AP Unit 4 Lesson 3 - Nerve Impulse Transmission

How do neurons produce electrical signals?

• Neurons generate electrical signals through brief, controlled changes in the permeability of their cell membranes to particular ions (NA+ / K+)

• Changes in the concentration of NA+ and K+ ions inside and outside the nerve cell create electrical signals, called action potentials, which allow nerve impulses to travel along the cell.

Resting Potential of a Neuron

• potential difference (voltage) across axomembrane when not conducting an impulse = ~ -65 mV

Cause of Negative Polarity

• Negative polarity caused by:

  • 1) Large negative anions (organic proteins) in axoplasm

  • 2) Uneven distribution of ions:

  • Na+/K+ pump pushes 3 Na+ out for every 2 K+ brought in when returning axon to resting potential

Resting Potential of a Neuron pt. 2

• At rest:

  • More Na+ on the outside of neuron

  • More K+ on the inside of neuron

  • Overall - Negative change on the inside is due to large negative proteins

What is Action Potential - The Nerve Impulse

• 1) an electrochemical change that moves in one direction along the length of the nerve fiber

• 2) caused by nerve stimulation (sensory stimuli)

Action Potential Steps: 1) Depolarization

• 1a) Axomembrane becomes permeable to Na+ ions:

  • Na+ ions moves from outside to inside of axon (via facilitated transport)

Action Potential Steps: 1) Depolarization pt. 2

• 1b) More Na+ channels open and axomembrane becomes even more polarized

  • the membrane potential changes: -65 mV to +40 mV

Action Potential Steps: 2) Repolarization

• 2a) Axomembrane becomes permeable to K+ ions:

  • voltage gated Na+ channels close

  • voltage gated protein channels specific to K+ open

  • K+ ions move from inside axon to outside (facilitated transport)

Action Potential Steps: 2) Repolarization pt. 2

• 2b) Axomembrane becomes repolarized:

• the inside of axon returns to resting potential, membrane potential changes from +40 mV to -65 mV

Action Potential Steps: 3) Recovery (Refractory Period)

• 3) Refractory Period: small amount of time when a new impulse cannot be sent

  • between nerve impulse transmissions, the protein pump moves Na+ and K+ ions across the membrane to restore their original concentration levels

Action Potential Steps: 3) Recovery (Refractory Period) pt. 2

• both Na+ and K+ are moved against the [ ] gradient

Action Potential Graph

Nerve Impulse Transmission Speed

• Myelin Sheath: lipid covering that allows for rapid speed of nerve impulse transmission (via insulation)

Nerve Impulse Transmission Speed pt. 2

• Transmission in 2 ways:

  • 1) Myelinated Fibers (Saltatory Conduction): nerve impulse “jumps” from node of Ranvier to next node

  • Speed: ~ 200 m/s

  • 2) Non-myelinated fibers: impulse must depolarize & repolarize at each point along the fiber

  • Speed: ~ 0.5 m/s

Nervous System Diseases

• Multiple Sclerosis:

  • The immune system attacks the myelin sheath and causes communication problems between your brain and the rest of your body

  • Eventually, the disease can cause permanent damage or deterioration of the nerve fibers

Nervous System Diseases pt. 2

• 2) Alzheimer’s:

  • causes changes in the brain that leads to deposits of certain proteins

  • Alzheimer’s disease causes the brain to shrink and eventually causes brain cells to die