Anatomy: cell body and processes
The nerve cell body, or soma (perikaryon), acts as the trophic center, providing nutrition and vital activity. It contains the nucleus, nucleolus, Barr body, and various organelles. This is precisely where enzymes and neurotransmitters are synthesized.
Two types of processes extend from the cell body:
- Dendrites: multiple, densely branching structures. Their main task is to receive incoming information. The number and branching patterns vary significantly across different cells.
- Axon (neurite): a long process that is always single. It originates from the axon hillock and transmits nerve impulses away from the cell. Along its path, the axon may give off side branches called collaterals. It terminates in numerous terminal branches, ending in button-shaped synaptic terminals. These swellings store synaptic vesicles containing neurotransmitters to communicate with the next cell.
Exceptions to the rule: pseudounipolar neurons
Special cells known as pseudounipolar neurons are located in the spinal ganglia. Their peripheral processes collect and carry information about temperature, pain, and tactile sensations to the central nervous system. Due to their structural similarity, these processes are traditionally called axons, although functionally they act as dendrites since they function exclusively to receive signals.
Myelination and nodes of Ranvier
To prevent the dissipation of the electrical signal, axons are covered by the myelin sheath, which acts as an insulator. In the central nervous system, it is formed by oligodendrocytes, and in the peripheral nervous system by Schwann cells (lemmocytes).
The myelination process proceeds as follows: the glial cell membrane extends and wraps around the axon multiple times. A single process is sequentially wrapped by many such cells. Small bare areas called nodes of Ranvier remain between adjacent myelinating cells.
Due to this insulation, the action potential causes depolarization exclusively at the nodes. As a result, the nerve excitation propagates in a saltatory manner (saltatory conduction), which vastly accelerates impulse transmission.
Classification of nerve fibers by speed
Signal conduction speed directly depends on the degree of fiber myelination and the distance between nodes of Ranvier. There are three types of axons:
- Type A: thick myelinated fibers with large intervals between nodes. They conduct impulses at maximum speed.
- Type B: thin myelinated fibers. They occupy an intermediate position in terms of transmission speed.
- Type C: unmyelinated fibers. Excitation travels relatively slowly along them because the signal has to "spread" across the entire membrane without the ability to jump between nodes.
Axonal transport
Substances synthesized in the perikaryon must be delivered to the synapses. This occurs along axonal microtubules in two directions:
- Anterograde (forward) transport: movement of neurotransmitters from the cell body to the axon terminals.
- Retrograde transport: backward movement from the terminals to the neuron cell body.
Based on speed, fast transport (200–400 mm/day, energy-dependent) and slow axoplasmic flow (1–5 mm/day) are distinguished.
Clinical significance: transport systems form the basis of tracing methods. By introducing fluorescent dyes into the cell body or terminal zone, researchers can visualize the entire course of neuronal processes.
Briefly: answers to topical questions
What types of neurons are distinguished by the number of processes?
Based on the number of processes, four main morphological types of neurons are distinguished.
- Unipolar — classified as an independent type.
- Bipolar — functionally polar cells in which information transmission occurs strictly in one direction.
- Pseudounipolar — a variant of bipolar neurons. These include sensory neurons whose cell bodies are located outside the CNS (in spinal ganglia) and which have a long dendrite.
- Multipolar — not described in detail in the sources, only mentioned.
How not to confuse processes: Dendrites — Dozens (they are numerous, branching, and receive signals), while the Axon — Absolutely one (transmits information out of the cell).
Class and exam questions
What is the difference between dendrites and an axon?
Dendrites are multiple branching processes that receive information. There is always a single axon, which can reach up to a meter in length and serves to transmit impulses from the neuron cell body to other cells.
Why does an axon need a myelin sheath?
It performs the function of electrical insulation. Thanks to myelin, the nerve impulse does not dissipate but rather jumps saltatory between the nodes of Ranvier, which significantly increases conduction velocity.
What is the trophic center of a neuron?
This is the cell body (soma or perikaryon). It contains the nucleus and organelles responsible for cell nutrition, as well as the biosynthesis of enzymes and neurotransmitters.
How do neurotransmitters enter the synaptic button?
They are synthesized in the neuron cell body and then transported via anterograde axonal transport along microtubules to the axon terminal, where they are stored in synaptic vesicles.
What else to study on this topic
- The mechanism of signal transmission at synapses using chemical neurotransmitters.
- Structure of the synaptic terminal: terminals, boutons, and vesicles.
- Differences in myelin formation by oligodendrocytes and Schwann cells.
- Laboratory methods for tracking the course of processes using fluorescent labels.
- Features of retrograde transport from multiple projection zones.
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