HEAL 1502 s2

• Central nervous system (CNS)

  Consists of the brain and spinal cord

  
• Peripheral nervous system (PNS)

  Consists of nerves, which lie outside the CNS
• Functions of the nervous system

  • Sensory
  • Integration
  • Motor

  
• Nervous tissue

  • Contains two types of cells: neurons and neuroglia
  • Neurons transmit nerve impulses
  • Neuroglia support and nourish neurons

  
• Types of neuroglia in the CNS

  • Microglia
  • Astrocytes
  • Oligodendrocytes
  • Ependymal

  
• Types of neuroglia in the PNS

  • Schwann cells
  • Satellite cells

  
• Microglia

  Phagocytic cells that remove bacteria and debris
• Astrocytes

  Provide metabolic and structural support directly to the neurons
• Oligodendrocytes

  Form myelin sheaths in CNS
• Ependymal

  Line ventricles (brain) and central canal (spine) and are involved in the production of cerebrospinal fluid in CNS
• Satellite cells

  Regulate nutrient and neurotransmitter levels around neurons in ganglia
• Schwann cells

  Form myelin sheaths in PNS

  
• Types of neurons

  • Sensory neurons
  • Interneurons
  • Motor neurons

  
• Sensory neuron

  Carries nerve signals from a sensory receptor to the CNS
• Interneuron

  Found only within the CNS, receive input from sensory neurons and other interneurons, sum up signals received from other neurons, then communicate with motor neurons

  
• Motor neuron

  Carries nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland), which carries out the response to the environmental change

  
• Structures of a neuron

  • Cell body
  • Dendrites
  • Axon

  
• Cell body

  Contains the nucleus, other organelles
• Dendrites

  Short extensions off the cell body that receive signals from sensory receptors or other neurons
• Axon

  The portion of a neuron that conducts nerve impulses, individual axons are termed nerve fibers, and collectively they form a nerve

  
• Longest axons found in Sciatic nerve: from base of spine to end of toes
• Myelin sheath

  Covers some axons, formed when Schwann cells (PNS) or oligodendrocytes (CNS) wrap around an axon many times, node of Ranvier is the space between myelin sheaths
• Long axons usually have a myelin sheath, short axons usually don't
• Gray matter of the CNS is gray because it contains no myelinated axons; white matter is white because it does
• Multiple sclerosis (MS)

  Myelin breaks down, neurons can't transmit information

  
• Myelin sheath is important in nerve regeneration within the PNS
• Nerve regeneration is happening in PNS not CNS

  
• Resting potential

  The potential energy of a neuron at rest, exists because the plasma membrane is polarized: there is a more positive charge outside the cell, more negative charge inside, measured in millivolts (mV)

  
• Sodium-potassium pump


  Protein carrier that pumps Na+ out and K+ into the neuron to maintain the resting potential

  
• Action potential

  Nerve signals that occur in axons, a stimulus is a change that activates the neuron, threshold is the minimum voltage that must be reached for an action potential to occur

  
• Action potentials are "all-or-nothing", increasing the strength of a stimulus does not change the size of an action potential
• Depolarization

  When an action potential begins, voltage dependent Na channels open and Na+ rushes into the cell, adding positively charged Na+ causes the inside of the axon to become positive compared to the outside

  
• Repolarization

  Immediately after depolarization, Na+ channels close and K+ channels open, K+ flows out of the cell, the inside of the cell becomes negative again, the sodium–potassium pump completes the action potential by returning K+ to the inside of the cell and Na+ to the outside, restoring the resting potential

  
• Saltatory conduction

  In myelinated fibers, action potentials only occur at nodes of Ranvier, allowing for much faster conduction (100 m/s) compared to unmyelinated axons (1 m/s)
• Refractory period

  The period of time immediately after an action potential during which the axon is unable to conduct another action potential, ensures the one-way direction of the signal from the cell body down the length of the axon

  
• Types of synapses

  • Electrical
  • Chemical
  • Mixed
• Electrical synapse

  Transmits signals through gap junctions, fastest type
• Chemical synapse

  Transmits signals via neurotransmitters released into the synaptic cleft
• Autapse

  Synapse connecting the cell to itself
• Synaptic cleft

  Small gap that separates the sending neuron from the receiving neuron