Neural Drive - SPSC 3275 PDF

NEURAL DRIVE SPSC 3275: Advanced Physiology of Exercise and Training Douglas College Andrew Kanerva  LEARNING OBJECTIVES After this class, students will be able to Describe motor unit recruitment Describe the size principle D...

NEURAL DRIVE SPSC 3275: Advanced Physiology of Exercise and Training Douglas College Andrew Kanerva  LEARNING OBJECTIVES After this class, students will be able to Describe motor unit recruitment Describe the size principle Define the all or none principle Describe rate code Define central and peripheral fatigue Outline the mechanisms of central and peripheral fatigue Describe a reflex arc Explain the reflex action initiated by muscle spindles and Golgi tendon organs Describe the length-tension relationship Describe the force-velocity curve  NEURAL DRIVE Neural drive represents the magnitude of motor output to produce muscle force and perform movement  GRADATION OF FORCE Gradation of force refers to process of muscles producing different amounts of force to meet the demands of different tasks. The amount of force produced by muscle is dictated by Motor unit recruitment Rate code  MOTOR UNIT RECRUITMENT Motor unit recruitment refers to the number of motor units recruited during muscle action. Motor unit recruitment depends on the intensity of a task. Force Speed  SIZE PRINCIPLE The size principle states that motor units are recruited in order of size from smallest to largest. Low intensity muscle actions recruit fewer, smaller motor units. More, larger motor units are recruited as exercise intensity increases  SIZE PRINCIPLE Smaller motor neurons, which innervate type I fibres, have low threshold requirements and therefore require small degrees of neural drive to fire Larger motor neurons, which innervate type IIa and IIx fibres, have bigger threshold requirements and therefore require larger degrees of neural drive to fire GRADATION OF FORCE  ALL OR NONE When a motor unit fires, all the muscle fibres of that motor unit contract simultaneously. Either the stimulus elicits an action potential or it does not  RATE CODE Rate code is the frequency of motor unit firing An increase in the rate of motor unit firing increases the amount of force generated by the muscle fibres in a given motor unit  RATE CODE A muscle twitch is the smallest unit of muscle contraction and occurs in response to a single nerve impulse  RATE CODE If the motor unit fires after the muscle has returned to a relaxed state, then the magnitude of the next muscle twitch will be the same as the first twitch  RATE CODE If the motor unit fires before the muscle has returned to a relaxed state, then the second action potential produces a greater amount of tension This is known as twitch summation  RATE CODE Tetanus is a sustained muscle contraction produced by a maximal rate of motor unit firing Ex. Erector spinae while standing  NEURAL DRIVE PLACES DEMAND ON THE NERVOUS What happens when SYSTEM. AS neural load accumulates? EXERCISE INTENSITY INCREASES, NEURAL DRIVE INCREASES.  Neural load refers to the amount of stress experienced by the nervous system during physical NEURAL activity. LOAD Specifically, neural load refers to the amount of neural activity and energy expenditure required by the central nervous system to coordinate muscle action.  FATIGUE During exercise, there is a progressive reduction in the ability to produce muscle force due to fatigue. Central fatigue Processes within the central nervous system that reduce neural drive. Peripheral fatigue Processes at or distal to the neuromuscular junction that suppress muscle action.  Substrate depletion Muscle glycogen Phosphocreatine Metabolite accumulation Inability to remove lactate efficiently via MCT4 transporters Accumulation of hydrogen ions Inhibition of calcium binding PERIPHERAL FATIGUE Calcium handling Reduced calcium release from sarcoplasmic reticulum Impaired calcium uptake by sarcoplasmic reticulum Impaired excitation Decreased sensitivity of postsynaptic nicotinic receptors Accumulation of hydrogen ions disrupts ion (Na+/K+)  Motor unit recruitment Afferent inhibitory feedback Afferent neurons detect mechanical loading and metabolite accumulation in muscle. Duration and intensity of afferent inhibitory feedback can vary. Afferent inhibitory feedback decreases lower motor neuron excitability CENTRAL FATIGUE Motor cortex activity Decreased upper motor neuron excitability