Nervous Tissue and the Neuron: The Body's Communication Network
Think about what happens when you touch a hot pan. Before you even think "that's hot," your hand has already pulled away. That split-second reaction is the work of nervous tissue — the fastest communication system in your body. It's what lets you feel, think, remember, and move.
Nervous tissue is built from two kinds of cells: neurons, which carry the actual signals, and neuroglia (or glial cells), which support, protect, and nourish the neurons. Without the glia, neurons would starve or get tangled up. Without neurons, there's no signal at all.
The Neuron: The Signal Carrier
A neuron is a cell specialised for electrical signalling. Its shape is unlike any other cell in your body — it's built for distance. A single neuron can stretch from your spinal cord all the way down to your big toe.
Every neuron has three main parts:
1. The cell body (soma) — This is the neuron's headquarters. It contains the nucleus and most of the organelles. It's where the cell makes proteins and keeps itself alive. The soma is usually small, but it's packed with the machinery needed to maintain the long extensions.
2. Dendrites — These are short, branching fibres that stick out from the soma like the branches of a tree. Their job is to receive signals from other neurons. The more dendrites a neuron has, the more inputs it can collect. Think of them as the neuron's "ears."
3. The axon — This is a single, long fibre that carries the signal away from the cell body toward the next neuron or a muscle/gland. Axons can be very long (up to a metre in humans) and are often wrapped in a fatty layer called the myelin sheath.
The myelin sheath is not part of the neuron itself — it's made by glial cells (oligodendrocytes in the CNS, Schwann cells in the PNS). It acts like the plastic coating on an electrical wire: it insulates the axon and makes the signal travel much faster.
The axon ends in a set of tiny branches called axon terminals (or synaptic knobs). These are where the signal gets passed to the next cell.
How a Signal Moves: The Impulse
A neuron at rest has a slight electrical difference across its membrane — the inside is negative relative to the outside (about -70 mV). This is the resting membrane potential.
When a strong enough stimulus arrives at the dendrites, it triggers a sudden reversal: sodium ions rush in, making the inside briefly positive. This is the action potential — the nerve impulse itself. This reversal travels down the axon like a wave, regenerating itself at each point.
The impulse is all-or-nothing. Either the stimulus is strong enough to trigger the full action potential, or nothing happens. There is no "half" a signal.
When the impulse reaches the axon terminals, it triggers the release of chemical messengers called neurotransmitters into the tiny gap (synapse) between neurons. These chemicals cross the gap and bind to receptors on the next neuron's dendrites, starting the process all over again.
Neuroglia: The Unsung Support Crew
Neurons get all the glory, but they can't function without glial cells. There are several types, each with a specific job:
| Glial Cell | Location | Main Job |
|---|
| Astrocytes | Brain & spinal cord | Feed neurons, clean up extra ions, form the blood-brain barrier |
| Oligodendrocytes | Brain & spinal cord | Make myelin sheaths for CNS axons |
| Schwann cells | Peripheral nerves | Make myelin sheaths for PNS axons; help repair damaged nerves |
| Microglia | Brain & spinal cord | Immune cells — eat pathogens and dead cells |
| Ependymal cells | Brain cavities | Line the fluid-filled spaces; help circulate cerebrospinal fluid |