What is its role in the reflex arc?
The autonomic nervous system follows the structural principle of a two-neuron pathway (with switching occurring in ganglia). The preganglionic fiber serves as the connective link between the central and peripheral parts of the system.
- Central neuron — located in the brain structures. Its axon is precisely the preganglionic fiber.
- Synaptic transmission — takes place in the autonomic ganglion, where the axon of the first neuron contacts the body of the second.
- Ganglionic neuron — receives the signal and sends its own (postganglionic) fiber to the effector (muscle or gland).
What is the structure of a preganglionic fiber?
From a biological standpoint, a preganglionic fiber is the axon of an association neuron of the autonomic nervous system. All fibers of this type are myelinated.
On a histological specimen (hematoxylin and eosin staining) within a mixed nerve, it has a characteristic appearance:
- Axon (axis cylinder) — visible in the center as a darker structure.
- Myelin sheath — appears as a light ring around the axon. The reason for this appearance is that myelin lipids dissolve in alcohols during sample preparation, leaving a "void" in their place.
What is the difference between sympathetic and parasympathetic fibers?
The topography and length of preganglionic fibers depend on the division of the autonomic nervous system.
- Sympathetic fibers — short. They emerge from the thoracic and lumbar segments of the spinal cord (Th1–L2) and run to the sympathetic trunk or prevertebral ganglia (e.g., celiac or mesenteric). For example, fibers to the eye originate from Th1–Th2, to the heart from Th1–Th4, and to the kidneys from L1–L2.
- Parasympathetic fibers — originate from cranial nerve nuclei or the sacral segments of the spinal cord. For example, fibers to the eye originate from the Edinger-Westphal nucleus, to the heart and intestine from the dorsal nucleus of the vagus nerve, and to the pelvic organs from segments S2–S4.
How does signal transmission occur in ganglia?
The transmission of excitation from the preganglionic fiber to postganglionic neurons has several features that ensure widespread signal propagation:
- Divergence (impulse multiplication) — a single preganglionic neuron, thanks to axon collaterals, contacts multiple (up to 100) effector neurons simultaneously.
- Convergence — branches from several different preganglionic fibers can terminate on a single effector neuron.
- Distant synapses — the neurotransmitter is released from the fiber and, via diffusion, simultaneously excites the postsynaptic membranes of several cells (in sympathetic ganglia, a single fiber excites about 20 neurons).
What is the exception to the two-neuron rule?
There is an important exception to the rule of obligatory relay in ganglia within the autonomic nervous system. It concerns the innervation of the adrenal glands.
- Target — chromaffin cells of the adrenal medulla (Adrenal medulla).
- Mechanism — these cells share an embryological origin with sympathetic ganglia. Therefore, sympathetic preganglionic fibers (from segments Th11–L1) run directly to them without synapsing on a postganglionic neuron. The adrenal gland functionally replaces a postganglionic neuron by secreting adrenaline and noradrenaline.
Class and exam questions
Why does the myelin sheath of a preganglionic fiber appear light on a histological slide?
During slide preparation, the lipids that make up myelin dissolve in alcohols. A "void" remains in their place, forming a light rim around the dark axon.
Do preganglionic fibers innervate effector organs directly?
As a general rule, no — they relay onto postganglionic neurons in autonomic ganglia. The only exception is the adrenal medulla, which preganglionic fibers reach directly.
How do sympathetic preganglionic fibers differ from postganglionic fibers in structure and length?
Sympathetic preganglionic fibers are short and myelinated. Postganglionic fibers, by contrast, are long and unmyelinated.
What else to study on this topic
- Autonomic ganglion
- Postganglionic fiber
- Autonomic reflex arc
- Adrenal medulla
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