Nerve cell - structure, types of neurons, synapse

Introduction

Nerve cells, or neurons, are the basic building units of the nervous system. They are specialised to receive, process and send signals. Even if neurons look quite different depending on where they are found, they all share the same basic architecture that is adapted for fast communication. Understanding the simple structure and the types helps a lot when studying neurohistology.

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Structure of a Neuron

A typical neuron has three main parts: cell body, dendrites, and axon.

Cell Body (Soma, Perikaryon)

The cell body contains the nucleus and the main organelles. It is usually quite large and pale-staining because the nucleus has a lot of euchromatin.

Inside, we can see Nissl bodies – clumps of rough ER that stain strongly with basophilic dyes. They produce proteins for the neuron, especially neurotransmitters.

The cell body also contains mitochondria, Golgi complex, lysosomes and cytoskeleton.

Dendrites

Dendrites are shorter, branching processes which receive signals.

They are thicker near the soma and taper away as they branch. They have many dendritic spines, which increase the surface area for synapses.

Importantly, dendrites contain Nissl substance in their proximal parts (but axons do not).

Axon

The axon is a single, long process specialised for sending impulses.

It usually starts from a region called the axon hillock, which looks like a clear cone-shaped area of the soma without Nissl bodies.

The first segment after the hillock is the initial segment, where action potentials commonly start.

The axon may be myelinated or unmyelinated:

• In the PNS, myelin is made by Schwann cells.
• In the CNS, myelin is produced by oligodendrocytes.

At the end of the axon are the axon terminals or terminal boutons, which form synapses.

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Types of Neurons

Neurons can be classified in many ways, but the most common in histology is the number of processes.

Multipolar Neurons

• One axon and many dendrites.
• The most common type in CNS: motor neurons, interneurons.
• Typical in anterior horn of spinal cord. They have a star-like shape with lots of branching.

Bipolar Neurons

• One axon and one dendrite.
• Found in retina, olfactory epithelium, and some sensory ganglia. They look more “symmetrical” and elongated.

Pseudo-unipolar Neurons

• Originally bipolar but the two processes fuse into one that splits like a T.
• Found in dorsal root ganglia and sensory cranial nerve ganglia. One branch goes to peripheral receptors, the other to the spinal cord.

Unipolar Neurons

• Rare in humans; mainly in invertebrates.
• One single process leaving the soma.

Functional Classification

Sometimes neurons are grouped by what they do:

• Sensory (afferent)
• Motor (efferent)
• Interneurons, connecting other neurons in CNS

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Synapse

A synapse is the connection between two cells where information is passed. Most are chemical synapses, although electrical ones exist too.

Structure of a Chemical Synapse

1. Presynaptic terminal
   • Located at the end of the axon.
   • Contains synaptic vesicles filled with neurotransmitter.
   • Has mitochondria and voltage-gated Ca²⁺ channels.
2. Synaptic cleft
   • A small gap (about 20–30 nm).
   • Neurotransmitter diffuses across it.
3. Postsynaptic membrane
   • Membrane of the next cell (neuron, muscle, or gland).
   • Contains receptors for neurotransmitters.
   • Often appears thickened → postsynaptic density.

How a Synapse Works (simple steps)

1. Action potential reaches the presynaptic terminal.
2. Voltage-gated Ca²⁺ channels open → Ca²⁺ enters.
3. Vesicles fuse with membrane and release neurotransmitter.
4. Neurotransmitter binds to receptors on postsynaptic side.
5. Postsynaptic cell depolarises or hyperpolarises, depending on the transmitter.
6. Neurotransmitter is removed (reuptake, degradation, or diffusion).

Electrical Synapses

• Much rarer in humans.
• Connected by gap junctions.
• Very fast but less flexible.

Citation

Morphologia, E. O. O. J. (2019). Junqueira’s Basic Histology : Text & Atlas (15th ed.), 2018. Morphologia, 13(2), 101–104. https://doi.org/10.26641/1997-9665.2019.3.101-104

slides from Ph.D. Lucia Fraser Lantová