NERVOUS

• Nervous tissue


  Consists of cells, some of which are very large, and with their elongated processes, which are usually grouped as relatively isolated masses or bundles
• Nervous system


  • Widely distributed in the body
  • Anatomic division: CNS and PNS
  • Functional division: Somatic NS and Autonomic NS
• Neuron


  The functional units of the nervous system
• Parts of a neuron

  • Neurolemma (cell membrane)
  • Cell body
  • Axon
  • Dendrites
• Cell body


  • Soma; Perikaryon
  • Serves as the synthetic and trophic center for the entire neuron
  • Most cell bodies are in contact with a great number of nerve endings
  • "Gray matter" - peripheral/outer in brain, deep/inner in spinal cord
• Chromatophilic substances (Nissl bodies)


  Large masses or regions with concentrated RER and polyribosomes
• Axon


  • Transmits impulses away from the cell body
  • Generally branch less profusely
• Components of an axon

  • Axon hillock
  • Axoplasm
  • Collaterals
  • Terminal button (Presynaptic axon terminal)
  • Neurolemmal sheath
  • Myelin
• Axon hillock


  Pyramid-shaped region of cell body where the axon originates
• Axoplasm


  Cytoplasm; the contents
• Collaterals


  Major branches of axons found in interneurons and some motor neurons
• Terminal button (Presynaptic axon terminal)

  Dilated ends of small axonal branch which contains abundant mitochondria and synaptic vesicles where neurotransmitters are released
• Neurolemmal sheath


  Made of Schwann cells (PNS) or oligodendrocytes (CNS)
• Myelin


  Compacted layers of cell membrane internal to neurolemmal sheath
• Structural classes of neurons

  • Multipolar neurons
  • Bipolar neurons
  • Unipolar or Pseudounipolar neurons
  • Anaxonic neurons
• Multipolar neurons


  Have one axon and two or more dendrites, most common
• Bipolar neurons


  Have one dendrite, one axon, comprise the sensory neurons of the retina, the olfactory epithelium, and inner ear
• Unipolar or Pseudounipolar neurons


  Have a single process that bifurcates close to perikaryon, one branch extending to a peripheral ending, the other toward the CNS
• Anaxonic neurons


  Have many dendrites, no true axon, do not produce action potentials, but regulate electrical changes of adjacent CNS neurons
• Anterograde transport


  Forward movement from the cell body through axonal microtubules (via Kinesin) to the synaptic terminals
• Retrograde transport


  Backward movement from the periphery through axonal microtubules (via Dynein) to the cell body
• Dendrites


  • Principal signal reception and processing sites
  • Short, small processes emerging and branching off the soma
  • Extensive arborization or branching
  • Presence of dendritic spines where most synapses occur, important for neural plasticity
• Glial cell types


  • Oligodendrocyte
  • Astrocyte
  • Ependymal cell
  • Microglia
  • Schwann cell
  • Satellite cells of ganglia
• Oligodendrocyte


  • Origin: Neural tube
  • Location: CNS
  • Main functions: Myelin production, electrical insulation
• Astrocyte


  • Origin: Neural tube
  • Location: CNS
  • Main functions: Structural & metabolic support of neurons, especially at synapses, repair processes
• Ependymal cell


  • Origin: Neural tube
  • Location: Line ventricles and central canal of CNS
  • Main function: Aid production & movement of CSF
• Microglia


  • Origin: Bone marrow (monocytes)
  • Location: CNS
  • Main function: Defense & immune-related activities
• Schwann cell


  • Origin: Neural crest
  • Location: Peripheral nerves
  • Main functions: Myelin production, electrical insulation
• Satellite cells of ganglia

  • Origin: Neural crest
  • Location: Peripheral ganglia
  • Main function: Structural & metabolic support for neuronal cell bodies
• Central nervous system (CNS)

  • Major structures: Cerebrum, cerebellum, spinal cord
  • Completely covered by connective tissue layers (meninges)
  • Tissue contains very little collagen, relatively soft and easily damaged
  • Organized areas of white matter and gray matter
• Gray matter


  Made up of cell bodies, thus darker in color
• White matter


  Made up of myelinated axons, thus lighter in color
• Neuron types in cerebral cortex

  • Pyramidal cells
  • Stellate/Granule cells
  • Cells of Martinotti
  • Fusiform cells
  • Horizontal cells of Cajal
• Pyramidal cells


  Pyramid-shaped cell bodies that vary in size and arise from the 5th or ganglionic layer of the neocortex forming most of the efferent pathway, including the largest Betz cells in the motor cortex
• Stellate/Granule cells


  Small neurons with short vertical axon and several short branching dendrites, giving the cell body the shape of a star
• Cerebellum


  • Coordinates muscular activity and maintains posture and equilibrium
  • Thick outer molecular layer, thin middle Purkinje cell layer, thick inner granular layer
• Spinal cord


  • White matter is peripheral, gray matter forms a deeper, H shaped/butterfly-like mass
  • Anterior/Ventral Horn contains cell bodies of large alpha lower motor neurons
  • Posterior/Dorsal Horn contains cell bodies of small second-order sensory neurons
• Blood-brain barrier


  Functional barrier that allows tight control over passage of substances from blood into CNS tissue, protects neurons and glia, helps maintain ion balance
• Components of the blood-brain barrier

  • Capillary endothelium
  • Basement membrane
  • Limiting layer of perivascular astrocytic feet
• Choroid plexus


  Highly vascular tissue, elaborately folded and projecting into the large ventricles of the brain, removes water from blood and releases it as CSF