Nervous System

• Sensitivity


  The ability to respond to stimuli
• Sensitivity


  • a fly escapes a swat
  • antennae of crayfish detect food and organisms move towards it
• Nervous system


  • Receives information from stimuli
  • Integrates the information
  • Generates a signals
  • Transmits information
• Transduction


  The process of converting stimuli into electrical impulses
• Systems of coordination in animals


  • Nervous system
  • Endocrine system
• Nervous systems of Cnidarians

  • Simple neural networks provide direct lines of communication from sensory cells to effectors
  • Little or no integration or processing of signals
  • Nerve net is most developed around the tentacles and the oral opening
• More complex and active animals


  • Need to process and integrate larger amounts of information
  • Higher numbers of neurons organized into clusters called ganglia
  • Ganglia may become enlarged or fused together at the anterior end, forming a larger, centralized integrative center, or brain
• 3 functional categories of neurons

  • Sensory neurons
  • Interneurons/Association neurons
  • Motor neurons
• Parts of the nervous system

  • Central Nervous System (CNS)
  • Peripheral Nervous System (PNS)

  
• Cell body of a neuron

  • Contains a nucleus, most of the cell's other organelles, rough ER and Golgi apparatus, a variable number of nerve fibres extending from it called dendrites

  
• Dendrons


  Nerve fibres which conduct electric impulses towards the cell body, small, relatively wide, and break up into fine terminal branches called dendrites
• Axons


  Nerve fibres which conduct electric impulses away from the cell body, may be several metres long, end in synaptic knobs/axon terminals containing neurotransmitter
• 3 types of neurons

  • Sensory neuron
  • Interneuron/Relay neuron
  • Motor neuron
• Motor neurons


  • Somatic motor neurons
  • Autonomic motor neurons
  • Sympathetic system
  • Parasympathetic system

  
• Nerves


  • Bundles of axons from many different neurons, wrapped by glia to electrically isolate and increase conduction speed

  
• Glia


  • Astrocytes contribute to the blood-brain barrier, oligodendrocytes and Schwann cells physically support and orient neurons, supply nutrients, insulate axons, and provide immune functions

  
• Myelin


  Lipid covering produced by oligodendrocytes and Schwann cells, interrupts at nodes of Ranvier to allow saltatory conduction
• Demyelination diseases like multiple sclerosis and Tay-Sachs impair action potential conduction
• Generation and propagation of nerve impulses

  Caused by changes in electrical properties of the plasma membrane, due to fixed anions, Na+/K+ pump, and voltage-gated ion channels
• Resting potential


  Electrical potential difference across the plasma membrane, measured as -65mV inside relative to outside
• Excitable cells


  Sensory cells, neurons, and muscle cells with changing resting potentials
• Non-excitable cells


  All other cells with constant membrane potentials
• Gated ion channels


  Proteins with gates that open/close to control ion permeability
• Action potential initiation


  Caused by opening of gated sodium channels in receptor cells
• Action potential propagation


  Wave of depolarization passing along the axon

  
• Restoring resting potential after action potential

  Involves closing of sodium channels and activation of the Na+/K+ pump
• Membrane permeability


  20x more permeable to potassium
• Sodium gated channels


  • Chemical-gated sodium channels in tongue taste receptor cells open when a certain chemical in food binds to them
  • Mechanically-gated ion channels in the hair cells of the inner ear open when they are distorted by sound vibrations
• Action potential
• How nerve impulses travel

  Action potential passes along an axon as a wave of depolarization

  
• Characteristics of action potentials


  • All or none law
  • Action potentials are always separate events
• Strength of stimulus


  The STRONGER the stimulus, the greater the frequency of action potentials set up
• Refractory period


  • Absolute refractory period
  • Relative refractory period
• Nerve impulses pass in one direction, from an active region to a resting region
• Diameter, myelin, conduction velocity data

  
• Saltatory conduction


  • Action potential jumps from node to node
  • Faster than in non-myelinated neurons
• Axon diameter and conduction velocity in non-myelinated axons

  Depends on the resistance of the axoplasm

  
• Temperature and conduction velocity

  Higher temperature leads to higher conduction velocity
• Synapse


  Connection between neurons or between neuron and effector cell

  
• Types of synapses


  • Electrical synapses
  • Chemical synapses
  • Neuromuscular junctions