Fundamentals

• Nervous system


  Master controlling and communicating system of the body

  
• Every thought, action and emotion reflects the activity of the nervous system

  
• Electrical signals


  Rapid and specific, causing usually immediate responses
• Chemical signals

  Immediate responses
• The nervous system receives millions of bits of information from sensory nerves and organs, and integrates these to determine responses

  
• The olfactory epithelium is highly related to the limbic system of the brain, which is why smell is a sensation that takes a lot of emotions
• Functions of the nervous system

  • Sensory input
  • Integration
  • Motor output

  
• Central Nervous System (CNS)

  Integration and command center
• Peripheral Nervous System (PNS)

  Interpretation of sensory input and dictation of motor responses

  
• Cranial nerves arise from the brain, while spinal nerves arise from the spinal cord

  
• Somatic control

  Voluntary control of skeletal muscles
• Autonomic control

  Control of visceral organs
• Principal cell types of the nervous system

  • Neurons
  • Supporting cells

  
• Supporting cells (Neuroglia or Glial cells)

  • Provide supportive scaffolding for neurons
  • Segregate and insulate neurons
  • Guide young neurons to proper connections
  • Promote health and growth
  • Outnumber neurons

  
• Astrocytes


  Most abundant, versatile, and highly branched glial cells in the CNS that support and brace neurons, anchor neurons to nutrient supplies, allow exchange between capillaries and neurons, guide migration of young neurons and synapse formation, control the chemical environment, and release neurotransmitters

  
• Microglia


  Small, ovoid cells with spiny processes in the CNS that act as phagocytes to monitor the health of neurons
• Ependymal cells


  Cells that line the central cavities of the brain and spinal column, many of which are ciliated to help circulate cerebrospinal fluid

  
• Oligodendrocytes


  Branched cells in the CNS that wrap nerve fibers to produce insulating myelin sheaths

  
• Satellite cells


  Cells in the PNS that surround neuron cell bodies and have similar functions to astrocytes in the CNS

  
• Schwann cells


  Cells in the PNS that surround and form myelin sheaths around large nerve fibers, with the same functions as oligodendrocytes in the CNS

  
• Neurons


  • Highly specialized cells that conduct messages in the form of nerve impulses
  • Have extreme longevity
  • Are amitotic
  • Have an exceptionally high metabolic rate
  • Are composed of a body, axon, and dendrites
  • Function in electrical signaling and cell-to-cell signaling during development

  
• Neuron cell body (perikaryon or soma)

  Contains the nucleus and nucleolus, is the major biosynthetic center, and is the focal point for the outgrowth of neuronal processes

  
• Dendrites


  Short branching extensions that are the receptive, or input, regions of the neuron, conveying electrical signals as graded potentials

  
• Axons


  Generate and transmit action potentials (nerve impulses), and secrete neurotransmitters from the axonal terminals upon arrival of the impulse

  
• Anterograde movement


  Movement of mitochondria, cytoskeletal elements, components used to renew axon plasma membrane, and enzymes needed for synthesis of certain neurotransmitters toward the axon terminals
• Retrograde movement


  Movement of organelles returned for recycling or degradation, and vesicles containing signal molecules, toward the cell body
• Myelin sheath


  Whitish, fatty (protein-lipid), segmented sheath around most long axons that functions in protection of the axon, electrical insulation of fibers, and increasing the speed of nerve impulse transmission

  
• Neurilemma


  The remaining nucleus and cytoplasm of a Schwann cell that forms the myelin sheath in the PNS

  
• Myelin sheath


  Whitish, fatty (protein-lipid), segmented sheath around most long axons
• Dendrites are always unmyelinated
• Functions of myelin sheath

  • Protection of the axon
  • Electrically insulating fibers from one another
  • Increasing the speed of nerve impulse transmission

  
• Schwann cells


  Cells that form the myelin sheath in the peripheral nervous system (PNS)
• Formation of myelin sheath by Schwann cells

  1. Schwann cell envelopes an axon in a trough

  2. Schwann cell encloses the axon with its plasma membrane

  3. Concentric layers of membrane make up the myelin sheath
• Neurilemma


  Remaining nucleus and cytoplasm of a Schwann cell

  
• Nodes of Ranvier or myelin sheath gaps

  • Gaps in the myelin sheath between adjacent Schwann cells
  • Sites where collaterals can emerge
• Unmyelinated axons


  Axons where a Schwann cell surrounds nerve fibers (thin) but coiling does not take place

  
• Both myelinated and unmyelinated fibers are present in the CNS
• Myelin sheaths are formed by processes of the oligodendrocytes that can coil around axons

  
• Nodes of Ranvier are widely spaced in the CNS
• There is no neurilemma in the CNS