Chapter 7

The Nervous System: Structure and Control of

Movement

EXERCISE PHYSIOLOGYTheory and Application to Fitness and Performance,

6th edition

Scott K. Powers & Edward T. Howley

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

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

Anatomical Divisions of the Nervous System

Figure 7.1

Relationship Between PNS and CNS

Figure 7.2

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

Figure 7.3

The Parts of a Neuron

Synaptic Transmission

Figure 7.4

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

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 Ca+2

• Maintained by sodium-potassium pump– Potassium tends to diffuse out of cell– Na+/K+ pump moves 2 K+ in and 3 Na+ out

The Resting Membrane Potential in Cells

Figure 7.5

Concentrations of Ions Across a Cell Membrane

Figure 7.6

Illustration of Ion Channels

Figure 7.7

The Sodium-Potassium Pump

Figure 7.8

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

An Action Potential

Figure 7.9

Depolarization and Repolarization of a Nerve Fiber

Figure 7.10

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

Basic Structure of a

Chemical Synapse

Figure 7.11

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

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

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

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

The Crossed-Extensor Reflex

Figure 7.12

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

Illustration of a Motor Unit

Figure 7.13

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

Role of the Vestibular Apparatus in Maintaining Equilibrium and Balance

Figure 7.14

Motor Control Functions of the Brain

• Brain stem – Responsible for:

• Many metabolic functions• Cardiorespiratory control• Complex reflexes

– Major structures:• Medulla • Pons• Midbrain• Reticular formation

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

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

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

Structures and Processes Leading to Voluntary Movement

Figure 7.16

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

Neurotransmitters of the Autonomic Nervous System

Figure 7.17

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. Alzheimer’s) and certain types of brain injury (e.g. stroke)