Coordination of Body Functions: Neuron Structure and Function

Neuron Definition and Structure

A neuron, also known as a nerve cell, is the basic structural and functional unit of the nervous system. It is primarily responsible for transmitting signals throughout the body. The key components of a neuron include:

Dendrites: These structures increase the surface area of the cell body, allowing it to receive signals from other neurons.
Cell Body: This is the region of the neuron where the nucleus and most organelles are located, facilitating cellular maintenance and function.
Axon: Responsible for conducting electrochemical impulses away from the cell body. This is the pathway through which information is transmitted to other neurons, muscles, or glands.
Termini (Axon Terminals): The endpoints of the axon that transmit messages to the target cell, effectively enabling communication between neurons or to other types of cells.

Polarization of Axons

Resting Potential

The resting membrane potential of most neurons typically ranges from −65 to −85 mV, indicating a polarized state, which is crucial for nerve function.

A typical resting potential graph shows time (ms) on the x-axis and voltage (mV) on the y-axis, highlighting the resting potential at approximately −65 mV before any action occurs.

Changes in Membrane Potential

Depolarization:
If sodium (Na⁺) gates open, sodium ions enter the neuron, causing depolarization, which makes the inside of the cell less negative (closer to zero).
Hyperpolarization:
- If chloride (Cl⁻) gates open, chlorides enter the cell, or if potassium (K⁺) gates open, potassium exits the cell, leading to hyperpolarization, which makes the inside of the cell more negative than the resting potential.

Generator Potential

Action Potential Generation

The generation of an action potential occurs at the axon hillock, where if sufficient depolarization accumulates to reach a threshold, more sodium gates open due to positive feedback, resulting in an action potential. The typical voltage change during this process can be depicted as follows:

Time (ms) vs. Voltage (mV) plot reveals the behavior as:
- Resting potential at −65 mV.
- Threshold potential where action potential initiates.
- Spike potential representing the peak of the action potential, around +30 mV.

Impact of Generator Potential

Generator Potential is defined as the sum of Excitatory Post-Synaptic Potentials (EPSPs) and Inhibitory Post-Synaptic Potentials (IPSPs).
- EPSPs: Result from excitatory neurons that release neurotransmitters, opening Na⁺ gates and contributing to depolarization.
- IPSPs: Result from inhibitory neurons that open K⁺ or Cl⁻ gates, leading to hyperpolarization, which inhibits action potentials from being generated.
The generator potential is graded, meaning it can vary in magnitude based on the sum of excitatory and inhibitory inputs.
In contrast, an action potential operates on an “all-or-none” basis; once the threshold is reached, the action potential is always the same size, regardless of the strength of the initial stimulus.

Signal Conduction

Once an action potential is generated at the axon hillock, the electrical signal is conducted down the length of the axon. Notably, each action potential represents an “all-or-none” event, indicating that each nerve impulse is uniform in nature and that variance occurs only due to the magnitude of the generator potential prior to the threshold being reached.