General characteristics and classification
The primary task of the autonomic nervous system is the innervation of internal organs. Under its round-the-clock control are the tone of blood vessels and the bronchial tree, cardiac activity, and thermoregulation (including sweating and heat production). In addition, it controls the smooth muscles of the gastrointestinal tract, the urinary tract, the smooth muscles of the eye, and the lacrimal process.
The system is traditionally divided into three divisions:
- Sympathetic — has centers in the CNS.
- Parasympathetic — also has representations in the brain and spinal cord.
- Metasympathetic — located exclusively in the periphery, directly within the walls of internal organs.
Many organs receive triple innervation. The sympathetic and parasympathetic branches most often act as functional antagonists (exerting opposite effects), but in some cases, their influence can be synergistic.
Comparative physiology of divisions
The sympathetic system is responsible for the body's adaptation to acute stress. Its activation increases blood pressure (due to vasospasm of the skin and internal organs), increases heart rate, and dilates the bronchi to intensify gas exchange. Energy production increases, body temperature rises, and sweating begins. At the same time, digestion, GI motility, and bladder emptying are inhibited. Pupils dilate (mydriasis).
The parasympathetic system, in contrast, ensures recovery processes (trophotropic function). It decreases vascular tone, slows down the heart rate, and weakens breathing by constricting the bronchi. Digestion and GI motility are enhanced, and bladder function is stimulated. Pupils constrict (miosis), and the ciliary muscle of the eye contracts for accommodation.
The metasympathetic system acts as a local coordinator. It harmonizes external sympathetic and parasympathetic signals and is also capable of controlling an organ in their complete absence. It is thanks to this system that an isolated intestinal fragment retains the ability for peristalsis and secretion, and an isolated heart responds to mechanical stretch with increased contractions.
Morphology of neurons of the autonomic arc
The autonomic nervous system has a structural unit — the reflex arc. It consists of three links, each with its own morphological features:
- Sensory (afferent) neuron. This is a pseudounipolar cell with a long dendrite. The cell bodies of such neurons lie outside the CNS — in spinal ganglia or cranial nerve ganglia. Their processes run from internal organs as part of mixed nerves (for example, the vagus nerve, nervus vagus, consists 90% of sensory fibers).
- Associative (intercalated) neuron. Their cell bodies are gathered in specific nuclei of the spinal cord and brainstem. The axons of these cells (preganglionic fibers) head toward the autonomic ganglia. They are myelinated, but thinner than somatic ones, so impulses travel along them more slowly.
- Effector (efferent) neuron. The cell bodies of these neurons are always located outside the CNS (in autonomic ganglia). Their postganglionic axons lack a myelin sheath and go directly to the target organs. Signal conduction velocity here is the lowest.
Receptor apparatus and neurotransmitters
To provide feedback, the ANS uses an extensive sensory network. The most numerous group is mechanoreceptors, which respond to stretch, contraction, or displacement of organs. They are divided into fast (recording the dynamic phase) and slow (fixing static parameters such as volume or geometry). Chemoreceptors work as blood composition sensors, reacting to the partial pressure of oxygen and carbon dioxide. There are also specific receptors: thermoreceptors in the GI tract and osmoreceptors in the liver. As for pain, its source is not entirely clear: it can arise from specialized nociceptors or from excessive stimulation of any polymodal sensors.
Chemical transmission of impulses is carried out using more than 10 types of neurotransmitters. The main roles are played by:
- Acetylcholine — the main neurotransmitter in all ganglia (for transmission from the preganglionic to the effector neuron), as well as at target organs of most parasympathetic fibers.
- Norepinephrine — released at target organs by sympathetic postganglionic fibers.
Serotonin, ATP, and various amino acids may also participate in transmission.
Briefly: answers to topical questions
Where are the centers of the sympathetic nervous system located in the spinal cord?
The centers of the sympathetic nervous system are located in the thoracic and upper lumbar segments of the spinal cord (segments from Th1 to L2 or L3). Anatomically, these centers are localized in the lateral horns of the gray matter, forming the lateral intermediate nucleus, which is also called the lateral intermediate column. This structure is not present throughout the entire spinal cord, but only at the specified levels. Due to this localization of preganglionic neurons, the sympathetic system has a second name — the thoracolumbar system.
Which cranial nerves carry parasympathetic fibers?
Parasympathetic fibers run as part of four pairs of cranial nerves. These include:
- Oculomotor nerve (n. oculomotorius) — innervation of eye muscles (pupillary sphincter, accommodation muscle).
- Facial nerve (n. facialis) — innervation of salivary and lacrimal glands.
- Glossopharyngeal nerve (n. glossopharyngeus) — innervation of salivary and lacrimal glands.
- Vagus nerve (n. vagus) — provides innervation to most organs of the thoracic and abdominal cavities (heart, lungs, stomach).
Which organs receive parasympathetic innervation from the sacral region?
From the sacral region of the spinal cord, parasympathetic innervation is received by the organs of the lesser pelvis, the lower part of the abdominal cavity, and the external genitalia. The axons form pelvic nerves that go to the ganglia of these organs. Specific innervated structures include:
- Distal colon.
- Rectum.
- Urinary bladder.
- Genitals, including external genitalia.
Sympathetic division — "fight or flight" response (stress, mydriasis, increased BP). Parasympathetic — "rest and digest" (trophotropic function, miosis, active digestion).
Class and exam questions
Where are the cell bodies of ANS effector neurons located?
Their cell bodies are always located outside the central nervous system — in specialized autonomic ganglia.
Which neurotransmitter is the main one in autonomic ganglia?
In both the sympathetic and parasympathetic divisions, signal transmission from the associative to the effector neuron occurs via acetylcholine.
What is the feature of the metasympathetic system?
It is located entirely in the periphery (in the walls of organs) and is capable of ensuring the work of myocytes and glands autonomously, even without external signals from the CNS.
What else to study on this topic
- Theories of visceral pain generation and the role of specialized nociceptors
- Dynamics of excitation in the ANS: involvement of associative neurons during strong stimuli
- Specific receptors of internal organs (GI thermoreceptors and liver osmoreceptors)
- Interaction of non-classical neurotransmitters (ATP, serotonin, amino acids)
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the topic "Autonomic nervous system" and 3,485 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in "Nervous system: nerves, ganglia, spinal cord"
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.