Chapter 7The Nervous System: Structure and Control of MovementObjectivesDiscuss the general organization of the nervous systemDescribe the structure & function of a nerveDraw and label the pathways involved in a withdraw reflexDefine depolarization, action potential, and repolarizationObjectivesDiscuss the role of position receptors in the control of movementDescribe the role of vestibular apparatus in maintaining equilibriumDiscuss the brain centers involved in voluntary control of movementDescribe the structure and function of the autonomic nervous systemGeneral Nervous System Functions1. Control of the internal environment– Nervous system works with endocrine system2. Programming spinal cord reflexes3. Voluntary control of movement4. Assimilation of experiences necessary for memory and learningOrganization of the Nervous SystemCentral nervous system (CNS)– Brain and spinal cordPeripheral nervous system (PNS)– Neurons outside the CNSSensory divisionMotor divisionFig 7.1Organization of the Nervous System– Sensory divisionAfferent fibers transmit impulses from receptors to CNSSensation-heat, light, touch, smell, pressureBlood and lymph vessels, internal organsSkin, muscles, tendons– Motor divisionEfferent fibers transmit impulses from CNS to effector organsResults in appropriate responses– Muscle contraction– Reflex activityDivisions of the Nervous SystemFig 7.1Relationship Between PNS and CNSFig 7.2Structure of a NeuronCell body - SomaDendrites: receptor areas, conduct impulses toward cell bodyAxon-nerve fiber– Carries electrical impulse away from cell body toward another neuron, organ– Length – few millimeters to a meterBasic Nerve StructureNeuron– Cell body (soma)– Dendrites– AxonA. Motor neuron– Away from somaB. Sensory neuron– Toward somaBasic Nerve StructureLarge nerve fibers– Skeletal muscles– Myelin sheathComposed of fat and proteinSegments separated by spaces-nodes of RanvierStructure of a NeuronAxon-nerve fiber– May be covered by Schwann cellsForms discontinuous myelin sheath along length of axonGaps – Nodes of Ranvier– Nerve impulse is propagated-bounces from node to node– Saltatory conduction-increases conduction velocityLarge myelinated fibers-skeletal muscle– 60 to 100 m/sec (135 to 225 miles/hr)Non-myelinated fibers-mixed in with myelinated – 6 to 10 m/secStructure of a NeuronElectrical Activity in NeuronsNeurons are “Excitable Tissue”–Irritability: ability to respond to a stimulus and convert it to a neural impulse–Conductivity: transmission of the impulse along the axonIon ConcentrationsIon ECF ICFSodium 150 mmoles/L 15Chloride 110 10Potassium 5 150Ion [ ] set up potential difference across membrane– Resting membrane potentialResting Membrane PotentialResting membrane potential–More Na+outside–More K+inside– Electrical gradient (difference) between inside and outside cellAction PotentialCell membrane depolarizes (polarity is reversed)–Na+moves in–K+moves out– With depolarizationInside positiveOutside negative– Starts action potentialpropagatedAction Potential Repolarization –Na+inside cell–K+ pulled out of cell (more negative)– Outside becomes +– Inside goes negativeReturn to restingElectrical Activity in NeuronsResting membrane potential– At rest, the neurons are negatively charged– Determined by concentrations of ions (Na+, K+, Cl-) across membrane – Negative inside, K+leaving – Positive outside cell membrane– [ ] maintained by Na/K pumpAn Action PotentialAction potential– Stimulus - neural message-threshold– Permeability of the membrane changes, Na+gates open – interior positively chargedAction potential (nerve impulse) occursDepolarizationRepolarizationRepolarization– Change in membrane permeability, restoring resting membrane potentialNa+gates close-entry into cell is slowedK+ leaves cell-returning inside to negativeRepolarizationFig 7.6bSynaptic TransmissionNeurons communicate across synapses using neurotransmitters–Released from presynaptic membrane–Binds to receptor on post synaptic membraneSynaptic TransmissionSynapse: Contact points between axon of one neuron and dendrite of another neuron– Nerve to nerve– Nerve to muscle-Neuromuscular junctionMyoneural junctionMotor end plateSynaptic TransmissionBasic Structure of a Chemical SynapseSynapse-two neuronsAxon terminals of presynaptic neuronSynaptic cleft-spaceReceptors of second neuronVesicles release transmitter substanceNT diffuses across cleft and impulse continuesResponse?Neuron ResponseType of transmitter released– Excitatory-cause depolarization of membranesAcetylcholine-neuron to muscleNorepinephrine-peripheral sympathetic nerves– Adrenergic receptorsAcetylcholine-peripheral parasympathetic nerves– Cholinergic receptors– Inhibitory-cause hyperpolarization of membranesGamma-aminobutyric acid (GABA)-spinal cord/brainGlycine-spinal cordSynaptic TransmissionExcitatory postsynaptic potentials (EPSP)– Causes depolarization which may or may not reach threshold– Sufficient amounts of NT causes depolarization to threshold– Action potential is generatedSynaptic TransmissionExcitatory postsynaptic potentials (EPSP)– Temporal summation:Timing is importantSuccessive discharges from the same terminalSumming several EPSPs from one presynaptic neuronSynaptic TransmissionSpatial summation: summing from several different presynaptic neurons– Additive effect of several stimuliStimulus Inhibitory postsynpaticpotentials (IPSP)– HyperpolarizationNT inhibits responseInhibitory postsynaptic potential (IPSP)Sensory InformationProprioceptors–Proprioception: ability to determine position of joint–Kinesthesia: sensation of joint motion or acceleration (rate of movement)Muscle Chemoreceptors–Sensitive to changes in the chemical environment surrounding a muscleProprioceptorsProvide CNS with information about body position and joint angle– Free nerve endings – touch & pressure– Golgi-type receptors – in ligaments & joints– Pacinian corpuscles – in tissues around jointsStrongly stimulated at beginning of movement then adapt – Steady signal until move is completedProprioceptorsJoint receptors– Provide body with information of orientation of body parts– Feedback about rates of limb movementMuscle ChemoreceptorsProvide CNS with information regarding the metabolic rate of muscular activity – Hydrogen ion concentration– Carbon dioxide (CO2)– Potassium (K+)Chemoreceptor feedback