Structural and functional organization
The sympathetic nervous system has a distinct hierarchical structure. The cell bodies of preganglionic neurons are located in the grey matter of the lateral horns of the spinal cord at the Th1 to L2–L3 segment levels.
The axons of these neurons are myelinated B-fibers with a slow conduction velocity. They head toward peripheral autonomic ganglia, which are divided into two types:
- Paravertebral ganglia: located in chains along the sides of the spine, forming the right and left sympathetic trunks. They are interconnected by longitudinal and transverse (commissural) branches, as well as with spinal nerves to innervate skeletal muscles, blood vessels, and glands.
- Prevertebral ganglia: located further from the spine, but still at a significant distance from the effector organs themselves (e.g., the heart or stomach). Fibers that pass through the sympathetic trunk in transit approach them.
A crucial feature of the sympathetic division is divergence (multiplication). A single preganglionic axon forms synapses with 10–30 ganglion neurons, ensuring a widespread, generalized propagation of excitation. The ganglia themselves are protected from substances circulating in the blood by a special blood-ganglion barrier.
Neurotransmitters and receptor apparatus
Signal transmission in the sympathetic nervous system occurs in two stages and utilizes different neurotransmitters.
- In the ganglia (preganglionic synapses): acetylcholine is released, which binds to nicotinic cholinergic receptors (N-cholinergic receptors), causing a fast excitatory postsynaptic potential (EPSP). Slow inhibitory postsynaptic potentials (IPSP), caused by the action of dopamine released by small interneurons, are also recorded in the ganglia.
- At target organs (postganglionic synapses): axons of ganglionic neurons (unmyelinated C-fibers) release predominantly noradrenaline (90%), as well as adrenaline (7%) and dopamine (3%). Exceptions are sweat glands and some blood vessels, which are innervated by cholinergic sympathetic fibers (releasing acetylcholine).
Located on the postsynaptic membrane of organs are α1- and β1-adrenergic receptors, which trigger intracellular cascades (via cAMP and inositol trisphosphate). About 80% of the released noradrenaline undergoes reuptake into vesicles, and the remainder is degraded by the enzymes MAO and COMT.
Physiological effects
According to V. Cannon's theory, the sympathetic system mobilizes the body. Its effects are more prolonged compared to the parasympathetic system and have a pronounced catabolic character.
Effect on body systems:
- Cardiovascular: increased heart rate and contractility, elevated blood pressure.
- Respiratory: bronchodilation, increased ventilation volume, and improved oxygenation.
- Metabolism: hyperglycemia (increased blood glucose levels), enhancement of overall metabolism.
- Musculoskeletal: increased muscle tone and impulse conduction velocity.
- Gastrointestinal: inhibitory effect (decreased motility, reduced absorption and enzymatic breakdown).
- Sense organs: pupil dilation (mydriasis).
The system is actively involved in the formation of emotions, stress and pain responses, as well as aggression.
Briefly: answers to topical questions
What types of adrenergic receptors are localized on the postsynaptic membrane of effector organs?
Primarily $\alpha_1$- and $\beta_1$-adrenergic receptors are localized on the postsynaptic membrane of effector target organs, which are activated by noradrenaline to produce the physiological effect.
- $\alpha_1$-adrenergic receptors — located in vascular smooth muscle, the radial muscle of the iris, sphincters of the gastrointestinal tract and urogenital system, as well as the splenic capsule; upon activation, they trigger the formation of inositol trisphosphate.
- $\beta_1$-adrenergic receptors — upon interaction with the neurotransmitter, they activate the enzyme adenylate cyclase, which converts ATP to cAMP.
Which higher autonomic centers regulate the activity of the sympathetic nervous system?
According to sources, the key higher autonomic center is the hypothalamus. Additionally, the activity of mechanisms associated with it may depend on the influence of the limbic system and the cerebellum.
- Hypothalamus — the higher autonomic center coordinating the body's response; corticotropin-releasing hormone is involved in the central regulation of autonomic functions, including the activation of the sympathetic division of the ANS.
- Limbic system — activates neurosecretory cells of the hypothalamus, influencing the regulation of corticotropin-releasing hormone synthesis.
- Cerebellum — participates in the regulation of visceral functions through connections with the autonomic centers of the hypothalamus; its neurons assess the state of internal organs and participate in adapting organ activity to the current needs of the body.
To easily recall the effects of the sympathetic system, imagine the response to a sudden threat: pupils dilate (to see better), bronchi open up (to breathe), the heart beats faster (to pump blood), and digestion shuts down (eating is not the priority right now).
Class and exam questions
What accounts for the generalized effect of the sympathetic system?
Due to the phenomenon of divergence: a single preganglionic axon branches extensively and contacts 10–30 neurons of the autonomic ganglion simultaneously, encompassing several organs at once.
How is the release of noradrenaline into the synapse regulated?
Through presynaptic receptors on the fiber itself: $\alpha_2$-receptors inhibit further noradrenaline release (negative feedback), while $\beta_2$-receptors enhance it (positive feedback).
What neurotransmitters are released in sympathetic ganglia?
The main neurotransmitter is acetylcholine, acting on N-cholinergic receptors. Dopamine, secreted by interneurons to generate a slow inhibitory potential, is also involved.
What else to study on this topic
- Mechanisms of formation of fast and slow postsynaptic potentials (EPSP and IPSP).
- The role of second messengers: cAMP (upon $\beta_1$ activation) and inositol trisphosphate (upon $\alpha_1$ activation).
- Co-transmitters of the sympathetic nervous system: ATP, somatostatin, and neuropeptide Y.
- Presynaptic heteroregulation of neurotransmitter release involving $\text{M}_2$-cholinergic receptors.
- Enzymatic degradation of catecholamines involving MAO and COMT.
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Physiology of the Sympathetic Nervous System" and 4,128 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Other topics in the «Autonomic Nervous System» section
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.