Nervous System Nervous System Basic Divisions of the Nervous

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					Lecture 21 - Fundamentals of the Nervous
System




                                                                                             Nervous System
                            Nervous System
                                                                                Master control and communication system
                                 Part 1 -
                            Fundamentals of the                                 Has three overlapping functions
                             Nervous System                                       Sensory receptors monitor changes inside and
                                                                                  outside the body
                                                                                    Change – a stimulus
                                                                                    Gathered information – sensory input


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                                                                                 Basic Divisions of the Nervous
                            Nervous System
                                                                                            System
              Processes and interprets sensory input                            Central nervous system (CNS)
                Makes decisions – integration                                     Brain and spinal cord
                                                                                  Integrating and command center
              Dictates a response by activating effector organs
                Response – motor output




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                                                                                 Basic Divisions of the Nervous
              Basic Divisions of the Nervous                                                System
                         System
            Peripheral nervous system (PNS)
              Outside the CNS

              Consists of nerves extending from brain and spinal cord
                Cranial nerves
                Spinal nerves
                Ganglia
                   Ganglia are neuron cell bodies located outside CNS

              Peripheral nerves link all regions of the body to the CNS

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Lecture 21 - Fundamentals of the Nervous
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                  Sensory Input and Motor                                      Sensory Input and Motor
                          Output                                                       Output
            Sensory (afferent) Receives information                     Divided according to region they serve
            FROM the body and transmits it TO the CNS                     Somatic body region
            f processing
            for       i                                                   Visceral body region

              Carried by nerve fibers of PNS to the CNS                 Results in four main subdivisions
                                                                          Somatic sensory – from skin, joints, muscle, special senses
                                                                          Visceral sensory – from blood vessels and viscera (organs)
            Motor (efferent) Transmits information                        Somatic motor – innervates skeletal muscle
            FROM the CNS TO muscles and glands                            Visceral motor – (ANS) innervates smooth muscle,
              Innervate muscles and glands                                cardiac muscle and glands of viscera (organs)

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              Types of Sensory and Motor
                     Information                                           Basic Divisions of the Nervous
                                                                                      System
                                                                        Somatic sensory
                                                                           General somatic senses – receptors are widely
                                                                                d throughout the body
                                                                           spread th    h t th b d
                                                                             Touch
                                                                             Pain
                                                                             Vibration
                                                                             Pressure
                                                                             Temperature


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              Basic Divisions of the Nervous                               Basic Divisions of the Nervous
                         System                                                       System
            Visceral sensory                                            Somatic motor
              General visceral senses – stretch, pain,                     General somatic motor – signals contraction of
              temperature, nausea, and hunger
              t      t               dh                                     k l t l     l
                                                                           skeletal muscles
                Widely felt in digestive and urinary tracts, and             Under our voluntary control
                reproductive organs                                          Often called “voluntary nervous system”




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Lecture 21 - Fundamentals of the Nervous
System




              Basic Divisions of the Nervous
                                                                                           Nervous Tissue
                         System
            Visceral motor                                                 Cells are densely packed and intertwined
              Regulates the contraction of smooth and cardiac                Two main cell types
                   l    d l d f i
              muscle and glands of viscera                                     N
                                                                               Neurons – R
                                                                                         Respond to stimuli & conduct nerve impulses
                                                                                                 d     i li          d            i  l
              Makes up autonomic nervous system (ANS)                          Support cells - Neuroglial cells (glial cells) in CNS
                                                                                 Nonexcitable
              Controls function of visceral organs
                                                                                 Surround and wrap neurons
              Often called “involuntary nervous system”                          Support and protect neurons




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                               The Neuron                                                      The Neuron

            The human body contains billions of neurons                    Other special characteristics
              Basic structural unit of the nervous system                     Longevity – can live and function for a lifetime
                S i li d cells conduct electrical impulses along the
                Specialized ll    d     l    i li     l     l     h           Do not divide
                plasma membrane
                  Nerve impulse (action potential)
                                                                              High metabolic rate – require abundant oxygen
                                                                              and glucose
                                                                                Neurons die after 5 minutes without oxygen




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                            The Cell Body                                                   The Cell Body

            Cell body (soma)                                               Chromatophilic bodies (Nissl bodies)
              Focal point for outgrowth of neuron processes                   Found in the cytoplasm of the cell body
              Location of nucleus                                             Clusters of rough ER and free ribosomes
              Most are located within the CNS                                 Renew membranes of the cell and protein part of
                Clusters of cell bodies lie along nerves in the PNS           the cytosol
                  Ganglia (singular = ganglion)




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Lecture 21 - Fundamentals of the Nervous
System



                                                                          Structure of a Typical Large
                              The Cell Body                                          Neuron

            Most neuronal cell bodies
              Located within the CNS
              Protected by bones of the skull and vertebral
              column
            Ganglia – clusters of cell bodies
              Lie along nerves in the PNS



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                          Neuron Processes                                         Neuron Processes

            Dendrites                                                  Axons
              Extensively branching from the cell body                   Neuron has only one
              Transmit electrical signals toward the cell body           Impulse generator and conductor
              Chromatophilic bodies – only extend into the               Transmits impulses AWAY from the cell body
              basal part of dendrites and to the base of the
              axon hillock                                               Chromatophilic bodies are absent
              Function as receptive sites for receiving signals          No protein synthesis in axon
              from other neurons


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                          Neuron Processes                                         Neuron Processes

