Autonomic Nervous System: Sympathetic and Parasympathetic Divisions

Autonomic Nervous System (ANS)

The Autonomic Nervous System (ANS) is a critical component of the peripheral nervous system that regulates involuntary physiological functions, including heart rate, digestion, respiratory rate, and more. The ANS is divided into two main branches: the sympathetic division and the parasympathetic division, each with distinct anatomical pathways and physiological effects.

Sympathetic Division

The sympathetic division prepares the body for 'fight or flight' responses, increasing alertness and metabolic activity. It consists of a network of ganglia and nerves that innervate various organs and tissues.

• Sympathetic Ganglia: There are three main types of sympathetic ganglia:

  • Paravertebral (Sympathetic Chain) Ganglia: Located on either side of the vertebral column; these ganglia direct sympathetic outflow to the thoracic cavity, head, body wall, limbs, and skin.

  • Prevertebral (Collateral) Ganglia: Located anterior to the vertebral column, near major abdominal arteries; these ganglia direct sympathetic outflow to abdominopelvic organs.

  • Adrenal Medullary Ganglia: The adrenal medulla acts as a modified sympathetic ganglion with short axons that release neurotransmitters (mainly epinephrine and norepinephrine) directly into the bloodstream.

• Sympathetic Pathways:

  • To a Sweat Gland on the Back: Preganglionic neuron originates in the thoracolumbar spinal cord (T1-L2), synapses in the sympathetic chain ganglion, and the postganglionic fiber travels to the sweat gland.

  • To the Heart: Preganglionic neuron from T1-T4 spinal segments synapses in the cervical or upper thoracic sympathetic chain ganglia; postganglionic fibers innervate the heart.

  • To the Stomach: Preganglionic neuron from T5-T9 synapses in the celiac (prevertebral) ganglion; postganglionic fiber innervates the stomach.

  • To the Adrenal Medulla: Preganglionic neuron from T8-T11 passes through the sympathetic chain and celiac ganglion without synapsing, and directly innervates the adrenal medulla, stimulating release of hormones.

Parasympathetic Division

The parasympathetic division is also known as the craniosacral division and is responsible for 'rest and digest' activities, promoting maintenance and conservation of energy.

• Activation: The parasympathetic nervous system becomes active during restful states, such as after eating or during relaxation.

• Ganglia Location: Parasympathetic terminal and intramural ganglia are located near or within the walls of target organs.

• Cranial Nerves: The vagus nerve (cranial nerve X) sends parasympathetic outflow to visceral organs in the neck, thoracic cavity, and most of the abdominal cavity.

• Sacral Outflow: The sacral part of the parasympathetic division (S2-S4) sends fibers to pelvic organs.

• Parasympathetic Pathways:

  • To the Heart: Preganglionic neuron originates in the medulla oblongata (via the vagus nerve), synapses in intramural ganglia within the heart wall; postganglionic fiber innervates the heart.

  • To the Stomach: Preganglionic neuron from the vagus nerve synapses in intramural ganglia in the stomach wall; postganglionic fiber innervates the stomach.

  • To the Urinary Bladder: Preganglionic neuron from sacral spinal cord (S2-S4) synapses in intramural ganglia in the bladder wall; postganglionic fiber innervates the bladder.

Comparison Table: Sympathetic vs. Parasympathetic Divisions

The following table summarizes key differences between the sympathetic and parasympathetic divisions of the ANS.

Feature	Sympathetic Division	Parasympathetic Division
Preganglionic Axon Length	Short	Long
Ganglionic (Postganglionic) Axon Length	Long	Short
Preganglionic Neurotransmitter	Acetylcholine (ACh)	Acetylcholine (ACh)
Ganglionic Neurotransmitter	Norepinephrine (NE) (except sweat glands: ACh)	Acetylcholine (ACh)
General Response	Fight or flight	Rest and digest
Heart Rate, Respiration	Increase	Decrease
Digestion, Urination	Decrease	Increase

Key Terms and Definitions

• Ganglion: A cluster of neuronal cell bodies outside the central nervous system.

• Preganglionic Neuron: The first neuron in the autonomic pathway, originating in the CNS.

• Postganglionic Neuron: The second neuron in the pathway, located in the autonomic ganglion.

• Neurotransmitter: Chemical messenger released by neurons to transmit signals across a synapse.

• Acetylcholine (ACh): The primary neurotransmitter of preganglionic neurons in both divisions and postganglionic neurons in the parasympathetic division.

• Norepinephrine (NE): The primary neurotransmitter of postganglionic neurons in the sympathetic division (except sweat glands).

Example: Sympathetic Pathway to the Adrenal Medulla

• Preganglionic neuron originates in the thoracic spinal cord (T8-T11).

• Axon passes through the sympathetic chain and celiac ganglion without synapsing.

• Synapses directly on chromaffin cells in the adrenal medulla.

• Adrenal medulla releases epinephrine and norepinephrine into the bloodstream.

Example: Parasympathetic Pathway to the Heart

• Preganglionic neuron originates in the medulla oblongata (vagus nerve).

• Synapses in intramural ganglia within the heart wall.

• Postganglionic neuron innervates cardiac muscle, decreasing heart rate.

Additional info: Some details about specific spinal cord segments and neurotransmitter exceptions (e.g., sweat glands) were inferred based on standard anatomy and physiology knowledge.