The autonomic nervous system (ANS) is a crucial component of the peripheral nervous system responsible for regulating involuntary bodily functions. It consists of two main branches: the sympathetic nervous system and the parasympathetic nervous system. These two systems have complementary roles and work in harmony to maintain homeostasis in the body. Here’s a detailed note on the structure and functions of each system:
1. Sympathetic Nervous System (SNS)
Structure:
– The sympathetic nervous system comprises a chain of ganglia (clusters of nerve cell bodies) that runs alongside the spinal cord.
– It originates from the thoracic and lumbar regions of the spinal cord, and the preganglionic neurons are relatively short.
– Preganglionic fibers are myelinated, while postganglionic fibers are unmyelinated.
Functions:
– The SNS is often called the “fight or flight” system, as it prepares the body for action in response to stress or danger.
– Functions of the sympathetic system include:
– Increasing heart rate and the force of heart contractions to deliver more blood to vital organs.
– Dilating the airways to enhance oxygen intake.
– Redirecting blood flow away from non-essential organs (e.g., digestive system) to the muscles, heart, and brain.
– Mobilizing energy reserves by stimulating the release of glucose and fatty acids into the bloodstream.
– Dilating pupils to enhance vision.
– Inhibiting non-essential functions like digestion and reproduction.
2. Parasympathetic Nervous System (PNS)
Structure:
– The parasympathetic nervous system has a more localized organization compared to the SNS.
– It originates from the cranial nerves and the sacral region of the spinal cord.
– Preganglionic fibers are relatively long, and ganglia are situated close to or within target organs.
– Preganglionic and postganglionic fibers are both myelinated.
Functions:
– The PNS is often called the “rest and digest” system, as it promotes relaxation and recovery.
– Functions of the parasympathetic system include:
– Slowing heart rate and reducing blood pressure.
– Constricting airways to decrease oxygen intake.
– Increasing digestive processes, such as salivation and intestinal activity.
– Promoting energy storage and conservation.
– Stimulating the pupils to constrict for closer vision.
– Enhancing reproductive functions.
Balance and Homeostasis
– The sympathetic and parasympathetic systems operate in a dynamic balance to maintain homeostasis in the body.
This balance ensures that the body can rapidly respond to stress or danger (sympathetic) and can return to a resting state when the threat is removed (parasympathetic).