            Axons                                                      Nerve impulse
              Branches along length are infrequent                       Generated at the initial segment of the axon
                Axon collaterals
                A      ll     l                                          Conducted along the axon
              Multiple branches at end of axon                           Releases neurotransmitters at axon terminals
                Terminal branches                                          Neurotransmitters – excite or inhibit neurons
                    End in knobs called axon terminals
                                                                         Neuron receives and sends signals



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Lecture 21 - Fundamentals of the Nervous
System



                                                                            Two Neurons Communicating
                                    Synapses                                       at a Synapse

            Site at which neurons communicate
            Signals pass across synapse in one direction
            M synapses are i the CNS
            Most               in h
            Presynaptic neuron
              Conducts signal toward a synapse
            Postsynaptic neuron
              Transmits electrical activity away from a synapse

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                                                                           Neurons Classified by Structure
                   Classification of Neurons

            Structural classification
              Multipolar – possess more than two processes
                 Numerous dendrites and one axon
              Bipolar – possess two processes
                 Rare type of neurons
                 Found in some special sensory organs
              Unipolar (pseudounipolar) – single process comes off of
              the cell body and divides into two branches
                 Start as bipolar neurons during development


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             Neurons Classified by Structure                             Neurons Classified by Structure




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Lecture 21 - Fundamentals of the Nervous
System




                 Functional Classification of                                             Functional Classification of
                          Neurons                                                                  Neurons
           Functional classification is                                              Motor (efferent) neurons - Most are multipolar
             According to the direction the nerve impulse travels                      Carry impulses away from CNS to effector organs
           Sensory ( ff
           S       (afferent) neurons
                           t)                                                          Cell b di        ithi th
                                                                                       C ll bodies are within the CNS
           Transmit impulses toward the CNS                                            Form junctions with effector cells
             Virtually all are unipolar neurons
             Cell bodies in ganglia outside the CNS                                  Interneurons – most are multipolar
                Short, single process divides into                                     Lie between motor and sensory neurons
                   The central process – runs centrally into the CNS
                                                                                         Coordinate & integrate between sensory & motor neurons
                   The peripheral process – extends peripherally to the receptors
                                                                                       Confined to the CNS
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              Neurons Classified by Function                                                       Supporting Cells

                                                                                      Six types of supporting cells
                                                                                        Four in the CNS
                                                                                        Two in the PNS

                                                                                      Provide supportive functions for neurons

                                                                                      Cover nonsynaptic regions of the neurons


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                        Neuroglia in the CNS                                                   Neuroglia in the CNS

            Neuroglia                                                                Astrocytes - most abundant glial cell type
               Glial cells have branching processes and a                              Sense when neurons release glutamate
                  t l ll b d
               central cell body                                                       Extract blood sugar from capillaries for energy
               Outnumber neurons 10 to 1                                               Take up and release ions to control environment
               Make up half the mass of the brain                                      around neurons
               Can divide throughout life                                              Produce molecules necessary for neuronal growth
                                                                                       Involved with memory


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Lecture 21 - Fundamentals of the Nervous
System



                    Astrocytes in the CNS
                                                                                       Neuroglia in the CNS

                                                                            Microglia – smallest and
                                                                            least abundant glial cell
                                                                               h            h
                                                                              Phagocytes – the
                                                                              macrophages of the CNS
                                                                              Engulf invading
                                                                              microorganisms and also
                                                                              dead neurons
                                                                              Derive from blood cells
                                                                              called monocytes

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                                                                                  Neuroglia in the CNS
                     Neuroglia in the CNS

            Ependymal cells
                Line the central cavity of the spinal cord and brain
                Bear cilia – help circulate the cerebrospinal fluid


            Oligodendrocytes – have few branches
                Wrap their cell processes around axons in CNS
                  Produce myelin sheaths – insulate nerves




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                     Neuroglia in the PNS
                                                                                            Myelin Sheaths
            Satellite cells – surround neuron cell bodies
            within ganglia
                                                                             Segmented structures composed of the
            Schwann cells (neurolemmocytes) –
                                                                             lipoprotein myelin
            surround axons in the PNS
              Form myelin sheath around axons of the PNS                     Surround thicker axons
                                                                             Form an insulating layer
                                                                                Prevent leakage of electrical current
                                                                             Increase the speed of impulse conduction


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Lecture 21 - Fundamentals of the Nervous
System




                                 Nerves                                                      Nerves

            Nerves – cablelike organs in the PNS                        Endoneurium – layer of delicate connective
              Consists of numerous axons wrapped in                     tissue surrounding the axon
              connective tissue
                    ti ti                                               Perineurium – connective tissue wrapping
              Axon is surrounded by Schwann cells                       surrounding a nerve fascicle
                                                                          Nerve fascicles – groups of axons bound into
                                                                          bundles
                                                                        Epineurium – whole nerve is surrounded by
                                                                        tough fibrous sheath

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                      Structure of a Nerve
                                                                           Integration Between the PNS
                                                                                     and CNS
                                                                        The CNS and PNS are functionally
                                                                        interrelated
                                                                        Nerves of the PNS
                                                                          Information pathways to & from body periphery
                                                                            Afferent PNS fibers respond to sensory stimuli
                                                                            Efferent PNS fibers transmit motor stimuli from CNS
                                                                            to muscles and glands



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               Integration Between the PNS
                                                                                          Reflex Arc
                         and CNS
            Nerves of the CNS
              Composed on interneurons that
                P          d     i           i f     i
                Process and receive sensory information
                Direct information to specific CNS regions
                Initiate appropriate motor responses
                Transport information from one area of CNS to another




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