Principles of efferent innervation
Signals traveling from the central nervous system (CNS) to peripheral tissues and effectors are called efferent. All efferent innervation is functionally and anatomically divided into two major blocks depending on the specific structures it controls.
The first block is the somatic nervous system. Its main target is skeletal musculature. Structurally, this pathway is designed as direct and continuous: only a single neuron runs from the center to the effector organ. The central part (motor neuron cell bodies) is located directly in the brain and spinal cord. Long axons extend from them—motor nerve fibers that stretch to skeletal muscles without any synaptic interruptions. There, they form neuromuscular junctions directly on the muscle fibers.
The second block is the autonomic nervous system. It takes over the control of internal organs, glands, and blood vessels. Unlike somatic innervation, nature has established a fundamentally different, more complex scheme for delivering nerve impulses to target tissues here.
Two-neuron principle of the autonomic pathway
The main distinguishing feature of the autonomic system is the structural principle of the two-neuron pathway. The signal from the CNS never goes directly to the internal organ; it always undergoes a synaptic switch.
- First neuron (preganglionic). Its cell body is always located within the CNS. The axon of this cell, called the preganglionic fiber, exits the CNS and heads toward a specialized node—the autonomic ganglion.
- Synaptic switch. It is precisely within the autonomic ganglion that the pathway of the first neuron ends. Here, its axon makes contact with the cell body of the next nerve cell.
- Second neuron (ganglionic). The cell body of this neuron lies within the ganglion itself. The axon extending from it (postganglionic fiber) heads directly to the cells of the effector organs to transmit the final command.
There is only one important exception to this strict rule of two neurons—chromaffin cells of the adrenal medulla. The fact is that during embryogenesis, these cells develop from the same primordia as sympathetic ganglia (they are related to them). Due to this origin, the adrenal medulla is innervated directly by preganglionic fibers, without the classical switch to a second, postganglionic neuron.
Anatomical distinction between sympathetic and parasympathetic divisions
The autonomic nervous system is subdivided into two major divisions: sympathetic and parasympathetic. At the anatomical level, they differ radically in the localization of centers (where the cell bodies of the first neurons lie) and the arrangement of ganglia (where switching to the second neurons occurs).
| Sign | Sympathetic division | Parasympathetic division |
|---|---|---|
| Localization of centers | Thoracolumbar division (lateral horns of the thoracic and lumbar segments of the spinal cord) | Craniosacral division (brain: midbrain and medulla oblongata; spinal cord: sacral segment) |
| Arrangement of ganglia | Significantly distant from the innervated organs | Located in direct proximity to the organ or inside it (intramurally) |
Ganglia of the sympathetic system are divided into vertebral, which form two sympathetic trunks running along both sides of the spine, and prevertebral, which include the cervical ganglia, celiac plexus, as well as superior and inferior mesenteric ganglia.
In turn, parasympathetic nodes are called intramural ganglia, as they are embedded directly into the wall of the innervated internal organ or lie tightly adjacent to it.
Briefly: answers to topical questions
From which specific embryonic primordium do the chromaffin cells of the adrenal medulla develop?
Chromaffin cells of the adrenal medulla develop from neural crests (ganglion plates). During embryogenesis, the cells of this neuroectodermal primordium migrate into the sympathetic ganglion primordium. There, one part of the cells differentiates into sympathetic ganglia, while the other becomes chromaffinoblasts. Eventually, chromaffinoblasts invade the interrenal body, forming the medulla of the adrenal gland.
Which neurotransmitters are released in the synapses of autonomic ganglia?
The neurotransmitter acetylcholine is released in the synapses of autonomic ganglia. The mechanism of excitation transmission from the preganglionic fiber to the postganglionic neuron is completely identical for both systems of autonomic innervation—both sympathetic and parasympathetic. Acetylcholine released into the synaptic cleft interacts with specific nicotinic N-cholinergic receptors.
As part of which cranial nerves do parasympathetic fibers from the brain pass?
Parasympathetic fibers from the brain pass as part of four pairs of cranial nerves. The cell bodies of their preganglionic neurons are localized in the midbrain and medulla oblongata:
- Oculomotor nerve (III pair) — fibers go to the ciliary ganglion for innervation of the eyeball.
- Facial nerve (VII pair) — fibers head to the pterygopalatine and submandibular ganglia.
- Glossopharyngeal nerve (n. glossopharyngeus, IX pair) — fibers go to the otic ganglion.
- Vagus nerve (n. vagus, X pair) — preganglionic fibers go to the intramural ganglia of the thoracic and upper abdominal organs.
What types of receptors are located on the cells of effector organs of the sympathetic nervous system?
Adrenoceptors are located on the cells of effector organs that have sympathetic innervation. These include:
- α1-adrenoceptors — localized on the postsynaptic membrane of vascular smooth muscles, the radial muscle of the iris, as well as sphincters of the gastrointestinal tract and urogenital system.
- β1-adrenoceptors — mediate sympathetic innervation of the heart, causing an increase in automatism and conductivity.
- β2-adrenoceptors — participate in the regulation of vascular tone (cause vasodilation). At the same time, in the smooth muscles of the bronchi, they lack direct sympathetic innervation and are stimulated humorally by adrenaline circulating in the blood.
Abbreviations for memorizing node localization: SS (Sympathetic ganglia — Separated from organs) and PC (Parasympathetic ganglia — Close to organs or lie inside them).
What else to study on this topic
- Embryogenesis and development of the adrenal medulla
- Topography of paravertebral sympathetic trunks
- Anatomy of prevertebral ganglia: celiac plexus and mesenteric ganglia
- Localization of parasympathetic system centers in the brainstem
- Morphology of neuromuscular junctions of the somatic system
Master the entire topic in the «Pharmacology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on the topic "Autonomic Nervous System" and another 5,304 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 116 video lessons on the subject
Other topics in the "Special Pharmacology" section
The entire "Special Pharmacology" sectionFree video lessons: pharmacologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.