What is the role of the fiber in the autonomic reflex arc?
The autonomic nervous system functions according to the reflex principle. Its efferent part has a structural feature—a two-neuron pathway (transmission occurs in ganglia).
- Central neuron — located within the brain or spinal cord structures, its axon (preganglionic fiber) extends to the autonomic ganglion.
- Ganglionic neuron — the cell body is located directly within the ganglion. Its axon is the postganglionic fiber, which extends to the cells of effector organs (muscles or glands).
The mechanism of excitation transmission from the preganglionic fiber to the postganglionic neuron is identical for both systems (sympathetic and parasympathetic). The neurotransmitter acetylcholine is released in the ganglionic synapses and acts on nicotinic acetylcholine receptors (N-receptors).
Morphological Features
In mixed nerve microsections (stained with hematoxylin and eosin), axis cylinders (neuronal processes) exhibit distinct differences depending on the fiber type.
- Sympathetic postganglionic fibers are unmyelinated. A single Schwann cell encompasses several axis cylinders simultaneously. Additionally, they are characterized by considerable length, as preganglionic fibers are short and terminate in the sympathetic trunk (truncus sympathicus) or prevertebral ganglia.
- Exception to the two-neuron rule — chromaffin cells of the adrenal medulla. Embryologically related to sympathetic ganglia, they functionally replace the postganglionic neuron and are innervated directly by preganglionic fibers.
Sympathetic innervation of organs
Postganglionic fibers of the sympathetic nervous system originate from various ganglia and produce specific effects in target organs:
- Eye — fibers from the superior cervical ganglion cause mydriasis (pupillary dilation).
- Heart — fibers from the cervical and upper thoracic ganglia increase heart rate and dilate coronary arteries.
- Abdominal organs — fibers from the celiac and mesenteric ganglia inhibit motility and secretion of the stomach, intestines, and pancreas.
- Urinary bladder — fibers originate from cells of the inferior mesenteric ganglion (ganglion mesentericum inferius) and run within the inferior hypogastric plexus (plexus hypogastricus inferior). They cause relaxation of the detrusor muscle and contraction of the smooth muscle of the internal urethral sphincter (m. sphincter urethrae internus).
- Skin and limbs — fibers from the cervical, thoracic, lumbar, and sacral ganglia provide vasoconstriction, sweating, and piloerection (contraction of arrector pili muscles).
Parasympathetic innervation of organs
- Eye — fibers from the ciliary ganglion cause miosis (pupillary constriction) and contraction of the ciliary muscle (accommodation).
- Glands of the head — fibers from the pterygopalatine and otic ganglia stimulate lacrimation, salivation (secretion of watery saliva), and vasodilation.
- Heart — fibers from the cardiac plexus cause bradycardia and constriction of coronary arteries.
- Intestines — fibers from Auerbach's myenteric plexus and Meissner's submucosal plexus stimulate peristalsis, secretion, and evacuation.
- Male genital organs — fibers from the hypogastric (pelvic) plexus provide erection, vasodilation, and secretion.
Class and exam questions
Which neurotransmitter mediates signal transmission to the postganglionic neuron?
In autonomic ganglia (both sympathetic and parasympathetic), excitation is transmitted via acetylcholine, which binds to N-receptors on the body of the second (ganglionic) neuron.
What do sympathetic postganglionic fibers look like under a microscope?
They are unmyelinated. Unlike myelinated fibers, where a single axis cylinder is surrounded by a light rim (myelin lipids dissolved in alcohols), here a single Schwann cell encloses multiple axis cylinders at once.
Why do the adrenal glands lack postganglionic innervation?
The cells of the adrenal medulla are embryologically related to sympathetic ganglia. They functionally replace the postganglionic neuron themselves, which is why preganglionic fibers reach them directly.
What else to study on this topic
- Preganglionic fiber
- Autonomic reflex arc
- Nicotinic Receptors
- Sympathetic trunk
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