Autonomic Nervous System (ANS): Structure, Function, and Physiology

Autonomic Nervous System (ANS)

Overview of the 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. It is subdivided into the sympathetic and parasympathetic nervous systems, which often have opposing effects on target organs.

• ANS vs. SNS: The ANS controls involuntary functions, while the Somatic Nervous System (SNS) controls voluntary movements.

• Major effectors: Smooth muscle, cardiac muscle, and glands.

ANS Motor Neuron Structure

The ANS uses a two-neuron chain to reach its effectors:

• Preganglionic neuron: Cell body in CNS (brain or spinal cord); axon extends to autonomic ganglion.

• Postganglionic neuron: Cell body in autonomic ganglion; axon extends to effector organ.

Neurotransmitters in the ANS

• Acetylcholine (ACh): Released by all preganglionic neurons and most parasympathetic postganglionic neurons.

• Norepinephrine (NE): Released by most sympathetic postganglionic neurons.

• Effect: ACh is generally excitatory; NE can be excitatory or inhibitory depending on the receptor type.

Parasympathetic Division

The parasympathetic division is responsible for 'rest and digest' activities, conserving energy and promoting maintenance functions.

• Craniosacral outflow: Preganglionic neurons originate in the brainstem (cranial nerves III, VII, IX, X) and sacral spinal cord (S2-S4).

• Ganglia location: Near or within target organs.

• Functions: Stimulates digestion, reduces heart rate, promotes glandular activity.

• Neurotransmitter: Both pre- and postganglionic neurons release ACh.

Sympathetic Division

The sympathetic division prepares the body for 'fight or flight' responses, mobilizing energy and increasing alertness.

• Thoracolumbar outflow: Preganglionic neurons originate in the thoracic and lumbar spinal cord (T1-L2).

• Ganglia location: Sympathetic chain (paravertebral ganglia) and collateral ganglia.

• Functions: Increases heart rate, dilates bronchioles, inhibits digestion, stimulates sweat glands.

• Neurotransmitter: Preganglionic neurons release ACh; postganglionic neurons release NE (and some ACh for sweat glands).

• Adrenal medulla: Secretes epinephrine and norepinephrine into the bloodstream for widespread effects.

Sympathetic Ganglia

• Paravertebral (sympathetic chain) ganglia: Located lateral to the spinal cord.

• Collateral (prevertebral) ganglia: Located anterior to the spinal cord; innervate abdominal organs.

• Adrenal medulla: Functions as a modified sympathetic ganglion.

Receptors in the ANS

• Cholinergic receptors: Bind ACh.

  • Nicotinic receptors: Always excitatory; found on all postganglionic neurons and skeletal muscle (SNS).

  • Muscarinic receptors: Can be excitatory or inhibitory; found on all parasympathetic target organs and some sympathetic targets (e.g., sweat glands).

• Adrenergic receptors: Bind NE and epinephrine.

  • Alpha receptors (α): Generally excitatory (e.g., vasoconstriction).

  • Beta receptors (β): Can be excitatory or inhibitory (e.g., increased heart rate, bronchodilation).

Visceral Reflexes

Visceral reflexes are automatic responses of smooth muscle, cardiac muscle, or glands to stimuli. They typically involve a polysynaptic pathway:

• At least four neurons: sensory neuron, interneuron(s), two postganglionic neurons.

• Short reflexes: Bypass CNS, occur in autonomic ganglia.

• Long reflexes: Involve CNS integration.

Comparison Table: Parasympathetic vs. Sympathetic Divisions

Feature	Parasympathetic	Sympathetic
Origin	Craniosacral (brainstem & sacral spinal cord)	Thoracolumbar (thoracic & lumbar spinal cord)
Ganglia Location	Near or within target organs	Near spinal cord (paravertebral), or anterior to it (prevertebral)
Neurotransmitter (postganglionic)	ACh	NE (most), ACh (sweat glands)
Main Functions	Rest & digest, energy conservation	Fight or flight, energy mobilization
Effect on Heart Rate	Decreases	Increases
Effect on Digestion	Stimulates	Inhibits

Key Equations

• Nerve impulse transmission: $V = IR$ (Ohm's Law: Voltage = Current x Resistance; relevant for understanding nerve conduction)

Example: ANS Control of Heart Rate

• Parasympathetic stimulation: Releases ACh at the heart, decreasing heart rate.

• Sympathetic stimulation: Releases NE at the heart, increasing heart rate and force of contraction.

Additional info: The ANS also regulates pupil diameter, respiratory rate, and sexual function. Disorders of the ANS can lead to conditions such as orthostatic hypotension and autonomic neuropathy.