Autonomic Nervous System: Neuroanatomy | Medical Science Optional Notes for UPSC PDF Download

Autonomous Nervous System

The Autonomic Nervous System (ANS) regulates heart rate, contractile force, digestive secretions, urination, defecation, and various other bodily functions. It comprises the sympathetic division, known as the "fight or flight" division, and the parasympathetic division, known as the "rest and digest" division.

The ANS functions through a two-neuron chain connecting the Central Nervous System (CNS) and the effector organ. The first neuron in this pathway is the preganglionic neuron, with its cell body located in the CNS. The lightly myelinated axon of the preganglionic neuron, termed the preganglionic axon, exits the CNS through a spinal or cranial nerve. It then synapses with the dendrites or cell body of the second motor neuron, known as the ganglionic neuron, in an autonomic ganglion.

The axon of the ganglionic neuron, referred to as the postganglionic axon, is unmyelinated and extends to the effector organ. Autonomic preganglionic neurons consistently release Acetylcholine (ACh) as their neurotransmitter. In contrast, most sympathetic postganglionic neurons release norepinephrine (NE), while parasympathetic postganglionic neurons release Ach as their neurotransmitter.

Divisions

The two divisions of the Autonomic Nervous System (ANS), namely the sympathetic and parasympathetic divisions, typically innervate the same organs but exert opposing (antagonistic) effects. This phenomenon, known as dual innervation, allows each division to "check" the other, contributing to the maintenance of body activities in homeostasis.

Parasympathetic fibers emerge from the brain and the sacral spinal cord, leading to the designation of the Parasympathetic division as the craniosacral division. In contrast, sympathetic fibers emerge from the thoracolumbar region of the spinal cord, earning the sympathetic division the title of the thoracolumbar division.

The parasympathetic division is characterized by long preganglionic axons and short postganglionic axons. In contrast, the sympathetic division features short preganglionic axons and long postganglionic axons. Parasympathetic ganglia are situated within the visceral effector organs, while sympathetic ganglia are found adjacent to the spinal cord.

[Question: 942284]

Functions

Innervation

• Parasympathetic nerve fibers originate from both the brainstem and the sacral spinal cord.
• The cranial nerves responsible for conveying parasympathetic motor fibers include the oculomotor (CN III), facial (CN VII), glossopharyngeal (CN IX), and Vagus (CN X).
• Cranial nerves III, VII, and IX provide parasympathetic output to the head.
• The vagus nerve supplies parasympathetic output to thoracic organs such as the heart and lungs, as well as abdominal organs including the stomach, liver, gallbladder, kidneys, small intestine, and the proximal half of the large intestine.
• Parasympathetic outflow from the sacral spinal cord serves the distal half of the large intestine, the urinary bladder, and the genitalia (penis/clitoris, vagina).
• The sympathetic division innervates the viscera of both the thorax and abdomen.
• Additionally, it controls the sweat glands and arrector pili muscles in the dermis of the skin, along with the smooth muscles present in every artery and vein.
• Sympathetic preganglionic neurons have their cell bodies located in the lateral horns of spinal segments T1 to L2.
• The axons of these neurons exit the spinal cord through the ventral roots.

Fight and Flight Response

(a) Adrenergic receptors interact with and respond to norepinephrine (NE), and typically, to epinephrine as well.
(b) Two primary types of adrenergic receptors exist: alpha receptors and beta receptors.
(c) The response of these receptors to NE varies significantly based on the specific subtype of the receptor.
(d) Recognizing that NE is released during the "fight or flight" response aids in anticipating its effects on different organs. NE (and possibly epinephrine) induces:

• Increased heart rate and force of contraction
• Elevated blood pressure
• Suppression of gastrointestinal secretion and smooth muscle activity
• Dilation of airways
• Constriction of visceral blood vessels
• Widening of blood vessels supplying the skin and skeletal muscles
• Inhibition of the urinary reflex
• Induction of sweating
• Release of glucose into the bloodstream by the liver

Nerves of Tomorrow: The term "nerves of tomorrow" is attributed to parasympathetic nerves, as they play a crucial role in restoring the body's functions to a normal state after an emergency. These nerves are instrumental in maintaining homeostasis during periods of no stress.

Autonomic Nervous system - Repeats

• What are the general features of parasympathetic division of autonomic nervous* is it known as 'anabolic division' (2003)? 
• How sympathetic stimulation prepares the person for flight and fight reaction (2004)?
• What are the differences between sympathetic and parasympathetic division of automatic nervous system? Why are parasympathetic nerves called nerves of tommorrow (2009)? 
•  Describe the cervical part of the sympathetic chain. (2015)