5.8 bio notes

Synaptic Integration to Action Potential

• The action potential travels in one direction along the neuron membrane until it reaches the end of the axon.
• The vast majority of the size of a neuron is the length of its axon.
• For example, the axon of the neurons that control the muscles of your toes travels the entire distance from your spine to the toe muscle.

Microstructure of the Nervous System

• Axon structure:
  • There are two types of axons:
    • Myelinated axons are insulated and protected by a cell-based covering.
      • The covering is called a myelin sheath.
      • The sheath has periodic gaps between the cells of the sheath called nodes.
    • Unmyelinated axons have no covering around the axon.

Action Potentials in Nerves

• Nerve Conduction:
  • This process by which an action potential travels along a nerve cell membrane is called nerve conduction.
  • Nerve conduction happens because of the diffusion of sodium ions.
  • Lots of sodium enters the cell during the action potential’s depolarization phase.
• Diffusion of Sodium Ions:
  • Some of the sodium ions that enter in the depolarization phase will move by diffusion.
  • Diffusion of these sodium atoms slowly raises the membrane potential until it is above the threshold potential.
  • Once above the threshold, the next segment will fire an action potential.
  • This repeats until the action potential reaches the end of the axon.
• Direction of Action Potentials:
  • Action potentials only conduct in one direction because the sections of the axon close behind it are in their refractory period.
  • Even though sodium will diffuse into the refractory membrane, it cannot fire an action potential because the sodium channels are inactivated.
• Continuous Conduction:
  • This kind of nerve conduction happens in axons that are unmyelinated.
  • In unmyelinated axons, the action potential must conduct in a straight line down the entire length of the axon.
  • Because of this, it is called continuous conduction, because it continues in a straight line without breaking or jumping.
• Saltatory Conduction:
  • Myelinated axons use a different kind of conduction called saltatory conduction.
  • In saltatory conduction, the action potential appears to jump from node to node.
  • In myelinated neurons, the Na and K channels of the action potential are all crammed into the nodes instead of spread evenly along the axon like they are in unmyelinated axons.
  • This makes the concentration gradient of sodium between the nodes much larger.
  • Remember Fick’s law and diffusion: the larger the concentration gradient is, the faster diffusion happens.
  • In myelinated neurons, diffusion is about 100 times faster.

Nerve Signaling

• Process:
  • When action potentials reach the axon terminal, they will initiate nerve signaling.
  • In each axon terminal are small bubbles of membrane that contain neurotransmitter. These bubbles are called synaptic vesicles.
  • Synaptic vesicles sit in the axon terminal until an action potential arrives.
• Step-by-Step Process:
  1. An action potential is conducted to the axon terminal.
  2. The depolarization phase triggers the opening of a voltage-gated calcium channel.
  3. The open calcium channel allows calcium to diffuse into the cell.
  4. The calcium signal inside of the cell causes synaptic vesicles to combine (fuse) with the nerve cell membranes.
  5. Fusion of the vesicles releases the neurotransmitter into the synaptic cleft.
  6. Released neurotransmitter will bind to a receptor on the post-synaptic cell.
  7. Neurotransmitter binding to the post-synaptic cell will cause a physiological change.
     • Exactly what change depends on what the post-synaptic cell is.
     • Muscle cells will contract to cause movement.
     • Glands will release a secretion (like sweat) or a hormone.
     • If it connects to another neuron, that neuron will send an action potential signal.