Bio 224 them 6

• •Is very large
• –up to 1 mm in diameter; typically around 0.5 mm
• –controls part of the water jet propulsion system;
• •Is unmyelinated which decreases the conduction velocity potential substantially

• -Voltmeter records the voltage difference between the microelectrodes inside and outside the neuron
• -Measurement made as a function of time

Decrease in potential. Membrane becomes less negative

increase in potential. membrane becomes more negative

increase in potential, membrane more negative

Change in membrane potential relative to resting potential

• charge outside the cell is positive
• charge inside the cell is negative

A triggering event opens ion channels, usually leading to net Na+ entry that depolarizes the membrane locally

Local current flows between the active and adjacent inactive areas, resulting in  depolarization of the previously inactive areas

• -Leakage of charge-carrying ions (usually potassium) across the plasma membrane
• -potential dies out altogether within a few millimeters of its site of initiation

voltage gated: open and close in response to voltage changes

ligand gated: open and close in response to ligands or chemicals

• 1.Na+/K+-ATPase pump
• 2.Ion channels
• 3.Negatively charged molecules such as proteins more abundant inside the cell.

transports 3 Na+ out for every 2 K+ moved in

• –Tend to be open more frequently at the resting membrane potential;
• –Most neurons have fifty times more potassium channels than sodium channels
• –Thus at rest the membrane is more permeable to potassium than sodium
• -activation gate triggered at threshold with delayed opening. (opens 1 msec after Na+)

• Activation gate
• deactivation gate
• both triggered at threshold but deactivation gate takes longer leaving a small window when Na+ can cross membrane

the portion of the membrane that has just undergone an action potential cannot be restimulated

• -the membrane can be restimulated only by a stronger stimulus than is usually necessary
• -K+ gates not yet closed
• -lingering inactivation of the voltage-gated Na+ channels.

• –A further rise in the membrane potential(more positive)
• –More channels to open
• –A greater electric current, and so on
• -The polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate
• -Na+ then actively transported out of the plasma membrane

the refractory period

The only areas of the axon that have enough Na+ channels to elicit an action potential

action potential seems to “jump” from node to node

potentials that cause action potentials when they are big enough (or enough coincide) to cross the threshold

• Axon diameter- broadaxons provide less resistance and action potential moves faster-
• Myelination -myelinated faster then unmyelinated