Nervous System Cells and Communication

Cell Types of the Nervous System

Glial Cells (Glia): Provide physical and metabolic support to neurons. They outnumber neurons 10:1, build scaffolding, insulate axons, transport nutrients and waste, and mediate immune responses.

Neurons: Interconnected information-processing cells essential for nervous system function, numbering approximately 100 billion at birth.

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Anatomy of a Neuron

Semipermeable Membrane: Encloses the neuron, allowing small or uncharged molecules to pass while blocking larger or highly charged molecules.

Soma (Cell Body): The core of the neuron containing the cell nucleus.

Dendrites: Branching structures extending from the soma that serve as input sites to receive incoming signals.

Axon: Major extension that conducts signals away from the soma toward the terminal buttons.

Myelin Sheath: A fatty insulating layer formed by glial cells that increases signal transmission speed along the axon.

Clinical Correlation (Multiple Sclerosis): An incurable autoimmune disorder involving large-scale degradation of the myelin sheath, causing symptoms such as dizziness, fatigue, and loss of motor control.

Terminal Buttons: Structures at the axon’s end containing synaptic vesicles filled with neurotransmitters.

Synapse: The microscopic gap between neurons where chemical communication takes place.

Receptors: Surface proteins on the receiving cell that bind with neurotransmitters in a specific "lock-and-key" configuration.

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Electrochemical Membrane Potential: Resting State

Resting Potential: The baseline charge of a non-firing neuron, held at approximately -70 mV.

Fluid Separation: Intracellular fluid (cytoplasm) is kept separate from extracellular fluid, establishing a net electrical difference across the membrane.

Ion Distribution:

Outside (Extracellular): High concentration of sodium ($\text{Na}^+$), driven inward by both chemical concentration and electrical attraction to the negative interior.

Inside (Intracellular): High concentration of potassium ($\text{K}^+$), with the interior maintaining an overall negative net charge relative to the outside.

Sodium-Potassium Pump: An active transport mechanism that continuously moves ions across the membrane to maintain the resting potential.

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Stages of an Action Potential

1. Initial Signal & Threshold of Excitation

Neurotransmitter binding opens membrane pores, allowing $\text{Na}^+$ to enter the cell.

If the internal charge reaches the threshold of approximately -55 mV, voltage-gated channels open, triggering a massive influx of $\text{Na}^+$.

2. Peak Action Potential (Depolarization)

The rapid influx of $\text{Na}^+$ causes the electrical charge to spike to approximately +30 mV.

At this peak, sodium gates close and potassium channels open.

3. Repolarization

Positively charged $\text{K}^+$ ions rapidly exit the cell.

The internal charge plummets back toward a negative state.

4. Hyperpolarization & Return to Rest

The internal charge briefly drops below the baseline resting potential (more negative than -70 mV).

The membrane potential gradually stabilizes back to its normal resting state of -70 mV.