Chapter 7 The Nervous System: Structure and Control of Movement EXERCISE PHYSIOLOGY Theory 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 Axon Synapse 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

The Parts of a Neuron Figure 7.3

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 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)