Action Potential - Firing of a Neuron - Depolarization

Action potential refers to a brief electrical charge that travels down a neuron.
Other names include "neural impulse," "nerve impulse," or "spark."
Essential for neuron communication.

Parts of a Neuron:
- Soma (Cell Body): Contains the nucleus and organelles.
- Dendrites: Receive messages from other neurons.
- Axon: Transmits action potential away from the cell body.
- Axon Terminal: Contains neurotransmitters that communicate with other neurons.

Neuron is surrounded by an ionic environment (charged particles).
Key Ions:
- Sodium (Na+): High concentration outside the neuron.
- Potassium (K+): High concentration inside the neuron.
Memory Technique: Salty banana - Sodium on the outside, Potassium on the inside to remember the charges.

A stimulus (like reaching for a glass of water) causes the release of neurotransmitters (such as acetylcholine).
Neuromodulators bind to receptor sites on dendrites, causing sodium channels to open.
Sodium then flows into the neuron, making the charge more positive.

The internal charge becomes more positive as sodium floods in.
Threshold: If the charge reaches -55 millivolts, the action potential will fire (All-or-Nothing Principle).
If not reached, the neuron will not fire.

Once threshold is reached, voltage-gated sodium channels open along the axon.
Sodium rushes into the neuron, rapidly increasing internal charge to +30 millivolts.
Termed depolarization - inside of neuron becomes positively charged.

After reaching +30 mV, sodium gates close and potassium channels open.
Potassium exits the neuron, returning the charge to a more negative state.
This phase is called repolarization.
The voltage can undershoot to around -90 millivolts due to excess potassium leaving.

The overshooting of the voltage leads to a hyperpolarized state (refractory period).
Neuron cannot fire again until it returns to resting state.
This phase is compared to a toilet needing to refill before the next flush - no firing until recharged.

The final state after all phases returns to resting potential.
The ion distributions are re-established with potassium ions inside and sodium outside, preparing the neuron for another action potential.

Differences between Voltage-Gated and Ligand-Gated Ion Channels:

Activation Mechanism:
- Voltage-Gated Ion Channels: Open in response to changes in membrane potential. They are sensitive to the voltage across the membrane and typically open during depolarization.
- Ligand-Gated Ion Channels: Open when a specific chemical (ligand), such as a neurotransmitter, binds to the receptor site on the channel.
Location and Function:
- Voltage-Gated Ion Channels: Found along the axon of neurons and are essential for the propagation of action potentials.
- Ligand-Gated Ion Channels: Located primarily at the synapses on dendrites and cell bodies; they play a crucial role in synaptic transmission and neuronal signaling.
Ions Typically Involved:
- Voltage-Gated Ion Channels: Commonly allow sodium (Na+) or potassium (K+) ions to pass through to influence the action potential.
- Ligand-Gated Ion Channels: May allow various ions (like Na+, K+, Ca2+) depending on the specific ligand-receptor interaction and the type of ligand-gated channel.
Response Time:
- Voltage-Gated Ion Channels: Typically respond rapidly to changes in voltage and are involved in fast signaling.
- Ligand-Gated Ion Channels: Their response time can vary and may take longer due to the binding process of the ligand.