Autonomic Nervous System: Sympathetic and Parasympathetic Functions
Sympathetic System
This system consists of two strands with ganglia (23 pairs) located at intervals, extending from the neck to the abdomen, placed on either side of the spinal cord.
The ganglia are named after the regions of the spine: there are three cervical ganglia, 12 thoracic, and 4 sacral.
Nerve branches emerge from these ganglia, communicating with the spinal nerves. These include afferent and efferent limbs, which interconnect to form networks and plexuses. Several plexuses exist, with the most important being:
- The cardiac plexus, which supplies the heart.
- The celiac plexus, which supplies the spleen, liver, and pancreas.
- The lumbar or mesenteric plexus, which supplies the large intestine.
- The hypogastric plexus, which supplies the kidneys, rectum, and bladder.
Parasympathetic System
It comprises ganglia located in or near the innervated organs, and nerve fibers that originate from both the brain and the spinal cord at the sacral level.
It consists of two main divisions:
- Cranial Parasympathetic: Formed by nerves originating from the brain that regulate heart rate, respiratory secretions, and saliva production.
- Sacral Parasympathetic: Formed by nerves originating from the sacral region of the spinal cord, innervating organs such as the kidneys, bladder, and bowel.
Topography of the Parasympathetic Nervous System
Topographically, it is divided into four parts:
Hypothalamic Portion
Centers where preganglionic fibers arise include the supraoptic nucleus, paraventricular nuclei of the tuber, and the anterior hypothalamus. Fibers originating from these centers descend to terminate in secretory cells of the neurohypophysis, forming the supraoptic-pituitary, paraventricular-pituitary, and tubero-pituitary fascicles. Interruption of these fibers leads to diabetes insipidus.
Mesencephalic Portion
Preganglionic fibers originate in the Edinger-Westphal nucleus and the anterior median nucleus. Fibers arising from this ganglion form postganglionic fibers, which become the short ciliary nerves that reach the ciliary muscle and iris. These fibers function to produce miosis by contracting the iris sphincter and to facilitate eye accommodation by contracting the ciliary muscle.
Rhombencephalic Portion
Nerve fibers are associated with various cranial nerves, such as:
- Fibers associated with the facial nerve.
- Fibers associated with the glossopharyngeal nerve.
- Fibers associated with the vagus nerve.
Sacral Portion
Nerve nuclei are located within the gray matter of the sacral spinal cord, extending from the second sacral segment to its end. The sacral parasympathetic function is to produce sphincter relaxation and contraction of muscular walls, facilitating urination, defecation, and erection of the genital organs.
Role of the Parasympathetic Nervous System
Its primary function is to maintain a state of rest or relaxation in the body after exertion, or to facilitate important functions such as digestion, urination, and sexual intercourse.
The parasympathetic nervous system participates in regulating the cardiovascular, gastrointestinal, and genitourinary systems. Tissues such as the liver, kidneys, pancreas, and thyroid receive parasympathetic innervation, suggesting its involvement in metabolic regulation.
Cardiovascular System Regulation
Parasympathetic effects on the heart are mediated by the vagus nerve. Acetylcholine decreases heart rate and the force of myocardial contraction through multiple mechanisms:
- Decreased rate of depolarization of the sinus node.
- Retardation of impulse conduction through the atrial musculature.
- Shortening of the refractory period.
- Decreased conduction velocity through the atrioventricular node.
- Inhibition of nerve endings of the sympathetic nervous system on myocardial fibers.