Structure of the Autonomic Nervous System

Introduction to the Autonomic Nervous System

The autonomic nervous system (ANS) regulates involuntary body functions such as heart rate, digestion, respiratory rate, pupillary response, and urination. Operating largely below the level of conscious awareness, this system acts as a control network that maintains internal stability, or homeostasis, despite changes in the external environment. It coordinates smooth muscle contractions, cardiac muscle activity, and glandular secretions across various organ systems.

Division into Sympathetic and Parasympathetic Systems

Functionally, the autonomic nervous system splits into two main branches that usually exert opposing effects on target organs.

• Sympathetic division: Dominates during stressful situations, physical exertion, or emergencies, triggering the classic fight-or-flight response.
• Parasympathetic division: Dominates during quiet, resting conditions, supporting energy conservation and routine housekeeping tasks like digestion in a rest-and-digest state.

Two-Neuron Chain Organization

Unlike somatic motor pathways that use a single neuron from the spinal cord to skeletal muscle, autonomic pathways rely on a two-neuron chain to reach target organs.

1. Preganglionic neuron: Cell body resides in the central nervous system, and its axon extends out to an autonomic ganglion.
2. Postganglionic neuron: Cell body sits in the ganglion, and its axon extends all the way to the effector organ.

Structure and Pathways of the Sympathetic Division

The sympathetic system is also known as the thoracolumbar division because its preganglionic neurons originate from the thoracic and upper lumbar segments of the spinal cord (specifically T1 to L2). Because these pathways must reach distant organs throughout the body, sympathetic preganglionic fibers are typically short, synapsing in nearby ganglia, while postganglionic fibers are long.

Structure and Pathways of the Parasympathetic Division

The parasympathetic system is called the craniosacral division because its preganglionic neurons originate from brainstem cranial nerves (III, VII, IX, X) and the sacral spinal cord segments (S2 to S4). In contrast to the sympathetic layout, parasympathetic preganglionic fibers are long, extending all the way to terminal ganglia located close to or inside the target effector organs, resulting in short postganglionic fibers.

Autonomic Ganglia: Sympathetic vs. Parasympathetic

Neurotransmitters and Receptors of the Autonomic Nervous System

Communication across autonomic synapses depends on specific chemical messengers and receptors. All preganglionic fibers, both sympathetic and parasympathetic, release acetylcholine (ACh), which binds to nicotinic receptors on the postganglionic neuron.

At the target organ, parasympathetic postganglionic fibers release ACh acting on muscarinic receptors. Conversely, most sympathetic postganglionic fibers release norepinephrine, acting on adrenergic receptors (alpha and beta), though sweat glands receive sympathetic ACh stimulation.