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The Nervous System

  • Master controlling and communicating system of body
  • Cells communicate via electrical and chemical signals
    • Rapid and specific
    • Usually cause almost immediate responses

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Functions of the Nervous System

  • Sensory input
    • Information gathered by sensory receptors about internal and external changes
  • Integration
    • Processing and interpretation of sensory input
  • Motor output
    • Activation of effector organs (muscles and glands) produces a response

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Divisions of the Nervous System

  • Central nervous system (CNS)
    • Brain and spinal cord of dorsal body cavity
    • Integration and control center
      • Interprets sensory input and dictates motor output
  • Peripheral nervous system (PNS)
    • The portion of the nervous system outside CNS
    • Consists mainly of nerves that extend from brain and spinal cord
      • Spinal nerves to and from spinal cord
      • Cranial nerves to and from brain

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Peripheral Nervous System (PNS)

  • Two functional divisions
    • Sensory (afferent) division
      • Somatic sensory fibers—convey impulses from skin, skeletal muscles, and joints to CNS
      • Visceral sensory fibers—convey impulses from visceral organs to CNS
    • Motor (efferent) division
      • Transmits impulses from CNS to effector organs
        • Muscles and glands
      • Two divisions
        • Somatic nervous system
        • Autonomic nervous system

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Motor Division of PNS:�Somatic Nervous System

  • Somatic motor nerve fibers
  • Conducts impulses from CNS to skeletal muscle
  • Voluntary nervous system
    • Conscious control of skeletal muscles

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Motor Division of PNS:�Autonomic Nervous System

  • Visceral motor nerve fibers
  • Regulates smooth muscle, cardiac muscle, and glands
  • Involuntary nervous system
  • Two functional subdivisions
    • Sympathetic
    • Parasympathetic
    • Work in opposition to each other

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Histology of Nervous Tissue

  • Highly cellular; little extracellular space
    • Tightly packed
  • Two principal cell types
    • Neuroglia – small cells that surround and wrap delicate neurons
    • Neurons (nerve cells)—excitable cells that transmit electrical signals

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Histology of Nervous Tissue: Neuroglia

  • Astrocytes (CNS)
  • Microglial cells (CNS)
  • Ependymal cells (CNS)
  • Oligodendrocytes (CNS)
  • Satellite cells (PNS)
  • Schwann cells (PNS)

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Astrocytes

  • Most abundant, versatile, and highly branched glial cells
  • Cling to neurons, synaptic endings, and capillaries
  • Functions include
    • Support and brace neurons
    • Play role in exchanges between capillaries and neurons
    • Guide migration of young neurons
    • Control chemical environment around neurons
    • Respond to nerve impulses and neurotransmitters
    • Influence neuronal functioning
      • Participate in information processing in brain

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Microglial Cells

  • Small, ovoid cells with thorny processes that touch and monitor neurons
  • Migrate toward injured neurons
  • Can transform to phagocytize microorganisms and neuronal debris

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Ependymal Cells

  • Range in shape from squamous to columnar
  • May be ciliated
    • Cilia beat to circulate CSF
  • Line the central cavities of the brain and spinal column
  • Form permeable barrier between cerebrospinal fluid (CSF) in cavities and tissue fluid bathing CNS cells

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Oligodendrocytes

  • Branched cells
  • Processes wrap CNS nerve fibers, forming insulating myelin sheaths thicker nerve fibers

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Satellite Cells and Schwann Cells

  • Satellite cells
    • Surround neuron cell bodies in PNS
    • Function similar to astrocytes of CNS
  • Schwann cells (neurolemmocytes)
    • Surround all peripheral nerve fibers and form myelin sheaths in thicker nerve fibers
      • Similar function as oligodendrocytes
    • Vital to regeneration of damaged peripheral nerve fibers

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Neurons

  • Structural units of nervous system
  • Large, highly specialized cells that conduct impulses
  • Extreme longevity (→ 100 years or more)
  • Amitotic—with few exceptions
  • High metabolic rate—requires continuous supply of oxygen and glucose
  • All have cell body and one or more processes

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Neuron Cell Body (Perikaryon or Soma)

  • Biosynthetic center of neuron
    • Synthesizes proteins, membranes, and other chemicals
    • Rough ER - Most active and best developed in body
  • Spherical nucleus with nucleolus
  • In most, plasma membrane part of receptive region
  • Most neuron cell bodies in CNS
    • Nuclei – clusters of neuron cell bodies in CNS
  • Ganglia – cluster of cell bodies in PNS

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Neuron Processes

  • Armlike processes extend from body
  • CNS
    • Both neuron cell bodies and their processes
  • PNS
    • Chiefly neuron processes
  • Tracts
    • Bundles of neuron processes in CNS
  • Nerves
    • Bundles of neuron processes in PNS
  • Two types of processes
    • Dendrites
    • Axon

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Dendrites

  • In motor neurons
    • 100s of short, tapering, diffusely branched processes
    • Same organelles as in body
  • Receptive (input) region of neuron
  • Convey incoming messages toward cell body as graded potentials (short distance signals)

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Figure 11.4b Structure of a motor neuron.

