Role in the Reflex Arc
The autonomic nervous system follows a structural two-neuron pathway principle (with switching occurring in ganglia). The preganglionic fiber serves as the connecting link between the central and peripheral parts of the system.
- Central neuron — located within brain structures. Its axon constitutes the preganglionic fiber.
- Synaptic relay — occurs in the autonomic ganglion, where the axon of the first neuron contacts the cell body of the second.
- Ganglionic neuron — receives the signal and sends its own (postganglionic) fiber to the effector (muscle or gland).
Structure of the Preganglionic Fiber
From a biological standpoint, a preganglionic fiber is the axon of an associative neuron of the autonomic nervous system. All fibers of this type are myelinated.
On a histological preparation (hematoxylin and eosin staining) within a mixed nerve, it has a characteristic appearance:
- Axis cylinder (axon) — visible in the center as a darker structure.
- Myelin sheath — appears as a light halo around the axon. This appearance occurs because myelin lipids dissolve in alcohols during sample preparation, leaving an empty space.
Differences 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 travel to the sympathetic trunk or prevertebral ganglia (e.g., celiac or mesenteric ganglia). 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 arise from the Edinger-Westphal nucleus, to the heart and gut from the dorsal nucleus of the vagus nerve, and to the pelvic organs from segments S2–S4.
Signal Transmission in Ganglia
The transmission of excitation from the preganglionic fiber to postganglionic neurons has several features that ensure widespread signal distribution:
- Divergence (impulse multiplication) — a single preganglionic neuron, through axon collaterals, contacts multiple (up to 100) effector neurons simultaneously.
- Convergence — branches from several different preganglionic fibers can terminate on a single effector neuron.
- Distance synapses — the neurotransmitter is released from the fiber and, via diffusion, simultaneously excites the postsynaptic membranes of multiple cells (in sympathetic ganglia, a single fiber excites about 20 neurons).
Exceptions to the Two-Neuron Rule
There is a major exception to the mandatory ganglion-switching rule within the autonomic nervous system concerning the innervation of the adrenal glands.
- Target — chromaffin cells of the 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 the postganglionic neuron by secreting epinephrine and norepinephrine directly into the bloodstream.