Autonomic Nervous System: Structure and Function

Autonomic Nervous System: Structure and Function

The autonomic nervous system (ANS) is a part of the peripheral nervous system that controls automatic bodily functions such as heart rate, breathing, digestion, and excretion. The ANS comprises two main branches: the sympathetic nervous system and the parasympathetic nervous system.

Structure of the Autonomic Nervous System:

• Central Region:

• Includes nuclei in the brain and spinal cord.

• Nerve fibers from the central nuclei extend to the periphery, classified into two types:

• Preganglionic fibers: From the nuclei to the autonomic ganglia.

• Postganglionic fibers: From the autonomic ganglia to the target organ.

• Autonomic Ganglia:

• Located along the spinal cord and near the organs, including:

• Paravertebral ganglia: Located along the spinal cord, forming the sympathetic trunk.

• Prevertebral (previsceral) ganglia: Located near the organs.

• Terminal ganglia: Located very close to the organs.

• Autonomic Nerve Plexuses:

• Networks of sympathetic and parasympathetic fibers interwoven before reaching the organs.

Sympathetic Nervous System:

• Central Region: Intermediolateral column in the spinal cord segments T1-L3.

• Peripheral Region:

• Preganglionic fibers: Follow the anterior spinal nerve roots, enter the white rami communicantes to the sympathetic paravertebral ganglia, or pass through these ganglia to the prevertebral ganglia.

• Paravertebral ganglia: Two chains of ganglia located on the anterolateral side of the spinal column, divided into regions:

• Cervical region: Superior cervical ganglion, middle cervical ganglion, inferior cervical ganglion.

• Thoracic region: 11-12 thoracic ganglia.

• Lumbar region: 3-4 lumbar ganglia.

• Sacral region: 4-5 ganglia.

• Prevertebral ganglia: Include celiac ganglion, superior mesenteric ganglion, aorticorenal ganglion, inferior mesenteric ganglion.

• Postganglionic fibers: From the paravertebral or prevertebral ganglia, pass through the gray rami communicantes into the spinal nerves to reach the target organs.

Parasympathetic Nervous System:

• Central Region:

• Brain: Nuclei of cranial nerves 3, 7, 9, 10.

• Spinal cord: Intermediolateral column in the lateral horn of the spinal cord segments S2, 3, 4.

• Peripheral Region:

• Preganglionic fibers: Follow cranial nerves 3, 7, 9, 10 to the terminal ganglia.

• Terminal ganglia: Located near or within the organs, including:

• Facial region: Ciliary ganglion, pterygopalatine ganglion, submandibular ganglion, otic ganglion.

• Thoracic region: Cardiac plexus, pulmonary plexus.

• Gastrointestinal tract: Myenteric plexus, submucosal plexus.

• Postganglionic fibers: Extend from the ganglia to the organs.

Functions of the Sympathetic Nervous System:

• Dilates pupils.

• Increases salivary secretion.

• Increases heart rate and contractility.

• Dilates bronchi.

• Inhibits gastrointestinal motility and secretions.

• Constricts pyloric sphincter.

• Contracts vas deferens, seminal vesicles, prostate gland, and uterus, constricts blood vessels.

• Decreases blood flow to skeletal muscles.

Functions of the Parasympathetic Nervous System:

• Constricts pupils.

• Stimulates lacrimal gland secretion.

• Increases salivary secretion.

• Decreases heart rate.

• Constricts bronchi.

• Stimulates gastrointestinal motility and secretions.

• Dilates blood vessels of the genitals.

• Contracts bladder.

• Increases blood flow to skeletal muscles.

The sympathetic and parasympathetic nervous systems work antagonistically, regulating bodily functions to maintain homeostasis.