How is the sympathetic nervous system structured?
The sympathetic nervous system has central and peripheral divisions. Due to the localization of its central apparatus, it is also referred to as the thoracolumbar system.
- Central division: represented by neurons of the lateral intermediate columns (horns) of the spinal cord at the level of the thoracic and upper lumbar segments (Th1–L2).
- Peripheral division: the final efferent pathway consists of a two-neuron chain.
- First (preganglionic) neurons: their cell bodies lie within the CNS. Their processes are short (B-type fibers). They may terminate in synapses within paravertebral ganglia forming the sympathetic trunk, or pass through them without synapsing, terminating instead in prevertebral ganglia (celiac, superior and inferior mesenteric ganglia).
- Second (postganglionic) neurons: their cell bodies lie within autonomic ganglia. Their processes are long (C-type fibers) and run toward target organs, sometimes as part of somatic nerves, as in the case of skin innervation.
Neurotransmitters and Receptors
Nerve impulse transmission in the sympathetic nervous system is mediated by specific neurotransmitters:
- In the first synapse (in the ganglion): the preganglionic neuron releases acetylcholine, which binds to nicotinic acetylcholine receptors (nAChRs) on the body of the second neuron.
- In the second synapse (at the target organ): the postganglionic neuron releases norepinephrine, which acts on adrenergic receptors (α- and β-ARs) of the organ. For this reason, the system is termed adrenergic.
Exception: innervation of the sweat glands. The second sympathetic neurons supplying them are cholinergic. They release acetylcholine, which acts on muscarinic acetylcholine receptors, activating perspiration.
Effects on organs (ergotropic effects)
The sympathetic system places the body in a state of physical exertion. Its stimulation produces the following effects:
- Cardiovascular system: increased blood pressure, increased heart rate (tachycardia). Blood is redistributed: vessels of the skin and internal organs constrict, while vessels of skeletal muscles and lungs dilate; coronary arteries also dilate.
- Respiratory system: increased respiratory rate, bronchial dilation (bronchodilation), and inhibition of bronchial gland secretion.
- Digestion and excretion: suppression of gastrointestinal motility and secretion, inhibition of gallbladder contraction via α2-adrenergic receptors. In the urinary system, urinary retention occurs: the detrusor relaxes and the internal sphincter contracts.
- Eye: pupillary dilation (mydriasis) and widening of the palpebral fissures.
- Metabolism and skin: elevated blood glucose levels, increased energy production, elevated body temperature. Sweating and piloerection (contraction of the arrector pili muscles — m. arrector pili) are observed in the skin.
What regulates the sympathetic nervous system?
The highest regulatory center of the autonomic nervous system is the hypothalamus. Its caudal division is responsible for enhancing sympathetic (ergotropic) activity.
The connection between the hypothalamus and peripheral divisions is maintained via descending pathways:
- Medial forebrain bundle.
- Mammillotegmental tract.
- Posterior longitudinal fasciculus of Schütz.
Impulses travel through these pathways to the descending reticular formation system of the midbrain, and from there to the preganglionic neurons of the spinal cord.
Clinical significance
Disorders in the functioning of the sympathetic nervous system lead to a number of autonomic dysfunctions:
- Orthostatic hypotension: a drop in diastolic blood pressure of more than 10 mm Hg within 3 minutes of standing up. It occurs due to transient cerebral ischemia, manifesting as dizziness and weakness. It is common in the elderly due to involutive processes in the autonomic nervous system.
- Raynaud's phenomenon (syndrome): episodic pallor and cyanosis of the fingers upon cold exposure or stress. It is associated with a sharp decrease in local blood flow; one of the causes may be impaired sympathetic innervation of small arteries.
- Sweating disorders: can manifest as either excessive sweating (hyperhidrosis) or decreased/absent sweating (hypohidrosis/anhidrosis).
Class and exam questions
Why is the sympathetic nervous system called thoracolumbar?
This is due to the anatomical localization of its central apparatus: the cell bodies of the first (preganglionic) neurons are located in the lateral horns of the thoracic (Th1–Th12) and upper lumbar (L1–L2) segments of the spinal cord.
Which neurotransmitter is released by sympathetic fibers in the sweat glands?
Unlike most organs where postganglionic sympathetic fibers release norepinephrine, the innervation of sweat glands is carried out by cholinergic fibers. They release acetylcholine, which acts on muscarinic acetylcholine receptors.
How does the sympathetic nervous system participate in the mechanism of labor?
During maximal excitation of the sympathetic nervous system center, a large amount of norepinephrine is released. In response, the parasympathetic center is excited, whose neurotransmitters (acetylcholine) cause contraction of the circular uterine muscles.
What else to study on this topic
- Parasympathetic nervous system
- Autonomic ganglia and plexuses
- Hypothalamus and its nuclei
- Adrenergic and cholinergic receptors
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