Neurotransmitter imbalance Accumulation of serotonin (sleepiness, relaxation, lethargy) Reduced dopamine (motivation) and norepinephrine (alertness, focus)  Symptoms Increased Rate of Perceived Exertion Decreased muscular strength Decreased muscular endurance FATIGUE Impaired motor control Impaired decision-making Decreased motivation Decreased arousal CENTRAL FATIGUE MAY BE SYSTEMIC  NEURAL LOAD Exercise Volume, Intensity and Complexity High volume (especially efforts to failure), high intensity, and/or high complexity movements require greater neural drive and present large neural loads. Training Status The nervous systems of untrained individuals may need to generate more neural activity to perform a given exercise compared to experienced athletes due to differences in motor learning and coordination. With training and skill development, neural load may decrease as movements become more coordinated and efficient. Peripheral Fatigue As exercise duration increases, neural load may rise as the CNS works harder to maintain motor output and compensate for decreased muscle function.  What volume, intensity, and recovery strategies maximize adaptations and limit fatigue? High stimulus to fatigue ratios indicate STIMULUS effective/thoughtful exercise training programs which TO FATIGUE promote adaptation RATIO Low stimulus to fatigue ratios indicate that training stimulus may be insufficient and/or that fatigue is disproportionately high compared to the benefits of training  INSTABILITY TRAINING What is the neural load of instability training? Does instability training have a low stimulus to fatigue ratio?  REFLEXES A reflex is an involuntary and nearly instantaneous action in response to a stimulus Reflexes occur through a reflex arc Sensory neuron Interneuron Motor neuron  PROPRIOCEPTORS Muscles and tendons contain proprioceptors, which are specialized sensory receptors that produce reflex actions Muscle spindles Golgi tendon organs  MUSCLE SPINDLE Muscle spindles are specialized muscle fibres that detect muscle stretch, particularly rate of lengthening When activated, muscle spindles cause the muscle being stretched to contract and the opposing muscle to relax  STRETCH REFLEX Muscle spindles initiate the stretch reflex Ex. Knee jerk When a muscle is stretched, the stretch reflex regulates the length of the muscle by increasing its contractility  RECIPROCAL INHIBITION Muscle spindles initiate reciprocal inhibition Reciprocal inhibition refers to the relaxation of antagonist muscles to accommodate contraction of agonist muscles. STRETCH REFLEX The stretch reflex enhances performance  MUSCLE SPINDLE Muscle spindles are collections of intrafusal fibres aligned in parallel with extrafusal fibres Intrafusal fibres do not generate muscle force Extrafusal fibres generate force  MUSCLE SPINDLE Muscles involved in fine motor skills or complex movement patters contain more muscle spindles per gram of muscle than muscles involved in gross motor skills or less complex movement patterns  ALPHA AND GAMMA MOTOR NEURONS Alpha motor neurons innervate force-generating extrafusal fibres Gamma motor neurons innervate intrafusal fibres and control the length of muscle spindles to maintain sensitivity during muscle contraction  ALPHA GAMMA COACTIVATION When the muscle lengthens, the spindles also lengthens When the muscle contracts, the spindles also contracts Coactivation of alpha and gamma neurons maintains sensitivity  MUSCLE SPINDLES Two sensory neurons (afferent) and one gamma motor neuron (efferent) innervate each muscle spindle The firing rate of the sensory neurons increases in proportion to rate of stretch  GOLGI TENDON ORGANS Golgi tendon organs (GTO) are located in the tendon at the myotendinous junction GTOs detect muscle tension/force. They trigger a relaxation response when tension is too high.  Autogenic inhibition is a protective mechanism, preventing muscles from producing more force than AUTOGENIC bones and tendons can tolerate INHIBITION If excessive tension is detected when the muscle contracts, the GTO inhibits the motor neuron through autogenic inhibition  During prolonged stretching, GTOs detect tension. If AUTOGENIC the stretch is held for approx. 30-60 seconds, the GTO INHIBITION inhibits the motor neuron through autogenic inhibition, causing the muscle to relax. LENGTH-TENSIO N RELATIONSHIP FORCE-VELOCIT Y RELATIONSHIP FORCE-VELOCIT Y RELATIONSHIP