Core 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 gray matter of the spinal cord, housing interneurons and motor neurons.
A defining feature of this pathway is its continuity. Axons from neural centers run directly to the innervated muscle without forming peripheral effector ganglia. These fibers are long, thick, myelinated type A fibers, ensuring high conduction velocity. The innervation zone is strictly segmental, and acetylcholine serves as the sole neurotransmitter.
Structure of the Autonomic Nervous System (ANS)
The autonomic nervous system consists of sympathetic, parasympathetic, and enteric (metasympathetic) divisions. Unlike the somatic system, the efferent pathway of the ANS is disynaptic (two-neuron): fibers always synapse in autonomic ganglia.
Preganglionic fibers (type B) travel to the ganglion, while postganglionic fibers (type C) extend from the ganglion to the target organ. The neurotransmitter profile of the ANS is more diverse: alongside acetylcholine, norepinephrine and other signaling molecules are actively utilized.
Sympathetic and Parasympathetic Divisions: Key Differences
These two divisions often exert opposing effects on visceral organs, and their anatomical structures differ significantly.
| Feature | Sympathetic Division | Parasympathetic Division |
|---|---|---|
| Centers | Posterior hypothalamus; lateral horns of thoracic and lumbar segments | Anterior hypothalamus; brainstem; lateral horns of 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 visceral organs) | Limited (does not innervate adrenal medulla and most blood vessel walls) |
| Neurotransmitters | Acetylcholine, norepinephrine | Acetylcholine |
Enteric (Metasympathetic) Nervous System
The third division of the ANS is notable for its localization. The nuclei of the enteric nervous system reside directly within the walls of hollow visceral organs.
Its processes (types B and C) are short, synapsing and branching extensively within the organ itself. Consequently, its innervation zone is strictly limited to organs possessing intrinsic motor activity, such as the heart, gastrointestinal tract, uterus, and ureters. The neurotransmitter array here is the broadest: besides acetylcholine and norepinephrine, it utilizes ATP, ADP, adenosine, and vasoactive intestinal peptide (VIP).