Nervous 2

• Neurons can initiate and conduct signals called nerve impulses. They are described as a wave of electrical fluctuation that travels along the plasma membrane (action potential)
• The input zone (dendrites and soma) receives the input from other neurons or from sensory stimuli
• The summation zone (axon hillock) is where the nerve impulses combine and trigger and impulse which will be conducted along the conduction zone (axon)
• The output zone (synaptic knobs) is where the signal leaves the neuron
• Impulse of electrical signalsthat move along the surface of the neuron have 2 major functions

  • method for rapid information transmission over relatively long distance in neurons
  • control neural activity by causing release of chemical transmitter at synaptic junction
• AP have a all or none principle. Without hitting the threshold, there is not action potential. When the threshold is reached, you cannot stop the AP
• Generating an action potential requires

  • existence of a resting membrane potential
  • presence of specific types of ion channels
• The potential difference means the voltage VS the membrane potential means the voltage of the membrane
• Resting membrane potential is -70mv. This is obtained via active transport (Na/K pump) and selective ion diffusion (leaky Na+ and K+ channels)
• Specific ion channels open and close on command. They are voltage gated ion channels responsible for AP
• Sequence of AP

  1. Resting membrane potential (-70mv)
  2. Threshold (-55mv)
  3. Depolarization (-55mv to +30mv)
  4. Repolarization (+30mv to -70mv)
  5. Optional: hyperpolarization
• In resting membrane potential, the voltage gated channels are closed. The Na/K pump is responsible for the difference in charge (more sodium outside and more potassium inside)
• Depolarization

  • when the threshold voltage is reached, the sodium channels open rapidly resulting in an influx of Na+
  • the Na/K pump stops working
  • potassium channels remain closed and potassium cannot cross the membrane
  • the polarity of the membrane is inversed
• Repolarization

  • sodium gates close
  • potassium channels open slowly
  • loss of K+ repolarizes the membrane and the polarity is reversed back to a negative inside
  • can become a little extra negative (hyperpolarization)
• Absolute refractory period (during spike): cannot generate an AP
• Relative refractory period (during hyperpolarization) needs larger stimulus for immediate 2nd AP
• After repolarization, there is the reestablishment of the resting membrane potential
• Nerve impulse transmission

  • AP does not transmit passively down membrane
  • AP triggers generation of new impulse at adjacent membrane (other neuron)
  • Achieved via diffusion of Na+ along axon
  • AP only moves in one direction due to refractory period
• Continuous conduction: AP travels one after another (domino effect). It is slow and solely proportional to the axon diameter
• Two ways to increase velocity of conduction

  • axon has a larger diameter
  • axon is myelinated
• Myelin sheats produced by oligodendrocytes and schwann cells insulate axon and improve conduction (saltatory) The gaps in myelin are called Nodes of Ranvier and are the sites for AP
• Continuous conduction is slow VS saltatory conduction is very fast as the AP jumps from one node of Ranvier to the next node
• Multiple sclerosis is an autoimmune neurological disorder involving the progressive loss of myelin sheaths. It slows and short circuits conduction of nerve impulses and an cause heaviness, weakness of muscles, double vision, etc