What is the Parasympathetic Nervous System?
The
parasympathetic nervous system (PNS) is one of the two divisions of the autonomic nervous system (ANS), the other being the sympathetic nervous system. The PNS is responsible for the body's 'rest and digest' functions. It counteracts the 'fight or flight' response initiated by the sympathetic system, bringing the body back to a state of calm and maintaining homeostasis.
Histological Features of the Parasympathetic Nervous System
The PNS is primarily comprised of
preganglionic neurons and
postganglionic neurons. Preganglionic neurons originate in the
brainstem and the sacral spinal cord, and they project to ganglia located near or within the effector organs. These ganglia contain the postganglionic neurons, which then innervate the target tissues.
Neurotransmitters and Receptors
The primary neurotransmitter of the PNS is
acetylcholine (ACh). In preganglionic neurons, ACh is released and binds to
nicotinic receptors on postganglionic neurons. In postganglionic neurons, ACh is again released and binds to
muscarinic receptors on the effector organs, such as the heart, lungs, and digestive tract.
Histological Identification
In histological sections, parasympathetic ganglia can be identified by their small size and location near or within the target organs. These ganglia contain large, round cell bodies with prominent nuclei and nucleoli. The surrounding stroma is less dense compared to sympathetic ganglia, reflecting a lower fiber content.Function and Impact on Effector Organs
The PNS exerts a calming effect on the body. It decreases heart rate, increases digestive activities by stimulating peristalsis and secretion of digestive enzymes, and promotes relaxation of sphincters. In the eyes, it causes pupillary constriction and accommodation for near vision.Clinical Relevance
Dysfunction of the PNS can lead to several clinical conditions. For example,
vagal nerve dysfunction can cause issues with heart rate and digestive disturbances. Understanding the histological layout and the functional pathways of the PNS is crucial for diagnosing and treating such conditions.
Conclusion
The parasympathetic nervous system plays a vital role in maintaining homeostasis and promoting the 'rest and digest' state. Histological examination reveals its intricate organization and specific features, which are essential for its function. Knowledge of the PNS's structure and function is fundamental for understanding its role in health and disease.
