Nervous System

• What are the functions of the Nervous System?

  • Receives sensory input
  • Integrates information
  • Controls muscles and glands
  • Maintains homeostasis
  • Establishes and maintains mental activity
• Main Divisions of Nervous System


  • Central nervous system (CNS)
  • Peripheral nervous system (PNS)
• Divisions of Peripheral Nervous System


  • Sensory division
  • Motor division
• Divisions of Motor Division


  • Somatic nervous system
  • Autonomic nervous system
• Neurons


  • Receive stimuli, conduct action potentials, and transmit signals to other neurons or effector organs

  
• Glial cells


  • Supportive cells of the CNS and PNS, do not conduct action potentials, carry out functions that enhance neuron function and maintain normal conditions within nervous tissue

  
• Cell body


  Contains a single nucleus
• Dendrite


  Cytoplasmic extension from the cell body that usually receives information from other neurons and transmits the information to the cell body

  
• Axon


  Single long cell process that leaves the cell body at the axon hillock and conducts sensory signals to the CNS and motor signals away from the CNS

  
• Structural Types of Neurons


  • Multipolar neurons
  • Bipolar neurons
  • Pseudo-unipolar neurons

  
• Types of Glial Cells


  • Astrocytes
  • Ependymal cells
  • Microglial cells
  • Oligodendrocytes
  • Schwann cells
• Myelin sheath


  Specialized layers that wrap around the axons of some neurons, formed by oligodendrocytes in the CNS and Schwann cells in the PNS

  
• Nodes of Ranvier


  Gaps in the myelin sheath where ion movement can occur

  
• Myelination


  Increases the speed and efficiency of action potential generation along the axon

  
• Multiple sclerosis


  Disease of the myelin sheath that causes loss of muscle function
• Unmyelinated axons


  Lack myelin sheaths, rest in indentations of oligodendrocytes in the CNS and Schwann cells in the PNS

  
• Gray matter


  Consists of groups of neuron cell bodies and their dendrites, with very little myelin
• White matter


  Consists of bundles of parallel axons with their myelin sheaths, which are whitish in color

  
• Resting membrane potential


  Exists due to the concentration of K+ being higher on the inside of the cell membrane and the concentration of Na+ being higher on the outside, the presence of negatively charged molecules inside the cell, and the presence of leak protein channels that are more permeable to K+ than Na+

  
• Sodium-potassium pump


  Compensates for the constant leakage of ions through leak channels by actively transporting K+ into the cell and Na+ out of the cell

  
• Leak channels


  Always open, allowing ions to "leak" across the membrane down their concentration gradient

  
• Gated channels


  Closed until opened by specific signals, such as neurotransmitters or changes in membrane potential
• Action potential


  Allows conductivity along nerve or muscle membrane, caused by the opening of voltage-gated Na+ and K+ channels

  
• Depolarization


  Caused by the movement of Na+ into the cell, which makes the inside of the cell membrane positive

  
• Threshold depolarization


  Causes voltage-gated Na+ channels to open, leading to the generation of an action potential

  
• Action potential


  Allows conductivity along nerve or muscle membrane, similar to electricity going along an electrical wire
• Channels responsible for the action potential


  • Voltage-gated Na+ and K+ channels, which are closed during rest (resting membrane potential)

  
• Action potential generation


  1. Stimulus applied to nerve cell

  2. Na+ channels open briefly

  3. Na+ diffuses quickly into cell

  4. Inside of cell membrane becomes positive (depolarization)

  
• Depolarization is not strong enough

  Na+ channels close again, local potential disappears without being conducted
• Depolarization is large enough


  Reaches threshold, causes voltage-gated Na+ channels to open, generally at the axon hillock

  
• Action potential propagation


  1. Opening of Na+ channels causes massive increase in membrane permeability to Na+

  2. Voltage-gated K+ channels also begin to open

  3. More Na+ enters cell, depolarization continues faster

  4. Charge reversal, Na+ channels close, Na+ stops entering

  5. More K+ channels open, K+ leaves cell, repolarization

  6. Briefly more negative than resting potential (hyperpolarization)

  
• All-or-none


  If threshold is reached, an action potential occurs; if not, no action potential occurs
• Sodium-potassium pump


  Assists in restoring the resting membrane potential

  
• Action potential conduction


  • Slower in unmyelinated axons, more rapid in myelinated axons
  • In unmyelinated axons, occur along entire membrane
  • In myelinated axons, occur in jumping pattern at nodes of Ranvier (saltatory conduction)

  
• Saltatory conduction


  Action potential conduction in myelinated axons, occurring in a jumping pattern at the nodes of Ranvier

  
• Axon conduction speed


  • Varies based on axon fiber diameter
  • Medium-diameter, lightly myelinated axons conduct at 3-15 m/s
  • Large-diameter, heavily myelinated axons conduct at 15-120 m/s

  
• Neuroneuronal synapse


  Junction where the axon of one neuron interacts with another neuron
• Synapse structure


  • Presynaptic terminal, synaptic cleft, postsynaptic membrane
• Neurotransmitters


  Chemical substances stored in synaptic vesicles in the presynaptic terminal
• Neurotransmitter release


  1. Action potential reaches presynaptic terminal

  2. Ca2+ channels open, Ca2+ moves into cell

  3. Ca2+ influx causes neurotransmitter release by exocytosis

  4. Neurotransmitters diffuse across synaptic cleft and bind to receptors on postsynaptic membrane