Nerve Cells

• Nervous system


  Mission control for the body
• Functions of nervous system

  • Receives sensory input from both inside and outside the body
  • Processes, integrates, analyzes, and sums up input received...decides on a course of action to take in response to the sensory input
  • Generates motor output
• Cells communicate


  • Via electrical and chemical signals
  • Rapid and specific
  • Usually cause almost immediate responses
  • In contrast to hormones of the endocrine system that take a longer time to bring about an effect
• Nervous system functions


  • Sensory input
  • Integration
  • Motor output
• 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 nervous system outside CNS
  • Consists mainly of nerves that extend from brain and spinal cord
• Divisions of PNS


  • Sensory (afferent) division
  • Motor (efferent) division
• Somatic sensory fibers


  Convey impulses from skin, skeletal muscles, and joints to CNS
• Visceral sensory fibers


  Convey impulses from visceral organs to CNS
• Somatic motor nerve fibers


  Conduct impulses from CNS to skeletal muscle
• Somatic nervous system


  • Voluntary nervous system
  • Conscious control of skeletal muscles
• Autonomic nervous system


  • Regulates smooth muscle, cardiac muscle, and glands
  • Involuntary nervous system
  • Two functional subdivisions: Sympathetic and Parasympathetic
• Neuroglia


  Small cells that surround and wrap delicate neurons
• Main neuroglia of CNS

  • Astrocytes
  • Microglial cells
  • Ependymal cells
  • Oligodendrocytes
• Astrocytes


  • Most abundant, versatile, and highly branched of 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
• 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
• 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
• Oligodendrocytes


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


  • Structural units of nervous system
  • Large, highly specialized cells that conduct impulses
• Neuron characteristics


  • Extreme longevity (lasts a person's lifetime)
  • Amitotic, with few exceptions
  • High metabolic rate: requires continuous supply of oxygen and glucose
  • All have cell body and one or more processes
• Neuron cell body


  • Also called the perikaryon or soma
  • Biosynthetic center of neuron
  • Synthesizes proteins, membranes, chemicals
  • Rough ER (chromatophilic substance, or Nissl bodies)
  • Contains spherical nucleus with nucleolus
  • Some contain pigments
  • In most, plasma membrane is part of receptive region that receives input info from other neurons
• Neuron cell body locations

  • Most are located in CNS
  • Nuclei: clusters of neuron cell bodies in CNS
  • Ganglia: clusters of neuron cell bodies in PNS
• Neuron processes


  • Armlike processes that extend from cell body
  • CNS contains both neuron cell bodies and their processes
  • PNS contains chiefly neuron processes
  • Tracts: Bundles of neuron processes in CNS
  • Nerves: Bundles of neuron processes in PNS
  • Two types: Dendrites and Axon
• Dendrites


  • Motor neurons can contain 100s of these short, tapering, diffusely branched processes
  • Contain same organelles as in cell body
  • Receptive (input) region of neuron
  • Convey incoming messages toward cell body as graded potentials (short distance signals)
  • In many brain areas, finer dendrites are highly specialized to collect information
  • Contain dendritic spines, appendages with bulbous or spiky ends
• Axon


  • Each neuron has one axon that starts at cone-shaped area called axon hillock
  • In some neurons, axons are short or absent; in others, axon comprises almost entire length of cell
  • Some axons can be over 1 meter long
  • Long axons are called nerve fibers
  • Axons have occasional branches called axon collaterals
  • Axons branch profusely at their end (terminus)
  • Can number as many as 10,000 terminal branches
  • Distal endings are called axon terminals or terminal boutons
• Axon functions


  • Generates nerve impulses and transmits them along axolemma (neuron cell membrane) to axon terminal
  • Terminal: region that secretes neurotransmitters, which are released into extracellular space
  • Can excite or inhibit neurons it contacts
  • Carries on many conversations with different neurons at same time
  • Rely on cell bodies to renew proteins and membranes
  • Quickly decay if cut or damaged
  • Have efficient internal transport mechanisms: Anterograde (away from cell body) and Retrograde (toward cell body)
• Certain viruses and bacterial toxins damage neural tissues by using retrograde axonal transport
• Myelin sheath


  • Composed of myelin, a whitish, protein-lipid substance
  • Protects and electrically insulates axon
  • Increases speed of nerve impulse transmission
• Myelinated fibers


  Segmented sheath surrounds most long or large-diameter axons
• Nonmyelinated fibers


  • Do not contain sheath
  • Conduct impulses more slowly
• Myelination in PNS


  • Formed by Schwann cells
  • Wraps around axon in jelly roll fashion
  • One cell forms one segment of myelin sheath
  • Outer collar of perinuclear cytoplasm (formerly called neurilemma): peripheral bulge containing nucleus and most of cytoplasm
  • Plasma membranes have less protein
  • No channels or carriers, so good electrical insulators
  • Interlocking proteins bind adjacent myelin membranes
• Myelin sheath gaps


  • Gaps between adjacent Schwann cells
  • Sites where axon collaterals can emerge
  • Formerly called nodes of Ranvier
• Nonmyelinated fibers in PNS

  Thin fibers not wrapped in myelin; surrounded by Schwann cells but no coiling; one cell may surround 15 different fibers
• Myelination in CNS


  • Formed by processes of oligodendrocytes, not whole cells
  • Each cell can wrap up to 60 axons at once
  • Myelin sheath gap is present
  • No outer collar of perinuclear cytoplasm
• Thinnest fibers are unmyelinated, but covered by long extensions of adjacent neuroglia
• Saltatory conduction


  Action potential propagation along a myelinated axon, ~100 meters/second
• 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