The neuron is the fundamental signaling unit of the nervous system, responsible for processing and transmitting information throughout the body. These cells are microscopically complex and possess unique shapes that facilitate their electrical and chemical functions. Neurons are minute and optically transparent, meaning that viewing their intricate structure requires powerful magnification and specialized laboratory methods. Visualizing these cells under a microscope has been paramount to understanding how the brain and nerves operate. This article describes the visual appearance of the neuron, covering its basic parts, the techniques that make it visible, and the variations in its shape.
The Universal Architecture: Key Visible Features
Every neuron possesses a common set of features that create its characteristic appearance under magnification. The central, largest part of the cell is the soma, or cell body, which often appears rounded, oval, or pyramidal in shape. Within the soma, a large, prominent nucleus is typically visible, sometimes appearing like a centralized “eye” due to its clear boundary and the densely stained nucleolus inside. This central location and distinct appearance make the soma the clearest reference point for identifying a neuron on a slide.
Extending outward from the soma are numerous, highly branched projections known as dendrites. These processes resemble the complex canopy of a tree, branching repeatedly into finer extensions. Dendrites function as the primary receptive surface for incoming signals from other neurons, appearing relatively short and clustered around the cell body under the microscope.
In contrast to the multiple, branching dendrites, the neuron features a single, long, slender projection called the axon. The axon emerges from the soma at the axon hillock and extends away, sometimes traveling vast distances. Visually, the axon is often smooth and uniform in diameter, lacking the complex branching seen in the dendritic tree. Specialized fatty layers called myelin sheaths often wrap around the axon, appearing as distinct, segmented coverings along its length under certain preparations.
How Staining Reveals Neuronal Structure
In their natural state, neurons are nearly invisible under a light microscope because they lack natural pigmentation and are mostly composed of water. To overcome this transparency, scientists employ specialized chemical staining techniques that selectively color specific parts of the cell, allowing researchers to study neuronal anatomy.
One of the oldest and most revealing techniques is the Golgi stain, which uses a silver compound to impregnate and entirely fill the internal structure of a neuron. This method stains only a small percentage of the total cell population, sometimes as few as one to five percent. The result is a striking, complete visualization of the entire neuron—the soma, all dendrites, and the axon—appearing as a dark silhouette against a clear background. This contrast allows for detailed morphological analysis of a single cell without interference from the dense surrounding tissue.
Another technique, the Nissl stain, provides a complementary view of the tissue. This stain uses basic dyes, such as cresyl violet, which bind strongly to nucleic acids like RNA and DNA. Because these molecules are concentrated in the nucleus and the rough endoplasmic reticulum, the stain primarily highlights the cell body. The Nissl stain is useful for counting cells and analyzing the organization and density of neuronal layers, but it does not reveal the fine details of the dendrites or the axon.
Diversity in Morphology: Different Types of Neurons
While the general structure of soma, dendrites, and axon is universal, the overall shape, or morphology, of a neuron varies significantly depending on its function and location. These variations are defined by the number of processes—axons or dendrites—that extend directly from the cell body, leading to distinct appearances under the microscope.
Multipolar Neurons
The most common type found throughout the brain and spinal cord is the multipolar neuron, characterized by one axon and numerous dendrites projecting from the soma. Under magnification, these cells often appear star-shaped or polygonal, including pyramidal cells in the cerebral cortex which have a distinct triangular cell body. Another multipolar example is the Purkinje cell, which features an enormous, fan-like dendritic tree that is visibly flat and highly branched.
Bipolar Neurons
In contrast, bipolar neurons are characterized by two distinct processes that extend from opposite ends of an elongated oval soma. One process serves as the axon and the other as the dendrite, giving the cell the appearance of a spindle or stretched ellipse. These cells are typically found in sensory organs, such as the retina of the eye.
Pseudounipolar Neurons
A third category includes the pseudounipolar or unipolar neurons, which are most often found in the sensory ganglia near the spinal cord. These cells appear round, with a single stalk emerging from the soma that immediately splits into two branches. This unique configuration makes the cell body look like a sphere with a single, short T-shaped extension.