ELECTRIC SIGNALS

• • Electrical conditions of a resting neuron’s membrane (1)

  ▪ The plasma membrane at rest is inactive (polarized)

  ▪ Fewer positive ions are inside the neuron’s plasma membrane than outside

  ▪ K+ is the major positive ion inside the cell

  ▪ Na+ is the major positive ion outside the cell

  ▪ As long as the inside of the membrane is more negative (fewer positive ions) than the outside, the cell remains inactive
• • Action potential initiation and generation (2)

  ▪ A stimulus changes the permeability of the neuron’s membrane to sodium ions

  ▪ Sodium channels now open, and sodium (Na+) diffuses into the neuron

  ▪ The inward rush of sodium ions changes the polarity at that site and is called depolarization
• • Action potential initiation and generation (3)

  ▪ A graded potential (localized depolarization) exists where the inside of the membrane is more positive, and the outside is less positive

  ▪ If the stimulus is strong enough and sodium influx great enough, local depolarization activates the neuron to conduct an action potential (nerve impulse)
• • Propagation of the action potential (4)

  ▪ If enough sodium enters the cell, the action potential (nerve impulse) starts and is propagated over the entire axon

  ▪ All-or-none response means the nerve impulse either is propagated or is not

  ▪ Fibers with myelin sheaths conduct nerve impulses more quickly
• • Repolarization (V)

  ▪ Membrane permeability changes again—becoming impermeable to sodium ions and permeable to potassium ions

  ▪ Potassium ions rapidly diffuse out of the neuron, repolarizing the membrane

  ▪ Repolarization involves restoring the inside of the membrane to a negative charge and the outer surface to a positive charge
• Sodium-potassium pump - restores initial condition of sodium and potassium ions