The Nervous System, Part 2 - Action! Potential!: Crash Course Anatomy & Physiology #9

Key Concepts

• Action Potential: The electrical impulse neurons use to communicate.
• Resting Membrane Potential: The voltage difference across a neuron's membrane when it's at rest (-70 mV).
• Depolarization: A change in a cell's membrane potential, making it more positive.
• Repolarization: The return of a cell's membrane potential to its resting state.
• Hyperpolarization: A state where the membrane potential is more negative than the resting potential.
• Voltage-Gated Channels: Ion channels that open or close in response to changes in membrane potential.
• Sodium-Potassium Pump: A protein that actively transports sodium and potassium ions across the cell membrane, maintaining the electrochemical gradient.
• Electrochemical Gradient: The combined difference in concentration and electrical charge of an ion across a membrane.
• Refractory Period: A period after an action potential when a neuron cannot fire another impulse.
• Myelin Sheath: An insulating layer around axons that speeds up signal transmission.
• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.
• Saltatory Conduction: The "leaping" of action potentials from one Node of Ranvier to the next in myelinated axons.
• Graded Potential: A small change in membrane potential that is localized.

Electricity and the Body

• The body is electrically neutral overall, but certain areas have different concentrations of positive and negative charges.
• Voltage: Measure of potential energy generated by separated charges, measured in millivolts (mV) in the body.
• Membrane Potential: The difference in charge across a cell membrane.
• Current: The flow of electricity from one point to another.
• Resistance: Anything that impedes the flow of current; high resistance = insulator, low resistance = conductor.
• Cell membranes act as barriers, separating charges to create potential energy.

Neuron at Rest

• A resting neuron has a resting membrane potential of approximately -70 mV.
• The inside of the neuron is more negative than the outside.
• Sodium ions (Na+) are more concentrated outside the neuron.
• Potassium ions (K+) are more concentrated inside the neuron, along with negatively charged proteins.
• The neuron is said to be polarized when it has a negative membrane potential.

Sodium-Potassium Pump

• The sodium-potassium pump maintains the electrochemical gradient by pumping 3 Na+ ions out of the cell for every 2 K+ ions pumped in.
• This creates a concentration gradient and a charge difference, making the outside of the neuron more positive.
• Nature hates gradients, so ions want to move across the membrane to even out the concentrations and charges.

Ion Channels

• Ion channels are proteins in the membrane that allow ions to pass through.
• Voltage-gated channels open and close in response to changes in membrane potential (e.g., sodium channels open around -55 mV).
• Ligand-gated channels open when a specific neurotransmitter or hormone binds to them.
• Mechanically-gated channels open in response to physical stretching of the membrane.
• When channels open, ions diffuse across the membrane down their electrochemical gradient.

Graded Potentials vs. Action Potentials

• Graded potentials are small, localized changes in membrane potential caused by the opening of a few ion channels.
• Action potentials are large, rapid changes in membrane potential that can travel long distances along the axon.

Action Potential: Step-by-Step

1. Resting State: All ion channels are closed, and the membrane potential is at -70 mV.
2. Stimulus: An environmental stimulus triggers some sodium channels to open, increasing the charge inside the membrane.
3. Threshold: If the stimulus is strong enough to depolarize the membrane to the threshold of -55 mV, an action potential is triggered.
4. Depolarization: Voltage-gated sodium channels open, and Na+ ions rush into the cell, causing the membrane potential to become positive (up to +40 mV).
5. Repolarization: Voltage-gated potassium channels open, and K+ ions flow out of the cell, attempting to rebalance the charges.
6. Hyperpolarization: The membrane potential briefly becomes more negative than the resting potential (around -75 mV).
7. Restoration: All gates close, and the sodium-potassium pump restores the resting membrane potential.

Refractory Period

• During the refractory period, a part of the axon cannot respond to any other stimulus, preventing signals from traveling in both directions.

Action Potential Communication

• Neurons communicate using a single, monotone "buzz" (action potential).
• The strength of the action potential is always the same.
• The frequency of the action potentials varies depending on the strength of the stimulus.
• Weak stimulus = less frequent action potentials.
• Strong stimulus = more frequent action potentials.

Conduction Velocity

• Conduction velocity is the speed at which an action potential travels down an axon.
• Action potentials are fastest in pathways governing reflexes and slower in glands, guts, and blood vessels.
• Myelin sheath increases conduction velocity.

Myelination and Saltatory Conduction

• Myelin is an insulating layer around axons.
• Nodes of Ranvier are gaps in the myelin sheath.
• In myelinated axons, action potentials "leap" from one Node of Ranvier to the next, a process called saltatory conduction.

Conclusion

The action potential is a fundamental process that allows neurons to communicate. It involves a series of precisely orchestrated changes in membrane potential, driven by the movement of ions across the cell membrane. The frequency and speed of action potentials can vary, allowing the nervous system to transmit a wide range of information. Myelination significantly increases the speed of action potential propagation through saltatory conduction.