Lecture on Autonomic Nervous System

Overview

• The Autonomic Nervous System (ANS) is crucial and has three branches:
  • Sympathetic Nervous System
  • Parasympathetic Nervous System
  • Enteric Nervous System (to be covered in another lecture)
• ANS operates under involuntary control

Divisions of the Nervous System

• Central Nervous System: Brain and Spinal Cord
• Peripheral Nervous System: Sensory System and Motor System
  • Motor System further divided into:
    • Somatic Nervous System: Somatomotor, under voluntary control
    • Autonomic (Visceromotor) Nervous System: Involuntary control

Somatic vs Autonomic Nervous System

• Somatic Nervous System:
  • Single alpha motor neuron extends from spinal cord (anterior gray horn) to effector (skeletal muscles)
  • Neurotransmitter: Acetylcholine, acts on nicotinic receptors
• Autonomic Nervous System:
  • Two neurons: Preganglionic and Postganglionic
  • Preganglionic: From brainstem or spinal cord
  • Postganglionic: Extends to effector organ

Sympathetic Nervous System (SNS)

• Known as "fight or flight"
• Thoracolumbar Outflow: T1 to L2 spinal segments
  • Preganglionic cell bodies located here
  • Short preganglionic and long postganglionic neurons
  • Neurotransmitter: Acetylcholine (preganglionic), Norepinephrine (postganglionic) exception sweat glands which had ACh

Parasympathetic Nervous System (PNS)

• Craniosacral Outflow: Comprised of cranial nerves III, VII, IX, X and sacral nerves S2 to S4

Oculomotor (III)

• Facial (VII)

• Glossopharyngeal (IX)

• Vagus (X)

• Vagus nerve provides major parasympathetic innervation in thoracic and abdominal cavities.

  • Long preganglionic and short postganglionic neurons
  • Neurotransmitter: Acetylcholine (both pre and postganglionic)

• Parasympathetic ganglia: Terminal or intramural

Neural Pathways and Ganglia

• Ganglia: Group of cell bodies in PNS
• Sympathetic Ganglia:types
  • Chain Ganglia (Paravertebral): Alongside vertebral column(pre ganglio move to anterior grey horn then to ventral root then to ventral ramus(spinal nerves here) then to white rami communicans and grey rami communicans(as part of spinal nerves) to

1. arrector pili) pilo motor fibres
2. blood vessels)vaso motor fibres
3. sweat glands)pseudo motor fibers

• **Collateral Ganglia (Prevertebral or pre aortic or sub diaphagmetic **: In front of vertebrae, near aorta as splanchnic Nerve
• Fibers can ascend/descend through chain, or pass directly to collateral ganglia as Splanchnic Nerve (thoracic) These include celiac superior mesentric and inferior mesentric ganglia There is no synapse

(Inter medial lateral grey column to ventral root either same as chain or directly)

Neurotransmitters and Receptors

• Sympathetic Pathway:
  • Preganglionic: Cholinergic (Acetylcholine)
  • Postganglionic: Adrenergic (Norepinephrine)
  • Exception: Some sympathetic postganglionic (e.g., sweat glands) release Acetylcholine
• Parasympathetic Pathway:
  • Both pre and postganglionic:cholinergic

Pathway Overview: 1. Preganglionic Neurons: These neurons originate in the lateral horn of the spinal cord from T1 to L2 (the thoracolumbar region). They travel through the ventral root and enter the spinal nerve.

2.	White Rami Communicantes: From the spinal nerve, the preganglionic fibers pass through the white rami communicantes (which are myelinated fibers) to reach the sympathetic trunk (paravertebral ganglia).
3.	Synapse in Sympathetic Ganglia:
•	Direct Synapse: Some of the preganglionic fibers synapse directly in the paravertebral ganglia at the same level.
•	Ascending/Descending: Some fibers travel up or down the sympathetic trunk to synapse in ganglia at different levels. For example, fibers originating at the T5 level might travel to the cervical ganglia or fibers originating in the T1 region may travel down to the lumbar ganglia.





4.	Postganglionic Neurons: The postganglionic neurons (which are unmyelinated) exit the ganglia via grey rami communicantes, travel back to the spinal nerve, and then go to the target organs (like the heart, blood vessels, etc.).
•	These fibers usually innervate structures in the head, neck, thorax, and abdomen, including the heart, lungs, and blood vessels.

Key Function: • The paravertebral pathway controls many of the immediate, local effects of the sympathetic nervous system, like increasing heart rate or dilating the pupils.

Collateral (Prevertebral) Ganglia Pathway:

The collateral or prevertebral ganglia are located anterior to the vertebral column and are closer to the organs they control, like the digestive organs. These ganglia include the celiac, superior mesenteric, and inferior mesenteric ganglia.

Pathway Overview: 1. Preganglionic Neurons: Similar to the paravertebral pathway, the preganglionic neurons originate in the T1 to L2 region of the spinal cord and travel via the white rami communicantes to enter the sympathetic trunk.

2.	Pass Through Sympathetic Trunk: Instead of synapsing in the sympathetic trunk (paravertebral ganglia), some preganglionic fibers pass through the sympathetic trunk and travel as a splanchnic nerve to reach the prevertebral ganglia.




3.	Synapse in Prevertebral Ganglia:
•	The preganglionic fibers synapse in one of the major prevertebral ganglia like the celiac ganglion, superior mesenteric ganglion, or inferior mesenteric ganglion.
•	These ganglia are located close to major blood vessels like the aorta, which supply abdominal and pelvic organs.




4.	Postganglionic Neurons: The postganglionic fibers (unmyelinated) travel from the prevertebral ganglia to the target organs, such as the stomach, liver, pancreas, small intestine, and kidneys.

Key Function:

•	The collateral pathway is responsible for controlling the function of abdominal and pelvic organs during the fight-or-flight response, like inhibiting digestion or regulating blood flow to digestive organs.

Summary of Pathways: 1. Paravertebral Pathway (Sympathetic Trunk): • Preganglionic neurons enter the sympathetic trunk via white rami communicantes. • They synapse in the sympathetic ganglia along the trunk. • Postganglionic neurons exit via grey rami communicantes and innervate thoracic and head organs (like the heart, lungs, blood vessels).

2.	Collateral Pathway (Prevertebral Ganglia):
•	Preganglionic neurons pass through the sympathetic trunk and travel via splanchnic nerves to the prevertebral ganglia.
•	They synapse in the celiac, superior mesenteric, or inferior mesenteric ganglia.
•	Postganglionic neurons innervate abdominal and pelvic organs (like the stomach, liver, intestines).