The Autonomic Nervous System: Structure, Function, and Organization

Chapter 14: The Autonomic Nervous System

Overview of the Autonomic Nervous System (ANS)

The autonomic nervous system (ANS) is a division of the peripheral nervous system that regulates involuntary physiological processes, including heart rate, blood pressure, respiration, digestion, and sexual arousal. The ANS operates largely below the level of consciousness to maintain homeostasis.

• Involuntary Functions: The ANS controls smooth muscle, cardiac muscle, and glands.

• Examples: Regulation of heart rate, pupil dilation, and digestive motility.

• Principal Divisions: The ANS is divided into the sympathetic and parasympathetic divisions, each with distinct anatomical and functional characteristics.

General Structure and Pathways of the ANS

The ANS consists of a two-neuron chain: a preganglionic neuron (cell body in the CNS) and a postganglionic neuron (cell body in an autonomic ganglion). The axon of the preganglionic neuron synapses with the postganglionic neuron, which then innervates the target organ.

• Sympathetic Division: Preganglionic neurons originate in the thoracolumbar region (T1-L2) of the spinal cord.

• Parasympathetic Division: Preganglionic neurons originate in the brainstem and sacral spinal cord (craniosacral outflow).

• General Pathway: CNS → Preganglionic neuron → Autonomic ganglion → Postganglionic neuron → Effector organ

Sympathetic Division: Organization and Pathways

The sympathetic division prepares the body for 'fight or flight' responses. Its preganglionic neurons are located in the lateral horns of the thoracic and upper lumbar spinal cord. Axons exit via the ventral roots and synapse in the sympathetic chain ganglia or collateral ganglia.

• Sympathetic Chain Ganglia: Run parallel to the vertebral column; provide widespread distribution of sympathetic fibers.

• Collateral Ganglia: Located anterior to the vertebral column; innervate abdominal and pelvic organs.

• Pathways: Preganglionic fibers may synapse at the same level, ascend or descend the chain, or pass through to collateral ganglia.

• Neurotransmitters: Preganglionic neurons release acetylcholine (ACh); postganglionic neurons typically release norepinephrine (NE).

Parasympathetic Division: Organization and Pathways

The parasympathetic division supports 'rest and digest' activities. Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2-S4). Ganglia are located near or within target organs.

• Craniosacral Outflow: Parasympathetic fibers travel with cranial and pelvic nerves.

• Ganglia: Terminal (intramural) ganglia are close to or within the effector organs.

• Neurotransmitters: Both pre- and postganglionic neurons release acetylcholine (ACh).

Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic Division	Parasympathetic Division
Origin	Thoracolumbar (T1-L2)	Craniosacral (Brainstem, S2-S4)
Ganglia Location	Near spinal cord (chain/collateral)	Near or within target organs
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Main Neurotransmitter (Postganglionic)	Norepinephrine (NE)	Acetylcholine (ACh)
General Function	Fight or flight	Rest and digest

Specific Pathways and Effects

• Sympathetic Pathways: Include innervation of the heart (increased rate and force), bronchioles (dilation), blood vessels (vasoconstriction), and sweat glands (secretion).

• Parasympathetic Pathways: Include innervation of the heart (decreased rate), digestive tract (increased motility and secretion), and pupils (constriction).

Reflexes and Integration

Autonomic reflexes involve sensory input from visceral organs, integration in the CNS, and motor output via the ANS. These reflexes help maintain homeostasis (e.g., baroreceptor reflex for blood pressure regulation).

Enteric Nervous System

The enteric nervous system is a specialized network of neurons within the walls of the gastrointestinal tract. It can function independently but is modulated by the sympathetic and parasympathetic divisions.

• Functions: Regulates digestive motility, secretion, and blood flow.

Summary Table: ANS Divisions and Effects

Organ/System	Sympathetic Effect	Parasympathetic Effect
Heart	Increases rate and force	Decreases rate
Bronchioles	Dilates	Constricts
Pupils	Dilates	Constricts
Digestive Tract	Decreases motility	Increases motility
Sweat Glands	Stimulates secretion	No effect

Key Terms and Definitions

• Preganglionic Neuron: The first neuron in the ANS pathway, with its cell body in the CNS.

• Postganglionic Neuron: The second neuron, with its cell body in an autonomic ganglion and axon extending to the effector organ.

• Autonomic Ganglion: A cluster of neuronal cell bodies outside the CNS where pre- and postganglionic neurons synapse.

• Dual Innervation: Most organs receive input from both sympathetic and parasympathetic divisions, allowing fine control of organ function.

Example: Pupillary Reflex

• Sympathetic Stimulation: Dilates pupils (mydriasis) in low light or stress.

• Parasympathetic Stimulation: Constricts pupils (miosis) in bright light or during rest.

Formulas and Equations

• Nernst Equation (for membrane potential):

$E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• Goldman-Hodgkin-Katz Equation (for resting membrane potential):

$V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Additional info: Equations included for academic context regarding membrane potentials relevant to ANS neuron function.