Copyright © 2012 American College of Sports Medicine Chapter 3 Neural Adaptations to Training

Copyright © 2012 American College of Sports Medicine Functions of the Nervous System Receives sensory information –Pain –Pressure –Hot/cold temperatures –Joint position –Muscle length Integrates information in appropriate places Controls output or response from tissues, glands, organs Controls emotions, personality, & other cerebral functions

Copyright © 2012 American College of Sports Medicine Magnitude: Strength The magnitude of signal is critical in determining the final output—muscular strength & power.—This quantity is termed neural drive and an increase in neural drive is critical to the individual striving to maximize performance. The increase in neural drive is thought to occur via increases in agonist muscle recruitment firing rate, and the timing, pattern of discharge during high-intensity muscular contractions. A reduction in inhibitory mechanisms is thought to occur.

Copyright © 2012 American College of Sports Medicine Functional Organization of the Nervous System I.Central nervous system(CNS) A.Brain B.Spinal cord II.Peripheral nervous system (PNS) (31 pairs of spinal nerves) A.Sensory division (posterior) [afferent] B.Motor division (anterior) 1.Somatic nervous system (muscle contraction—and fatigue) [efferent] 2.Autonomic nervous system (smooth & cardiac muscle, glands, tissue, organs) a.Sympathetic nervous system b.Parasympathetic nervous system

Copyright © 2012 American College of Sports Medicine Divisions of the Nervous System

Copyright © 2012 American College of Sports Medicine

Nerve Cells Nerve tissue comes in 2 forms: A-Supporting Cells –Play stability roles throughout CNS B-Neurons –Actual nerve cells –Communicate with other tissues & nerves –Sensory neurons: unipolar –Motor neurons: multipolar

Copyright © 2012 American College of Sports Medicine Nerve Cells (cont’d) Key Features of Neurons –Dendrites: receive input form other nerve cells –Cell body Contains organelles Plays critical role in integrating stimuli from other neurons –Axons: long processes that communicate with target tissues –Axon hillock: area where action potential is initiated –Myelin sheath: wraps axons, increases transmission speed –Presynaptic terminal: end of axon branches

Copyright © 2012 American College of Sports Medicine Motoneuron Anatomy

Copyright © 2012 American College of Sports Medicine Nerve Cells (cont’d) Neural Communication –Action potential: generation of an electrical current (signal) Integration: cell body integrates charges from other neurons Propagation: ion movement down axon at nodes of Ranvier Neurotransmitter release: from nerve terminal, allowing communication

Copyright © 2012 American College of Sports Medicine The Action Potential

Copyright © 2012 American College of Sports Medicine The Brain

Copyright © 2012 American College of Sports Medicine Higher Brain Centers: Brainstem Components –Medulla oblongata –Midbrain –Pons –Reticular formation Functions –Cardiac heart rate & force of contraction –Blood pressure –Blood vessel diameter –Breathing –Hearing –Vision –Sleep –Consciousness

Copyright © 2012 American College of Sports Medicine Higher Brain Centers: Diencephalon Components –Thalamus –Hypothalamus –Pineal body Functions –Relay area of brain –Release of hormones –Homeostasis –Autonomic control –Body temperature –Emotions –Most functions in body

Copyright © 2012 American College of Sports Medicine Higher Brain Centers: Cerebrum Components –Cerebral cortex Primary sensory area Premotor cortex Primary motor cortex Functions –Sensory integration –Voluntary muscle contraction –Memory bank for skilled motor activities

Copyright © 2012 American College of Sports Medicine Higher Brain Centers: Cerebellum Functions –Integrates sensory information –Coordinates skeletal muscle activity –Provides blueprint of how motor skill should be performed

Copyright © 2012 American College of Sports Medicine Higher Brain Centers: Basal Ganglia Functions –Planning & control of: Muscle function Posture –Controlling unwanted movements

Copyright © 2012 American College of Sports Medicine Descending Corticospinal Tracts Collection of axons linking cerebral cortex to spinal cord Motor Pathway –Neurons in brain form synapses with other nerves –Nerves exit spinal cord for innervation of skeletal muscle –Site of many potential neural changes

Copyright © 2012 American College of Sports Medicine Motor Units Definition –Functional unit of nervous system –A single alpha motor nerve & all muscle fibers it innervates Location –Cell bodies & dendrites: in spinal cord –Axons Extend beyond spinal cord Innervate skeletal muscles in periphery

Copyright © 2012 American College of Sports Medicine The Motor Unit

Copyright © 2012 American College of Sports Medicine Motor Units (cont’d) Recruitment –Voluntary activation of motor units during effort –Based on size principle: smaller to larger –Selective recruitment Preferential recruitment of type II motor units Can occur during change in direction of exerted forces & explosive muscle actions –Muscle mass activation Depends on magnitude of muscle hypertrophy May decrease when muscle size increases

