Chapter 7 The Nervous System: Structure and Control of Movement

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Chapter 7The Nervous System Structure and
Control of Movement

• EXERCISE PHYSIOLOGY
• Theory and Application to Fitness and
  Performance, 6th edition
• Scott K. Powers Edward T. Howley

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General Nervous System Functions

• Control of the internal environment
• With the endocrine system
• Voluntary control of movement
• Programming spinal cord reflexes
• Assimilation of experiences necessary for memory
  and learning

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Organization of the Nervous System

• Central nervous system (CNS)
• Brain and spinal cord
• Peripheral nervous system (PNS)
• Neurons outside the CNS
• Sensory division
• Afferent fibers transmit impulses from receptors
  to CNS
• Motor division
• Efferent fibers transmit impulses from CNS to
  effector organs

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Anatomical Divisions of the Nervous System
Figure 7.1
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Relationship Between PNS and CNS
Figure 7.2
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Structure of a Neuron

• Cell body
• Dendrites
• Conduct impulses toward cell body
• Axon
• Carries electrical impulse away from cell body
• May be covered by Schwann cells
• Forms discontinuous myelin sheath along length of
  axon
• Synapse
• Contact points between axon of one neuron and
  dendrite of another neuron

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The Parts of a Neuron
Figure 7.3
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Synaptic Transmission
Figure 7.4
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Electrical Activity in Neurons

• Neurons are excitable tissue
• Irritability
• Ability to respond to a stimulus and convert it
  to a neural impulse
• Conductivity
• Transmission of the impulse along the axon

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Resting Membrane Potential

• Negative charge inside cells at rest
• -5 to -100 mv (-40 to -75 mv in neurons)
• Determined by
• Permeability of plasma membrane to ions
• Difference in ion concentrations across membrane
• Na, K, Cl-, and Ca2
• Maintained by sodium-potassium pump
• Potassium tends to diffuse out of cell
• Na/K pump moves 2 K in and 3 Na out

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The Resting Membrane Potential in Cells
Figure 7.5
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Concentrations of Ions Across a Cell Membrane
Figure 7.6
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Illustration of Ion Channels
Figure 7.7
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The Sodium-Potassium Pump
Figure 7.8
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Action Potential

• Occurs when a stimulus of sufficient strength
  depolarizes the cell
• Opens Na channels and Na diffuses into cell
• Inside becomes more positive
• Repolarization
• Return to resting membrane potential
• K leaves the cell rapidly
• Na channels close
• All-or-none law
• Once a nerve impulse is initiated it will travel
  the length of the neuron

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An Action Potential
Figure 7.9
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Depolarization and Repolarization of a Nerve Fiber
Figure 7.10
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Neurotransmitters and Synaptic Transmission

• Synapse
• Small gap between presynaptic neuron and
  postsynaptic neuron
• Neurotransmitter
• Chemical messenger released from presynaptic
  membrane
• Binds to receptor on postsynaptic membrane
• Causes depolarization of postsynaptic membrane

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Basic Structure of a Chemical Synapse
Figure 7.11
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Neurotransmitters and Synaptic Transmission

• Excitatory postsynaptic potentials (EPSP)
• Causes depolarization
• Temporal summation
• Summing several EPSPs from one presynaptic neuron
• Spatial summation
• Summing from several different presynaptic
  neurons
• Inhibitory postsynaptic potentials (IPSP)
• Causes hyperpolarization

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Proprioceptors

• Provide CNS with information about body position
  and joint angle
• Free nerve endings
• Sensitive to touch and pressure
• Initially strongly stimulated then adapt
• Golgi-type receptors
• Found in ligaments and joints
• Similar to free nerve endings
• Pacinian corpuscles
• In tissues around joints
• Detect rate of joint rotation

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Muscle Chemoreceptors

• Sensitive to changes in the chemical environment
  surrounding a muscle
• H ions, CO2, and K
• Provide CNS about metabolic rate of muscular
  activity
• Important in regulation of cardiovascular and
  pulmonary responses

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Reflexes

• Rapid, unconscious means of reacting to stimuli
• Order of events
• Sensory nerve sends impulse to spinal column
• Interneurons activate motor neurons
• Motor neurons control movement of muscles
• Reciprocal inhibition
• EPSPs to muscles to withdraw from stimulus
• IPSPs to antagonistic muscles
• Crossed-extensor reflex
• Opposite limb supports body during withdrawal of
  injured limb

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The Crossed-Extensor Reflex
Figure 7.12
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Somatic Motor Function

• Somatic motor neurons of PNS
• Responsible for carrying neural messages from
  spinal cord to skeletal muscles
• Motor unit
• Motor neuron and all the muscle fibers it
  innervates
• Innervation ratio
• Number of muscle fibers per motor neuron

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Illustration of a Motor Unit
Figure 7.13
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Vestibular Apparatus and Equilibrium

• Vestibular apparatus
• Located in the inner ear
• Responsible for maintaining general equilibrium
  and balance
• Sensitive to changes in linear and angular
  acceleration

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Role of the Vestibular Apparatus in Maintaining
Equilibrium and Balance
Figure 7.14
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Motor Control Functions of the Brain

• Brain stem
• Responsible for
• Many metabolic functions
• Cardiorespiratory control
• Complex reflexes
• Major structures
• Medulla
• Pons
• Midbrain
• Reticular formation

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Motor Control Functions of the Brain

• Cerebrum
• Cerebral cortex
• Organization of complex movement
• Storage of learned experiences
• Reception of sensory information
• Motor cortex
• Motor control and voluntary movement
• Cerebellum
• Coordinates and monitors complex movement

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Motor Functions of the Spinal Cord

• Withdrawal reflex
• Other reflexes
• Important for the control of voluntary movement
• Spinal tuning
• Voluntary movement translated into appropriate
  muscle action

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Control of Motor Function

• Subcortical and cortical motivation areas
• Sends a rough draft of the movement
• Cerebellum and basal ganglia
• Coverts rough draft into movement plan
• Cerebellum fast movements
• Basal ganglia slow, deliberate movements
• Motor cortex through thalamus
• Forwards message sent down spinal neurons for
  Spinal tuning and onto muscles
• Feedback from muscle receptors and proprioceptors
  allows fine-tuning of motor program

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Structures and Processes Leading to Voluntary
Movement
Figure 7.16
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Autonomic Nervous System

• Responsible for maintaining internal environment
• Effector organs not under voluntary control
• Smooth muscle, cardiac muscle, and glands
• Sympathetic division
• Releases norepinephrine (NE)
• Excites an effector organ
• Parasympathetic division
• Releases acetylcholine (ACh)
• Inhibits effector organ

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Neurotransmitters of the Autonomic Nervous System
Figure 7.17
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Exercise Enhance Brain Health

• Exercise improves brain function and reduces the
  risk of cognitive impairment associated with
  aging
• Regular exercise can protect the brain against
  disease (e.g. Alzheimers) and certain types of
  brain injury (e.g. stroke)