week 7 central fatigue

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.