Copyright © 2012 American College of Sports Medicine The Size Principle

Copyright © 2012 American College of Sports Medicine Motor Units (cont’d) Recruitment (cont’d) –Postactivation potentiation Time after maximal or near-maximal muscle contraction when it is easier to recruit type II motor units Examples: Swinging a weighted bat prior to batting in baseball Performing a few sets of squats prior to vertical jump Using weighted vest prior to speed or agility events Performing a few sets of weight training prior to sport

Copyright © 2012 American College of Sports Medicine Motor Units (cont’d) Firing Rate –Number of times per second a motor unit discharges –Affected by nerve’s conduction velocity –Conduction velocity higher in power than in endurance athletes –Lower at rest –Positive relationship with amount of force produced –Role of increasing firing rate depends on muscle size

Copyright © 2012 American College of Sports Medicine Motor Units (cont’d) Motor Unit Synchronization –Occurs when two or more motor units fire at fixed time intervals –May be advantageous for bursts of strength or power needed in a short period of time –Occurs in greater measure after resistance training (RT) –May be advantageous for timing of force production

Copyright © 2012 American College of Sports Medicine Motor Units (cont’d) Antagonist Muscle Activation –Increases joint stability & movement coordination –Reduces risk of injury –May counteract effects of agonist muscles –Magnitude affected by: Muscle group Velocity & type of muscle action Intensity Joint position Injury status

Copyright © 2012 American College of Sports Medicine Communication With Skeletal Muscle: The Neuromuscular Junction Neuromuscular Junction –Gap between nerve & muscle –Crossed by neurotransmitter, allowing action potential to reach muscle –Acetylcholine Key neurotransmitter Released between motor nerve & skeletal muscle

Copyright © 2012 American College of Sports Medicine The Motor Unit and Neuromuscular Junction

Copyright © 2012 American College of Sports Medicine Sensory Nervous System Golgi Tendon Organs (GTOs) –Proprioceptors located at muscle-tendon junction –Convey information regarding muscle tension to CNS –Amount of stretch to GTOs increases with increased muscle tension –Cause agonist muscle relaxation & antagonist muscle excitation –Defense mechanism to protect body from excessive damage

Copyright © 2012 American College of Sports Medicine What is a Proprioceptor? In the limbs, the proprioceptors are sensors that provide information about joint angle, muscle length, and muscle tension, which is integrated to give information about the position of the limb in space. The muscle spindle is one type of proprioceptor that provides information about changes in muscle length. 2: GTO and Muscle Spindle

Copyright © 2012 American College of Sports Medicine Golgi Tendon Organ

Copyright © 2012 American College of Sports Medicine Sensory Nervous System (cont’d) Muscle Spindles –Proprioceptors located within muscle fibers –Consist of nuclear chain & nuclear bag fibers –Respond to: Magnitude of change in muscle length Rate of change of length –Convey information to CNS regarding static changes in muscle length or joint angle –Enhance human performance –Initiate stretch reflex

Copyright © 2012 American College of Sports Medicine Muscle Spindle

Copyright © 2012 American College of Sports Medicine Training Studies EMG, or Electromyography, has shown neural function: –To be higher for high-intensity muscular effort vs. low-intensity –To be higher during ballistic or explosive resistance exercise compared with slower velocities –To be higher for concentric (CON) vs. eccentric (ECC) muscle actions when matched for intensity –To increase as fatigue ensues in CON & ECC muscle actions –To be reduced following a workout compared with preworkout –To be lower for high-volume, high-intensity training –To be reduced during periods of detraining

Copyright © 2012 American College of Sports Medicine Training Studies (cont’d) Unilateral Training –Cross education: strength & endurance gained in non-trained limb during unilateral training –Adaptations are carried over to opposite limb –Induces novel stimulus to nervous system –Improves functional performance –Useful in injured individuals due to partial training effect in injured limb Bilateral Deficit –Max force produced in bilateral contraction is smaller than sum of limbs contracting unilaterally: You CAN’T jump twice as high with 2 legs as compared to jumping with just 1!

Copyright © 2012 American College of Sports Medicine Autonomic Nervous System Controls: –Heart rate –Force of contraction –Respiration rate –Digestion –Blood pressure & flow –Fuel mobilization Sympathetic Branch (“Fight or Flight”)[stress or exercise] Parasympathetic Branch: Returns body to normal [homeostasis]

Copyright © 2012 American College of Sports Medicine Autonomic Nervous System (cont’d)