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Neuron cell body

Dendritic

spine

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The Axon: Structure

  • One axon per cell arising from axon hillock
    • Cone-shaped area of cell body
  • In some axon short or absent
  • In others most of length of cell
    • Some 1 meter long
  • Long axons called nerve fibers
  • Occasional branches (axon collaterals)
  • Branches profusely at end (terminus)
  • Can be 10,000 terminal branches
  • Distal endings called axon terminals or terminal boutons

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The Axon: Functional Characteristics

  • Conducting region of neuron
  • Generates nerve impulses
  • Transmits them along axolemma (neuron cell membrane) to axon terminal
    • Secretory region
    • Neurotransmitters released into extracellular space
      • Either excite or inhibit neurons with which axons in close contact
  • Carries on many conversations with different neurons at same time
  • Lacks rough ER and Golgi apparatus
    • Relies on cell body to renew proteins and membranes
    • Efficient transport mechanisms
    • Quickly decay if cut or damaged

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Transport Along the Axon

  • Molecules and organelles are moved along axons by motor proteins and cytoskeletal elements
  • Movement in both directions
    • Anterograde—away from cell body
      • Examples: mitochondria, cytoskeletal elements, membrane components, enzymes
    • Retrograde—toward cell body
      • Examples: organelles to be degraded, signal molecules, viruses, and bacterial toxins

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Myelin Sheath

  • Composed of myelin
    • Whitish, protein-lipoid substance
  • Segmented sheath around most long or large-diameter axons
    • Myelinated fibers
  • Function of myelin
    • Protects and electrically insulates axon
    • Increases speed of nerve impulse transmission
  • Nonmyelinated fibers conduct impulses more slowly

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Myelination in the PNS

  • Formed by schwann cells
    • Wrap around axon in jelly roll fashion
    • One cell forms one segment of myelin sheath
  • Myelin sheath
    • Concentric layers of schwann cell plasma membrane around axon
  • Outer collar of perinuclear cytoplasm (formerly called neurilemma)
    • Peripheral bulge of schwann cell containing nucleus and most of cytoplasm

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Figure 11.5b Nerve fiber myelination by Schwann cells in the PNS.

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Myelin sheath

Outer collar

of perinuclear

cytoplasm

(of Schwann

cell)

Cross-sectional view of a myelinated axon (electron

micrograph 24,000x)

Axon

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Myelination in the PNS

  • Plasma membranes of myelinating have cells less protein
    • No channels or carriers
    • Good electrical insulators
    • Interlocking proteins bind adjacent myelin membranes
  • Nodes of ranvier
    • Myelin sheath gaps between adjacent schwann cells
    • Sites where axon collaterals can emerge
  • Nonmyelinated fibers
    • Thin fibers not wrapped in myelin; surrounded by schwann cells but no coiling; one cell may surround 15 different fibers

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Myelin Sheaths in the CNS

  • Formed by multiple, flat processes of oligodendrocytes, not whole cells
  • Can wrap up to 60 axons at once
  • Nodes of ranvier are present
  • No outer collar of perinuclear cytoplasm
  • Thinnest fibers are unmyelinated
    • Covered by long extensions of adjacent neuroglia
  • White matter
    • Regions of brain and spinal cord with dense collections of myelinated fibers – usually fiber tracts
  • Gray matter
    • Mostly neuron cell bodies and nonmyelinated fibers

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Figure 11.3d Neuroglia.

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Myelin sheath

Process of

oligodendrocyte

Nerve

fibers

Oligodendrocytes have processes that form myelin

sheaths around CNS nerve fibers.

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Structural Classification of Neurons

  • Grouped by number of processes
  • Three types
    • Multipolar – 3 or more processes
      • 1 axon, others dendrites
      • Most common; major neuron in CNS
    • Bipolar – 2 processes
      • 1 axon and 1 dendrite
      • Rare, e.G., Retina and olfactory mucosa
    • Unipolar – 1 short process
      • Divides T-like – both branches now considered axons
        • Distal (peripheral) process – associated with sensory receptor
        • Proximal (central) process – enters CNS

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Functional Classification of Neurons

  • Grouped by direction in which nerve impulse travels relative to CNS
  • Three types
    • Sensory (afferent)
    • Motor (efferent)
    • Interneurons

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Functional Classification of Neurons

  • Sensory
    • Transmit impulses from sensory receptors toward CNS
    • Almost all are Unipolar
    • Cell bodies in ganglia in PNS
  • Motor
    • Carry impulses from CNS to effectors
    • Multipolar
    • Most cell bodies in CNS (except some autonomic neurons)
  • Interneurons (association neurons)
    • Lie between motor and sensory neurons
    • Shuttle signals through CNS pathways; most are entirely within CNS
    • 99% of body's neurons
    • Most confined in CNS

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