自律神経系：概要

The autonomic nervous system divides into sympathetic and parasympathetic divisions. The sympathetic division originates from thoracic and lumbar spinal segments and triggers a fight-or-flight response, increasing heart rate and redirecting blood flow. The parasympathetic division originates from cranial and sacral nerves and induces a rest-and-digest response, promoting digestion and lowering heart rate. Most organs receive signals from both divisions.

Preganglionic neurons originate in the central nervous system and extend to autonomic ganglia, where they synapse with postganglionic neurons. Postganglionic neurons then extend from ganglia to innervate target organs. In the sympathetic division, preganglionic neurons are short and postganglionic neurons are long. In the parasympathetic division, preganglionic neurons are long and postganglionic neurons are short, located within target organs.

Sympathetic ganglia include paravertebral ganglia located near the spinal cord and prevertebral ganglia in the abdominal cavity. Paravertebral ganglia neurons innervate organs above the diaphragm, such as the heart and lungs. Prevertebral ganglia neurons innervate organs below the diaphragm, including the gastrointestinal tract and urinary system. This anatomical arrangement allows coordinated sympathetic control of visceral organs.

The autonomic nervous system uses two primary neurotransmitters: acetylcholine and noradrenaline. Acetylcholine is released by parasympathetic neurons and some sympathetic neurons, binding to cholinergic receptors. Noradrenaline is released by most sympathetic postganglionic neurons, binding to adrenergic receptors. These neurotransmitters mediate the opposing physiological effects of sympathetic and parasympathetic activation.

Parasympathetic ganglia are located within or near target organs because parasympathetic preganglionic neurons are long, extending directly from cranial and sacral nerves to these ganglia. This anatomical arrangement allows postganglionic neurons to be very short, providing precise, localized control of individual organs. This contrasts with sympathetic organization, where ganglia are near the spinal cord, enabling more widespread coordinated responses.

The autonomic nervous system regulates involuntary functions essential for homeostasis, including cardiac output, glandular secretion, and digestion. The sympathetic division prepares the body for stress through fight-or-flight responses, while the parasympathetic division promotes rest-and-digest functions. Most organs receive dual innervation from both divisions, allowing balanced control that maintains stable internal conditions despite external changes.

The autonomic nervous system is one of three divisions of the peripheral nervous system, alongside the somatic and enteric nervous systems. While the somatic nervous system controls skeletal muscles and the enteric nervous system manages gastrointestinal functions, the autonomic nervous system specifically regulates involuntary visceral functions. All three divisions work together to maintain overall nervous system function and homeostasis.