Main organizational principles of the somatic nervous system
The somatic division provides conscious control over the body. The nuclei of the somatic nervous system are localized in the grey matter of the spinal cord, where interneurons and motor neurons are situated.
A distinctive feature of this division is the continuity of the pathway. Axons from neural centers go directly to the innervated muscle without forming effector ganglia in the periphery. These fibers are long and belong to thick myelinated type A, which ensures a high impulse conduction velocity. The innervation zone is segmental in nature, and acetylcholine acts as the sole signal transmitter (neurotransmitter).
Structure of the autonomic nervous system (ANS)
The autonomic nervous system consists of sympathetic, parasympathetic, and metasympathetic divisions. Unlike the somatic system, the efferent pathway of the ANS is disynaptic (two-neuron): the fibers invariably synapse in autonomic ganglia.
Preganglionic fibers (type B) run up to the ganglion, and postganglionic fibers (type C) run from it toward the organ. The neurotransmitter profile of the ANS is more diverse: alongside acetylcholine, noradrenaline and other substances are actively utilized.
Sympathetic and parasympathetic divisions: key differences
These two divisions often exert opposite effects on organs, and their anatomical structure differs significantly.
| Sign | Sympathetic division | Parasympathetic division |
|---|---|---|
| Centers | Posterior hypothalamus; lateral horns of the thoracic and lumbar segments | Anterior hypothalamus; brainstem; lateral horns of the sacral segments |
| Ganglia | Paravertebral and prevertebral | Intramural and terminal |
| Fibers | Preganglionic are short, postganglionic are long | Preganglionic are long, postganglionic are short |
| Innervation | Diffuse (all internal organs) | Limited (does not innervate the adrenal glands and the walls of many blood vessels) |
| Neurotransmitters | Acetylcholine, noradrenaline | Acetylcholine |
Metasympathetic nervous system
The third division of the ANS is distinguished by its locality. The nuclei of the metasympathetic nervous system are located directly within the walls of hollow internal organs.
Its processes (types B and C) are short, interrupted, and branch abundantly within the organ itself. Therefore, the innervation zone is strictly limited to organs with intrinsic motor activity, such as the heart, intestine, uterus, and ureter. The neurotransmitter range here is the widest: in addition to acetylcholine and noradrenaline, ATP, ADP, adenosine, and vasoactive intestinal peptide (VIP) are utilized.
Briefly: answers to topical questions
Which receptor types (cholinoceptors, adrenoceptors) perceive neurotransmitters on target organs?
Neurotransmitters on target organs are perceived by specific postsynaptic receptors:
- M-cholinergic receptors (Muscarinic) — located in neuroeffector synapses of the parasympathetic nervous system.
- N-cholinergic receptors (Nicotinic) — found in neuroeffector cholinergic synapses of the sympathetic nervous system, as well as in neuromuscular synapses of the somatic nervous system and ganglionic synapses of the ANS.
- Adrenergic receptors — located in neuroeffector synapses of the sympathetic nervous system. Depending on localization, $\alpha$-, $\beta_1$-, and $\beta_2$-adrenergic receptors are distinguished.
For instance, in the heart, $\beta_1$-adrenoceptors mediate sympathetic influences, while $M_2$-cholinoceptors mediate parasympathetic ones.
What links make up the reflex arc of an autonomic reflex?
The reflex arc of an autonomic reflex consists of three links:
- Afferent (sensory) link — includes a receptor (thermo-, mechano-, and chemoreceptors) and an afferent fiber of a sensory neuron carrying the impulse to the cells of spinal or peripheral ganglia.
- Second link (associative) — represented by interneurons and central (preganglionic) neurons located in the lateral horns of the spinal cord, medulla oblongata, or midbrain. They send preganglionic fibers to the autonomic ganglia.
- Third link (efferent) — formed by ganglionic (effector) neurons of the peripheral ganglion, which send postganglionic fibers directly to the working organ (effector — muscle or gland).
To remember the fiber length in the ANS, imagine the path to the ganglion: Sympathetic division — ganglia are near the spine, so PREganglionic are short (short run). Parasympathetic — ganglia are directly in the organ, so PREganglionic are long (long run).
Class and exam questions
What is the main anatomical difference between the somatic and autonomic pathways?
The somatic pathway consists of a single long neuron (running without interruption all the way to the muscle). The autonomic pathway is always two-neuron and synapses in an autonomic ganglion.
Where are the autonomic ganglia of the parasympathetic division located?
They are located directly within the innervated organs (intramurally) or in their immediate vicinity.
Which organs are not innervated by the parasympathetic system?
The parasympathetic nervous system does not innervate the adrenal glands and the walls of most blood vessels.
What else to study on this topic
- Types of nerve fibers (A, B, C) and impulse conduction velocity
- Structure and functions of the metasympathetic nervous system
- Role of the hypothalamus in the regulation of autonomic functions
- Signal transmission mechanisms in ANS synapses
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