Transcript Chapter 7
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 – 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 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)