Download Motor Units (cont`d)

Chapter 3
Neural Adaptations to Training
Copyright © 2012 American College of Sports Medicine
Functions of the Nervous System
• Receives sensory information
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Pain
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Pressure
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Hot/cold temperatures
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Joint position
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Muscle length
• Integrates information in appropriate places
• Controls output or response from tissues, glands, organs
• Controls emotions, personality, & other cerebral functions
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• 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.
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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
a.
(smooth & cardiac muscle, glands, tissue, organs)
Sympathetic nervous system
b. Parasympathetic nervous system
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Divisions of the Nervous System
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Nerve Cells
Nerve tissue comes in 2 forms:
A-Supporting Cells
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Play stability roles throughout CNS
B-Neurons
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Actual nerve cells
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Communicate with other tissues & nerves
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Sensory neurons: unipolar
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Motor neurons: multipolar
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Nerve Cells (cont’d)
• Key Features of Neurons
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Dendrites: receive input form other nerve cells
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Cell body
• Contains organelles
• Plays critical role in integrating stimuli from other neurons
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Axons: long processes that communicate with target tissues
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Axon hillock: area where action potential is initiated
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Myelin sheath: wraps axons, increases transmission speed
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Presynaptic terminal: end of axon branches
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Motoneuron Anatomy
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Nerve Cells (cont’d)
• Neural Communication
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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
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The Action Potential
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The Brain
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Higher Brain Centers: Brainstem
• Components
• Functions
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Medulla oblongata
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Midbrain
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Pons
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Reticular formation
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Cardiac heart rate & force
of contraction
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Blood pressure
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Blood vessel diameter
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Breathing
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Hearing
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Vision
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Sleep
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Consciousness
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Higher Brain Centers: Diencephalon
• Components
• Functions
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Thalamus
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Relay area of brain
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Hypothalamus
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Release of hormones
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Pineal body
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Homeostasis
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Autonomic control
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Body temperature
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Emotions
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Most functions in body
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Higher Brain Centers: Cerebrum
• Components
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• Functions
Cerebral cortex
• Primary sensory area
• Premotor cortex
• Primary motor cortex
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Sensory integration
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Voluntary muscle
contraction
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Memory bank for skilled
motor activities
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Higher Brain Centers: Cerebellum
• Functions
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Integrates sensory information
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Coordinates skeletal muscle activity
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Provides blueprint of how motor skill should be performed
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Higher Brain Centers: Basal Ganglia
• Functions
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Planning & control of:
• Muscle function
• Posture
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Controlling unwanted movements
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Descending Corticospinal Tracts
• Collection of axons linking cerebral cortex to spinal cord
• Motor Pathway
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Neurons in brain form synapses with other nerves
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Nerves exit spinal cord for innervation of skeletal muscle
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Site of many potential neural changes
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Motor Units
• Definition
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Functional unit of nervous system
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A single alpha motor nerve & all muscle fibers it innervates
• Location
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Cell bodies & dendrites: in spinal cord
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Axons
• Extend beyond spinal cord
• Innervate skeletal muscles in periphery
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The Motor Unit
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Motor Units (cont’d)
• Recruitment
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Voluntary activation of motor units during effort
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Based on size principle: smaller to larger
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Selective recruitment
• Preferential recruitment of type II motor units
• Can occur during change in direction of exerted forces &
explosive muscle actions
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Muscle mass activation
• Depends on magnitude of muscle hypertrophy
• May decrease when muscle size increases
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The Size Principle
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Motor Units (cont’d)
• Recruitment (cont’d)
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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
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Motor Units (cont’d)
• Firing Rate
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Number of times per second a motor unit discharges
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Affected by nerve’s conduction velocity
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Conduction velocity higher in power than in endurance athletes
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Lower at rest
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Positive relationship with amount of force produced
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Role of increasing firing rate depends on muscle size
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Motor Units (cont’d)
• Motor Unit Synchronization
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Occurs when two or more motor units fire at fixed time intervals
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May be advantageous for bursts of strength or power needed in
a short period of time
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Occurs in greater measure after resistance training (RT)
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May be advantageous for timing of force production
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Motor Units (cont’d)
• Antagonist Muscle Activation
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Increases joint stability & movement coordination
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Reduces risk of injury
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May counteract effects of agonist muscles
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Magnitude affected by:
• Muscle group
• Velocity & type of muscle action
• Intensity
• Joint position
• Injury status
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Communication With Skeletal Muscle: The
Neuromuscular Junction
• Neuromuscular Junction
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Gap between nerve & muscle
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Crossed by neurotransmitter, allowing action potential to reach
muscle
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Acetylcholine
• Key neurotransmitter
• Released between motor nerve & skeletal muscle
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The Motor Unit and Neuromuscular
Junction
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Sensory Nervous System
• Golgi Tendon Organs (GTOs)
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Proprioceptors located at muscle-tendon junction
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Convey information regarding muscle tension to CNS
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Amount of stretch to GTOs increases with increased muscle
tension
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Cause agonist muscle relaxation & antagonist muscle excitation
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Defense mechanism to protect body from excessive damage
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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
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Golgi Tendon Organ
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Sensory Nervous System (cont’d)
• Muscle Spindles
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Proprioceptors located within muscle fibers
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Consist of nuclear chain & nuclear bag fibers
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Respond to:
• Magnitude of change in muscle length
• Rate of change of length
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Convey information to CNS regarding static changes in muscle
length or joint angle
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Enhance human performance
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Initiate stretch reflex
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Muscle Spindle
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Training Studies
• EMG, or Electromyography, has shown neural function:
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To be higher for high-intensity muscular effort vs. low-intensity
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To be higher during ballistic or explosive resistance exercise
compared with slower velocities
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To be higher for concentric (CON) vs. eccentric (ECC) muscle
actions when matched for intensity
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To increase as fatigue ensues in CON & ECC muscle actions
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To be reduced following a workout compared with preworkout
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To be lower for high-volume, high-intensity training
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To be reduced during periods of detraining
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Training Studies (cont’d)
• Unilateral Training
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Cross education: strength & endurance gained in non-trained limb
during unilateral training
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Adaptations are carried over to opposite limb
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Induces novel stimulus to nervous system
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Improves functional performance
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Useful in injured individuals due to partial training effect in injured limb
• Bilateral Deficit
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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!
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Autonomic Nervous System
• Controls:
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Heart rate
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Force of contraction
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Respiration rate
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Digestion
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Blood pressure & flow
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Fuel mobilization
• Sympathetic Branch (“Fight or Flight”)[stress or exercise]
• Parasympathetic Branch: Returns body to normal [homeostasis]
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Autonomic Nervous System (cont’d)
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