Neurons and Action Potentials

• Neurons facilitate communication in the brain and nervous system via action potentials.
• Neurons enable thinking, movement, and sensory perception.
• Communication involves the exchange of ions like Sodium and Potassium across the neuron's membrane.

Sodium Ions (Na+)

• Sodium ions (Na+) have a positive charge.
• They are more concentrated outside the neuron.
• Only a few sodium ions are found inside the neuron.
• This creates a concentration gradient.

Potassium Ions (K+)

• Potassium ions (K+) also carry a positive charge.
• Potassium is more concentrated inside the neuron.
• Potassium ions leak out of the neuron.
• They move down the concentration gradient.
• This leakage is less than sodium leaks.

Membrane Potential

• The inside of the neuron is negatively charged compared to the outside due to ion concentration differences.
• A greater amount of potassium leaks out than sodium leaks in.
• This charge difference is the electrical/membrane potential, which is measured in millivolts (mV).
• Membrane potential indicates the electrical potential across a neuron's membrane over time.
• Resting neuron membrane potential is typically -70 mV, and is when a neuron isn't firing.
• Depolarization happens when the neuron becomes less negative.
• Repolarization is when the neuron returns to its resting membrane potential.

Sodium-Potassium Pump

• The sodium-potassium pump actively transports three sodium ions out for every two potassium ions it brings in.
• The pump maintains ion gradients necessary for membrane potential.
• The pump counteracts ion leakage to maintain the resting potential.
• The pump helps maintain the charge difference (electrical potential) across the neuron's membrane.
• This pump counteracts sodium leaking in and potassium leaking out, restoring them to their original sides of the membrane.

Action Potential Generation

• Neurons use sodium and potassium gradients and channels to generate a digital pulse called the action potential.
• Action potentials travel along the neuron's axon.
• They transmit signals to other neurons.
• Sodium and potassium channels in the axon membrane facilitate ion flow.
• The axon membrane contains many sodium and potassium channels.
• Sodium channels act like hallways with doors that allow only sodium ions into the neuron.
• Potassium channels act like hallways with doors that allow only potassium ions out of the neuron.
• At rest, the sodium channel's outside door is closed, but the inside door is open; the potassium channel door is closed.

Sodium Channels

• When a neuron is at rest, the sodium channel outside door is closed but its inside door is open
• Depolarization opens the fast outside door of sodium channels allowing sodium ions to rush into the cell.
• The influx of sodium ions causes further depolarization.
• This leads to the opening of more sodium channels.
• A positive feedback loop is created until the threshold for starting an action potential is reached.
• Depolarization also slowly closes the sodium channel inside door.

Potassium Channels

• After sodium influx, potassium channels open, allowing potassium ions to flow out of the cell.
• The efflux of potassium ions makes the membrane potential more negative, causing repolarization.
• Potassium channel doors close slowly causing hyperpolarization for a moment, which is below the resting membrane levels.
• Afterwards, the membrane potential returns to resting levels.
• As the membrane potential repolarizes, potassium channels slowly close, sodium channel outside doors rapidly close, and sodium channel inside doors slowly open.
• Potassium channels stay open longer, causing the membrane potential to briefly hyperpolarize (go below resting levels).
• The membrane potential eventually returns to resting levels.
• The sodium-potassium pump then restores the original ion balance.
• The action potential process occurs rapidly, taking only 2-3 milliseconds at one point in the axon.

Action Potential Propagation

• The influx of sodium ions at one location on the axon depolarizes the adjacent section.
• If the depolarization is sufficient, it triggers an action potential in the adjacent section.
• The action potential is regenerated along the axon, propagating the signal.
• To send a message, the action potential must travel along the axon.
• As sodium ions rush into the axon at one location, they depolarize the next section of the axon.
• If the next piece of axon depolarizes enough, an action potential occurs there as well.
• The action potential is regenerated repeatedly along the axon, appearing to travel from the cell body to the axon terminal.

Neurotransmitter Release

• The action potential triggers the release of neurotransmitters when it reaches the nerve terminal.
• When the nerve terminal depolarizes, it releases neurotransmitters (chemical signals) that are received by the dendrites of the next neuron.
• Neurotransmitters are detected by the dendrites of the next neuron.
• The signal transmits to the next cell, and the entire process then repeats.
• This process repeats in the next neuron, propagating the signal.

Explore neurons, the fundamental units of the nervous system, and their communication via action potentials. Learn about the roles of sodium and potassium ions in creating membrane potential. Understand how ion concentration differences lead to a negative charge inside the neuron relative to its outside environment.