neurons

Neurons are the fundamental units of the nervous system.
- Comprised of three main parts:
- Dendrites: Little branches that receive signals from other neurons.
- Soma (Cell Body): Contains the neuron's organelles like the nucleus.
- Axon: A long extension wrapped in fatty myelin that transmits signals.

Dendrites receive signals through neurotransmitters.
- When neurotransmitters bind to receptors on the dendrite:
- They act as chemical signals.
- Binding opens ion channels, allowing ions to flow in and out, converting chemical signals into electrical signals.
A neuron can possess numerous dendrites, receiving inputs from various sources.
- If the summed charge from these inputs changes the overall membrane potential sufficiently, it triggers an action potential.
- Action potential is an electrical signal that travels down the axon at speeds up to 100 meters per second.
- This, in turn, triggers the release of neurotransmitters at the axon terminal, facilitating communication with other neurons.

The cell maintains an electrical charge based on ion concentration differences:
- Typically, there is a higher concentration of:
- Sodium ( ext{Na}^+), Chloride ( ext{Cl}^-), Calcium ( ext{Ca}^{2+}) outside the cell.
- Potassium ( ext{K}^+), and other negatively charged anions ( ext{A}^-) inside the cell.
- This distribution leads to a resting membrane potential of approximately −65 millivolts (mV) relative to the external environment.

When a neurotransmitter binds to a receptor and opens ligand-gated ion channels:
- Ligand-gated channels respond to neurotransmitters.
- Example: Ligand-gated sodium channel allows sodium to enter the cell.
- The influx of positive sodium reduces the negative charge making the cell less polarized, a process known as depolarization.
Upon simultaneous neurotransmitter activity:
- Ions like sodium ( ext{Na}^+) and calcium ( ext{Ca}^{2+}) flow into the cell, while potassium ( ext{K}^+) may flow out.
If the net influx of positive charge exceeds a threshold (typical value around −55 mV), an excitatory postsynaptic potential (EPSP) is generated. In contrast, if chloride channels open, producing a net influx of negative charge, it results in an inhibitory postsynaptic potential (IPSP), making the cell more negative.

A single EPSP or IPSP produces a minor change in the resting membrane potential.
Sufficient EPSPs across multiple dendrites can lead to reaching the threshold potential.
Voltage-gated sodium channels open in response to the threshold being met:
- Sodium rushes into the cell creating a rapid change in voltage,
- Resulting in action potential generation, described as the neuron “firing.”
- The inside of the cell may reach a potential up to +40 mV.

Inactivation of Sodium Channels:
- Sodium channels cease sodium influx after an action potential, entering an inactivated state characterized by the presence of an inactivation gate. This state differs from being merely open or closed.
- Inactivation occurs shortly after depolarization and persists until repolarization.
When depolarization ceases, sodium channels remain inactivated while the membrane repolarizes, eventually returning to a closed state.

Potassium voltage-gated channels open around the same time the sodium channels inactivate but respond slower:
- Allow potassium ions to exit the cell, moving down their electrochemical gradient, thus reducing the positive charge inside, which promotes repolarization.
Unlike sodium channels, potassium channels lack an inactivation gate and remain open longer, contributing to a hyperpolarized state of the neuron following an action potential.

Absolute Refractory Period: The time during which a neuron cannot fire another action potential due to inactive sodium channels. This period ensures that action potentials do not occur too closely together in time and that they only move in one direction.
Relative Refractory Period: Following the absolute period, sodium channels return to a closed state but can be activated again. However, due to the lingering effects of potassium channels remaining open, a larger stimulus is required to initiate another action potential during this phase.
The sodium-potassium pump also plays a crucial role:
- Actively transports 2 sodium ions out and 2 potassium ions into the cell, contributing to the restoration of the resting membrane potential.

After the repolarization and slight hyperpolarization, the neuron returns to its resting membrane potential as all channels return to their original states, preparing the neuron for the next potential action.