferents?

To activate motoneuron when a muscle is elongated. Signup and view all the answers

What is primarily activated during intense exercise by Group III and IV afferents?

Increased heart rate and respiration rate. Signup and view all the answers

How does sustained submaximal isometric contraction affect afferent activity of Group Ia spindle afferents?

It causes a progressive decrease in afferent activity. Signup and view all the answers

What is the effect of fatigue on gamma activation in relation to spindle sensitivity?

Gamma activation decreases, leading to decreased spindle sensitivity. Signup and view all the answers

What mechanism is suggested to be involved in the reduced motoneuron firing rates during repeated activation?

Increased descending drive needed for maintenance. Signup and view all the answers

Which process leads to the inhibition of motoneuron pools during high-intensity exercise?

Direct inhibition by Group III and IV afferents. Signup and view all the answers

What best describes central fatigue?

Alterations in brain and spinal cord functions Signup and view all the answers

Which of the following is a characteristic of peripheral fatigue?

It is related to changes at the neuromuscular junction and beyond. Signup and view all the answers

What is the primary reason voluntary effort may not achieve maximal muscle power?

Volitional muscle activation is often limited due to central factors. Signup and view all the answers

Which of the following is NOT typically included in the measurement of voluntary activation?

Total time spent exercising Signup and view all the answers

What does the ratio used in calculating voluntary activation reflect?

The difference between maximal force outputs under different conditions. Signup and view all the answers

Which of the following factors can stimulate force production beyond voluntary activation?

Supramaximal electrical stimulation applied during MVC Signup and view all the answers

What outcome is expected from a superimposed twitch during maximal voluntary contraction?

It reveals how much additional force can be produced. Signup and view all the answers

Which statement best describes the difference between central and peripheral fatigue mechanisms?

Central fatigue is influenced by brain and spinal cord activity, while peripheral fatigue involves muscular junctions. Signup and view all the answers

Study Notes

Isolated Motoneuron Excitability

Repeated current injection into motoneurons results in decreased firing rates and reduced excitability.
During consistent firing rate maintenance, other motor units (MUs) are recruited, indicating increased descending drive required for motoneuron firing.
Magnetic stimulation of the cervical spinal cord during maximal voluntary contraction (MVC) leads to reduced muscle response.
Mechanisms behind decreased motoneuron excitability during repetitive activation remain unclear.

Group III and IV Afferents

Group III afferents respond to mechanical stimuli; Group IV to chemical stimuli like lactate and ATP, with some sensitive to noxious metabolites.
Increased activity of Group III and IV afferents observed during intense exercise.
Inhibition of motoneuron pools occurs directly and indirectly via Ia afferent neuron inhibition.
Activation of heart rate and respiration is noted with increased afferent activity.

Group Ia Spindle Afferents

Ia spindle afferents activate motoneurons when muscle is stretched; they decrease in activity during sustained submaximal contractions.
Gamma-alpha coactivation normally occurs; during fatigue, reduced gamma activation decreases spindle sensitivity, leading to reduced H-reflex.
Functions include no activation of antagonists, synergist activation, and stabilization of joints.

Recovery from Fatigue

Isotonic MVC induces fatigue that can be slightly recovered by electrical stimulation, which restores central but not peripheral fatigue.
Brief rest (1 minute) leads to full recovery from fatigue effects.

Motor Unit Firing Patterns

Firing rates of motor units (5-10 Hz for basic, 50-60 Hz for large bursts) regulate force output through recruitment.
During long endurance and low-intensity efforts, motor units rotate, being de-recruited and later reactivated.
Firing rates of motor units can decrease by up to 50% during MVC, indicating less drive from the motor cortex and adaptations at the spinal level.

Twitch Interpolation Method

Superimposed twitch during MVC can indicate how much additional force can be produced.
Decrease in voluntary activation during central fatigue can be measured through twitch interpolation.
The method verifies the extent of central fatigue, revealing that superimposed twitches grow as voluntary activation decreases.

Motor Cortex and Central Fatigue

Surface EMG activity rises during submaximal contractions; increased motor evoked potentials (MEPs) suggest motor cortex facilitation.
Motor cortex drive is transient; prolonged activity results in reduced or suboptimal output.
Superimposed twitch method indicates supraspinal fatigue contributes significantly to force loss during extended efforts.

Pharmacological Influences

Pharmacological modulation of neurotransmitter states has inconsistent effects on central fatigue.
Agents like 5-HT exhibit dual effects depending on concentration, influencing excitatory or inhibitory outcomes.
Methylphenidate increases dopamine but does not significantly alter perceived effort, raising safety concerns in performance enhancements.

Summary of Motoneuron Activity

Repeated firings of motoneurons lead to a decline in excitability; exact mechanisms are not fully understood.
Muscle fatigue indicates reduced ability to produce maximal force, characterized by a decline in maximum voluntary contractions, not task failure.
Central fatigue refers to changes occurring within the brain, spinal cord, and peripheral nerves, while peripheral fatigue pertains to actions distal to the neuromuscular junction.

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Description

Explore the concepts of motor unit firing rates and motoneuron excitability from Stanfield's 'Principles of Human Physiology'. This quiz covers the effects of low intensity on motoneuron activity and the recruitment of muscle units during constant firing. Test your understanding of these physiological principles.